United States Environmental Protection Agency

Office of Wastewater Management

Washington, DC 20460

EPA‐800‐R‐11‐006

November 2011

Exhaust Gas Scrubber

Washwater Effluent
Exhaust Gas Scrubber Washwater Effluent Contents 

i 

CONTENTS 

Page 

SECTION 1 INTRODUCTION ............................................................................................................... 1

SECTION 2 EXHAUST GAS SCRUBBER TECHNOLOGY ....................................................................... 4

2.1 Seawater Scrubbers ................................................................................................. 5

2.2 Freshwater Scrubbers .............................................................................................. 7

2.3 Hybrid and Dry Scrubbers ...................................................................................... 8

SECTION 3 EXHAUST GAS SCRUBBER WASHWATER TREATMENT .................................................... 9

SECTION 4 IMO GUIDELINES FOR EGCSS, WASHWATER MONITORING AND DISCHARGE 

CRITERIA ........................................................................................................................... 10

4.1 Washwater Monitoring ......................................................................................... 10

4.2 Continuous Discharge Limits ............................................................................... 10

4.2.1 pH .............................................................................................................. 10

4.2.2 PAHs ......................................................................................................... 11

4.2.3 Turbidity/Suspended Particulate Matter (Solids) ...................................... 11

4.3 Other Discharge Limits ......................................................................................... 12

4.3.1 Nitrates ...................................................................................................... 12

4.3.2 Washwater Additives and Other Substances ............................................ 12

4.3.3 Washwater Residue ................................................................................... 12

4.4 Periodic Monitoring .............................................................................................. 12

SECTION 5 CHARACTERIZATION OF POLLUTANTS IN EGCS WASHWATER DISCHARGES ............... 13

5.1 The MS Zaandam Seawater Scrubber Trials ........................................................ 13

5.2 The Pride of Kent Seawater Scrubber trials .......................................................... 18

5.3 The Suula Freshwater Scrubber Trial ................................................................... 23

5.4 Summary of Marine EGCS Scrubber Trials ......................................................... 25

SECTION 6 ASSESSMENT OF POLLUTANTS DISCHARGED IN SCRUBBER WASHWATER AND 

PROTECTIVENESS OF IMO GUIDELINES ............................................................................. 26

6.1 pH .......................................................................................................................... 28

6.2 PAH....................................................................................................................... 30

6.3 Turbidity ............................................................................................................... 32

6.4 Nitrate ................................................................................................................... 33

6.5 Sulfate, COD and Dissolved Oxygen ................................................................... 34

6.6 Metals .................................................................................................................... 35

6.7 Summary ............................................................................................................... 36

SECTION 7 ACRONYMS ................................................................................................................... 38

SECTION 8 REFERENCES ................................................................................................................. 40
Exhaust Gas Scrubber Washwater Effluent Contents

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LIST OF TABLES 

Page 

1 PAH Discharge Concentration Limit by Flow Rate ..........................................................11

2 Concentrations of Metals in the Washwater Discharge from the Zaandam 

Scrubber Trial ....................................................................................................................17

3 Concentrations of PAHs in the Seawater Intake and Washwater Discharge from 

the Pride of Kent Scrubber Trial ........................................................................................22

4 Approximate Amount of Water (t/hr) Affected by a Scrubber for a 12 MW 

Engine ................................................................................................................................30

LIST OF FIGURES 

Page 

1 SOx Scrubber Components ..................................................................................................4

2 The EcoSilencer® Scrubber Installed on the Pride of Kent .................................................5

3 Open Type Wet Scrubber .....................................................................................................6

4 Closed Type Wet Scrubber ..................................................................................................7

The EPA contacts for this document are Ryan Albert (202) 564-0763 and Shirley Fan (202) 564-

2425.
Exhaust Gas Scrubber Washwater Effluent Executive Summary 

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EXECUTIVE SUMMARY 

This document contains information about effluent washwater discharges associated with 

the operation of exhaust gas cleaning systems (EGCSs), also known as sulfur oxide (SOx) 

scrubbers, onboard vessels. The use of scrubbers to clean the exhaust from marine engines using 

high sulfur residual oil and diesel fuels is an option for reducing SOx air emissions required by 

Annex VI of the MARPOL International Convention for the Prevention of Pollution from Ships. 

Washwater generated by an EGCS can contain contaminants from three sources: 

1. Pollutants scavenged from the exhaust gas exiting the engine (combustion 

products, fuel and lubricants); 

2. The source of washwater used to clean the exhaust (seawater or freshwater); and 

3. The scrubber itself (dissolution of materials, possible reaction products and/or 

chemical additives). 

The deployment of SOx scrubbers to treat emissions from diesel engines on large ocean 

going vessels has been so far limited to a handful of ships. However, the use of SOx scrubbers in 

combination with high sulfur residual oil fuels may be an economically attractive option in 

Sulfur Oxide Emission Control Areas (SECAs) and worldwide. As the deadlines for SOx 

reduction1 draw nearer, some vessels may begin using scrubbers to reduce SOx emissions rather 

than burning low sulfur fuel oil. 

This document describes the basic technology of EGCS, treatment processes used to 

remove pollutants from washwater prior to discharge, guidelines established by the International 

Maritime Organization (IMO) for washwater monitoring and discharge criteria, available 

onboard sampling data characterizing the constituent concentrations in washwater discharge, and 

impacts of scrubber washwater discharge on water quality and aquatic life and protectiveness of 

IMO guidelines. 

 

1 The revised Annex VI entered into force on July 1, 2010. The sulfur content of fuel will fall in the Emission 

Control Areas (ECAs), which include the Baltic Sea, the North Sea and the English Channel, from 1.5% to 1% in 

July 2010 and to 0.1% in January 2015. A North American ECA, which includes waters adjacent to the Pacific, 

Atlantic and Gulf coasts and the 8 main Hawaiian Islands, will become enforceable in 2012. Globally, the highest 

permitted sulfur content of fuel will fall from 4.5% to 3.5% in January 2012 and to 0.5% in January 2020. 
Exhaust Gas Scrubber Washwater Effluent Section 1 - Introduction 

1 

SECTION 1 

INTRODUCTION 

The International Convention for the Prevention of Pollution from Ships (MARPOL), 

1973 as modified by the Protocol of 1978, also known as MARPOL 73/78, Annex VI Prevention 

of Air Pollution from Ships, requires the reduction of SOx emissions either by burning low sulfur 

fuel oil or cleaning the exhaust gas. Cleaning, or scrubbing, the exhaust gas is generally accepted 

to be more economical. According to EGCS manufacturer Wärtsilä, the cost saved by using 

heavy fuel oil instead of low sulfur fuel oil can pay back the cost of installing and operating 

EGCS in about one year (Hatley, 2010). The EGCSs used today for sulfur control are commonly 

known as SOx scrubbers. In a wet scrubber, the exhaust gas is mixed with washwater, and the 

water-soluble components of the exhaust gas are removed by dissolution into the washwater. 

SOx scrubbers are capable of removing up to 95 percent of SOx from ship exhaust (EGCSA, 

2010). By removing sulfur from the exhaust, the scrubber also removes most of the direct sulfate 

particulate matter (PM). Sulfates are a large portion of the PM from ships operating on high 

sulfur fuels. By reducing the SOx emissions, the scrubber also controls much of the secondary 

PM that is formed in the atmosphere from these emissions. 

There are two main types of wet scrubber technologies. The first wet scrubber 

technology, referred to as seawater scrubbing, is an open-loop design which uses seawater to 

scrub the exhaust before discharging the washwater back to the sea following treatment. In a 

seawater scrubber, the exhaust gases are brought into contact with seawater, either through 

spraying seawater into the exhaust stream or routing the exhaust gases through a water bath. The 

sulfur dioxide (SO2) in the exhaust gas dissolves in the washwater, where it is ionized to 

bisulphate and sulfite, which are then readily oxidized to sulfate (Hassellöv and Turner, 2007). 

The ionization of SO2 and the sulfuric acid formed from sulfur trioxide (SO3) also produces 

acidity. The sulfuric acid in the water then reacts with carbonates and other salts in the seawater 

to form sulfates (USEPA, 2009a). The washwater is treated to remove solids and raise the pH 

prior to discharge back to sea. The removed solids are stored onboard for shoreside disposal. 

The second wet scrubber technology is a closed-loop system. Freshwater is used as 

washwater, and caustic soda is injected into the washwater to neutralize the sulfur in the exhaust. 

A small portion of the washwater is bled off and treated to remove suspended solids. The 

suspended solids are held as sludge and disposed of shoreside as with the open loop design. The 

treated bleed-off washwater can be discharged at open sea or held onboard for disposal 

shoreside. Additional freshwater is added to the system as needed. While this design is not a 

completely closed loop, it can be operated in zero discharge mode for a period of time (USEPA, 

2009a). 
Exhaust Gas Scrubber Washwater Effluent Section 1 - Introduction 

2 

Wet scrubbers are a well-established technology for SOx and PM reduction that are 

widely used in stationary sources such as land‐based oil‐ and coal‐fired power plants. In these 

applications, lime and caustic soda are typically used to neutralize the sulfuric acid in the 

washwater. The first use of exhaust gas scrubbers in the marine industry was not for SOx 

emission control but rather as an inexpensive means to produce inert gas for reducing the fire 

hazard in the cargo tanks of tankers while unloading. By the 1970s, their use was widespread 

(Entec, 2005). 

The first prototype seawater scrubber system for exhaust gas control onboard a vessel 

was installed in 1991 (Entec, 2005). A comprehensive seawater scrubber field trial was 

conducted in May 1998 onboard the Canadian ice breaker Louis S. St.-Laurent. The trials were 

conducted during 22 days of a 6-week transatlantic voyage. Around the same time, a different 

scrubber prototype, the Eco-Silencer®, was fitted for testing onboard the Canadian freight and 

passenger ferry Leif Ericson. This was a full-scale plant and treated the entire exhaust stream 

from the engine. One of the specific aims of the Leif Ericson trials was to investigate washwater 

quality though a water treatment plant and, if necessary, apply alternate water treatment 

processes (Entec, 2005). More recent prototype SOx scrubber installations aboard three ships, 

the Zaandam, the Pride of Kent and the Suula, have demonstrated their viability in this 

application and provided data to characterize pollutant concentrations in scrubber washwater 

discharges. These demonstrations have shown that scrubbers can replace and fit into the space 

occupied by the exhaust silencer units and can work well in marine applications (USEPA, 

2009a). 

The deployment of SOx scrubbers to treat emissions from the diesel engines on large 

ocean going vessels is currently limited to a handful of ships. Although the installation of an 

EGCS allows a ship operator to continue to use high sulfur fuel after the 2015 and 2020 

deadlines for ECAs and worldwide, respectively, there is uncertainty associated with this 

compliance solution (Kehoe et al., 2010). The continued use of EGCS depends on the successful 

completion of monitoring specified in the IMO Guidelines (see Section 4) and long-term 

scientific studies to prove they do not negatively impact the environment. In the interim, a ship 

owner installing EGCS undertakes a risk that the EGCS may need to be removed in favor of 

using low-sulfur fuel if in the future it is found that scrubbers have unacceptable impacts.

The use of SOx scrubbers in combination with high sulfur residual oil and diesel fuels 

may be economically beneficial in SECAs and worldwide; therefore, a large number of vessels 

may begin using scrubbers. In June 2010, the Motorship Newsletter announced the first 

commercial order for seawater scrubbers capable of meeting new European Union (EU) 

regulations on fuel emissions from ships, without requiring low sulfur fuel oil (Motorship 

Newsletter, 2010). Seawater scrubbers were installed on four new 45,000-ton ferries burning 

residual fuel oil to meet rules demanding sulfur emissions equivalent to 0.1 percent fuel-sulfur 

content. 
Exhaust Gas Scrubber Washwater Effluent Section 1 - Introduction 

3 

This document describes the constituent concentrations in the washwater discharged by 

vessels operating wet scrubber EGCSs. This discharge can contain material from: 

 Pollutants scavenged from the exhaust gas exiting the engine (combustion 

products, fuel and lubricants); 

 The seawater used to clean the exhaust (in the case of a seawater scrubber); and 

 The scrubber itself (dissolution of system materials, possible reaction products 

and/or chemical additives). 

The use of scrubbers to clean the exhaust from marine engines using high sulfur residual 

oil and diesel fuels may lead to high concentrations of a number of harmful compounds in the 

water body around the ships (AEA, 2009). Several trials were conducted onboard vessels to 

characterize the constituent concentrations in washwater discharge. Section 5 summarizes the 

results of the trials and assessments of the potential impacts of scrubber washwater discharge on 

water quality and aquatic life. Section 6 presents a comparison of the results of the trials with the 

2009 Guidelines for Exhaust Gas Cleaning Systems (IMO Annex 9, Resolution MEPC.184(59), 

adopted July 17, 2009), criteria intended to protect aquatic resources from adverse impacts 

resulting from scrubber washwater discharges. 
Exhaust Gas Scrubber Washwater Effluent Section 2 - Exhaust Gas Scrubber Technology 

4 

SECTION 2 

EXHAUST GAS SCRUBBER TECHNOLOGY 

As described in Section 1, there are several different designs of SOx scrubbers that 

remove sulfur oxides from the exhaust gases of vessel engines and boilers. The majority of wet 

scrubber systems have three basic components, illustrated in Figure 1 and described below. 

Source: EGCSA. 2010 

Figure 1. SOx Scrubber Components 

1. Exhaust gas cleaning unit serves as a contact chamber that enables the exhaust 

stream from an engine or boiler to be intimately mixed with water, either 

seawater, freshwater, or both. Due to space and access limitations, the exhaust gas 

cleaning units tend to be high up in the ship, in or around the funnel area. 

2. Wash water treatment to remove water-soluble pollutants, such as sulfur dioxide 

(SO2), sulfur trioxide (SO3) and nitrogen dioxide (NO2), which form sulfates and 

nitrates that dissolve in the washwater after the scrubbing process. Scrubber 

washwater also contains suspended solids, heavy metals, hydrocarbons and 

polycyclic aromatic hydrocarbons (PAHs). Before the washwater is discharged, it 

must be treated to remove solids. The treatment process typically includes a 

multicyclone, a cyclonic separator similar to that used to separate water from 
Exhaust Gas Scrubber Washwater Effluent Section 2 - Exhaust Gas Scrubber Technology 

5 

residual fuel prior to delivery to the engine. Heavier particles may also be trapped 

in a settling or sludge tank for disposal. 

3. Sludge handling to retain sludge removed by the washwater treatment process for 

disposal shoreside. 

Figure 2 shows the EcoSilencer® scrubber incorporated into the exhaust train on the ferry 

Pride of Kent. 

Source: Entec, 2005 

Figure 2. The EcoSilencer® Scrubber Installed on the Pride of Kent

2.1 SEAWATER SCRUBBERS

In a seawater or “open” type scrubber (see Figure 3), seawater is used as wash water for 

scrubbing, and the resulting wastewater is treated and discharged back to sea. The natural 

alkalinity of the seawater is used to neutralize the acidity that results from SOx removal.2

 

 

2 Alkalinity in the open ocean is typically 2,200 to 2,300 μmol CaCO3/L (Hassellöv and Turner, 2007). Alkalinity in 

some coastal areas, ports, rivers and estuaries can be affected by the different drainage areas of the inflowing rivers, 

resulting in large variations in the chemistry. Rivers running through soil rich in carbonates will be high in 

alkalinity. For example, the southern rivers of the Baltic Sea run through calcite bedrock resulting in high carbonate 

concentrations with consequently high alkalinity (approximately 1,650 -1,950 μmol/L), whereas the northern rivers 

run through granite bedrock resulting in low alkalinity (approximately 800-1,300 μmol/L) (Hassellöv and Turner, 

2007). In general, the alkalinity in the Baltic Sea is lower than the open sea because of the minimal exchange of 

water through the Danish straits. Seawater scrubbers can operate at low alkalinity levels, but in some cases the SO2

removal efficiency may be reduced (Henriksson, 2007).
Exhaust Gas Scrubber Washwater Effluent Section 2 - Exhaust Gas Scrubber Technology 

6 

Source: EGCSA. 2010 

Figure 3. Open Type Wet Scrubber 

Typically, seawater systems use 45 m3 of washwater per megawatt-hour3 (MWh) for 

scrubbing (MEPC, 2008). Seawater scrubbing of the exhaust from a representative 10 MWh 

engine would generate 450 m3/hr or 2.8 million gallons per day (MGD) of washwater effluent. 

The volume of washwater required for a given scrubbing efficiency increases with elevated 

alkalinity and with higher water temperature (DNV, 2009). The energy consumption of a 

seawater scrubber is 2 to 3 percent of the engine power output (Filancia, 2009). 

Seawater scrubbing requires the exhaust gases to be mixed with seawater to dissolve the 

sulfur oxides. Manufacturers use various techniques to achieve mixing without unduly 

obstructing the passage of exhaust gas, which could result in a “back pressure” outside of the 

engine builder’s limits and adversely affect engine operation. The sulfur oxides of engine 

exhausts typically consist of approximately 95 percent SO2 and 5 percent SO3 (EGCSA, 2010). 

When dissolved, a reaction occurs whereby the sulfur dioxide is ionized to bisulfite and sulfite, 

which is then readily oxidized to sulfate in seawater containing oxygen (Hassellöv and Turner, 

2007). 

The ionization to bisulfite and sulfite and the sulfuric acid formed from sulfur trioxide 

produces acidity (Karle and Turner, 2007). The acidity is neutralized initially by the alkalinity of 

the seawater, due to its natural bicarbonate content. After the initial buffering capacity is 

consumed, the pH of washwater is reduced to approximately 3. At low pH, the ionization of 

sulfur to sulfite is negligible and exhaust gas cleaning is limited (EGCSA, 2010). 

 

3

 Maximum continuous rating (MCR) or 80% of the power rating of the fuel oil combustion unit. 
Exhaust Gas Scrubber Washwater Effluent Section 2 - Exhaust Gas Scrubber Technology 

7 

2.2 FRESHWATER SCRUBBERS

In a freshwater or “closed” type scrubber, freshwater treated with an alkaline chemical 

such as caustic soda is used for neutralization and scrubbing. As shown in Figure 4, washwater is 

recirculated and any evaporative loss is replaced with additional freshwater. A small quantity of 

the washwater is bled off to a treatment plant and then discharged. Typically, closed freshwater 

systems have much smaller discharge rates than seawater scrubbers (approximately 0.1 to 0.3 

m3

/MWh) and treat a smaller volume of effluent (MEPC, 2008). Closed freshwater systems can 

periodically be operated in a “zero discharge mode” without discharging any washwater 

overboard (SSG, 2007). 

Source: EGCSA. 2010 

Figure 4. Closed Type Wet Scrubber 

Freshwater scrubbers are used when high efficiency cleaning is needed or when the 

variable alkalinity associated with seawater precludes use of seawater scrubbers. The ability to 

operate an EGCS without discharging washwater for short periods of time, using available 

holding tank capacity, may be advantageous in water bodies that are especially sensitive or 

vulnerable, such as enclosed water bodies and/or those with low alkalinity. The SOx exhaust gas 

removal efficiency of a freshwater scrubber is typically greater than 90 percent and removal 

efficiencies as high as 97 percent can be obtained for exhaust from generator engines. The 

energy consumption of freshwater scrubbers is reported to be about 0.5 percent of the engine 

power output (Hatley, 2010). 
Exhaust Gas Scrubber Washwater Effluent Section 2 - Exhaust Gas Scrubber Technology 

8 

2.3 HYBRID AND DRY SCRUBBERS

Hybrid scrubbers can operate using either seawater or freshwater as source water, while 

dry scrubbers operate using chemisorption. Hybrid systems can operate in either an open loop 

(seawater) at sea or closed loop (freshwater) in harbors, estuaries, and other sensitive water 

bodies (Aalborg, 2010). The advantages of a hybrid EGCS include the ability to operate in either 

freshwater or seawater and the ability to maintain scrubber efficiency in low alkalinity water 

bodies, such as the Baltic Sea, estuaries and rivers. A hybrid scrubber was installed on the 21 

MW diesel engine aboard the ferry Tor Ficaria. The Tor Ficaria’s scrubber has operated over 

1,000 hours since May 2010 and is reported to be the largest EGCS installed onboard a ship 

(Aalborg, 2010). 

Dry scrubbers use solid media rather than washwater to capture sulfur oxides from the 

exhaust gas. Exhaust gas in a dry scrubber is passed through a bed of granular solid media, such 

as calcium carbonate (CaCO3), burnt lime (CaO), or hydrated lime (Ca(OH)2), to which the 

sulfur oxides absorb and react to form gypsum (CaSO4) (Couple Systems, 2010). The advantage 

of dry scrubbing technology is that pollutants are not transferred from air to water, as they are in 

wet scrubbers, but instead react with the solid media to form a byproduct. The byproduct can be 

reused for high-temperature desulfurization at power plants, as a raw material for cement and 

steel making, or as fertilizer (Couple Systems, 2010). There are over 500 dry scrubber 

installations for exhaust gas capture at power plants. A dry scrubber was installed to clean the 

exhaust gas from the 3.6 MW main engine aboard the German cellulose freighter MS Timbus in 

2009. 
Exhaust Gas Scrubber Washwater Effluent Section 3 - Exhaust Gas Scrubber Washwater Treatment 

9 

SECTION 3 

EXHAUST GAS SCRUBBER WASHWATER TREATMENT 

Operating or cleaning a wet exhaust gas scrubber for marine diesel engines and boilers 

generates effluent (washwater) that must be treated prior to discharge. Washwater contains 

gaseous and particulate emissions removed from the exhaust in the funnel by the scrubber. The 

SOx and nitrogen oxides (NOx) dissolved in the scrubber washwater react to form sulfuric and 

nitric acids, which significantly reduce the pH of the washwater leaving the scrubber. Washwater 

constituents can also include PM emissions captured by the scrubbers, as well as traces of oil, 

PAHs, heavy metals and nitrate. 

Washwater effluent from seawater systems is treated, blended with reaction water to 

neutralize the pH, and discharged overboard. For freshwater scrubbers, only a fraction (e.g., 10 

percent) of the washwater is bled off for treatment and discharged. Alternatively, freshwater 

scrubber washwater can be routed to a bilge water tank not destined for discharge or other 

suitable holding tank for treatment and discharge, or for disposal shoreside. 

Treatment of scrubber washwater normally consists of suspended solids removal and pH 

neutralization. In current marine EGCS installations, such as the scrubbers aboard the Pride of 

Kent and Zaandam, the washwater leaving the scrubbers is passed through a multicyclone, a 

prefiltration device that predominantly separates particles larger than 30-40 μm from the 

washwater using centrifugal force (WATERCO, 2010). The solids from the multicyclone are 

collected and concentrated in a sludge tank. The amount of sludge generated by washwater 

treatment depends on the amount of solids captured and the water content of the sludge 

following settling. Solids generation for a seawater scrubber system are approximately 0.6 

percent by weight of residual fuel consumed (EGCSA, 2010). 

The effectiveness of centrifugal separation and settling depends on the particle size 

distribution and particle density (USEPA, 2009a). Additional treatment processes can be added 

to improve the efficiency of solids removal, including various filtration methods and/or 

coagulation and flocculation. Coagulation and flocculation remove particles too small for 

gravitational settling by aggregating them into large, more readily separable particles. Such 

additional treatment has been used to treat the washwater bled from closed freshwater scrubbers, 

such as the EGCS installed on the Suula. 

After the washwater is treated to remove solids, it is blended with reaction water, 

typically seawater used for engine cooling (EGCSA, 2010). The function of the reaction water is 

to neutralize the pH of the washwater prior to discharge overboard. Reaction water and 

washwater can be mixed in nearly equal proportions, although increasing the proportion of 

reaction water can improve the neutralization of the acidic washwater (HA & H-K, 2010). 
Exhaust Gas Scrubber Washwater Effluent Section 4 – IMO Guidelines for EGCSs, Washwater 

Monitoring and Discharge Criteria 

10 

SECTION 4 

IMO GUIDELINES FOR EGCSS, WASHWATER MONITORING 

AND DISCHARGE CRITERIA 

On October 9th, 2008, the 168 member states of the International Maritime Organization 

(IMO) adopted more stringent standards to control exhaust emissions from the engines that 

power ocean going vessels. The engine and fuel standards are included in amendments to Annex 

VI of MARPOL. The revised Annex VI entered into force on July 1, 2010. The sulfur content of 

fuel decreased in the ECAs, including the Baltic Sea, the North Sea and the English Channel, 

from 1.5 percent to 1 percent in July of 2010 and will decrease to 0.1 percent in January of 2015. 

A North American ECA (including waters adjacent to the Pacific, Atlantic and Gulf coasts and 

the 8 main Hawaiian Islands) will become enforceable in 2012 (USEPA, 2010b). Globally, the 

highest permitted sulfur content of fuel will fall from 4.5 to 3.5 percent in January of 2012 and to 

0.5 percent in January of 2020. The IMO developed guidance criteria for the use of exhaust gas 

cleaning devices, such as SOx scrubbers, as an alternative to operating on low sulfur fuel. As a 

component of their analyses, the IMO also set scrubber washwater monitoring and discharge 

criteria in Section 10 of the Guidelines for EGCSs (Resolution MEPC.170(57)). The current 

Guidelines for EGCSs were adopted on July 17th, 2009 though Resolution MEPC.184(59), and 

are summarized in Sections 4.1through 4.3 below. 

4.1 WASHWATER MONITORING

The IMO guideline recommends pH, PAH concentration, turbidity and temperature 

should be continuously monitored and recorded when the EGCS is operated in ports, harbors, or 

estuaries. In other areas, these parameters should be continuously monitored and recorded 

whenever the EGCS is in operation, except for short periods of maintenance and cleaning of the 

equipment. 

4.2 CONTINUOUS DISCHARGE LIMITS

4.2.1 pH

The washwater pH must meet one of the following requirements and the results recorded 

in the vessel’s EGCS Technical Manual (ETM) as applicable (Resolution MEPC.184(59)): 

 The discharge washwater should have a pH of no less than 6.5 measured at the 

ship’s overboard discharge with the exception that during maneuvering and 

transit, a maximum difference of 2 pH is allowed between the ship’s inlet and 

overboard discharge; or 

 During commissioning of the unit(s) after installation, the discharged washwater 

plume should be measured externally from the ship (at rest in a harbor) and the 
Exhaust Gas Scrubber Washwater Effluent Section 4 – IMO Guidelines for EGCSs, Washwater 

Monitoring and Discharge Criteria 

11 

discharge pH at the ship’s overboard pH monitoring point will be recorded when 

the plume at a distance of 4 meters from the discharge point equals or exceeds a 

pH of 6.5. This discharge pH, which is found to achieve a minimum pH of 6.5 in 

the washwater plume 4 meters from the ship, will become the overboard pH 

discharge limit. 

4.2.2 PAHs

The maximum continuous PAH concentration in the discharged washwater should not be 

greater than 50 μg/L phenanthrene equivalence (PAHphe) above the inlet water PAH 

concentration. For the purposes of this criteria, the PAH concentration in the washwater should 

be measured downstream of the water treatment equipment but upstream of any washwater 

dilution or other reactant dosing unit, if used, prior to discharge. The 50 μg/L limit is normalized 

for a washwater flow rate through the EGCS unit of 45 tons/MWh. This limit should be adjusted 

for washwater flow rates, as shown in Table 1. 

Table 1. PAH Discharge Concentration Limit by Flow Rate 

Flow Rate 

(t/MWh) 

Discharge Concentration Limit 

(μg/L PAHphe equivalents) 

Measurement 

Technology 

0 - 1 2250 Ultraviolet Light 

2.5 900 Ultraviolet Light 

5 450 Fluorescencea

11.25 200 Fluorescencea

22.5 100 Fluorescencea

45 50 Fluorescencea

90 25 Fluorescencea

Source: Resolution MEPC.184(59)

a. Fluorescence technology should be used for any flow rate > 2.5 t/MWh. 

For a 15-minute period in any 12-hour period, the continuous PAHphe concentration 

limit may exceed the limit described above by up to 100 percent, to allow for an abnormal start 

up of the EGCS (Resolution MEPC.184(59)). 

4.2.3 Turbidity/Suspended Particulate Matter (Solids)

The washwater treatment system should be designed to minimize suspended PM, 

including heavy metals and ash. The maximum continuous turbidity in washwater should not be 

greater than 25 formazin nephlometric units (FNU) or 25 nephlometric turbidity units (NTU) or 

equivalent units, above the inlet water turbidity. However, during periods of high inlet turbidity, 

the precision of the measurement device and the time lapse between inlet measurement and 
Exhaust Gas Scrubber Washwater Effluent Section 4 – IMO Guidelines for EGCSs, Washwater 

Monitoring and Discharge Criteria 

12 

discharge measurement are such that the use of a difference limit is unreliable. Therefore, all 

turbidity difference readings should be a rolling average over a 15-minute period to a maximum 

of 25 FNU. For the purposes of this criteria, the turbidity in the washwater should be measured 

downstream of the water treatment equipment but upstream of washwater dilution (or other 

reactant dosing) prior to discharge. For a 15-minute period in any 12-hour period, the continuous 

turbidity discharge limit may be exceeded by 20 percent (Resolution MEPC.184(59)). 

4.3 OTHER DISCHARGE LIMITS

4.3.1 Nitrates

The washwater treatment system should prevent the discharge of nitrates beyond that 

associated with a 12 percent removal of NOx from the exhaust, or beyond 60 mg/L (1 mM) 

normalized for a washwater discharge rate of 45 t/MWh (similar to the adjustment tabulated 

above for PAHs), whichever is greater. All systems should be tested for nitrates in the discharge 

water. 

4.3.2 Washwater Additives and Other Substances

An assessment of the washwater is required for those EGCS technologies which make 

use of chemicals, additives, preparations or create relevant chemicals in situ. 

4.3.3 Washwater Residue

Residues (sludge) generated by the EGCS should be delivered shoreside to adequate 

reception facilities. Such residues should not be discharged to the sea nor incinerated onboard. 

4.4 PERIODIC MONITORING

The IMO washwater discharge criteria are intended to act as initial guidance for 

implementing EGCS designs. The IMO Guidelines state that the criteria should be revised in the 

future as more data become available on the contents of the discharge and its effects, taking into 

account any advice given by the Group of Experts on the Scientific Aspects of Marine 

Environmental Protection (GESAMP). Administrations (i.e., signature nations) should therefore 

provide for collection of relevant data. To this end, IMO requests ship owners, in conjunction 

with the EGCS manufacturer, to sample inlet water (for background), water after the scrubber 

(but before any treatment system) and discharge water and to analyze these samples using EPA 

or ISO test procedures for the following parameters (Resolution MEPC.184(59)): 

 pH; 

 PAH and oil (detailed GC-MS analysis); 

 Nitrate and nitrite; and 

 Metals (Cd, Cu, Ni, Pb, Zn, As, Cr and V). 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

13 

SECTION 5 

CHARACTERIZATION OF POLLUTANTS IN EGCS 

WASHWATER DISCHARGES 

Prototype SOx scrubbers have been installed onboard three large ocean-going vessels: the 

Zaandam, the Pride of Kent, and the Suula (USEPA, 2009a). The scrubber trials aboard these 

vessels also provided data to characterize constituent concentrations in washwater discharges. 

These data represent the best available information regarding pollutant concentrations that can be 

expected in scrubber washwater discharges, including: 

 pH, 

 Temperature, 

 Turbidity, 

 Nutrients, 

 Metals, and 

 PAHs. 

The results from the scrubber trials (summarized below) are published as project reports, 

which have not been peer reviewed. Samples were analyzed by certified laboratories or by 

academic institutions. 

5.1 THE MS ZAANDAM SEAWATER SCRUBBER TRIALS

In November 2006, the Puget Sound Clean Air Agency received funding from the EPA to 

evaluate whether a seawater scrubbing system could be successfully designed, 

installed/retrofitted and operated within the tight confines of an existing cruise ship. In April 

2007, Holland America Line installed a seawater scrubber in the stack of one of the five 9 MW 

diesel generators on the cruise ship MS Zaandam when she was in drydock in Victoria, Canada 

(HA & H-K, 2010). 

The study was designed to answer a number of basic questions about seawater scrubbing, 

including: 

 How much washwater is generated by this scrubber? 

 What pollutants are present in the washwater discharge and what are their 

concentrations or values? 

 How are these pollutants dissipated into the environment when the ship is at dock? 

 How much sludge is generated by this scrubber (in grams per ton of fuel burned)? 

 What are the pollutants that end up in the sludge and what are their concentrations? 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

14 

Both continuous and periodic sampling of washwater was conducted onboard the 

Zaandam. In‐line monitoring occurred nearly continuously between August 2007 and January 

2008, and during scrubber operations between February and September 2008 and in February 

2009. During these periods, the Zaandam operated in the Pacific Ocean, including coastal waters 

of Alaska, British Columbia, California, and Hawaii. Periodic samples were also collected as 

follows: 

 August 18, 2007 ‐ Skagway, Alaska 

 August 24, 2007 ‐ Juneau, Alaska 

 September 12, 2007 ‐ Vancouver, British Columbia 

 August 30, 2008 ‐ Vancouver, British Columbia 

 September 3, 2008 ‐ Juneau, Alaska 

 September 4, 2008 ‐ Skagway, Alaska. 

Washwater sample collection followed the procedures from the Environmental Impact 

Assessment work performed on the ferry the Pride of Kent by Newcastle University and the 

Terramare Institute. Samples of raw seawater and scrubber washwater were collected at the inlet 

and discharge of the system, respectively, to assess the changes that occurred as a result of 

scrubbing. Samples were analyzed by certified laboratories in the jurisdictions where they were 

collected. 

The average washwater flow rate during the scrubber trials aboard the Zaandam was 390 

m3/hr (2.5 MGD). The following is a summary of the pollutant concentrations and other 

parameters measured in the raw seawater intake and the washwater samples, based upon the 

presentation of data in Appendix F of the final project report to EPA (HA & H-K, 2010). 

 pH

The washwater discharge pH was approximately 2 standard units lower than the intake. 

The mean pH measured in the washwater discharged in coastal waters of California and Hawaii 

in 2008 was 5.8; the minimum measured value was 5.4. Over this period, it was noted that 

problems with pumps reduced the flow rates in the scrubber system. Higher discharge pH values 

were measured in Alaskan coastal waters in 2008; the mean discharge pH was 6.3. The increase 

in pH was achieved by raising the volume of reaction water being blended with washwater and 

lower engine loads (HA & H-K, 2010). 

Temperature

The increase in washwater temperature between the inlet and the discharge was less than 

3.5°C throughout the duration of the project (HA & H-K, 2010). 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

15 

Turbidity

In‐line continuous measurement of turbidity was found to be unreliable as air bubbles in 

the washwater interfered with the function of the turbidity probe. Results from periodic (discrete) 

sampling in 2007 indicated that turbidity and total suspended solids (TSS) decreased in 

concentration between the intake and the discharge. Maximum turbidity in the discharge was 8 

NTUs, and the maximum change between intake and discharge was ‐9 percent (HA & H-K, 

2010). 

Dissolved Oxygen (DO)

DO concentration in the discharge ranged from 4.0 mg/L in the warmer waters off 

California and Hawaii to 10.0 mg/L in the colder waters of Alaska. Although these 

concentrations represent varying degrees of saturation, they also suggest that re‐oxygenation of 

washwater prior to overboard discharge was generally successful (HA & H-K, 2010). 

Alkalinity

As expected, alkalinity decreased between the seawater inlet and the washwater discharge 

as carbonates and bicarbonates were consumed during scrubbing. Alkalinity was reduced 

between 76 to 98 percent with a minimum alkalinity measured in the discharge of 21 mg 

CaCO3/L (HA & H-K, 2010). 

Chemical Oxygen Demand (COD)

COD generally increased between inlet and discharge as a result of the presence of 

sulfites in the discharge. COD increased between the discharge and the intake between 33 and 91 

percent with a maximum concentration in the discharge of 130 mg/L (HA & H-K, 2010). 

Conductivity

There was no clear trend of increasing or decreasing conductivity between the inlet and 

discharge. The scrubber did not add new salts into the marine environment (HA & H-K, 2010). 

Total Suspended Solids

TSS decreased between inlet and discharge samples. In 2007, the maximum TSS in the 

discharge was 17 mg/L, and the maximum change was ‐73 percent. Results of periodic sampling 

in 2008 indicated no consistent trend for TSS between inlet and discharge samples (HA & H-K, 

2010). 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

16 

Ammonia, Nitrate/Nitrite, Kjeldhal Nitrogen

According to the project report (HA & H-K, 2010), the scrubber installed aboard the 

Zaandam did not remove NOx from engine emissions. For the most part, nitrogen compounds 

(ammonia, nitrite/nitrate and total Kjeldahl nitrogen (TKN)) were not detected in the washwater 

of this EGCS. Nitrite/nitrate was measured in the washwater discharge in Skagway at a 

concentration of 146 μg/L, while a TKN concentration of 45 μg/L was measured in the discharge 

sampled in Vancouver (HA & H-K, 2010). 

Sulfites

Sulfites (compounds containing sulfide ion, SO2

-3) were analyzed in Alaska in 2008 using 

a Hach field test kit. Sulfites were not detected in the intake, but were measured in the discharge 

at concentrations ranging from 32 to 48.6 mg/L. These are the compounds likely contributing to 

the COD measured in the discharge (HA & H-K, 2010). 

PAHs

The in-line discharge probe detected PAHs at approximately 100 parts per billion (ppb, 

equivalent to μg/L) in California and Hawaii, and 134 ppb in Alaska. The study authors believe 

that the in-line measurements were not reliable as the air bubbles in the washwater interfered 

with the probe’s ability to detect the PAHs. PAHs were not detected in any of the washwater 

discharge samples collected in Alaska in 2007 and analyzed by the laboratory. PAHs were 

detected in one discharge sample in Vancouver, at a concentration of 1.3 ppb phenanthrene 

equivalents (HA & H-K, 2010). 

Organic Compounds

Volatile organics were not detected in washwater discharge samples collected in 2008 

(HA & H-K, 2010). 

BTEX, HEM and TPH-D/G

BTEX (benzene, toluene, ethylbenzene and xylene), hexane extractable material (HEM), 

and TPH-D/G (total petroleum hydrocarbon –diesel and gasoline components) were not detected 

in the washwater discharge (HA & H-K, 2010). 

Metals

 Dissolved (filtered) and total metals concentrations were measured in 7 paired 

influent/effluent samples4

. For several metals (arsenic, copper, lead, nickel and selenium), total 

 

4

 Data from samples collected in 2008 during tests using a flocculant chemical to treat washwater were excluded 

from this analysis. 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

17 

and/or dissolved concentrations were measured in the washwater discharge at concentrations 

exceeding EPA’s National Recommended Water Quality Criteria for the protection of aquatic 

life (NRWQC) for saltwater organisms (USEPA, 2005). In general, when exceedances were 

detected in the discharge, they were also detected in the intake at similar concentrations. This 

indicates that the ambient seawater was a likely source of the metals. Discharge concentrations 

exceeding the intake concentration by more than 50 percent were interpreted as a stronger 

indication that a metal was being added to the washwater by the scrubbing process. Details of the 

exceedances of water quality criteria for metals are described below and presented in Table 2 

(HA & H-K, 2010): 

Table 2. Concentrations of Metals in the Washwater Discharge from 

the Zaandam Scrubber Trial 

Metal 

Median Total 

Discharge 

Concentration 

(μg/L) 

(n=7) 

Median 

Filtered 

Discharge 

Concentration

(μg/L) (n=7) 

# Samples 

Exceeding 

150% of 

Intake 

Concentration

EPA 

NRWQC 

Saltwater 

Acute 

(μg/L) 

# Samples 

Exceeding 

Acute 

Criterion 

EPA 

NRWQC 

Saltwater 

Chronic 

(μg/L) 

# Samples 

Exceeding 

Chronic 

Criterion 

Arsenic 81 81 1 69 6 36 7 

Chromium 22 12 0 570 0 74 0 

Copper 18 15 1 4.8 7 3.1 7 

Lead 0.4 0.4 0 210 0 8.1 2 

Nickel 20 12 5 74 2 8.2 7 

Selenium 100 94 2 290 0 71 6 

Source: HA & H-K. 2010 

n – Number of samples 

 Arsenic concentrations in all samples5 (intake and discharge) exceeded acute and 

chronic criteria. Discharge concentrations exceeded intake concentrations in half of 

the samples; in one sample the total discharge concentration exceeded the intake 

concentration by more than 50 percent. 

 All intake and discharge samples exceeded acute and chronic criteria for copper. In 

the majority of samples, influent copper concentrations exceeded discharge 

concentrations. In one sample the total effluent copper concentration exceeded the 

influent concentration by more than 50 percent. 

 

5

 In EPA’s 2009 Study of Discharges Incidental to Normal Operation of Commercial Fishing Vessels and Other 

Non-Recreational Vessels Less Than 79 Feet (USEPA, 2010), the high concentrations of chloride and other cations 

in seawater were found to interfere with the analytical determination of arsenic, as well as selenium. It is possible 

that a similar problem occurred in this study, and would explain the unexpectedly high concentrations of these 

metals in these samples of seawater and washwater. 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

18 

 Effluent lead concentrations sampled in Juneau and Skagway in 2007 exceeded acute 

and chronic criteria, although the paired influent samples had similar or higher lead 

concentrations. Lead criteria were not exceeded in samples collected at Juneau and 

Skagway in 2008. 

 Total and dissolved nickel effluent concentrations exceeded the chronic criterion in 

all samples. In two discharge samples, total nickel concentrations also exceeded the 

acute criterion. In 5 of 7 paired data sets, the total discharge concentration exceeded 

the intake concentration by more than 50 percent. 

 Six total and 4 dissolved effluent samples exceeded the chronic criterion for 

selenium. In 2 of the sample pairs, the total discharge concentration exceeded the 

intake concentration by more than 50 percent. 

 Washwater samples were also analyzed for antimony, beryllium, cadmium, 

chromium, mercury, thallium and zinc; no exceedances of water quality criteria were 

noted for these metals. 

During operation of the seawater scrubber on the Zaandam in ports in Hawaii and 

Alaska, sooty deposits were observed rising to the surface in the washwater discharge plume, and 

a slight sheen was witnessed. These visible pollutants appeared to be caused from combustionderived carbon deposits. These carbon deposits, which were nearly neutral in buoyancy, passed 

with the washwater through the multicyclone, attached to entrained air bubbles, and rose to the 

sea surface at the point of discharge. The washwater discharge contained very little petroleum 

hydrocarbon, but it appeared to float to the surface resulting in the slight sheen observed. The 

addition of an oil-water separator to the washwater treatment process ahead of the multicyclone 

in later trials appeared to prevent the reoccurrence of this visible soot/sheen (HA & H-K, 2010). 

5.2 THE PRIDE OF KENT SEAWATER SCRUBBER TRIALS

Trials of seawater scrubbers were conducted aboard the P&O Lines Pride of Kent, a ferry 

operating between the harbors of Dover and Calais. Seawater scrubbers were installed on four 

1.2 MW auxiliary engines onboard the Pride of Kent. The trials lasted over 16 months, during 

which the auxiliary engines ran almost continuously (Hufnagl et al., 2005). The ability of 

seawater scrubbers to reduce atmospheric emissions was examined, and the impacts of scrubber 

washwater discharges on the water quality in the harbors were studied. When all engines 

operated at full power, the washwater flow rate was 216 m3

/hr (1.4 MGD). Sampling was 

conducted during five events held in February6, March, July, September and November of 2004, 

when the seawater scrubbers for the auxiliary engines were partially in use (Hufnagl et al., 2005). 

 

6

 The scrubbers were not operating during sampling in the ports of Dover and Calais in February, so data for those 

samples have been excluded from consideration in this document. 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

19 

In each sampling event, discrete grab samples were collected at various locations in each harbor, 

including a transect 5 to 700 meters from the washwater discharge, at the seawater intake, and in 

the washwater discharge. Additional samples were collected at 6 locations throughout the 

seawater/washwater treatment system during the Pride of Kent’s crossings of the English 

Channel. The ambient and washwater samples were analyzed for a suite of constituents and other 

parameters similar to those measured on the Zaandam. Results from the final project report 

(Hufnagl et al., 2005) are summarized in Sections 0 through 0 (Hufnagl et al., 2005). 

Temperature

Washwater temperatures in the discharge were significantly higher than those measured 

at the seawater inlet. The temperature increases ranged from 1.9 to 4.1°C in March and July, and 

from 11.9 to 16.9°C in September and November. Discharge temperatures were greater than 

37°C (99°F) in both ports and the channel during the September sampling event. In the July and 

September sampling events, a 1 °C temperature increase was measured in the ambient harbor 

water closest to the ship. Although the report authors expected no adverse effects to result from 

the discharge of heated washwater, they also noted that the buoyancy of the discharge plume 

resulting from the difference in temperature (and hence density) with the ambient water would 

have an effect on mixing in the upper water column. Note that the IMO does not provide 

guidelines for washwater discharge temperature (Hufnagl et al., 2005). 

pH

The SOx and NOx dissolved in the scrubber washwater form sulfuric and nitric acids, 

which significantly reduce the pH of the washwater leaving the scrubber. The washwater 

samples taken downstream from the scrubbers, prior to blending with reaction water, had low pH 

values, ranging from 2.67 to 3.79. The buffering capacity of seawater used as reaction water was 

partially effective in neutralizing the washwater acidity. The average decrease in washwater pH 

between inlet and discharge samples was 1.39. Discharge pH values in all of the September 

samples were less than 6.5, as were two-thirds of the November samples. The lowest pH 

measured in the overboard discharge was 6.15. These values are below the lower limit of 6.5 in 

the IMO Guidelines for washwater discharge, which is also the lower limit of the EPA NRWQC 

pH criteria for marine aquatic life. However, ambient water sampled along the discharge 

transects of the harbor (including directly in front of the washwater discharge at 1 to 5 m 

distance) indicated no observable decrease in pH (Hufnagl et al., 2005). 

Sulfate

In addition to washwater acidification, the dissolved SOx increased the sulfate 

concentration in the effluent. Sulfate concentrations measured in washwater downstream from 

the scrubber were 14 to 19 percent higher than those measured at the seawater intake. In the 

discharge samples, sulfate concentrations ranged between 2,600 and 3,052 mg/L, which accounts 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

20 

for an increase of 0.4 to 6 percent. This may not be a significant increase, since the analytical 

error associated with these measurements was reported to be approximately 6 percent (Hufnagl 

et al., 2005). 

Nitrate

Nitrate, an important aquatic nutrient, is formed in the scrubbing process when NOx is 

dissolved in the washwater. In the washwater samples taken downstream from the scrubber, the 

nitrate concentrations were two to thirteen times higher than in the seawater inlet samples. The 

washwater treatment processes (multicyclone and filtration) did not reduce these concentrations 

because the treatment is designed to remove suspended solids and particulate pollutants, not 

dissolved pollutants such as nitrate. Nitrate concentrations increased between the influent and 

effluent discharge in 10 of 12 washwater samples. The average discharge nitrate concentration 

was 638 μg/L (or 144 μg NO3-N/L), 21 percent greater than the average influent concentration 

(Hufnagl et al., 2005). 

Ammonia concentrations in washwater increased between the inlet and discharge in 4 of 

10 samples; the average discharge concentration was 60 μg NH3-N/L, 5 percent higher than the 

average inlet concentration. Dissolved inorganic nitrogen (combined nitrite, nitrate, and 

ammonia) concentrations increased between the inlet and discharge in 8 of 10 samples; the 

average discharge concentration was 215 μg N/L, 21 percent higher than the average inlet 

concentration (Hufnagl et al., 2005). 

Metals

The washwater samples collected onboard the Pride of Kent were analyzed for total 

concentrations of barium, lithium, strontium, calcium, potassium and magnesium, vanadium, 

chromium, manganese, iron, cobalt, nickel, lead, copper, zinc and molybdenum. Copper, lead, 

nickel and zinc exceeded EPA’s NRWQC for saltwater aquatic organisms in the washwater 

discharge. When exceedances were detected in the discharge, they were often also detected in the 

intake in similar or even higher concentrations, indicating that the ambient seawater was a likely 

source of the metals. In addition, the sampling conducted at several points within the washwater 

system onboard the Pride of Kent provided further identification of likely sources of the metals 

that were detected (Hufnagl et al., 2005). 

Sampling conducted throughout the seawater/washwater treatment system revealed that 

the seawater scrubbing process itself did not influence the concentrations of metals in washwater. 

No metals, except for iron, were detected directly downstream of the scrubbers. Metals present in 

the washwater in particulate form, including iron and vanadium, were removed by the treatment 

system and retained in the sludge settling tank (Hufnagl et al., 2005). 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

21 

The reduced pH of the washwater leaving the scrubber affected the solubility of metal 

ions. The highest metals concentrations downstream of the scrubber were measured for iron, 

which may be contributed by contact with the ship’s steel components. Elevated copper, nickel 

and zinc concentrations were also detected. The copper may originate from ship coatings that 

contain copper as antifouling biocide while nickel, like iron, is a compound of ship steel. 

Details of the exceedances of water quality criteria for metals are provided below 

(Hufnagl et al., 2005): 

 Copper was detected in intake and discharge samples at Dover in March and 

November, and in November at Calais. Copper was also detected in the discharge 

samples collected on the channel crossing in November. All intake and discharge 

samples exceeded acute and chronic saltwater criteria. Intake copper concentrations 

ranged from 37 to 129 μg/L, while concentrations in discharges ranged from 32 to 

129 μg/L. 

 Lead was detected during November in discharge samples from the channel crossing 

and Calais, and both intake and discharge samples from Dover. Lead concentrations 

in these samples ranged from 18 to 34 μg/L, all of which exceeded the chronic 

saltwater criterion for lead. 

 Nickel was detected in a single discharge sample, from Calais in November, but was 

not detected in the corresponding intake sample. The measured concentration, 34 

μg/L, exceeded the saltwater chronic criterion. 

 Zinc was detected in each of the discharge samples collected in Dover, but in only 

one intake sample (at one-fifth of the discharge concentration). At Calais, zinc was 

detected in one discharge sample. Of the samples collected during the channel 

crossings, zinc was detected in one intake and one discharge sample from different 

sampling events. All of the discharge samples exceeded the acute and chronic criteria; 

however, the occurrence and increase in zinc concentrations was suspected to be an 

artifact introduced by sample contamination. 

PAHs

Samples were analyzed for dissolved and particulate concentrations of the 16 PAHs 

defined to be environmentally relevant by the EPA7, including both small (4-6 rings) and large 

(>6 rings) PAHs. Total PAH concentrations (i.e., the sum of dissolved and particulate 

concentrations for the 16 PAHs) increased significantly from intake to discharge in all samples. 

 

7

 EPA’s priority pollutant list of 16 PAHs includes: naphthalene, acenaphthylene, acenaphthene, fluorene, 

phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, 

benzo[k]flouranthene, benzo[a]pyrene, dibenz(ah)anthracene, benzo[ghi]perylene, and indeno(1,2,3-cd)pyrene. 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

22 

The average total PAH discharge concentration was 3,080 ng/L, twelve times the average intake 

concentration of 250 ng/L. The PAH composition was dominated by phenanthrene, fluorene and 

pyrene in all samples (Hufnagl et al., 2005). 

For individual PAHs, concentrations in the washwater discharge samples were higher 

than concentrations measured in the seawater inlet by factors of one to two orders of magnitude. 

EPA has classified seven PAH compounds as probable human carcinogens, and NRWQC 

have been established for nine PAHs to protect human health. A summary of the concentrations 

measured for these PAH compounds and for phenanthrene, the most abundant PAH in these 

samples, in intake seawater and washwater discharge samples is presented in Table 3. Five of the 

PAH compounds (benz[a]anthracene, benzo[a]pyrene, benzo[b]+ benzo[b]fluoranthene and 

chrysene) exceed the human health criterion in all or nearly all discharge samples. Two others 

(dibenz[a,h]anthracene and indeno[1,2,3,c,d]pyrene) exceeded the criterion in about half of the 

samples (Hufnagl et al., 2005). 

Table 3. Concentrations of PAHs in the Seawater Intake and Washwater Discharge from 

the Pride of Kent Scrubber Trial 

PAH 

Median Total 

Influent 

Concentration

(ng/L) 

(n=13) 

Median Total 

Discharge 

Concentration

(ng/L) 

(n=13) 

Dissolved 

Fraction, fd

Human 

Health 

Criterion 

(Organism 

Consump. 

Only, ng/L, 

Except As 

Noted) 

Number of 

Discharge 

Concentrations

Exceeding 

Criterion 

Benz[a]anthracene 6 262 0.36 18 12 

Benzo[a]pyrene 5 136 0.12 18 12 

Benzo[b]fluoranthene + 

benzo[k]fluoranthene 4 49 0.10 18 12 

Chrysene 12 373 0.40 18 13 

Dibenz[a,h]anthracene 8 9 0.15 18 6 

Fluorene 14 137 0.97 5.3 mg/L 0 

Indeno[1,2,3,c,d]pyrene 5 18 0.13 18 8 

Phenanthrene 54 1,230 0.88 

Pyrene 23 408 0.53 4 mg/L 0 

Source: Hufnagl et al., 2005 

Even higher PAH concentrations were measured inside the seawater scrubber washwater 

treatment system. The total PAH concentrations in samples collected prior to seawater dilution 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

23 

ranged from 11,900 to 20,400 ng/L. Phenanthrene concentrations prior to seawater dilution were 

5,100 to 8,200 ng/L, six to ten times lower than the 50 μg/L IMO Guideline. Most of the total 

PAHs were bound to soot particles, and concentrations were reduced by multicyclone treatment. 

Note, however, that improving the treatment efficiency would further reduce particulate PAH 

concentrations in the washwater discharge. The relatively high PAH concentrations in the 

discharge samples were predominantly the low molecular weight PAHs (such as the two and 

three benzene ring compounds: fluorene and phenanthrene) that are more soluble in water and 

not effectively removed by particle separation. Based on the average total PAH concentrations in 

intake and discharge samples, and assuming a washwater flow rate of 216 m3

/hr and 2.25x 

dilution by reaction water, the EGCS on the Pride of Kent discharged about 33 grams of total 

PAH per day (Hufnagl et al., 2005). 

No increase in PAH concentrations was observed in ambient harbor samples collected in 

front of the seawater scrubber discharge. Principal component analyses indicated that the PAH 

compounds measured in the seawater scrubber samples originated from a petroleum source 

rather than a combustion source, as would be expected from the funnel. Incomplete combustion 

of the fuel was suspected (Hufnagl et al., 2005). 

5.3 THE SUULA FRESHWATER SCRUBBER TRIAL

In 2008 a Finnish maritime consortium installed and tested an EGCS aboard the tanker 

MT Suula (Wärtsilä, 2010). This scrubber trial offers a contrast to the other studies of seawater 

scrubbers because the Suula was outfitted with a closed freshwater scrubber. A medium speed 

680 kW auxiliary engine fed exhaust gas to the scrubber, which was smaller than what would be 

used in a full scale EGCS. In the freshwater scrubber, sulfur oxides in the exhaust gas are 

transferred to washwater containing sodium hydroxide and sulfates are formed. The washwater 

leaving the scrubber was dosed with sodium hydroxide, cooled by a seawater heat exchanger to 

limit evaporative loss, and then re-circulated back to the scrubber. A fraction of the washwater 

was bled off for treatment and discharge, at a rate of about 0.1 m3/MWh or 0.07 m3

/hr (430 

gallons per day, GPD), based upon the auxiliary engine size (personal communication with 

Wärtsilä; see Attachment A). The bleed-off washwater was treated prior to discharge to remove 

pollutants in a high efficiency treatment plant that combined coagulation and flocculation, 

dissolved air flotation, oil and sludge skimming, and granular activated carbon adsorption 

(Wärtsilä, 2007). Pollutants removed from the bleed-off washwater were retained as sludge, 

which was collected into a residual sludge tank and held for disposal at a shore-side waste 

reception facility. The rate of sludge generation was approximately 0.1 to 0.4 kg/MWh (Wärtsilä, 

2010). 

As a part of certification tests that were conducted during the scrubber trial, water quality 

for the bleed-off washwater was demonstrated to the classification societies Germanischer Lloyd 

and Det Norske Veritas (DNV). The freshwater scrubber fulfilled all the IMO Guideline 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

24 

discharge quality criteria for pH, turbidity, PAHphe, and nitrate due to the efficiency of the water 

treatment unit (Wärtsilä, 2010). The bleed-off treatment unit was found to have a very high 

cleaning efficiency regardless of the fuel sulfur content (both low- and high-sulfur content fuels 

were tested). Results from the project report are summarized below (Wärtsilä, 2010): 

pH

Sodium hydroxide (NaOH) was added to the scrubbing water circulation to maintain the 

process pH and the efficiency of SOx removal. Due to efficient NaOH dosing and monitoring in 

this system, the pH of the discharge was maintained at a value of 7.65 (Wärtsilä, 2010). 

Nitrate 

Certification tests demonstrated that the nitrate concentration in the bleed-off discharge 

was less than 2,000 mg/L. When normalized for a washwater discharge rate of 45 t/MHw8

, this 

concentration is reduced to 9 mg/L, which is below the IMO limit of 60 mg/L (Wärtsilä, 2010). 

Turbidity

The turbidity of the discharge bleed-off following water treatment was 0.5 FNU, also 

under the IMO limit of 25 (Wärtsilä, 2010). 

PAHs and Organic Compounds

The PAHphe concentration of the discharged bleed-off water was 14 μg/L. This is 

considerably lower than the flow-normalized IMO limit of 2,250 μg/L. The bleed-off treatment 

unit achieved PAHphe reductions of 97 to 98 percent when the scrubber was tested with fuels of 

different sulfur contents. Hydrocarbon (C10-C40) concentrations were also reduced by nearly 

100 percent, to discharge concentrations of less than 1 mg/L (Wärtsilä, 2010). 

The authors of this study noted that continuous measurement of the PAHphe equivalence 

in a reliable way is challenging using existing technology. Traditional sampling and laboratory 

analyses onshore are a more reliable approach to monitoring PAH content in discharge. 

Additional research and development work was suggested to find the best methodology for PAH 

monitoring in the discharge (Wärtsilä, 2010). 

Metals 

Metals were also removed from the bleed-off washwater by the treatment unit prior to 

discharge. The sum of metals9

 specified by IMO Resolution MEPC.184(59) was reduced by 66 

 

8

 See Section 4 for an explanation of the IMO Guideline discharge limit flow normalization. 
Exhaust Gas Scrubber Washwater Effluent Section 5 – Characterization of Pollutants in 

EGCS Washwater Discharges 

25 

to 83 percent, to a concentration less than 60 mg/L (total metals concentration). The discharge 

concentrations for individual metals were not reported. The IMO Guidelines indirectly limit the 

permissible amount of metals in the washwater discharge by its the turbidity criterion. As this 

case illustrates, a considerable concentration of metals can be discharged even though the 

discharge turbidity is well under the limits established by the IMO Guidelines (Wärtsilä, 2010). 

5.4 SUMMARY OF MARINE EGCS SCRUBBER TRIALS

The trials conducted with wet scrubbers onboard marine vessels have demonstrated the 

capabilities of this technology to remove sulfur emissions from exhaust gas. They have also 

provided limited data characterizing constituent concentrations in washwater discharges. The 

trials aboard the Zaandam, Pride of Kent and Suula provided measurements of several 

washwater constituents including pH and pollutants removed from the exhaust gas (SOx and 

NOx, and the products of their transformation: acidity, SO4, NO3 and COD) and PM (PAHs, 

hydrocarbons and metals). Neutralization of washwater was achieved by blending with sufficient 

seawater “reaction water”. Washwater was also treated to remove the suspended solids that were 

attributed to PM removed by the scrubbers. Treatment was accomplished using multicylones 

alone or in combination with filtration, or, more effectively, using an advanced treatment system 

incorporating coagulation and filtration, flotation and adsorption. 

Issues with the technology for continuous monitoring of PAHs, as required by the IMO 

Guidelines, were noted in two of the trials. One trial also reported difficulties with continuous 

monitoring of turbidity. Washwater monitoring results also suggested that some constituents 

warrant further consideration, including pH, metals, PAHs and nitrate (see Section 6). Although 

all of the constituents are addressed in the IMO scrubber washwater guidelines, the guideline 

limits may be inadequate for metals and PAHs. 

It should also be noted that the trials aboard the Zaandam, Pride of Kent and Suula were 

limited, because the scrubbers were installed on auxiliary engines that were not always operating 

at full power when washwater was sampled. When exhaust is scrubbed at higher power output 

levels, or when larger propulsion engine exhaust is scrubbed, more washwater will be generated 

and discharged. Due to this and the fact that constituent concentrations in exhaust and/or 

washwater may be different at higher power outputs, the representativeness of the trial results 

may be uncertain. In addition, the performance and effects of seawater scrubbers may be 

different in areas with brackish water or freshwater (Hufnagl et al., 2005). Freshwater may have, 

for example, less pH buffering capacity, unless the water drains a carbonaceous area; therefore, 

the ability of such a water body to assimilate acidic washwater discharge may be limited. 

 9

 These metals include cadmium, copper, nickel, lead, zinc, arsenic, chromium and vanadium. 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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26 

SECTION 6 

ASSESSMENT OF POLLUTANTS DISCHARGED IN SCRUBBER 

WASHWATER AND PROTECTIVENESS OF IMO GUIDELINES 

In Section 5, data were presented that characterize the constituent concentrations that can 

be expected in scrubber washwater discharges. These concentrations were compared to EPA 

NRWQC as a preliminary screen for all discharge data with the potential to cause or contribute 

to the nonattainment of a water quality standard in a given receiving water body. The pollutant 

concentrations were also compared to IMO Guideline limits, when available. In addition, an 

analysis of the protectiveness of the IMO Guideline washwater limits offered in an impact 

assessment prepared for the European Commission on the revision of Directive 1999/32/EC10

was considered and incorporated in this discussion where appropriate (AEA, 2009). 

EPA’s NRWQC are recommended concentrations of analytes for a water body that are 

intended to protect human health, aquatic organisms and the water body uses from unacceptable 

effects from exposures to these pollutants. The NRWQC are not directly related to analyte 

discharge concentrations for a number of reasons (USEPA, 2010a). First, NRWQC are based not 

only on concentration, but also on duration and frequency. Second, it is not always necessary to 

meet all water quality criteria within the discharge pipe to protect the integrity of a water body 

(USEPA, 1991). Under EPA’s regulations at 40 CFR 122.44(d)(1)(ii), when determining 

whether a discharge causes, has the reasonable potential to cause, or contributes to an in-stream 

excursion above a narrative or numeric criteria within a state water quality standard, the 

permitting authority will use procedures that account for, where appropriate, the dilution of the 

discharge in the receiving water. A mixing zone allows for ambient concentrations above the 

criteria in small areas near outfalls while dilution occurs. To ensure mixing zones do not impair 

the integrity of the water body, the permitting authority will determine the mixing zone such that 

it does not cause lethality to passing organisms and, considering likely pathways of exposure, 

significant human health risks. 

Nevertheless, comparing analyte concentrations in vessel discharges to NRWQC 

provides a conservative screen of whether these discharges cause, have the reasonable potential 

to cause, or contribute to nonattainment of the water quality standards in a water body. If the 

concentration of a given analyte in vessel discharge is less than the applicable screening criterion 

values, the discharge would likely not cause, have the reasonable potential to cause, or contribute 

to nonattainment of a water quality standard based on that value, particularly after considering 

assimilation and/or dilution by the receiving water. On the other hand, if the concentration of a 

given analyte in vessel discharge is greater than the applicable screening criterion value, then 

there is the possibility of ecological or human health risk. 

 

10 Sulfur Content of Certain Liquid Fuels 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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27 

Because the washwater discharges were measured at the “end of pipe” before being 

released into a harbor where they are subsequently diluted, discharge concentrations greater than 

the applicable criterion do not necessarily indicate that a discharge poses a significant risk or 

would be likely to cause or contribute to a water quality standard exceedance. However, 

exceedance of a screening benchmark indicates that the discharge warrants further consideration 

regarding the potential to cause or contribute to nonattainment of water quality standards. 

Mobile sources such as vessels complicate the analysis because they discharge into many 

different water bodies. In general, greater mixing and dilution would be expected for discharges 

from vessels that are in motion while discharging than from stationary sources. Vessel discharges 

to areas with high vessel traffic, areas with a low degree of flushing, or impaired water bodies 

could reduce mixing and dilution. With these factors in mind and assuming the discharge data 

presented in this document are representative of scrubber washwater discharges in general, a 

discharge concentration marginally above the applicable screening criterion value is most likely 

not of significant concern. On the other hand, a discharge concentration substantially above the 

applicable screening criterion value (e.g., by a factor of 10 or 100) may be more likely to be of 

concern, particularly if the discharge is of significant volume, is in an area of low flushing, is in 

an area where there is a high degree of vessel traffic, or is in a waterbody that is already impaired 

or under other stress. 

The dispersion (mixing) of pollutants discharged from a ship has been evaluated using 

numerical hydrodynamic models. The overall dispersion has been modeled for different 

scenarios regarding ship size, washwater discharge point and speed (MARINTEK, 2006). The 

dispersion in the wake behind a moving vessel has been identified as the dominant dilution 

process (MARINTEK, 2006). For a 163 m length vessel moving at a constant speed of 14.5 

knots with a washwater discharge of 440 m3

/hour at 8 m depth and 20 m ahead of the stern, the 

washwater was diluted by a factor of 1:30 before the discharge passed the stern of the ship and 

by a factor 1:2,000 at a distance 50 m behind the stern. At slower vessel speeds, such as while 

maneuvering in a confined harbor, less dilution is expected. The power output of the propulsion 

engines is also expected to be lower at slower vessel speeds, which would also reduce the rate of 

washwater discharge (assuming the vessel is scrubbing propulsion engine exhaust). 

Dilution in port may be influenced by local conditions, but much less mixing is expected 

in comparison to the discharge from a moving vessel. One study assumed a speed of 0 knots, a 

washwater temperature of 7.5°C, and an ambient current speed of 5 centimeters per second 

(cm/s) (MARINTEK, 2006). A dilution factor of 1:100 in the center of the plume was predicted 

less than 20 m downstream from the discharge, corresponding to an average dilution factor of 

approximately 1:150. The dilution factor will probably be smaller than this if the washwater 

discharge is warmer than 7.5°C (for comparison, the maximum washwater discharge 

temperatures measured during the Pride of Kent trials were 37°C), because heated effluent will 

be buoyant and may tend to “float” on top of the ambient water instead of mixing. 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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The pollutant concentrations were also compared to IMO Guideline limits, when 

available. The IMO washwater criteria are stipulated in Section 10 of Annex 9 of Resolution 

MEPC.184(59) and regard pH, PAH content, turbidity and nitrate content. These concentration 

limits do consider the dilution of washwater discharges in the ambient water in a mixing zone 

surrounding the vessel. The IMO Guideline limits also reflect the results of monitoring, which 

generally did not detect increases in pollutant concentrations in the discharge plume, even in 

close proximity to the point of discharge. The current IMO Guidelines imply that the washwater 

criteria apply everywhere; however, in earlier versions the criteria were only applied to harbors, 

estuaries and ports (AEA, 2009). It is uncertain, at present, if separate limits will be developed 

for operation at open sea. 

6.1 PH 

There is some concern that sending seawater back into the ocean with acidic sulphurcontaining wastewater is harmful enough to the marine ecosystem that it “renders the technology 

impractical” (Kehoe et al., 2010). The washwater from a seawater scrubber unit can have a pH as 

low as 3; therefore, to avoid a negative impact on ecosystems and potential corrosion issues, the 

washwater must be further neutralized and diluted with additional seawater to increase the pH 

level to greater than 6.5, as required under the IMO EGCS Guidelines. Alternatively, the IMO 

Guidelines allow the pH limit for the discharge to be determined during commissioning by 

measuring pH in the plume at a distance of 4 m from the discharge point, and determining the 

discharge pH that corresponds to a plume pH equal or exceeding a value of 6.5. This alternative 

compliance method of measuring the pH is consistent with EPA CORMIX principles where 4 m 

is considered the boundary of the initial mixing zone between water discharged overboard and 

water surrounding the vessel (Jirka,1996 and EGCSA, 2010). 

Low pH water can adversely impact marine organisms such as shellfish and, as many 

organisms are only able to survive when environmental conditions are stable, a decrease in pH 

may be a risk for those organisms. The IMO indicates that, in general, no environmental harm is 

considered to arise from short-term exposure of seawater with pH values as low as 6.5 

(Hassellöv and Turner, 2007). A different pH discharge criteria is applied for moving vessels: 

during maneuvering and transit, a maximum difference of 2 pH units is allowed between the 

ship’s inlet and overboard discharge. As previously noted, for vessels underway, the mixing of 

discharged washwater with fresh seawater by the turbulence of the ship’s wake is highly 

effective, so that recovery of the pH to that of the surrounding water is rapidly achieved 

(EGCSA, 2010). 

Most studies of the pH reduction associated with scrubber washwater discharges have 

focused on local receiving water impacts. As seawater scrubbing cuts short the cycle of SO2

directly deposited in the ocean, this technology may also accelerate the pH decrease in surface 

waters of the oceans. However, this effect is minor compared to ocean acidification due to 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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29 

increased carbon dioxide concentrations in the atmosphere (Kehoe et al., 2010). One study 

estimated that the pH of the ocean would decrease by 0.02 within the next 100 years, assuming 

that sulfur is evenly distributed in the 100m deep surface layer (Kehoe, 2010). In comparison, 

another study predicted that average pH of the surface ocean may decline by 0.255 by year 2100 

(Kehoe, 2010). 

EPA’s NRWQC criteria for marine aquatic life is expressed as a pH in the range of 6.5 to 

8.5, but not more than 0.2 standard units outside of normally occurring range (USEPA, 1976). 

EPA guidelines concerning the introduction of acids state that, within the initial mixing zone, the 

pH change is not allowed to be higher than 0.2 (EGCSA, 2010). In comparison to the EPA pH 

criteria, the IMO Guidelines have the same lower limit, but lack the limit on pH change. 

Problems were encountered during two of the seawater scrubber trials with neutralization 

of the acidic washwater resulting in the discharge of washwater with pH values below 6.5. On 

the Zaandam, the discharge washwater pH was more than 2 standard units lower than the intake 

values. The mean pH of the discharged washwater was 5.8 while in the coastal waters of 

California and Hawaii and 6.3 in Alaska. The discharge of low pH washwater effluent was 

associated with low washwater and/or reaction water flow rates. On the Pride of Kent, the 

average decrease in washwater pH was only 1.39. However, all of the discharged washwater pH 

values were below 6.5 during one sampling event; two-thirds of the pH values were below this 

limit in another. 

A 2007 study calculated the amount of water needed for scrubbing SO2 and dilution to 

first obtain pH of 6.5 and then a pH change within the initial mixing zone of 0.2 (Hassellöv and 

Turner, 2007). The nominal flow rate of scrubbing water used by MEPC is around 45 t/MWh 

giving about 540 t/hr for a ship with a 12 MW engine. The estimated quantities of water needed 

for scrubbing are presented in Table 4. With the present criteria for pH, the dilution in the sea 

and natural buffering may rapidly achieve acceptable levels, even in confined areas. 

Furthermore, vessels use substantially less propulsion power when traveling at low speeds and 

maneuvering in harbors and other confined areas. Vessels using EGCS for propulsion engines 

should therefore discharge less washwater in harbors, further limiting the impact of seawater 

scrubbers on receiving water pH. As previously stated, operation of a seawater scrubber in 

brackish or freshwater may have different effects (e.g., fresh waters with less pH buffering 

capacity). Almost 7 times more water for dilution is needed when scrubbing takes place in 

freshwater, compared to the open sea. 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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Table 4. Approximate Amount of Water (t/hr) Affected by a Scrubber for a 

12 MW Engine 

Quantity of Water 

Open Ocean Baltic Sea Freshwater River 

Water for scrubbing 700 900 2,500 

Water for dilution to pH = 6.5 1,400 1,700 15,000 

Factor for dilution to ∆pH of 0.2 3 3.5 2.5 

Total amount of water 6,300 9,100 44,000 

Source: Hassellöv and Turner, 2007 

In an environmental impact assessment of the Pride of Kent seawater scrubber trials, 

samples were taken directly in front of the washwater discharge from a stationary vessel in each 

harbor at distances of 1 to 5 m, 50 m, 350 m and 700 m. While washwater samples taken from 

inside the ship had pH values of 6.2 to 6.5, in no sample taken within the discharge plume was 

the pH below that of the surrounding water (EGCSA, 2010). 

The IMO criterion for pH limits the amount of water that may be affected by low pH 

during scrubber operation to a reasonable amount. This is dependent on the dilution, volume of 

water and the specific area of concern. As noted, specific attention may be needed for waters 

with low alkalinity. Increasing the volume of reaction water used to neutralize washwater in 

open seawater systems, or operating closed freshwater systems in zero-discharge mode as 

needed, may be practical solutions to limit pH change in vulnerable (i.e., low alkalinity) 

receiving waters. 

6.2 PAH 

PAHs are the largest known group of carcinogenic substances and include many 

individual chemical substances containing two or more condensed aromatic rings. PAHs occur 

naturally in petroleum and are also produced as by-products of fuel combustion. PAHs are an 

important class of environmental contaminants that are known to accumulate in ecosystems. The 

EPA has identified 16 PAH compounds as priority pollutants. Some of these compounds are 

carcinogenic and/or mutagenic to mammals; in addition, they have both acute toxicity and sublethal effects on some aquatic organisms. PAHs may also bioaccumulate in edible shellfish, 

which gives them a pathway to humans (Hartmann and Quinn, 1999). 

The IMO has set limits for the online monitoring of phenanthrene, based on studies to 

date which have shown no negative influences of EGCS washwater on port environments. 

Phenanthrene is the most prevalent of the 16 EPA PAHs found in the vessel washwater systems 

(Behrends et al., 2005). Furthermore, the IMO asserts that, as PAHs are also found naturally in 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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petroleum, their monitoring ensures that unburned oil or hydrocarbons do not enter the sea 

(EGCSA, 2010). The IMO also requests ship owners to sample inlet and untreated and 

discharged washwater, and to employ gas chromatography-mass spectrometry analysis for 

PAHs. 

Sediments can be disturbed during the maneuvering of a ship in shallow water, and as a 

result may enter the washwater system. Since harbor sediments are often contaminated with 

PAHs, PAHs can enter the washwater system along with the sediment. The IMO therefore 

requires the background concentration of PAH at the washwater inlet be taken into account when 

measuring the PAH concentration at system discharge. It is also required that washwater 

monitoring take place following water treatment, but before any dilution for correction of the 

washwater pH. 

While a set of 16 PAHs is customarily analyzed and measured as individual chemicals, 

the IMO Guidelines set the washwater criteria for PAH in phenanthrene equivalents. The 

rationale for this is unclear but may be that measuring PAH is a surrogate for hydrocarbons and 

phenanthrene was found to be the most abundant PAH in the analysis of washwater during trials 

on the Pride of Kent. However, the concept of phenanthrene equivalents is not well established 

and needs to be explained or replaced (AEA, 2009). Assuming that it simply means that the 

phenanthrene concentration is measured, the 50 μg/L criteria correspond to about 2.2 g/MWh of 

PAHphe. Thus, the washwater criterion may be high, especially considering that the scrubber 

washwater should be treated before monitoring for PAHphe. A 12 MW engine could, with the 

suggested limit, emit around 27 g/hr phenanthrene in addition to unknown amounts of other 

PAHs. This emission limit may be unacceptably high, and widespread use of scrubbers emitting 

washwater with PAH concentrations close to this limit could pose a risk to the environment 

(AEA, 2009). 

The Oslo and Paris Conventions for the protection of the marine environment of the 

northeast Atlantic (OSPAR) have developed Ecotoxicological Assessment Criteria (EAC) for a 

large range of chemicals, including PAHs. The EAC can be applied as Predicted No Effect 

Concentration (PNEC) values (MARENTEK, 2006). The EAC for phenanthrene in water is 

proposed as a range: 0.5 to 5 μg/L (provisional value). The PNEC value is the maximum 

permissible concentration that can be sustained over time. The Predicted Environmental 

Concentration (PEC) equals the proposed washwater criteria multiplied by the dilution (if inlet 

concentration is assumed to be zero). Since the washwater criterion is set to 50 μg/L PAHphe 

(for a scrubber washwater flow of 45 t/MWh), the washwater must be diluted by a factor of 10 to 

100 to reach this PNEC concentration (AEA, 2009). Furthermore, the background concentration 

should also be considered, which may lead to further limits on the emissions. 

The median concentration of phenanthrene measured in the washwater following 

treatment but before mixing with reaction water on the Pride of Kent was 6.1 μg/L, well below 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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32 

the 50 μg/L PAHphe IMO Guideline limit. Comparison of the other PAHs measured in 

washwater discharged by the Pride of Kent during scrubber trials to the EPA NRWQCs suggests 

that scrubbers may emit washwater with concentrations of other PAHs that could pose a risk to 

the environment. The EGCS on the Pride of Kent was discharging about 33 grams of total PAH 

per day. Dividing the median discharge concentrations by the applicable human health criteria 

(values tabulated in Section 5), the dilution required to reach the criteria is calculated to be as 

high as 21 for chrysene, 15 for benz(a)anthracene, and 7 for benzo(a)pyrene. Phenanthrene 

concentrations measured in the washwater discharge on the Zaandam and Suula were <1.3 μg/L 

and 14 μg/L; concentrations for the other PAHs were not reported. 

The PAH criterion appears to have been selected to allow continuous monitoring of the 

washwater discharge. While it may be appropriate to use phenanthrene for monitoring purposes, 

the criteria should, nevertheless, ensure that all of the environmentally relevant PAHs are 

controlled. How the measured phenanthrene equivalents relates to the 16 EPA PAHs, for 

example, or its correlation to total hydrocarbons needs to be established during certification. 

Furthermore, it remains to be established that the PAH IMO Guideline limit will ensure that the 

levels of total hydrocarbon compound emissions are at safe levels (AEA, 2009). 

6.3 TURBIDITY

Turbidity is a measure of the amount of suspended solids in the water, based upon the 

loss of optical transparency (i.e., cloudiness) of the water. When combined with PAH, the 

measurement of turbidity is intended to demonstrate that the scrubber and washwater treatment 

system is operating correctly (EGCSA, 2010). However, this method of continuous monitoring 

has not been demonstrated to be a reliable measure of the concentration of PM (including metals) 

emitted with the washwater (AEA, 2009). Turbidity can be a useful indicator of the status of the 

scrubber (i.e., that suspended particles are not being formed in the scrubbing process and/or the 

washwater treatment process is functioning properly). Turbidity, however, is not a direct method 

of determining the amount of exhaust particles that end up in the washwater because there is no 

direct correlation between turbidity and particle concentration. Further, turbidity values are 

strongly dependent on the size of particles in the water; smaller particles are likely to have 

significantly less influence on the measured turbidity than larger ones. Studies are required to 

correlate the turbidity measurements with concentrations of PM, ash, metals, etc. in the 

washwater before turbidity can be considered as a reliable indicator of particulate levels in the 

washwater (AEA, 2009). These uncertainties make it difficult to make an environmental 

assessment of the turbidity criteria. 

Turbidity was measured in the scrubber trials onboard the Zaandam and the Suula. 

During the Zaandam trials, the maximum turbidity was 8 NTUs, well below the 50 NTU IMO 

Guideline limit. Turbidity levels actually declined between the inlet and discharge in the first 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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33 

year of that trial (i.e., the scrubber system removed turbidity from the seawater used as scrubber 

washwater). On the Suula, the turbidity was 0.5 FNU, also well below the guideline limit. 

6.4 NITRATE

Nitrate is the most highly oxidized form of nitrogen, and excess nitrate concentrations in 

aquatic systems can result in a rapid increase or accumulation in the population of algae, possibly 

leading to algae blooms and eutrophication. This can disrupt functioning of an aquatic system, 

causing a variety of problems such as a lack of oxygen in the water needed for fish and shellfish 

to survive (Behrends and Liebezeit, 2003). In near-shore or harbor situations, where phosphorous 

is available (e.g. from river inputs, runoff from agriculture or direct input of domestic sewage), 

addition of nitrates may lead to enhanced biomass production. 

The IMO Guidelines include two criteria for nitrate concentration in the washwater: 60 

mg/L or a limit based on removing 12 percent of the NOx from the exhaust stream, based on a 

hypothetical scrubber design capable of removing more NOx than the soluble NO2 fraction. A 

scrubber unit on a ship operating close to this 12 percent limit may, depending on the sea area, 

increase the ship’s contribution to the nitrate load in the sea (AEA, 2009). In principle, 

washwater discharges from numerous ships operating EGCS could lead to large additional 

loadings of nitrates to sensitive aquatic ecosystems. 

A 2009 report analyzed the impact of the 12 percent exhaust NOx removal criteria in the 

Baltic Sea (AEA, 2009). The Baltic is heavily trafficked by vessels, is a SECA, and is highly 

impacted by eutrophication. Serious efforts have been made to decrease the flow of nutrients into 

the Baltic and the total emission of NOx from shipping in the Baltic is currently around 400 

kilotons per year. Modeling was used to estimate that about 13 percent of the NOx emitted from 

shipping in the Baltic Sea is also deposited in that water body; the rest is deposited over land or 

(to a smaller extent) in other sea regions. Consequently, approximately 107 kilotons per year 

(calculated as nitrates) is deposited in the Baltic Sea from Baltic shipping. If all of these ships 

were to use scrubbers, and all of these scrubbers captured 12 percent of the NOx in the exhaust 

and released it as nitrates in the scrubber washwater, the total nitrate loading from vessels would 

increase to 193 kilotons (an increase of about 86 kilotons per year). Notwithstanding several 

unrealistic assumptions, this estimate does give a maximum possible increase in nitrate loading. 

Twenty-five percent of the total load of nitrogen to the Baltic Sea comes from atmospheric 

deposition; the rest comes from rivers and direct waterborne discharges. Thus, the hypothetical 

maximum increase in the load of nitrates to a water body of the size of the Baltic Sea would be a 

nearly-negligible, 2 percent, if all ships were to use scrubbers.

A 2009 report estimated the potential for additional biomass production due to the 

discharge of nitrate from the washwater of an EGCS, using the nitrate levels monitored over the 

ten-month period of the Pride of Kent trials (Hufnagl, 2005). Adding 9.59 μM of nitrate to the 

10-60 μM ambient concentrations measured in harbors, when converted by stoichiometry, equals 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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34 

an increase in plankton biomass of 2.13 mg/L over the 2.2-13.3 mg/L biomass produced by 

ambient nutrients. The potential for increase in annual production was equated to less than the 

actual production during one sunny day (Hufnagl, 2005). Thus, these calculations also 

demonstrate that the impact of the nitrates discharged in washwater from seawater scrubbers is 

likely to be minimal (EGCSA, 2010). 

Nitrate concentrations were measured in all three of the scrubber trials. Low nitrate 

concentrations (>146 μg/L and 638 μg/L) were measured in the Zaandam and the Pride of Kent

washwater discharge. Onboard the Suula, a significantly higher concentration (<2,000 mg/L) was 

detected. However, since the IMO Guideline limit of 60 mg/L for nitrate is normalized by the 

washwater discharge rate, the normalized nitrate concentration of 9 mg/L is below the IMO 

Guideline (the Suula, using a closed scrubber system, discharges bleed-off washwater at about 

1/500th of the customary rate for an open seawater scrubber). 

Nitrate loadings from washwater discharges can be calculated from the Pride of Kent and 

Suula EGCS trial data. Based on the average concentration increases between intake and 

discharge samples, the nitrate washwater loading from the seawater scrubber EGCS on the Pride 

of Kent was 0.35kg NO3-N per day. The dissolved inorganic nitrogen washwater loading was 

0.53 kg N/d. The freshwater scrubber aboard the Suula generated a nitrate loading of less than 

3.4 kg NO3-N per day. 

6.5 SULFATE, COD AND DISSOLVED OXYGEN

Sulfate is an abundant and conservative component of seawater; therefore, the discharge 

of this parameter does not represent a limiting factor for seawater scrubbing. Studies and field 

testing confirm that the sulfate increase from exhaust gas scrubbing would be insignificant when 

compared with the quantity already in the oceans (Hassellöv and Turner, 2007 and Hufnagl, 

2005). For example, washwater sampling aboard the Pride of Kent demonstrated that scrubbers 

added a negligible amount of sulfate. 

When SO2 is dissolved in water, a reaction occurs whereby the sulfur dioxide is ionized 

to bisulfite and sulfite, which is then readily oxidized to sulfate in seawater containing oxygen 

(Hassellöv and Turner, 2007). This process increases the COD, which could potentially have an 

adverse impact on aquatic systems. COD is a measure of the theoretical oxygen consumption of 

a water sample (i.e., the amount of oxygen required for degradation of the organic matter). There 

are no environmental standards for COD (MARINTEK, 2006). 

Tolerance to reduced dissolved oxygen varies between organisms, life stages and 

environmental conditions. The Norwegian Marine Technology Resrach Institute concluded that 

COD was not a matter of concern under the circumstances of their study (MARINTEK, 2006). 

Using worse case scenarios, a study evaluated how much dilution of washwater would be 

required to return oxygen levels to within 1 percent of those of the ambient water. They found 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

in Scrubber Washwater and Protectiveness of IMO Guidelines 

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that if the pH of the washwater had already been corrected to within 0.2 standard units of 

ambient values, then no further dilution would generally be required to maintain DO levels 

(Hassellöv and Turner, 2007) 

It is known from various in-field tests and modeling of discharge plumes that the pH and 

oxygen of discharged water very rapidly returns to that of the surrounding water, especially 

when the vessels is underway; thus, the IMO does not require dissolved oxygen to be monitored 

(EGCSA, 2010). 

6.6 METALS

Metals are a diverse group of pollutants, many of which are toxic to aquatic life and 

humans. While some metals, including copper, nickel, and zinc, are known to be essential to 

organism function, many others, including thallium and arsenic, are nonessential and/or are 

known to have only adverse impacts. Even essential metals can do serious damage to organism 

function in sufficiently elevated concentrations. Adverse impacts can include impaired organ 

function, impaired reproduction, birth defects, and at extreme concentrations, acute mortality. 

There are a number of sources for metals to enter the washwater in wet scrubber systems: 

 System materials, typically iron, copper and zinc may be a source of metals. The reduced 

pH of washwater, which can be as low as pH 3 within the system, will increase the 

solubility of metal ions and therefore the choice of materials is very carefully considered 

by exhaust gas cleaning system designers. 

 System inlet water may contain metals found in seawater or from electrochemical 

protection to prevent fouling of seawater pipes and from antifouling paints (typically 

copper). Electrochemical protection is often installed at the inlet to seawater pumping 

systems to prevent the parasitic growth of organisms that may foul pipework, filters and 

coolers of ship’s machinery. An EGCS may take a washwater supply from a point with 

electrochemical protection, but will not have a direct influence on it (EGCSA, 2010). 

 The combustion of fuel and lubricant is another source of metals, typically vanadium, 

nickel, calcium and zinc. Although it makes up a relatively small amount of the overall 

PM, ash represents the incombustible residue of burning fuel oil and lubricant. The 

majority of fuel oil ash content consists of metal compounds that occur naturally in 

petroleum; principally vanadium and nickel, which are oil soluble and so cannot be 

removed by onboard pre-treatment such as filtration and centrifugal purification 

(EGCSA, 2010). These metals can also enter ecosystems from unscrubbed exhaust 

emissions into the atmosphere. 

 The IMO Guidelines do not contain any limits for the concentrations of metals in 

washwater discharge, although the IMO requests ship owners to sample and analyze washwater 

for a suite of metals. Turbidity is monitored as a surrogate for suspended solids. Presumably, 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

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turbidity values below the limit of 5 FTU/NTU indicate that the washwater treatment system is 

effectively removing particulates from the washwater, as well as particulate metals. Washwater 

samples taken from various points within the scrubber system during the Pride of Kent trials 

showed that that the majority of metals were bound to particles or were particulates themselves 

and were effectively removed by the washwater treatment plant. 

Monitoring conducted during scrubber trials demonstrates that metals are present at 

concentrations above EPA NRWQC for several metals, even though the washwater treatment 

plants were functioning and turbidity limits were being met. The monitoring during scrubber 

trials also considered the contribution of metals detected in the washwater intake. Comparison of 

the metals measured in washwater discharged by the Zaandam during scrubber trials to the EPA 

acute and chronic NRWQCs suggests that scrubbers may emit washwater with arsenic, copper, 

lead, nickel and selenium concentrations that could pose a risk to the environment. During 

scrubber trials aboard the Pride of Kent, copper, lead, nickel and zinc11 were measured in 

washwater discharge at concentrations exceeding NRWQCs. Washwater discharged by the Suula

was reported to contain less than 60 mg/L of total metals (the sum of cadmium, copper, nickel, 

lead, zinc, arsenic, chromium and vanadium). This concentration greatly exceeds the NRWQCs 

for any of these metals (or even the approximately 1 mg/L sum of the NRWQCs for all of these 

metals except vanadium); therefore, exceedances of NRWQCs for the several metals parameters, 

in each of the scrubber trials, suggest that scrubbers may emit washwater with concentrations of 

metals that could pose a risk to the environment. 

6.7 SUMMARY

A number of constituents found in EGCS washwater discharges have been evaluated to 

determine whether these discharges could cause an exceedance of any NRWQC. The 

protectiveness of the IMO Guideline washwater limits were also evaluated in terms of whether 

they adequately mitigate the potential for washwater discharges to result in ecological or human 

health risk. 

Based on this evaluation, several metals (including arsenic, copper, lead, nickel and 

selenium) and PAHs (chrysene, benz(a)anthracene and benzo(a)pyrene) were measured in 

washwater discharges at end-of-pipe concentrations that exceeded the NRWQC for that 

chemical. Accordingly, these parameters have the potential to contribute to an exceedance of 

water quality standards on a localized scale. The IMO Guideline washwater limits intended to 

address turbidity and PAH-phenanthrene equivalents may not be sufficiently protective, based 

upon the available monitoring data (EGCSA, 2010). 

The pH of EGCS washwater discharges may also be a concern, because wet scrubbing 

involves the transfer of SOx from exhaust gasses to washwater, which is accompanied by a 

 

11 Zinc concentrations were suspected to be an artifact introduced by sample contamination. 
Exhaust Gas Scrubber Washwater Effluent Section 6 -Assessment of Pollutants Discharged 

in Scrubber Washwater and Protectiveness of IMO Guidelines 

37 

significant increase in acidity. The increased acidity must be neutralized, either by the natural 

alkalinity in seawater or by adding an alkaline chemical to freshwater. The monitoring data from 

scrubber trials onboard ships demonstrate that washwater neutralization is generally effective, 

although large volumes of reaction water may be required in open systems depending on the 

alkalinity of the water body. The IMO Guideline washwater limits for pH may be protective, 

both at sea and in confined harbors (EGCSA, 2010). 

The IMO Guidelines require continuous measurement of washwater turbidity to monitor 

the operation of the scrubber and the washwater treatment system. For example, washwater 

turbidity declined from inlet to discharge onboard one vessel conducting scrubber trials; 

however, it is not clear that monitoring turbidity serves as a surrogate for other washwater 

constituents of concern (HA & H-K, 2010). 

Total hydrocarbon concentrations in washwater discharges were low or below detection 

in the scrubber trials, despite the observation of a visible sheen accompanied by sooty deposits in 

the washwater discharge plume in one of these trials. Two other washwater parameters, nitrate 

and sulfate, appear unlikely to cause a concern because the quantities of these parameters that 

have been measured in EGCS washwater discharges should be readily assimilated in marine and 

estuarine receiving waters. In the case of nitrate, most of the EGCS washwater loading (1 to 3 

kg/d/vessel, based on monitoring during trials) would be deposited to the receiving water via the 

atmosphere if scrubbers were not used12. 

 

12 The nitrate in scrubber washwater is derived from the oxidation of atmospheric nitrogen as well as nitrogen in fuel 

that occurs during combustion. As such, NOx and nitrate emissions are unaffected by the sulfur content of the fuel 

and switching to low-sulfur fuels will not reduce these emissions. Consequently, compliance with MARPOL Annex 

VI via the two SOx control strategies (fuel switching and EGCS) result in roughly comparable loadings of nitrate to 

receiving waters (see calculation and discussion of Baltic Sea nitrate loading in Section 6.4). 
Exhaust Gas Scrubber Washwater Effluent Section 7 - Acronyms 

38 

SECTION 7 

ACRONYMS 

Organizations and Associated Initiatives 

AEA AEA Energy & Environment Consulting Group

DNV Det Norske Veritas

EGCSA Exhaust Gas Cleaning Systems Association 

EPA Environmental Protection Agency

MEPC Marine Environment Protection Committee 

MARPOL International Convention for the Prevention of Pollution From Ships

SECA Sulfur Oxide Emission Control Area

EU European Union

IMO International Maritime Organization

NRWQC National recommended water quality criteria

GESAMP Group of Experts on the Scientific Aspects of Marine Environmental 

Protection

VGP Vessel General Permit

ETM EGCS Technical Manual 

CORMIX Cornell Mixing Zone Expert System 

OSPAR Administrator of the Oslo and Paris Conventions for the protection of the 

marine environment of the northeast Atlantic 

Chemical abbreviations 

SOx Sulfur oxides (SO2 and SO3) 

NOx Nitrogen oxides (NO and NO2) 

PAH Polycyclic aromatic hydrocarbon 

BETX Benzene, toluene, ethylbenzene and xylene 

PAHphe Polycyclic aromatic hydrocarbon phenanthrene equivalence concentration 

TPH-D/G Total petroleum hydrocarbon –diesel and gasoline components 

DIN Dissolved inorganic nitrogen 

THC Total hydrocarbon 

TKN Total Kjeldahl nitrogen 

HEM Hexane extractable material 

COD Chemical oxygen demand 

TSS Total suspended solids 

Units of measure 

MGD Million gallons per day 

MW Megawatt 
Exhaust Gas Scrubber Washwater Effluent Section 7 - Acronyms 

39 

MWh Megawatt-hour 

PPB Parts per billion 

t Metric ton 

FNUkt Formazin nephlometric units Kilotons 

NTUFNU Nephlometric turbidity units Formazin nephlometric units 

NTU Nephlometric turbidity units 

EGCS Exhaust gas cleaning system 

PMEGCS Particulate matter Exhaust gas cleaning system 

PECPM Predicted environmental concentration Particulate matter 

PNECPEC Predicted no effect concentration Predicted environmental concentration 

EACPNEC Ecotoxicological assessment criteria Predicted no effect concentration 

GC-MSEAC Gas chromatography-mass spectrometry Ecotoxicological assessment 

criteria 

AWPGC-MS Advanced wastewater purification Gas chromatography-mass spectrometry 

MSAWP Motor ship Advanced wastewater purification 

MS Motor ship 

 
Exhaust Gas Scrubber Washwater Effluent Section 8 - References 

40 

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