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Preface 

This report has been written in cooperation between: 

MARINTEK – Energysystems & Environment 

SINTEF - Water and Environment and 

NIVA – Norwegian Institute for Water Research 

Some supporting dispertion calculations has been undertaken by 

CFD Norway A/S 

SINTEF Materials and Chemisty 

Furthermore, information has been exchanged in common interest with Wärtsilä Finland Oy, VTT 

Technical research centre of Finland and DNV. 

The work has been sponsored jointly by Norwegian shipowners and the Research Council of 

Norway as Work Package 3 of the MARULS project. 
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Summary 

Background 

Regulation 14(4)(b) of MARPOL Annex VI requires ships to document that discharges from a 

possible exhaust gas-SOx cleaning systems (EGCS-SOx) have no adverse impact based upon 

criteria communicated by the authorities of the port State to the Organization. 

IMO Guidelines for on-board exhaust gas-SOx cleaning systems (MEPC resolution 130.53) 

contain some information on discharges from the EGCS-SOx. At MEPC 53, however, IMO is in 

the process of developing more specific recommendations and criteria relevant to EGCS-SOx 

wash water discharges. 

Taking advantage of data from trials using exhaust gas seawater scrubbers conducted in Norway 

in the early nineties as well as up to date toxcisity data and discharge modeling tools, a set of 

possible criteria have been developed and submitted as a proposition IMO MEPC 55. This report 

provides background information regarding the development of this proposition. 

Outline of discharge criteria development 

The development of discharge criteria consists of three main steps; 

 

.1 Identification of harmful compounds and effects. 

.2 Identification of the maximum permissible concentration that can be sustained over time 

for each of the compounds identified, the Predicted No Effect Concentration (PNEC). 

.3 Evaluation of the expected environmental concentration of each discharge compound, 

designated as Predicted Environmental Concentration (PEC). 

A similar procedure is used to assess other possible effects of the discharge such as change in 

temperature, pH, Dissolved Oxygen (DO), Chemical Oxygen Demand (COD) etc. 

The PNEC-concentration is derived from scientific toxicology studies, either by use of species 

sensitivity distribution curves or by applying safety factors to realistic worst case toxicity levels. 

These factors are very high when the amount of relevant studies is low and less conservative when 

the toxicity to several species is better known. 

The PEC is usually derived from a study where discharge measurement data and predictions is 

combined with distribution models. 

A risk to the marine environment is indicated if the PEC is higher than the PNEC. However, 

according to international practice set out by UN, discharges resulting in local concentrations that 

are higher than no-effects limits for long term (chronic) exposure can be acceptable in the 

immediate vicinity of the discharge point if the discharge is rapidly diluted. 
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The concentration of substances and variations in parameters in the discharge water depends on 

the specific wash water rate, which may vary by a factor of 300 between EGCS-SOx principles. 

However, pollutant mass flow may be virtually the same. Hence concentration limits must be 

related to a nominal wash water rate. 

Predicted No Effect Concentrations (PNECs) can be obtained from regulations (such as EU, USEPA, UK) or in scientific literature when standards are not available. 

Differences in sensitivity to contaminants and other environmental stressors between freshwater 

and marine organisms are small, particularly when compared to the safety factors commonly used 

to derive PNEC values. For practical risk assessment purposes one may regard freshwater, 

brackish water and marine species as equally sensitive. PNEC values could thus be suitable 

world-wide with few exceptions. The principal difference between locations would be the amount 

of discharge (i.e. traffic density) and how rapidly the water is exchanged. 

Port States will have to consider both the extent of ship traffic, and natural dilution effects such as 

provided by wind, current and rivers when defining their local criteria. To avoid proliferation of 

criteria it is proposed that IMO develops a set of default criteria that represent typical situations. 

(Level 1, 2 and 3) The Port State would then have the opportunity to choose one of the criteria on 

the list which would significantly ease verification of compliance. 

Proposed criteria 

Waste water dispersion modelling shows that discharges are rapidly dilluted when the ships are in 

motion. The most critical situation is when the ship is at quay. In this case, it is likely that an 

average plume dilution of 1:150 may be obtained within the near field (20 m) and within 6-7 

minutes. Taking these dilution factors and the PNEC values for the actual scrubber components 

into account we have estimated the worst case discharge concentrations that may be allowed 

without the PEC/PNEC ratio exceeding 1 outside the near field. 

In those cases where the scrubber discharge measured in trials was lower than the proposed 

criteria by a factor of ten, that particular substance was considered not to require a 

dischargecriteria. The modelling further indicated that PNEC values can be obtained even with 

pH=0 and DO = O at outlet, indicating that no regulation would generally be needed for pH and 

COD. 

As previously argued, to avoid proliferation of criteria it is advised that IMO develops a set of 

default criteria that represent typical situations. (Level 1, 2, 3, etc) The Port State would then have 

the opportunity to choose one of the predefined criteria on the list which would significantly ease 

verification of compliance and provide much needed design targets for EGCS-SOx manufacture. 

The resulting criteria proposition following this logic is shown in Table 1 
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Table 1 Proposed discharge water limits (μg/liter) 

Criteria level* 1 2 3 

“Safety Margin” 1 10 100 

Nickel - 30 3 

Vanadium - - 150 

Copper - 40 4 

Lead - - 12 

THC 4500 (4.5 ppm) 450 (0.45 ppm) 45 (0.045 ppm) 

*At nominal wash water rate of 44 kg/kWh (representative of trials and dispersion models used) 

These proposals are made based on results from two specific trials. They should be seen in 

conjunction with experiences from other trials, studies as well as capabilities of water cleaning 

systems and means of verification. It is anticipated that some revision of the existing EGCS-SOx 

guideline, particularly chapter 7 (wash water monitoring) and chapter 17 (wash water), could be 

appropriate when using this approach. An additional cosmetic criterion demanding no visible 

discharge and absolute limit values irrespective of wash water rate could also be considered. 

Selection of predefined discharge criteria by port States 

In order to select predefined discharge criteria, the port State would have to identify the rate of 

water exchange and the anticipated discharge from scrubbers in the area in question. The 

concentration of contaminants in the area can then be calculated assuming different discharge 

levels and compared with PNECs as provided above or other as deemed necessary. The port state 

could then select the least stringent set of criteria which provide sufficient environmental 

protection. 
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Table of Contents 

1. Introduction ......................................................................................................................................7

2. Exhaust Gas SOx Cleaning Systemes (EGCS-SOx).........................................................................8

3. Principles for the development of EGCS-SOx discharge criteria................................................10

4. Identification of harmful compounds in EGCS-SOx discharge water........................................12

4.1 Harmful compounds from the exhaust gas .................................................................................12 

4.1.1Exhaust gas hydrocarbon compounds ................................................................................12 

4.1.2Exhaust gas Particulate Matter (PM)..................................................................................13 

4.2 Harmful compounds from the the sea water used to clean the exhaust ......................................15 

4.3 Harmful compounds and effect from the EGCS-SOx ................................................................15 

4.4 Results of discharge water analysis from ECGS-SOx trials.......................................................16 

4.4.1Acidity ................................................................................................................................18 

4.4.2Oxygen demand..................................................................................................................19 

4.4.3Particulate matter in untreated discharge water..................................................................19 

4.4.4Metals in untreated discharge water ...................................................................................19 

4.4.5Hydrocarbons......................................................................................................................20 

4.5 General remark – concentrations vs flux ....................................................................................20 

5. Toxicity of scrubber discharges and safe environmental levels....................................................20

5.1 Risk assessment of discharge components - the PEC/PNEC approach ......................................20 

5.2 PNEC derivation for relevant metals..........................................................................................21 

5.3 Compliance with other water regulation.....................................................................................22 

5.4 PNEC derivation of organic compounds ....................................................................................23 

5.5 PNEC derivation of other discharge compounds........................................................................25 

6. Prediction of environmental concentration ....................................................................................26

6.1 Modelling of discharge dilution from a single ship ....................................................................26 

6.1.1Dilution of discharges from a single ship in open sea ........................................................26 

6.1.2Dilution of discharges from a single ship at port operation................................................28 

6.1.3Dilution of discharges from a single ship at quay side manoeuvring.................................28 

6.1.4Summary: Dilution of discharges from a single ship .........................................................28 

6.2 Accumulation of discharges – local and regional effects ...........................................................29 

6.2.1The Oslo Fjord Case study – regional effects.....................................................................29 

7. Current discharge criteria................................................................................................................31

8. Proposed waste water discharge criteria for exhaust gas scrubbers............................................31

8.1 Sensitivity of marine and brackish water organisms to contaminants........................................31 

8.2 Proposed waste water criteria for seawater scrubber discharges from vessels at sea .................32 

8.3 Proposed waste water criteria for seawater scrubber discharges in closed waters .....................33 

9. GLOSSARY ....................................................................................................................................36

10. REFERENCES..................................................................................................................................37

APPENDIX 1: Report CFD-Norway 

APPENDIX 2: Memo Sintef Materials & Chemistry 
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1. Introduction 

Emissions to air from international shipping are regulated by IMO in MARPOL Annex VI which 

entered into force on May 19th 2005. Regulation 14 of MARPOL Annex VI limits sulphur 

emissions by placing a global limit on the sulphur content of marine fuels of 4.5% by mass. 

Regulation 14 further limits sulphur emissions in certain geographical areas, so called Sulphur 

Emission Control Areas (SECAs). In these areas, ships may only use fuels containing up to 1.5% 

sulphur or employ an exhaust gas SOx cleaning system (EGCS-SOx) that delivers an equivalent 

reduction in sulphur emissions. The SECA regulations will be effective for the Baltic Sea from 

May 2006 and for North Sea & English Channel from November 2007. 

 

Discharges from exhaust gas SOx cleaning system are regulated in regulation 14(4)(b) which inter 

alia reads: ...Waste streams from the use of such equipment shall not be discharged into enclosed 

ports, harbours and estuaries unless it can be thoroughly documented by the ship that such waste 

streams have no adverse impact on the ecosystems of such enclosed ports harbours and estuaries, 

based upon criteria communicated by the authorities of the port State to the Organization. The 

Organization shall circulate the criteria to all parties to the Convention... 

No port State has, at the time of writing, communicated such criteria to the IMO. 

Some guidance to the monitoring and handling of discharges are included as part pf the 

Guidelines for on-board exhaust gas-SOx cleaning systems adopted by IMO in resolution 

MEPC.130(53). 

According to this guideline, the EGCS-SOx unit’s wash water systems should: 

(a) eliminate, or reduce to a level at which they are not harmful, hydrocarbons, carbon 

residue, ash, vanadium, other heavy metals, and other substances contained within 

EGCS-SOx unit’s wash water that may have an adverse impact on ecosystems if 

discharged overboard, 

(b) ensure that the approach adopted, to control wash water quality and residual waste 

is not achieved in a way that causes pollution in other areas or environmental 

media, 

(c) also taking into account guidelines to be developed by the Organization. 

As of today, the guidelines referred to in bullet (c) of the Guidelines for on-board exhaust gasSOx cleaning systems have not been developed. At its last meeting, the appropriate committee, 

MEPC 54, failed to make progress in the development of these guidelines, principally due to lack 

of information. MEPC 54 thus decided to extend its call for information on this matter. 
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The lack of IMO guidance and port State criteria makes the specification of EGCS-SOx unit’s 

difficult. Such criteria are thus urgently needed. 

The work contained in this report has been done in an effort to contribute to the development of 

discharge criteria. The result of this work will be communicated to the Norwegian Maritime 

Directorate who in turn will submit results to the IMO as deemed appropriate. 

EU regulations are not of primary interest in this report which is written in the interest of the IMO 

process. It should, however, be mentioned that the European Comission has also implemented 

1.5% sulphur SOx emission regulation for ships in SECA area through Directive 2005/33/EC. The 

European Union basically impose the same requirements as IMO within SECA and the additional 

requirement that: 

1. the same 1.5% sulphur limit applies to fuels used by passenger vessels on regular services 

between EU ports from 19 May 2006; 

2. fuel used by inland vessels and by seagoing ships at berth in EU ports is limited to 0.1 % 

sulphur, from 1 January 2010 

Directive 2005/33/EC also opens for exhaust gas SOx cleaning as long as equivalent or higher 

reduction in sulphur emissions can be demonstrated with no adverse effect on the environment. 

2. Exhaust Gas SOx Cleaning Systemes (EGCS-SOx) 

It is generally acknowledged that EGCS-SOx units, at least in the short term, will be based on the 

principle of sea water scrubbing with or without the use of a chemical additive. These units have 

the potential to remove a very high fraction of the SO2 in the exhaust. The use of such equipment 

can thus permit much larger emission reductions than is achievable with low the use of sulphur 

residual fuels. 

Exhaust gas scrubbing is a process where the exhaust gas is contacted with a liquid (most often 

seawater) for selective absorption of SO2. Once the SO2 is absorbed into the sea water, the SO2 is 

converted by reaction with alkaline material in the liquid to SO3 and finally SO4. The conversion 

of the SO2 helps the sulphur to stay in the water and allows more SO2 to be absorbed. For this 

reason, the alkalinity of the water is important to the efficiency of the scrubber. The alkalinity in 

seawater is normally a function of salinity according to the relation 

alkalinity (mmol/l) = 0.0697* salinity (psu) 1

1

 PSU = Practical Salinity Unit, in practice the same as per mille 
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This relation may be modified and even reversed in areas of strong freshwater input to the sea 

where the alkalinity of the freshwater is high due to mineral composition in the watershed. Rivers 

running trough a calcite area will have increased alkalinity. In such situations the calculation 

above gives an underestimate of the alkalinity, and hence represents a worst case. 

Since the alkalinity of sea water is comparatively low, large quantity of sea water is needed. Even

more scrubbing water would be needed in brackish water regions such as the Baltic Sea. The 

alkalinity of sea water can in certain regions also vary significantly with time due to variabtions in

winds, currents, melting of ice etc. 

Different scrubber designs have been developed to obtain maximum contact area between the sea 

water and exhaust gas. The exhaust gas may be significantly cooled in the scrubber. To preven 

white smoke and ensure a buyoyant exhaust, the exhaust may be reheated. One option is heat 

exchange between the hot exhaust from the engine and the cold exhaust leaving the scrubber. A 

principal arrangement of a sea water scrubber is shown in Figure 1. 

Figure 1 Principal arrangement of seawater scrubber unit 
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The amount of water needed to clean the exhaust will depend on a number of factors including 

system design, fuel sulphur content, SOx emission target, alkalinity of the water and use of 

chemicals. Some field testing experience indicates that pure sea water consumption needed for 

near-complete (~ 90%) SOx removal will be in the range of 40-100 kg/kWh. This water 

requirement can be 

• Approximately halved if the system is designed according to current minimum 

requirements (emission equivalent to 1.5% sulphur fuel) 

• Significantly increased in fresh water and brackish water 

• Very significantly reduced if alkaline chemicals are added to the sea water 

Sea water scrubbing will also remove some of the particulate matter generated during combustion. 

Sea water scrubbing is also capable of removing some NO2 from the exhaust. Since the fraction of 

NO2 in NOx is usually about 5% (the remainder being NO which is not appreciably affected), the 

potential for NOx removal is limited. (The use of chemical additives could potentially alter this) 

The principal challenge with sea water scrubbing is not solely to reduce the SOx emissions, but to 

do so while: 

• maintaining a buoyant exhaust 

• maintaining minimum consumption of space, weight and energy 

• maintaining minimum pressure drop in the exhaust 

• keeping discharge water emissions at a level where it is not harmful 

• preventing corriosion 

• prevent water droplets in exhaust 

Discharge of wash water is regulated in ports, harbours and estuaries. In case of scrubbers using 

chemical additives, temporary storage may be possible; however in case of scrubbers relying on 

sea water only, storing used wash water on-board, even termporarily, will be difficult due to the 

large quantities needed and because the wash water will be corrosive. Hence the wash water will 

generally have to be cleaned (as neccesary) and discharged continuously. 

As indicated in the introduction to this report, discharge criteria for EGCS-SOx wash water 

composition must be developed. 

3. Principles for the development of EGCS-SOx discharge criteria 

Man-made impact on the marine ecosystem is expressed at all levels of biological organisation, 

from sub-individual (biochemical and physiological effects), via individual and population levels 

to the ecosystem level. Covering all these levels of effects in a risk assessment context is 

impractical and will not necessarily result in a better protection of marine resources. 
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GESAMP (the Joint Group of Experts on the Scientific Aspects of Marine Pollution in the UN 

system) has given the following definition of marine pollution: “Pollution means the introduction 

by man, directly or indirectly, of substances or energy into the marine environment (including 

estuaries) resulting in such deleterious effects as harm to living resources, hazards to human 

health, hindrance to marine activities including fishing, impairment of quality for use of seawater 

and reduction of amenities” (GESAMP 1991). 

Since man made effects on individual organisms not necessary results in effects on a higher 

ecological level, the definition limits pollution to those perturbations which have an impact on 

populations and/or ecosystems. This limitation is very important when assessing significant 

spatial end temporal impacts from scrubber discharges. 

In practical terms, this means that discharges resulting in local concentrations that are higher than 

no-effects limits for chronic exposure can be acceptable if the discharge is rapidly dilluted. Part of 

the challenge when defining discharge criteria is deciding how rapid the dilution must be or – in 

other words – how large water volume can be permitted to be ‘affected’ without resulting in 

effects on any marine populations. Naturally, this may vary from one location to another. The 

following steps must generally be followed in order to develop such discharge criteria: 

The first step in the process of the development of discharge criteria is the identification of 

harmful compounds that must be controlled. 

The second step is identifying the maximum permissible concentration that can be sustained over 

time for each of the compounds identified. This level is often designated as PNEC (Predicted No 

Effect Concentration). The PNEC-concentration is derived from scientific toxicology studies, 

either by use of species sensitivity distribution curves or by applying safety factors to realistic 

worst case toxicity levels. These factors are very high when the amount of relevant studies is low 

and less conservative when the toxicity to several species is better known. 

The third step is to evaluate the expected environmental concentration of each discharge 

compound, often designated as PEC (Predicted Environmental Concentration). A risk to the 

marine environment is indicated if the PEC is higher than the PNEC. According to the GESAMP 

principle the risk will be acceptable if PEC exceedes PNEC only in the immediate vincinity of the 

discharge point, which may result in effects on individuals but not on populations. However a 

PEC>PNEC value cannot be accepted for a larger area where the risk for population effects is 

present. 

Port States will have to consider both the extent of ship traffic, and natural dillution effects such 

as provided by wind, current and rivers when defining their local criteria. To avoid prolifieration 

of criteria it is advised that IMO develops a set of default criteria that represent typical situations. 

(Level 1, 2, 3, etc) The Port State would then have the opportunity to choose one of the criteria on 

the list which would significantly ease verification of compliance. 
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4. Identification of harmful compounds in EGCS-SOx discharge water 

The discharge water from the EGCS-SOx unit contains material from three sources: 

1. the exhaust gas exiting the engine (air, fuel, lubricant and combustion products); 

2. the sea water used to clean the exhaust 

3. the scrubber itself (possible reaction products and possible chemical additives) 

4.1 Harmful compounds from the exhaust gas 

The composition of the exhaust gas will vary with engine type, engine age, operating conditions, 

fuel and lubricating oil composition. Table 1 indicates concentration levels of the main 

constituents in the exhaust gas from 4 stroke diesel engines. The concentration can vary 

considerably for some of the constituents depending on engine load (fuel/air ratio) and fuel 

composition 

Table 2 Typical exhaust gas concentration levels in exhaust gas from 4 stroke diesel engines. 

Component Concentration level (Vol.-%) 

Nitrogen - N2 75,6 (74,3-77) 

Oxygen - O2 12,5 (11,25-15,5) 

Carbon dioxide - CO2 6,2 (4,0-6,5) 

Water vapour - H2O 4,5 (4-8) 

Argon - Ar 0,9 

Nitrogen oxides - NOx 0,15-0,20 

Sulphur oxides - SOx 0,01-0,15 

Hydrocarbon compounds HC, <0,03 

CO, <0,03 

Particulate matter 0.5-1.5 g/kWh on heavy fuel 

All substances in Table 2 can be captured or to some extent entrained in the EGCS-SOx

washwater. Of the elements on this list, however, it is mainly the particulate matter and 

hydrocarbons that is of interest as potentially harmful pollutants. Particulate matter is of concern 

primarily due to its content of potentially harmful contaminants (cf chapter 4.1.2.). 

4.1.1 Exhaust gas hydrocarbon compounds 

The hydrocarbon compounds are principally derived from fuel that has not been burned or is only 

partly burned. Hydrocarbon compounds can also originate in lubricating oil. Hydrocarbon 

compounds are generally biodegradable. Certain hydrocarbon compounds are more harmful than 

others. In particular, the polyaromatic compounds PAH. Some of the hydrocarbon compounds, 

and particularily the larger molecules such as PAHs, will contribute to the formation of particles 
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or associate strongly with soot particles. The issue of PAHs is discussed futher in conjunction 

with particulate matter. 

4.1.2 Exhaust gas Particulate Matter (PM) 

Particulate matter is small, solid or liquid substances that can be collected on a filter in the exhaust 

stream. The formation of the particles is an accumulation process. The amount and size of the 

particles will depend upon where and how the particulate matter is collected. There are two 

fundamentally different standard procedures for the measurement of PM. 

ISO 8178 – where particles are collected after dilution in air which promotes particle 

growth by condensation 

ISO 9096 – where particles are collected on a hot filter in order to avoid condensed 

matter 

The total amount of PM measured depends on the measurement method and fuel quality. An 

example is given in Table 3. 

Table 3 Particulate mass PM in exhaust gas as a function of the fuel sulphur content. 

Fuel sulphur content ISO 8178 ISO 9096

% g/kWh g/kWh 

1.0 0.6 0.2 

2.5 1.2 0.4 

Particulate emissions are affected substansially by the ash and sulphur content of the fuel used. 

Particle size is variable, but of great importance because it affects dispersion and how harmful 

they are. Figure 2 illustrates classification and variation in particle size. 

The composition of particles of different sizes is different. Generally, the smallest particulates are 

condensed water, sulphuric acid and hydrocarbons. Intermediate particles contain more 

carbonaceous compounds, metal oxides, and ash. Larger particles contain relatively less 

carbonaceous compounds and are dominated by metal oxides and ash. 

The particulate matter contains a variable mixture of compounds, most importantly 

• Soot 

• Ashes 

• PAH and PAH-derivatives. 

• Aldehydes, alkanes and alkenes 

• Unburned fuel or incombustible elements in the fuel 

• Metal salts like sulphates 
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Figure 2 Particulate size and classification 

Fine particles PM 2,5 Particulate matter PM10

0,01 0, 1 2,5 10μm

10 100 1000 2500 10000

Many of the PAHs and PAH-derivatives, especially nitro-PAHs, have been found to be potent 

mutagens and carcinogens. Analysis data of particles generated in a medium speed engine using 

residual fuel oil 380 cSt/50 o

C at 100 % load is shown in Table 4. The analyses refer to dry 

particles. 

Table 4 Analysis of marine diesel particulate matter 

Metal oxides and sulfates 65 % 

Carbon (soot) 25 % 

Hydrocarbons (fuel and lube oils) 10 % 

including traces of 

 PAH 17 ppm

Plychlorinated biphenyls (PCB) 17 ppm

 Chlorophenols 40 ppm

 Chlorobenzenes 44 ppm

Polychlorinated dibenzodioxins (PCDD) 3,9 ppb 

Polychlorinated dibenzofurans (PCDF) 2,8 ppb 

Table 5 gives an example of a measurement performed with the ISO 9096 method of a W6L20 

medium speed engine operating on high ash HFO (2.6 % S) and 100 % load, in %-mass. 

nm

Ultrafine particles
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Table 5 Elemental composition of particles of different size (%) 

90 nm particles 8 micron particles

Silicon Si 0.5 0.3

Sulphur S 11.2 11.8

Nickel Ni 1.9 1.1

Sodium Na 3.5 0.8

Calcium Ca 0.3 0.1

Oxygen O 43.3 34.6

Carbon C 10.6 41.8

Iron Fe 0.4 -

Vanadium V 28.2 9.6

TOTAL 100.0 100.0

Some fuel constituents are uncombustible. In general, these uncombustible compounds will be 

emitted from the engine as particulate matter. Therefore, some aspects of the paticle composition 

will mirror the fuel composition. 

4.2 Harmful compounds from the the sea water used to clean the exhaust 

Naturally, any harmful compounds entering the EGCS-SOx unit with the seawater used for 

scrubbing will be expected to come out as part of the wash water. This is, however, not an 

additional pollution, and the issue is not generally expected to be problematic, however 

background levels must be considered when determining discharge limits and monitoring. 

4.3 Harmful compounds and effect from the EGCS-SOx 

Harmful compounds can enter the washwater as result of chemical reactions in the EGCS-SOx 

unit. The absorption of SOx will also cause the seawater pH to drop and consume dissolved 

oxygen. 

Chemicals may be added to improve the SOx removal or reduce the water consumption needed to 

achieve a given SOx reduction rate. This is particulary attractive when the alkalinity of the sea 

water is low. The most common additives in land based plants are lime and sodium hydroxide. 

Sodium hydroxide (NaOH) can be supplied either as granulate or as concentrated solutions (up to 

40 %). Handling and dosing is more convenient in the liquid form. The use of NaOH does not 

result in increased precipitation since sodium sulphate and sulphite are very soluble. 

Neutralization with lime or Ca(OH)2 results in the formation of large amounts of gypsum 

(CaSO4). Although the product is considered harmless to the environment it could create a visible 

plume after the ship which may be unwanted. Aslo, the product is insoluble in water and tends to 
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settle in tanks and pipes, particularily where the flow is slow. For practical reasons lime is 

generally considered unsuitable for use onboard a marine vessel. 

Small amounts of wear metals from the scrubber may also show up on analysis of scrubber water. 

4.4 Results of discharge water analysis from ECGS-SOx trials 

MARINTEK has coordinated two ship trials of ECGS-SOx units. Both units were based on 

scrubbing exhaust with pure seawater, and neither used any after-treatment of the discharge water. 

Some key information from these two trials is shown in Table 6. 

Table 6 Key information from ECGS-SOx trials coordinated by MARINTEK 

Ship Kronprins Harald Fjordshell 

Propulsion Engines Wärtsilä Sulzer 12ZAV40 * 2, 

6ZAL40 * 2 (4 stroke) 

Sulzer 6RND76, 10400 kW (2 

stroke) 

Ship Owner Color Line Norske Shell Shipping 

Test Year 1991 1993 

Description Part flow prototype using spray, 

high swirl (cyclone) and 

packaged bed with counter 

current water flow. 

1:10 scale prototype with inlet 

venturi/ spray and mist catchers 

based on inert gas generator. 

Combined with SCR 

Water consumption 64 m3/h at 8 bar ~ 50 kg/kWh at maximum load 

(constant water rate) 

Measured SO2 reduction 57 % in port 

 (from 70 to 30 ppm) 

73% in transit 

(from 454-121 ppm) 

90 % over a wide range 

The scrubbers and analysis have been made at different operating conditions: 

Transit: Loaded ship at service speed 

Port operation: Approaching port at reduced speed (4 knots) 

Quay side maneuvering: (0 knots), same power as port operation 
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Table 7 Discharge water analysis data 

Washing 

water 

characteristic 

Unit Intake/Outlet Fjordshell M/S Kronprins Harald 

Transit Port 

operation 

Transit Port 

operation 

Salinity p.p.t 19,1 19,0 17,4 14,5 

Temperature Intake 12,6 9,8 n.a. n.a. oC 

Outlet 23 17,2 n.a. n.a. 

SO4 g/l Intake 2,71 2,76 1,4 1,18 

Outlet 2,91 2,82 1,83 1,46 

SO2 mg/l Intake n.a n.a <0,5 <0,5 

Outlet n.a n.a <0,5 57 

pH Intake 8,01 7,96 8,11 7,95 

Outlet 2,95 3,76 3 2,7 

TOC mg/l Intake 2,1 n.a. n.a. n.a. 

Outlet 2,7 1,5 n.a. n.a. 

COD mg/l Intake n.a n.a. n.a. n.a. 

Outlet 33,3 10 n.a. n.a. 

Particles and Intake 0,42 4,3 0,6 2,3 

soot 

mg/l 

Outlet 0,68 0,88 14,7 14,7 

Hydrocarbons mg/l Intake 0,031 0,053 <0,1 <0,1 

Outlet 0,081 0,045 1,0 1,7 

AOX μg/l Intake 62,5 43,9 17 38 

Outlet 25,3 33,2 700 51 

PAH μg/ Intake <0,1 <0,1 n.a. n.a. 

Outlet <0,1 <0,1 n.a. n.a. 

PCDD pg/l Intake 59,8 n.a. n.a. n.a. 

Outlet 13,7 n.a. n.a. n.a. 

PCDF pg/l Intake 50,3 n.a. n.a. n.a. 

Outlet 291 n.a. n.a. n.a. 

Table 8 Measurement principles used in trials – discharge water 
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Parameter Measurement principle 

Water sample particulate 

material weight 

Norwegian Standard NS 4733 

Salinity Titration by silver 

pH Electrodes 

SOx Gravimetry(BaSO4) 

THC Gas Chromatography 

PAH Gas Chromatography 

AOX SCAN-W9-89 

Cu, Pb, Cd, Ni, V Atomic absorption, NS 47771 

As, Hg, Zn Atomic absorption – hydride method 

Cl Titration by silver 

Cr6+ Photometric (diphenylcarbazid) 

PCDD (polychlorinated 

dibenzo-p-dioxines) 

Gas chromatography, Mass spectroscopy, conforming to EN-450001 

PCDF (polychlorinated 

dibenzofurans) 

Gas chromatography, Mass spectroscopy, conforming to EN-450001 

Temperature Chromel-Alumel thermocouple 

Flow Electromagnetic flowmeter (Fisher&Porter ND100) 

Table 9 Measurement principles used in trials – other parameters 

Parameter Measurement principle 

Gas flow Pitot tube conforming to ISO 3966 

CO NDIR, Rosemount Binos 100, calibrated at 804 ppm 

CO2 NDIR, Rosemount Binos 100, 0-20%, calibrated at 4,13% 

O2 Paramagnetism, Signal 8000, 0-25%, calibrated in air (20.95%) 

NOx CLA, Signal 4000, Calibrated at 855 ppm 

HC FID, IPM Ratfish calibrated at 84 ppm 

SO2 * Absorbtion(H2O2), distillation and titration 

Particulate material 

(exhaust) 

Collected in dillution tunnel (ISO 8178-1), Weighted in lab using 

Mettler Microweight AE163. 

NH3 * Absrorption (5% H2SO4 ), distillation and titration 

N2O * Absrorption (3M NaOH), drying (Mg(ClO4)) 

Noise Hand portable instrument 

* Analysed in laboratory 

4.4.1 Acidity 

Sulphur dioxide and NOx are known as the main constituents of acid rain. In the scrubber a very 

small part of NOx (less than 5 %) will be absorbed, unless specific absorbents (e.g. ammonia) is 
19

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added to the water. Carbon dioxide does not contribute to the acidity. The acidity of the wash 

water is therefore mainly due to the sulphur compounds and especially sulphurous and sulphuric 

acids. Environmental aspects of low pH are discussed in chapter 5.5. 

Example: Assuming a fuel with 4 % sulphur (m/m) and a specific wash water consumption of 80 

kg/kWh and 100 % absorption efficiency a sulphur concentration of 100 mg/l (3.125 .

 10-3 mol/l) 

can be expected. If sea water with salinity 35 PSU is used, the pH of the water before dilution can 

be calculated to about 3.25 

4.4.2 Oxygen demand 

It is expected that the oxidation of sulphite (HSO3

-

) to sulphate (SO4

2-) ion is the main contributor 

to the Chemical Oxygen Demand (COD) of the washing water. However, there may also be other 

components, e.g NOx, uncombusted fuel, VOC or others that contribute to the COD. 

Example: Assuming the same condition as in the previous example (4 % S and 80 kg water/kWh ) 

and provided that 100 % of the sulphur is on the bisulphite form, it can be calculated that the COD 

resulting from sulphur alone will be 50 mg /l before dilution. 

In the Fjordshell experiments the COD was found to be 33 mg/l under transit operation and 10 

mg/l under port operation. In the M/S Kronprins Harald experiments COD was not analysed, but 

under transit operation no SO2 was detected and under port operation only a small part of the 

absorbed sulphur was detected as SO2. The concentration of SO2 was found to be 57 mg/l, 

representing only 23 % of the total absorbed sulphur. This is equivalent to a COD of 14 mg/l. 

The results so far indicate that a substantial part of the SO2 is oxidized to sulphate during the 

combustion and/or scrubbing process. 

4.4.3 Particulate matter in untreated discharge water 

The amount and composition of particles generated by marine diesel engines are influenced by the 

combustion process and the type of fuel used. Some of the particulate matter will be trapped in the 

scrubber water, resulting in particulate pollution of the discharge. Particle concentration in the 

wash water from two different scrubbers were found to vary from less than 1 to about 15 mg/l 

(Table 7). 

Micropollutants are to a large extent associated with particles or suspended in the wash water. The 

composition of particles may be divided in the three main groups; 1) metal oxides and sulphates, 

2) carbon (soot), and 3) other organic compounds. 

4.4.4 Metals in untreated discharge water 

Table 10 gives examples of concentrations of specific heavy metals found in wash water from 

scrubbers. 
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Table 10 Examples of heavy metal concentrations observed in scrubber discharge water. 

Substance Unit Fjordshell 

Transit load Port load 

Vanadium (V) mg/l 0.035 0.023 

Nickel (Ni) μg/l 32.8 10.4 

Zinc (Zn) μg/l 6 15 

Chromium (Cr) μg/l <1 <1 

Lead (Pb) μg/l 5 0.6 

Copper (Cu) μg/l 41.6 15.3 

Mercury (Hg) μg/l <0.1 <0.1 

Arsene (As) μg/l <0.1 <0.1 

Cadmium (Cd) μg/l 0.05 0.08 

Specific water 

consumption 

kg/kWh ~ 50 ~ 150 

Generally an increase in the content of heavy metals across the scrubber is observed. 

4.4.5 Hydrocarbons 

As shown in Table 7, the concentration of hydrocarbons in the wash water varies from 0.05 to 1.7 

mg/l. 

4.5 General remark – concentrations vs flux 

The concentration of pollutants in the discharge water will naturally be reduced when the amount 

of wash water is increased, and the pH reduction will be smaller. In this case, the water can be 

considered partly diluted before discharge. However, the mass flow of many pollutants may be 

virtually the same. 

Especially for non-degradable constituents the concentration at the discharge point is less relevant 

than the long term concentration in a larger area. For this purpose, it is the flux or mass flow of 

pollutants that is important, not the concentration. 

5. Toxicity of scrubber discharges and safe environmental levels 

5.1 Risk assessment of discharge components - the PEC/PNEC approach 

In order to perform a risk assessment of scrubber water discharges to the sea it is necessary both 

to evaluate the expected environmental concentration of each discharge compound, the PEC 

(Predicted Environmental concentration) and the toxic properties of the same compound the 
21

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PNEC (Predicted No Effect Concentration). If the PEC/PNEC ratio is >1 then a risk to the marine 

environment is indicated. Usually, as a first approach both PEC and PNEC is estimated 

conservatively, meaning that realistic worst case assumptions are included in both PEC and PNEC 

derivation. Therefore if a PEC/PNEC<1 the risk to marine life should be negligible. If a 

PEC/PNEC>1 is arrived at during the first approach one should investigate whether it is possible 

to refine the PEC and PNEC estimates and make them more realistic for the actual situation. In 

this case refinement can be achieved by performing measurements either of actual releases in the 

case of PEC or performing chronic toxicity studies on relevant aquatic organisms in the case of 

PNEC. If the PEC/PNEC ratio is still >1 then risk reduction measurements have to be 

implemented leading to reduction in releases. Such a reduction can either be achieved by treating 

the effluent or if this is not effective, prohibiting the processes that give rise to the emissions. 

5.2 PNEC derivation for relevant metals 

The following metals have been found relevant for risk assessment concerning release of Scrubber 

effluent; Nickel (Ni), Copper (Cu), Zinc (Zn), Quicksilver (Hg), Lead (Pb), Arsen (As), Cadmium 

(Cd), Chromium (Cr) and Vanadium (Va). Most of these metals have other release soursces and 

have therefore been evaluated with regard to aquatic toxicity both nationally and internationally. 

UK and USA have developed quality standards for several of these metal compounds and within 

the EU Directive for existing chemicals (Commission regulation No 1488/94), PNEC values have 

also been derived. As EU PNEC values may have limited international acceptance, national 

quality standards may have to be employed when these are lower than the EU PNEC values in 

order to ensure international acceptance of the risk assessment. Table 11 contains a summary of 

available PNEC values for metals which has undergone evaluation within EU existing chemicals 

regulation. Notice that for some metals both a marine and freshwater PNEC has been derived. 

Lack of a specific marine PNEC is mainly due to that one has not found scientific basis that 

indicate a significant difference in the toxicity between organisms in these two compartments. 

Therefore use of a freshwater PNEC should adequately protect also marine organisms. 

Table 11 Metals with PNECs suggested by EU risk assessment program for existing 

chemicals (regulation no 1488/94) 

Metal PNEC consept HC5 AF PNEC(μg/l) 

Cd (fresh) SSD total 0.38 2 0.19 

Cu (fresh) SSD total 8.2 1 8.2 

Ni (fresh) SSD total 6.43 3 2.1 

Pb (fresh) SSD total 14.5 1 14.5 

Pb (marine) SSD total 8.3 1 8.3 

Zn (fresh) SSD added 15.6 2 7.8 

Zn (marine) SSD added 6.1 2 3.5 
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For those metals not having been assessed in the EU risk assessment program for existing 

chemicals the data from Crommentuijen et al (1997) may be used. The Maximum Permissible 

Concentrations (MPC) derived by Crommentuijen et al (1997) are assessed according to the same 

methods as in the EU risk assessment program for existing chemicals. The MPC is similarly 

derived as HC5 in Table 1, but with addition of a mean background level in the environment. In 

order that these values should be equal to PNEC it is necessary to divide by an assessment factor 

(AF). It seems that an AF=2 most closely match current practice in EU as indicated in Table 1. 

Except for As, Cr and V where only freshwater MPCs are available, these estimates include 

marine tests. 

Table 12 shows MPCs for some metals which have not been assessed in the EU risk assessment 

program for existing chemicals. The data is from Crommentuijen et al (1997). The PNEC is 

derived using an AF=2. 

Table 12 MPCs for some metals not assessed in EU risk assessment program for existing 

chemicals. 

Metall PNEC consept MPC (μg/l) PNEC (μg/l) 

As (fresh) SSD total 25 12.5 

Cr (fresh) SSD total 8.7 4.3 

Hg (fresh) SSD total 0.24 0.12 

Hg (marine) SSD total 0.23 0.11 

V (fresh) Lowest NOEC 

AF=1000 

4.3 4.3 

5.3 Compliance with other water regulation 

In order that the PNECs used in the risk assessment are internationally accepted they should also 

comply with national water quality standards. Both UK, EU (The EC Dangerous Substances 

Directive (76/464/EEC) and USA EPA have developed national standards for many compounds. 

These are shown in Table 13 for the relevant metals. The US EPA Water Quality Standards 

(WQS) are based on a scientific assessment of available ecotoxicological information, and deviate 

in some cases significantly from the European PNEC values. For Hg, US EPA has taken into 

account also risk to humans via biomagnification in the food chain. The Environmental Quality 

Standards (EQS) of EU are established through a similar procedure as US EPA. There is good 

agreement in levels between these two standards, except again with respect to Hg were EU has a 

10x higher value than US EPA. 
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Table 13 Relevant US EPA and UK Water Quality Standards. 

compound US EPA WQS 

μg/l freshwater 

US EPA WQS 

μg/l seawater 

EQS (UK) 

μg/l seawater 

Cd 0.66 8.8 2.5 

Ni 56 7.1 30 

Pb 1.3 5.6 25 

Zn 180 58 40 

As 190 36 25 

Cr 11 50 15 

Cu 6.5 2.9 (1 hr max) 5 

Hg 0.012 0.024 0.3 

V 100 

The derived metal PNECs are all below the shown aquatic WQSs, the only exception being the 

very low US WQS value for Hg. One should therefore proceed with the risk assessment using the 

PNECs presented here. In addition Hg should be assessed also using the US EPA derived WQS 

value of 0.024 μg/l for Hg in order to be able to conclude that also the human endpoint associated 

with the WQS value is assessed. 

5.4 PNEC derivation of organic compounds 

The organic compounds that might be of concern as identified in Chapter 4 are hydrocarbons 

(THC), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dioxins 

(PCDD/PCDF). Except to some extent the THC, these compounds together with the metals are 

associated with the total particulate matter (PM) being trapped in the scrubber water (cf Chapter 

4.1.2). 

OSPAR (1996) developed Ecotoxicological Assessment Criteria (EAC) for a large range of 

chemicals to facilitate evaluation of monitoring data on toxic substances in the marine 

environment. The EAC values were derived essentially in the same way as the PNEC values given 

above (based on available toxicity test data and use of application factors) and should therefore in 

principle be applied as PNEC values. This is proposed in the present context. The OSPAR EAC 

values for the relevant compounds are given in Table 14. Later on OSPAR (2004) provided EAC 

values for groups of PAH compounds according to molecular size (Table 15). PNEC values for 

THC, total PAH or sumPAH16 and for dioxins are not available from this source. 

Most of these organic substances will be adsorbed to particles in the discharge and due to their 

strong particle association (especially to soot particles) they will to a large extent be particle 

bound also after discharge. An impact assessment should thus focus on their behaviour as part of 

the PM, e.g they might settle to the sediment with other PM over time. The EAC-sediment might 

therefore be the most relevant assessment criteria, but modelling of PM settling behaviour will be 

highly unreliable. 
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Table 14 Ecotoxicological Assessment Criteria (EAC) for PCBs and PAHs as proposed by 

OSPAR (1996). 

Substance EAC-water (μg/l) EAC-sediment (mg/kg dw) 

PAHs 

Naphtalene 5 - 50 f 0,05 – 0,5 f 

Phenanthrene 0,5 – 5 p 0,12 – 1 f 

Anthracene 0,001 – 0,01 p 0,05 – 0,5 f 

Fluoranthene 0,01 – 0,1 p 0,5 – 5 p 

Pyrene 0,05 – 0,5 p 0,05 – 0,5 p 

Benzo(a)anthracene n.d. 0,1 – 1 p 

Chrysene n.d. 0,1 – 1 p 

Benzo(a)pyrene 0,01 – 0,1 p 0,1 – 1 p 

sumPCB7 n.r. 0,001 – 0,01 p 

f: firm p: provisional n.d.: no/insufficient data n.r.: not relevant vs OSPAR monitoring 

Table 15. Ecotoxicological Assessment Criteria (EAC) for PAH groups as proposed by 

OSPAR (2004). 

PAH group EAC-water 

(μg/l) 

EAC-sediment 

(mg/kg ww) 

2 rings: naphtalene 2,4 p 0,038 p* 

3 rings: phenanthrene& anthracene 0,17 p 0,031 p* 

4 rings: fluoranthene, pyrene, benzo(a)-

anthracene, chrysene

0,69 p 0,141 p* 

5 rings: benzo(k)fluoranthene, benzo(a)pyrene 0,03 p 0,021 p* 

6 rings: benzo(ghi)perylene, indeno(123-cd)-

pyrene 

0,016 p 0,0037 p* 

p: provisional * sediment normalised to 1% organic carbon 

Agreed PNEC values for THC have not been found in the available literature. A value of 40,4 μg/l 

has been proposed by Scholten et al 1993 for the aliphatic fraction of the oil (the aromatic fraction 

should be covered by the PAH criteria) and is adopted by the Norwegian Oil Operators 

Association (OLF) for risk assessment of offshore produced water (Johnsen et al 2000). The large 

wealth of toxicity data on oil hydrocarbons is difficult to condense due to variability in 

experimental conditions, analytical procedures and oil composition, but there appears to be 

consensus that chronic environmental impact of oil is observed when concentration of THC 

exceeds 50 μg/l, which is close to the aliphatic PNEC given above. We therefore propose that the 

PNEC for THC is set at 40 μg/l. 
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5.5 PNEC derivation of other discharge compounds 

The other discharge components of concern would be pH, total particulate matter (PM), total 

organic carbon (TOC) and potential chemical oxygen demand (COD). The PM and TOC are both 

typical features of the seawater and vary considerably in space and time. PM originates from a 

range of natural (e.g. river runoff, resuspension of bottom sediments, phytoplankton growth 

intensity) and manmade sources (e.g. industrial and sewage outfalls, land runoff, mining, 

dredging). PM levels may be extremely patchy in nearshore waters. We consider PNEC values for 

PM and TOC as of minor importance in the environmental risk assessment. 

Review of the literature on pH tolerance indicates that there is little evidence of harm to marine 

organisms from a decrease of 0.5 –1 pH units from ambient (Knutzen 1981). It is also suggested 

that marine organisms are more sensitive to a rise in pH than a decrease (Wolff et al 1988). Table 

13 shows some national EQS (Environmental Quality Standards) values for pH. The strictest is 

given by the US EPA allowing not more than 0.2 pH unit deviation from what is the normal pH 

range of the ambient water. This implies a variable PNEC depending on the natural pH-levels 

which normally range from 7.0 to 8.5 (partially correlated with primary production). Otherwise 

we would propose a PNEC for pH of 7.0 which according to the UK Water Research Centre 

should safeguard both fish and shellfish at chronic exposure. 

Table 16 Examples of national environmental quality standards (EQS) for pH in marine 

waters (From Wolfe et al 1988).

EQS given by value 

US EPA 6,5 – 8,5 1) 

Australia 6,5 – 8,5 

China 7,5 – 8,3 

Japan 7,8 – 8,3 

UK Water Research Centre, protection of shellfish 7,0 – 8,5 

UK Water Research Centre, protection of fish 6,0 – 8,5 

1) but not more than 0,2 units outside normally occurring range 

Although many countries have discharge limits for COD (typically 150 - 350 mg/l) there are no 

environmental standards. This is because COD will manifest itself through oxidation and hence by 

reducing the dissolved oxygen correspondingly. This process takes time which implies that there 

will be a gradual oxygen consumption taking place concurrent with dispersion and dilution. An 

assessment will therefore have to evaluate to what extent the COD will contribute to the risk of 

reduced dissolved oxygen in the receiving waters. A worst (but not entirely realistic) case is that 

the COD is oxidised immediately. 

There is a wealth of literature showing that tolerance to reduced dissolved oxygen in seawater 

varies considerably dependent on organism, life stage and environmental conditions. A 

concentration that should safeguard most marine temperate-coldwater organisms are proposed to 
26

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be 5 mgO2/l (Bakke et al. 2002). Fully saturated cold-temperate seawater contains about 9-10 

mgO2/l. 

6. Prediction of environmental concentration 

The Predicted Environmental Concentration (PEC) resulting from the operation of scrubbers 

depends on the dilution of the discharge water. Such dilution is difficult to measure in field; 

however it can be readily be modelled and numeric dillution models are commonly used to obtain 

a PEC. 

6.1 Modelling of discharge dilution from a single ship 

The dillution of the discharge from a single ship has been numerically modelled by SINTEF 

Material and Chemistry (2006, 1994) and by CFD Norway as (2006). Reports on these studies are 

included as appendix 1 and 2 to this report. The overall dispersion has been modelled for different 

scenarios regarding ship size, washwater discharge point and speed, however the results are in 

accordance and point towards the same general conclusion. 

The dilution of the wastewater plume after discharge during transit and port operations may be 

described as a combination of 3 main processes: 

• Dilution of the waste water in the exit jet emerging from the outlet 

• Turbulent mixing in the boundary layer along the hull 

• Dispersion in the propeller and the turbulent wake left behind the vessel. 

6.1.1 Dilution of discharges from a single ship in open sea 

SINTEF (2006) found that the dispersion in the wake behind the vessel was the dominant dilution 

process. Under the transit operation conditions of a discharge of 440 m3/hour wastewater at 8 m 

depth 20 m ahead of the stern from a 163 m length vessel at constant speed of 14,5 knot, the 

process water was diluted by a factor of 1:30 before the effluent passed the stern of the ship and 

by a factor 1:2000 at a distance 50 m behind the stern. (Appendix 2) 

By use of a different numerical model, CFD Norway investigated the dilution of a discharge 

behind a large container ship. (Appendix 1) Ship particulars are given in Table 17. Discharge 

point was at the stern just in front of the propellers. 
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Table 17 Ship particulars 

Ship length L=230 m

Ship speed 24 knots 

Propeller (single screw) diameter 7.9 m

Thrust 2600 kN 

Shaft power 32 MW

Washwater discharge rate 75 ton/MW (670 kg/s) 

Figure 3 shows the dillution as modelled. Some maximum concentrations behind the ship in are 

given in Table 18. 

Figure 3 Graphical illustration of dillution of discharge 
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Table 18 Dilution of original discharge 

Distance behind ship Maximum concentration 

x/L x [m] log10 (c/c0) ) 

max 

c/c0 max

0.004 0.92 -0.37 0.43 

0.035 8.05 -1.28 0.052 

0.1 23 -1.59 0.026 

0.25 57.5 -1.85 0.014 

0.5 115 -2.05 0.009 

1 230 -2.15 0.007 

C = Concentration, C0 = concentration at discharge point 

This model estimated dilution by a factor of 1:70 at 57 m behind the stern. If one assumes an 

additional 1:30 times dilution for a side outlet as used by SINTEF the total dilution at 57 m behind 

the stern will be 1:2150, i.e. essentially the same as for the SINTEF modelling. 

6.1.2 Dilution of discharges from a single ship at port operation 

This scenario has only been modelled by SINTEF (2006) for the same vessel as above, but with a 

speed of 4 knots and a discharge of 130 m3/h. Under such conditions the dilution factor at 50 m 

behind the stern was 1:1750, i.e. almost as large as during transit operation. 

6.1.3 Dilution of discharges from a single ship at quay side manoeuvring 

During quay side manoeuvring the following processes regulate dispersion: 

• Dilution of the waste water in the exit jet emerging from the outlet 

• Turbulent mixing during vertical transit of the plume to neutral buoyancy 

• Ambient turbulence such as caused by currents, winds and ship movement. 

Dillution at quay side will be strongly influenced by local conditions. One scenario has been 

modelled by SINTEF (1994) using the same discharge arrangement as for the port operation, 

speed 0 knots, temperature of the wastewater 7,5°C and ambient current speed of 5 cm/s. A 

dilution factor of 1:100 in the centre of the plume was reached at less than 20 m distance 

downstream from the outlet, corresponding to an average dilution factor of approximately 1:150. 

6.1.4 Summary: Dilution of discharges from a single ship 

In summary, the discharges from a single ship are rapidly diluted when the ship is moving. A 

small area behind the ship will be affected, however. The dilution at quay side is strongly 

influenced by local conditions, but should be expected to be much slower than when the ship is in 

moving operation. 
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6.2 Accumulation of discharges – local and regional effects 

The combined effect of the discharges from several ships must be considered in relation to the 

prevailing local conditions. The most important parameter is the exchange rate of the receiving 

surface water with neighbouring areas. 

6.2.1 The Oslo Fjord Case study – regional effects 

In this project we have made an evaluation of the risk that the total ship traffic in the land-locked 

Inner Oslo Fjord including Oslo harbour would gradually reduce the buffering capacity and other 

aspects of water quality of the local water masses if all ships were equipped with seawater 

scrubbers. 

The exchange of water in the Inner Oslo Fjord is achieved by an outflow of brackish surface water 

compensated by an inflow of saline water beneath the surface layer. This exchange is driven by 

river runoff and winds, and is strongest during periods with northerly wind. The annual mean 

water exchange of the surface layer (i.e. down to a depth of 10 m) is estimated to 900 m3

/s. 

Traffic data for Inner Oslo Fjord are based on data from Oslo and Drammen harbours and is 

shown in Table 19. 

Table 19 Marine vessels visiting Inner Oslo Fjord 2004. 

Harbour and ship type Number of ships Total dead weight 

 (1000 tonnes) 

Oslo international 3003 60775 

Oslo domestic cargo 98 378 

Oslo domestic tankers 38 95 

Drammen international 938 5436 

Total 4077 66684 

 

Local ferries, fishing boats and other vessels smaller than ca 2500 tonnes are assumed to use 

distillate fuel and hence have no incentive to clean their exhaust. 

Average tonnage of visiting ships included is 16.356 tonnes. It is assumed that these ships will 

have an average main engine power of 5200 kW. 

We consider the two different cases 1) transit operation and 2) port operation. Under port 

operation an engine operation of 30 % is expected for a total operation time of 1 hour. Under 

transit operation an average speed of 14 knots and an engine load of 90 % is assumed. The Oslo 

Fjord is about 80 km long giving a total transit time of 6 hours. 
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For the calculation of the sulphur emission an average consumption of 0.2 kg fuel per kWh is 

used. The calculation is based on an average sulphur content of 2.7 % in the fuel. One hundred % 

of the sulphur is removed in the scrubber and discharged to the sea as sulphurous acid. 

Estimated sulphur discharge: 

Transit operation 4077 · 5200 · 6 · 0.9 · 0.2 · 0.027/1000 = 618 t/year 

Port operation 4077 · 5200 · 1 · 0.3 · 0.2 · 0.027/1000 = 34 t/year 

Total discharge 652 t/year =74 kg/h 

It is seen from this calculation that the port operation discharge will contribute to about 5,5 % of 

the total sulphur discharge. In addition comes discharge at berth which is not included here. 

Neutralization proceeds according to the equation: 

H2SO3 + 2 HCO3

-

 = SO3

2- + 2 H2CO3 

Each mole of sulphur needs 2 moles of bicarbonate to be neutralized. Provided that the incoming 

water has a salinity of 20 ‰ the alkalinity can be assessed to 1.394 mmol/l. The rate of water 

exchange necessary to provide enough alkalinity for neutralization of the discharge is 

Q = 74/32 · 2 · 1000/1.394 = 3.318 m3

/h = 0.9 m3/s 

This is about 0.1 % of the actual exchange, which suggests that the risk of gradual reduction in 

buffering capacity is negligible. 

Based on the same water exchange regime as above for the Inner Oslo Fjord, the modelling of 

discharge dilution at port operation (chapter 6.1.2), and a traffic load of 6000 visits/year, SINTEF 

(1994) calculated the risk of the wastewater changing the water quality in the local surface waters. 

The total waste water discharge from these vessels was estimated to roughly 4000 m3/day or 167 

m3/hour. This amounts to 0.2 % of the annual mean freshwater input to the Inner Oslo Fjord of 

about 90.000 m3/hour, and 0.003 % of the total mean exchange of surface water between the fjord 

and the sea outside (6.4x106

 m3/hour). These fluxes suggest that the overall equilibrium dilution 

of the total wastewater volume in the surface water of the Inner Oslo Fjord is in the range of 

1:38000. This is way above the dilution necessary to reduce the PEC/PNEC ratio to less than 1 for 

all discharge components of concern, and shows clearly that the risk of gradual deterioration of 

the water quality of the Inner Oslo Fjord if all vessels were equipped with sea water scrubbers is 

negligible. 

The methods used in the assessments above ought to be of general value. It must, however, be 

pointed out that such assessments depend entirely on site-specific data on water exchange rates, 
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traffic densities (i.e. discharge water volumes and fluxes) and specific knowledge of the acidity 

and contaminant content of the actual waste water effluents. 

7. Current discharge criteria 

The content of petroleum hydrocarbons in discharges of bilgewater from ships is limited by 

MARPOL Annex I to 15 ppm. This limit is much higher than what would be expected from a 

regular seawater scrubber. However, this limit is not intended for regulation of scrubber 

discharges and the limit does not in intself imply that hydrocarbon concentrations up to 15 ppm 

would be acceptable. In fact hydrocarbon concentrations that are acutely lethal to marine 

organisms may be as low as 1 ppm. 

An effort has been made to identify other discharge standards, criteria or limits of relevance that 

may exist nationally or internationally. We have looked for standards for the most relevant 

components such as dissolved oxygen (DO), chemical oxygen demand (COD), pH, total 

suspended solids (TSS), total organic carbon (TOC), nickel, vanadium, zink, copper, lead, 

mercury, chromium, and total PAH/PAH16. 

The conclusion from structured internet searches and contacts to the Norwegian Pollution Control 

Authority (SFT) and the Ministry of Environment is that there are few if any such 

criteria/standards of direct relevance to scrubber discharge from ships. 

8. Proposed waste water discharge criteria for exhaust gas scrubbers 

8.1 Sensitivity of marine and brackish water organisms to contaminants 

Available literature suggests that there may be some systematic, compound-specific differences in 

sensitivity to contaminants and other environmental stressors between freshwater and marine 

organisms (Wheeler et al 2002). Metals appear to be most toxic to freshwater organisms while 

organic compounds seem more toxic to marine organisms. These differences are in general not 

large, and for all practical risk assessment purposes one may regard freshwater and marine species 

as equally sensitive. Brackish water species are in general more tolerant to natural large 

environmental fluctuation than their freshwater and marine counterparts, but there is no reason to 

assume that they also are more tolerant to toxicants. Still it is reasonable to expect that toxicant 

sensitivity in brackish water organisms does not deviate systematically from that of freshwater 

and marine organisms. We therefore find no arguments to develop ecosystem-specific waste water 

discharge criteria for scrubber components to safeguard the inherent organisms of marine and 

brackish water ecosystems, as long as agreed safety factors are applied to develop PNEC values. 
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8.2 Proposed waste water criteria for seawater scrubber discharges from vessels at sea 

The waste water dispersion modelling shows that a gradual dilution of the plume in the surface 

water will occur and can be described. There appears to be no discontinuity in this pattern that 

might serve as a definition of a near field impact area or volume in the wake of a ship. We will 

therefore rather arbitrarily define that a near field should be limited to 50 m behind the stern. 

Within this area or volume one must expect that organisms encountering the waste water will be 

affected, but this should be accepted on basis of the GESAMP principle since this area/volume in 

all aspects is small compared to the complete water body the ship is moving through. The 

modelling in transit and port operation shows that a plume dilution of 1:2000 and 1:1700 

respectively are achieved at this distance, and within less than 10 seconds. For quay side 

manoeuvring we have also arbitrarily defined a distance of 20 m from the outlet as the nearfield. It 

is likely that an average plume dilution of 1:150 may be obtained within the nearfield and within 

6-7 minutes. 

Taking these dilution factors and the PNEC values for the actual scrubber components into 

account we have estimated the worst case discharge characteristics that may be allowed without 

the PEC/PNEC ratio exceeding 1 outside the near field. This should then be proposed as the waste 

water discharge criteria values. These values are presented in Table 20 and Table 21. 

Table 20 shows that even at quay side manoeuvring with the lowest estimated dilution, the 

wastewater may have metal, THC, and PAH content that is 1-3 orders of magnitude higher than 

those reported as typical for scrubber wastewater effluents, and still the PEC/PNEC ratio would 

be <1 at the border of the near field. The component that comes closest to the proposed discharge 

criterion is THC, where the quay side criterion is only 3 times higher than the estimated typical 

discharge concentration, but still on the safe side. 

For pH and dissolved oxygen (Table 21) the outlet concentrations would theoretically have to be 

negative to cause a PEC/PNEC=1 at the border of the near fields. This implies that even with pH 

= 0 and no dissolved oxygen (DO) in the discharge, the PEC/PNEC ratio would be far below 1 at 

the border of the nearfield. For DO the table shows that even a COD of 150-350 mg/l (a typical 

range for national discharge limits for COD) could have been realised within the near field 

without raising the PEC/PNEC ratio for DO above 1 outside this field. 

One should point out that these discharge criteria are very conservative since the PNEC values 

chosen cover impact during chronic exposure, whereas the anticipated exposure time in the near 

fields is only 10 seconds during transit and port load and 6-7 minutes at quay side manoeuvring. 

Also the calculation is based on the strictest PNEC values where alternative values are given by 

different authorities (cf chapter 6). 
33

 / / 2006-10-

Table 20: Proposed discharge criteria (WDC) for scrubber waste water compounds that are 

elevated in the waste water (μg/liter). These criteria will ensure that the PEC/PNEC ratio 

does not exceed 1 at the border of the nearfields in the transit load, port load, and quay side 

manoeuvring scenarios. 

Compound Most 

conservative 

PNEC 

Ambient WDC 

transit 

load 

WDC 

port load 

WDC quay 

side 

manoeuvring 

Waste water 

concentration 

Nickel 2,1 0,1 4000 3400 300 32,8 

Vanadium 100 2 196000 166600 14700 35 

Copper 2,9 0,3 5200 4420 390 41,6 

Lead 8,3 0,05 16500 14025 1238 5 

Mercury 0,024 0,001 46 39 3 <0,1 

Cadmium 2,5 0,03 4940 4199 371 0,08 

THC 40 10 60000 51000 4500 1700 

PAH 3,3 0,1-0,2 6212 5280 466 0,25 

Table 21: Proposed discharge criteria (WDC) for scrubber waste water compounds that are 

elevated in the waste water. These criteria will ensure that the PEC/PNEC ratio does not 

exceed 1 at the border of the nearfields in the transit load, port load, and quay side 

manoeuvring scenarios. 

Compound Most 

conservative 

PNEC 

Ambient WDC transit 

load 

WDC port load WDC quay 

side 

manoeuvring 

pH 6,8 7,0 -392 -92 -38 

DO mg/l 5 8-10 -9990 -8190 -755 

*NOTE: Negative WDC values indicates that PNEC values can be obtained even with pH=0 and 

DO = O at outlet, indicating that no regulation would generally be needed for pH and COD. 

The large safety margin between the trial results and the indicated criteria, and considering the 

conservative approach used in defining them, indicates that there is no apparent need for scrubber 

wastewater criteria in the open sea. This is in good harmony with MARPOL Regulation 14 which 

places no restrictions on discharges outside enclosed ports, harbours and estuaries. 

8.3 Proposed waste water criteria for seawater scrubber discharges in closed waters 

To protect the water quality, the port State needs to limit the total discharges of pollutants within 

the area of concern. The total discharge of pollutants depend both on the amount of exhaust that is 

being scrubbed and the discharge criteria. 

Even though the Inner Oslo Fjord case concluded that there is no apparent risk for gradual 

deterioration of the water quality even if all present vessel traffic uses seawater scrubbing of their 
34

 / / 2006-10-

exhaust, this may not be the case for other enclosed water bodies with high ship traffic. As said 

earlier each site and situation is unique and has to be evaluated separately to generate site specific 

waste water discharge criteria, however, the risk procedure used in the Oslo Fjord case is 

applicable to other enclosed ship traffic areas as well. 

As previously argued, to avoid prolifieration of criteria it is advised that IMO develops a set of 

default criteria that represent typical situations. (Level 1, 2, 3, etc) The Port State would then have 

the opportunity to choose one of the predefined criteria on the list which would significantly ease 

verification of compliance and provide much needed design targets for EGCS-SOx manufacture. 

Table 22 Comparison of Possible discharge water limits and worst case trial results (μg/liter) 

Criteria level 1 2 3 

“Safety Margin” 1 10 100 Trial worst case 

Nickel 300 30 3 32.8 

Vanadium 15000 1500 150 35 

Copper 400 40 4 41.6 

Lead 1200 120 12 5 

Mercury 3 0.3 0.03 <0.1 

Cadmium 350 35 4 0.08 

THC 4500 450 45 1700 

PAH 450 45 5 0.25 

As seen in Table 22, there is a considerable difference between the worst case results observed in 

the trials and proposed criteria in many cases. It is suggested that a limit value is not needed when 

the worst case observed in the trial is about one order of magnitude or more below the proposed 

criteria. The resulting criteria proposition following this logic is shown in Table 23. 

Table 23 Proposed discharge water limits (μg/liter) 

Criteria level* 1 2 3 

“Safety Margin” 1 10 100 

Nickel - 30 3 

Vanadium - - 150 

Copper - 40 4 

Lead - - 12 

THC 4500 (4.5 ppm) 450 (0.45 ppm) 45 (0.045 ppm) 

*At nominal wash water rate of 44 kg/kWh (representative of trials and dispersion models used) 

The safety margin indicates how much more discharge an area can sustain compared to the 

unregulated situation, all other aspects being equal. 
35

 / / 2006-10-

These proposals are made based on results from two specific trials. They should be seen in 

conjunction with experiences from other trials, studies as well as capabilities of water cleaning 

systems and means of verification. An additional cosmetic criterion demanding no visible 

discharge and absolute limit values irrespective of wash water rate could also be considered. 

In order to select predefined discharge criteria, the port State would have to identify the rate of 

water exchange and the anticipated discharge from scrubbers in the area in question. The 

concentration of contaminants in the area can then be calculated assuming different discharge 

levels and compared with PNECs as provided above or other as deemed necessary. The port state 

could then select the least stringent set of criteria which provide sufficient environmental 

protection. 
36

 / / 2006-10-

9. GLOSSARY 

COD chemical oxygen demand 

AOX absorbate organic chloride 

PAH polyaromatic hydrocarbons 

PCDD polychlorinated dibenzo-p-dioxines 

PCDF polychlorinated dibenzofurans 

PEC predicted environmental concentration 

PNEC predicted no-effect concentration 

THC Total Hydrocarbon 

HC Hydrocarbon 

WDC water discharge criteria 

DO dissolved oxygen 

EQS environmental quality standard 

TSS total suspended solids 

TOC total organic carbon 

SPM suspended particular matter 

PCB polychlorinated biphenyls 

EAC Ecotoxicological Assessment Criteria 
37

 / / 2006-10-

10. REFERENCES 

Bakke T, B Braaten, J Molvær and MT Scaanning, 2002. Seawater flue gas desulfurization FGD 

may require new regulations on ambient dissolved oxygen. Paper no 71226 at the 14th

Conference of the Electric Power Supply Industry (CEPSI), Fukuoka Japan. 7p. 

Crommentuijn, T., Polder, M.D. & van de Plasse, E.J. (1997). Maximum permissible 

concentrations and neglible concentrations for metals, taking background concentrations 

into account. RIVM report No. 601501 001. 

IMO resolution MEPC.130(53) Guidelines for on-board exhaust gas SOx cleaning systems 

MARPOL 73/78 Annex VI – Regulations for the prevention of air emissions from ships. IMO 

Publication – Sales number I664E 

EU directive 2005/33/EC 

MARINTEK div. Machinery, MT22-F93-0220: Exhaust gas seawater scrubbing, ‘SSN Project’ 

Ives R,, SN Klokk, 1993. Exhaust gas sulphur removal by sea water washing, marine diesel 

engines. 20th international congress on combustion engines, (CIMAC) London 1993 

Johansen, Ø, 1993: Exhaust Gas Sea Water Washing System – Dispersion of Process Water. 

OCEANOR Report OCN R-93069, 38 pp. 

Johansen, Ø, K. Tangen, and F. Moy, 1994: Exhaust Gas Sea Water Washing System – 

Environmental Impact Assessment. OCEANOR Report OCN R-94003, 71 pp. 

Johnsen et. al. (2000) Johnsen S, Frost TK, Hjelsvold M, Statoil, and Utvik, TR. 2000. The 

environmental impact factor - a proposed tool for production water impact reduction, 

management, and regulation. SPE 61178. Society of Petroleum Engineers. 

Knutzen (1981) Effects of decreased pH on marine organisms. Mar.Poll.Bull., 12, 25-29. 
38

 / / 2006-10-

Wolff et. al (1988) Wolff EW. et al 1988. Proposed environmental quality standards for List II 

substances in water. pH. WRC Technical Report TR 259. Water Research Centre UK. 

66pp.

Wheeler et al 2002 Wheeler JR, Grist EPM, Leung KMY, Morrit D and Crane M. 2002. Species 

sensitivity distributions: data and model choice. Mar. Poll. Bull., 45, 192-202. 
39

 / / 2006-10-

APPENDIX 1 

Report CFD-Norway 

APPENDIX 2 

Memo Sintef Materials & Chemistry 
 CFD Analysis of Process Water Dispersion

from

KRISO Container Ship (KCS)

Ivar Øye

June 2006

CFDn Report 325:2006

Preliminary, 12.06.2006-1

CFD norway as
CFD norway as

CFDn report 325:2006 i

REPORT DOCUMENTATION PAGE

Principal:

MARINTEK

Classification:

Konfidensiell

Address of principal:

Norsk Marinteknisk Forskningsinstitutt A/S

Otto Nielsens vei 10

Postboks 4125 Valentinlyst

7450 Trondheim

Local co-ordinator:

Øyvind Buhaug

Title:

CFD Analysis of Process Water Dispersion from KRISO

Container Ship (KCS)

Author(s):

Ivar Øye

Abstract:

By applying a CFD code for computing viscous, incompressible

flows, the dispersion of process water from a container ship of

length 230 meters and of speed 24 knots was analysed. The

results should be regarded as a first assessment.

The computational results show a rapid dispersion of the

wastewater concentrations. At 1 m aft of aft perpendicular (just

behind the propeller) the maximum concentration had dropped

to 43% of its initial value, at 8 meters the corresponding value

was 5.2%. Due to the swirl in the propeller slip stream the

concentration iso-lines became rapidly asymmetric. At distances

of 0.5 L and above, the wastewater concentrations are confined

to a 30-50 cm thick layer near the surface.

Approved by:

Keywords: Ship hydrodynamics, CFD, Pollutant, Dispersion CFD norway as

Report number:

325:2006
CFD norway as

CFDn report 325:2006 ii

Table of Contents

Table of Contents ........................................................................................................ii

1 Introduction............................................................................................................1

2 Numerical Modelling.............................................................................................2

2.1 Grid Generation ........................................................................................2

2.2 Propeller Model.........................................................................................2

3 Discussion of Results...........................................................................................3

4 Conclusions...........................................................................................................4

List of Figures..............................................................................................................5

Figures..........................................................................................................................6

A Description of the Numerical Method Used.....................................................16

A. Bibliography..........................................................................................................16
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1 Introduction

By injecting water spray into the exhaust stack some pollutants may be washed out from the

gas and hence resulting in cleaner air. However, the dispersion of the pollutants now contained

in the wastewater and their impact on the environment must be investigated.

The objective of this study was to predict the dispersion of process water from a representative

ship type to make first assessments of its decay and spreading rates. The actual ship was the

Kriso Container Ship (KCS) of length 230 m which has been widely used as a reference test

case for CFD codes in hydrodynamics, with supplementary experimental data1

. The dispersion

of pollutants is well modelled in the atmosphere, but more complicated in the wake of a ship

including the swirling propeller slip stream. Computations were performed at a ship speed of 24

knots, at an estimated power requirement of 32 MW.

The computations were carried out with a Finite-Volume RANS code solving the incompressible

Navier-Stokes equation coupled with a k-epsilon turbulence model. The code utilises structured

multi-block grids well adopted for high Reynolds number flows about complex geometric

shapes.

In the next sections we present the ship model and grid generation, followed by a discussion of

the results. Finally, we arrive at some concluding remarks.

 1

 See http://hyperteam.kriso.re.kr for test case definition and data.
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2 Numerical Modelling

2.1 Grid Generation

The CFD grid was constructed by using a total of 1.44 million grid points shared among 14

blocks, of which 9,200 was located on the hull surface, as observed in Figure 1. The surface

grid was projected onto the NURBS patches from the supplied CAD geometry file.

Since the low-Reynolds number version of the k-epsilon model requires integration of the

turbulence quantities in the inner layer of the boundary layer, the grid is highly clustered in the

wall proximity, with a wall normal distance of y+ equal to 1 in the first cell layer. The far field

boundaries were located at a distance of 5 ship lengths away.

2.2 Propeller Model

The propeller slipstream was computed with a simplified propeller model implemented as an

actuator disc model for the thrust including the effect of swirl. The actuator disc model is a

widely used simplification of rotor-stator configurations. By assuming a constant velocity in

circumferential direction, the necessity of resolving the flow about each blade is avoided.

Instead, momentum theory is applied which yield the relationship between total thrust T [N], the

pressure jump ∆p over the propeller disc, the ship speed Vs, and the induced velocities uC in

the propeller plane.

, 2 ( ) T = ∆pA ∆p = ρuC VS + uC

In the present computations the tangential velocity component (swirl) was given by propeller

data. The propeller main dimensions were a diameter of 7.9 m, net thrust of 2,622 kN and a

power consumption of 32 MW.
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3 Discussion of Results

A finite-volume method for solving the Navier-Stokes (RANS) equations was applied to for

computing the dispersion of wastewater from a container ship. A detail on the numerical method

is found in Appendix A.

The mass flow rate of wastewater from the process was specified at 75 tons/MWh, or 667 kg/s

for the current case. The wastewater was released at a depth of 8 meters, just ahead of the

propeller. Due to the coarseness of the grid, the exact position of the outlet should be regarded

as approximate. A sensitivity analysis of the location of the outlet was not a part of this study.

The results are presented as concentrations of wastewater at cross sections through the wake.

The cross sections are defined at distances aft of the aft perpendicular as listed in Table 1.

Table 1. Distance to specified cross section for visualisation of wastewater

concentrations, and the computed maximum value in each section.

x/L x [m] log10 (c/c0) max c/c0 max

0.004 0.92 -0.37 0.43

0.035 8.05 -1.28 0.052

0.1 23 -1.59 0.026

0.25 57.5 -1.85 0.014

0.5 115 -2.05 0.009

1 230 -2.15 0.007

The dispersion of the wastewater is visualised by concentration plots, its development in the

wake is shown in Figure 3. Note the logarithmic scale. At 1/250 of the initial concentration the

logarithmic value is -2.4. Investigating the individual cross section in Figure 4 - 9, we find the

maximum values of the normalised concentrations presented in Table 1. At a distance of 0.25 L

and less the pollutant distribution has a shape of a round plume with swirl, see Figure 10.

However, at further distance the maximum concentrations are located in a 30-50 cm thick layer

near the surface. In the real situation encountering wind-induced waves, the wave-induced

velocities will most likely increase the effective dispersion of the wastewater both in horizontal

and vertical direction.

The results should be regarded as a first assessment, and further studies should emphasis on

grid sensitivity and turbulence model sensitivity on the diffusion rate. Also, to make the validity

of the results more general, computations should be carried out for several ship sizes and

types, with various types of propulsion systems.
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CFDn report 325:2006 4

4 Conclusions

By applying a CFD code for computing viscous, incompressible flows, the dispersion of process

water from a container ship of length 230 meters and of speed 24 knots was analysed. The

results should be regarded as a first assessment.

The computational results show a rapid dispersion of the wastewater concentrations. At 1 m aft

of aft perpendicular (just behind the propeller) the maximum concentration had dropped to 43%

of its initial value, at 8 meters the corresponding value was 5.2%. Due to the swirl in the

propeller slip stream the concentration iso-lines became rapidly asymmetric. At distances of 0.5

L and above, the wastewater concentrations are confined to a 30-50 cm thick layer near the

surface.
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List of Figures

Figure 1 Wetted part of ship hull visualised by the CFD surface mesh.................................. 6

Figure 2 Definition of cross sections for plotting of results at (from left) 0.004 L, 0.035 L, 0.1L,

0.25 L, 0.5 L, and 1.0 L aft of aft perpendicular. .............................................. 7

Figure 3 Plot of wastewater concentration, log10 (c/c0), at cross sections defined in Figure 2.7

Figure 4 Computed normalized wastewater concentration at cross section 0.004 L aft of

vessel. Grid size equals 2 by 2 metres. ........................................................... 8

Figure 5 Computed normalized wastewater concentration at cross section 0.035 L aft of

vessel. Grid size equals 2 by 2 metres. ........................................................... 9

Figure 6 Computed normalized wastewater concentration at cross section 0.1 L aft of vessel.

Grid size equals 2 by 2 metres. ..................................................................... 10

Figure 7 Computed normalized wastewater concentration at cross section 0.25 L aft of

vessel. Grid size equals 2 by 2 metres. ......................................................... 11

Figure 8 Computed normalized wastewater concentration at cross section 0.5 L aft of vessel.

Grid size equals 2 by 2 metres. ..................................................................... 12

Figure 9 Computed normalized wastewater concentration at cross section 1.0 L aft of vessel.

Grid size equals 2 by 2 metres. ..................................................................... 13

Figure 10 Computed normalized wastewater concentration at cross sections (from top) 0.035

L, 0.1 L, 0.25 L , and 0.5 L. Grid size equals 2 by 2 metres........................... 14
CFD norway as

CFDn report 325:2006 6

Figures

Figure 1 Wetted part of ship hull visualised by the CFD surface mesh.
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Figure 2 Definition of cross sections for plotting of results at (from left) 0.004 L,

0.035 L, 0.1L, 0.25 L, 0.5 L, and 1.0 L aft of aft perpendicular.

Figure 3 Plot of wastewater concentration, log10 (c/c0), at cross sections defined in

Figure 2.
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Figure 4 Computed normalized wastewater concentration at cross section 0.004 L

aft of vessel. Grid size equals 2 by 2 metres.
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Figure 5 Computed normalized wastewater concentration at cross section 0.035 L

aft of vessel. Grid size equals 2 by 2 metres.
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Figure 6 Computed normalized wastewater concentration at cross section 0.1 L aft

of vessel. Grid size equals 2 by 2 metres.
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Figure 7 Computed normalized wastewater concentration at cross section 0.25 L aft

of vessel. Grid size equals 2 by 2 metres.
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Figure 8 Computed normalized wastewater concentration at cross section 0.5 L aft

of vessel. Grid size equals 2 by 2 metres.
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Figure 9 Computed normalized wastewater concentration at cross section 1.0 L aft

of vessel. Grid size equals 2 by 2 metres.
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Figure 10 Computed normalized wastewater concentration at cross sections (from

top) 0.035 L, 0.1 L, 0.25 L , and 0.5 L. Grid size equals 2 by 2 metres.
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CFD norway as

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A Description of the Numerical Method Used

The method used for the solution of the compressible or incompressible Reynolds-averaged

Navier-Stokes (RANS) equations is a finite volume method based on central differences in

space co-ordinates and an explicit Runge-Kutta method for the integration in time [A.1-3]. The

method starts from an initial field which is marched in time until a stationary condition is

reached. At this stationary condition is the conservation equations for mass and momentum

satisfied. The time step is calculated locally in each control volume to increase the rate of

convergence. The k-epsilon model was applied for the generation of turbulence [A.4].

The flow solver facilitates multi-block grids with a general and flexible specification of boundary

conditions. The cell centred finite volume discretisation stems from the integral form of the

Navier-Stokes equations which describes conservation of mass and momentum when viscous

forces is neglected. On a finite control volume Ω this reads

Ω + ⋅ = 0

∂

∂

∫ ∫

Ω

U d dS

t S

F n a)

where the vector U contains the variables

U [p c u v w] T , , , 2 = , b)

where p is static pressure and c is an artificial speed of sound. The variables u, v, and w are

the Cartesian velocity components in x-, y-, and z-direction respectively. The variables are

assumed to be averaged over each control volume. In Eq. a), F refer to the common flux vector

for non-viscous transport terms while the vector n is the outward unit normal of the surface with

area S.

A. Bibliography

[A.1] Jameson, A., Schmidt, W., Turkel, E., “Numerical Solutions of the Euler Equations Using

Runge-Kutta Time-stepping Schemes,” AIAA Paper 81-1259, 1981.

[A.2] Eriksson, L.-E., "A finite volume solution technique for the Navier-Stokes equations

governing viscous compressible flow,'' HOG Report 1987:102(A), Division of Hydro- and

Gas Dynamics, The Norwegian Institute of Technology, Trondheim, 1987.

[A.3] Øye, I.J., “On the Aerothermodynamic Effects on Space Vehicles,” dr.ing. thesis, MTFrapport 1996:140 (D), NTNU, Trondheim 1996.

[A.4] Øye, I.J., “Evaluation of Turbulence Models for Flow about Wings and Bodies”, in: Haase,

W., Chaput, E., Elsholz, E., Leschiner, M.A., Müller, D. (Eds.), “ECARP – European

Computational Aerodynamics Research Project: Validation of CFD Codes and

Assessment of Turbulence Models”, Notes on Numerical Fluid Mechanics, Volume 58.,

Vieweg Verlag, 1997.
MEMO 

MEMO CONCERNS 

Marine Use of Low Sulphur Heavy Fuel Oil, 

WP3 Wastewater Criteri, 

Dispersion Modelling 

FOR YOUR ATTENTION COMMENTS ARE INVITEDFOR YOUR INFORMATION AS AGREED

DISTRIBUTION

SINTEF Materials and Chemistry

Address: NO-7465 Trondheim, 

 NORWAY

Location: Brattørkaia 17B,

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Telephone: +47 4000 3730 

Fax: +47 930 70730 

Enterprise No.: NO 948 007 029 MVA 

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Henrik Rye, SINTEF Materialer og kjemi 

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PROJECT NO. DATE PERSON RESPONSIBLE / AUTHOR NUMBER OF PAGES

80401827 2006-05-29 Øistein Johansen 8

1 BACKGROUND 

This memo contains a brief description of a computer model for dispersion of aqueous effluents 

from vessels in the open sea. The model was developed as a part of a study conducted by 

MARINTEK in the 1990’ies on a scrubber system for removal of sulphur and particulate matter 

from exhaust gas emitted from vessel engines. The system was based on a scrubber where sea 

water was used to absorb the contaminants before the exhaust is released to the atmosphere. The 

washing water should be discharged to the sea through outlet ports below the water line. The 

dispersion model was developed at OCEANOR and presented in two reports. The first is a

description of the model concept, referred to later in this memo as the Model Report (Johansen 

1993). The second contains model applications in the context of environmental impact

assessments, later referred to as the EIA Report (Johansen 1994). 

The exhaust gas washing concept has gained renewed interest recently due to forthcoming 

regulations of permitted sulphur content of fuel oils used by vessels sailing in the North Sea 

region. On this background, an extensive research programme has been initiated (MAROFF), with 

contributions from SINTEF Byggforsk, MARINTEK and others. As dispersion model 

developments and applications have so far not been included in the programme, SINTEF 

Byggforsk has asked SINTEF Materials and Chemistry to give a brief description of the 

dispersion model developed in the before-mentioned project, and to indicate possible scaling laws

that could be applied to make use of model results from the previous study. The description of the 

model is given in Chapter 2 of this memo, while the discussion of scaling laws is given in Chapter

3. 

This memo contains project information and preliminary results as a basis for final report(s). 

SINTEF accepts no responsibility of this memo and no part of it may be copied. 
2

2 DISPERSION MODEL CONCEPTS

This chapter contains a brief description of the concepts used in the dispersion model that was 

developed at OCEANOR in conjunction with the previous exhaust gas washing system study. The 

model was developed as a particles-fluid-model, where the release of process water is represented

by model particles that are subject to advection and turbulent diffusion. The subsequent dilution 

of the process water was determined from the concentration of model particles in various crosssections downstream of the vessel. 

The model took into account three different mechanisms for turbulent diffusion, i.e.: 

 dilution of process water in the exit jet emerging from the outlet port 

 turbulent mixing in the boundary layer along the hull 

 dispersion in the turbulent wake left behind the vessel 

The influences of the exit jet and the boundary layer on the advection of the process water in the

vicinity of the outlet port were also included in the model. 

The sub-model for the exit jet was based on well known semi-empirical relations for momentum 

jets in cross-flow (Briggs formula). The sub-model for the velocity and turbulence profiles in the 

boundary layer was derived from classical theory of boundary layer flow. A sub-model for 

turbulent mixing behind the stern of vessel was developed on the basis of the theory for wakes 

behind self-propelled bodies, assuming stationary flow (the vessel is moving at constant velocity).

Figure 2.1 shows the results in normalized form. 

B

U'

K

0.001

0.01

0.1

1

10

0.1 1 10 100 1000

Normalized distance, X = x /b 0

N

ormalized wake variables, B , U', K

Figure 2.1. Normalized turbulent dispersion parameters for a wake behind a self propelled body.

X=x/b0 is the normalized downstream distance, scaled with the initial wake radius 

b0=0.5 R, where R is the half width (radius) of the hull. B=b/b0, U’=u’/U and 

K=D/(U b0) are normalized wake variables representing the wake radius b, turbulent 

velocity fluctuations u’ and turbulent diffusivity D as a function of the normalized 

downstream distance. 
3

One important aspect of this sub-model is that for a given vessel geometry (hull radius), the 

dilution of the effluent at a given downstream distance x will be independent of the vessel speed. 

It should also be noted that this model was found to produce significantly less dilution at a given 

downstream distance than a more conventional model for dispersion in a momentum wake, i.e. a 

wake formed behind an obstacle in a stationary flow. The momentum wake model might be 

relevant during manoeuvring and acceleration from rest, but the self propelled body model seems 

to be most appropriate for transport conditions (constant velocity). 

Simulations with different vessel speeds indicated that turbulent mixing in the exit jet dominated 

the dilution of the process water along the hull of the vessel at low vessel speeds, while turbulent 

mixing in the boundary layer dominated at high vessel speeds. In the former case, dilution tends to 

increase with increasing discharge rate and decrease with increasing vessel speed at a given down 

stream distance from the exit port. In the latter case, dilution tends to decrease with increasing 

discharge rate, and increase with increasing vessel speed. 

In general, however, turbulent dispersion occurring in the wake behind the vessel will be the 

dominant process governing the dilution of the process water. Example calculations in the 

environmental impact assessment report (see Figure 2.2) showed that the process water was 

diluted by a factor of 1:30 before the effluent passed the stern of the vessel, while dilution ratios 

of 1:2000 were found in the wake at a distance of 50 meters behind the stern of the vessel. This 

seems to imply that, as a first order estimate, one might neglect the initial dilution due to (a) 

turbulent entrainment in the exit jet and (b) turbulent dispersion in the boundary layer along the 

hull, and make dilution estimates entirely on turbulent dispersion in the wake. These observations 

will have certain implications on the possible scaling of the results to other discharge conditions 

and vessel speeds. This will be the main issue in the next chapter. 

However, the model results will also to a certain extent be sensitive to the discharge arrangement, 

particularly the location of the discharge port. In the simulations presented in the EIA-report, the 

discharge port was located 8 m below the water, 20 m ahead of the stern of the vessel. As the 

example taken from the Model Report indicates (Figure 2.2), this arrangement implies that the 

plume of diluted process enters into the periphery of the wake as it passes the stern of the vessel. 

A discharge port located in the stern of the vessel would bring the plume directly into the central 

parts of the wake, where the turbulent mixing would be stronger. Obviously, simplified 

approaches based on scaling laws can not account for such effects. 

It should also be noted that the dominant role of the turbulent mixing in the wake is limited to the 

cases where the vessel is moving at a presumed constant velocity, i.e. the for the port load and 

transport load cases defined in the previous study. For the quay side manoeuvring case, where the 

vessel was supposed to be almost at rest, turbulent entrainment in the exit jet will be the main 

cause of the dilution of the process water. 
4

Figure 2.2. Example of results from the process water dispersion model for the transport load 

case (discharge rate 440 m3

/h, vessel speed 14.5 knots). Solid lines indicate vessel hull 

with elliptical cross section, thin lines show the extent of the wake. Open markers 

represent model particles. Reproduction of Figure 9 in the Model Report.
5

3 SCALING LAWS 

The environmental impact assessment (EIA) that was made in the previous study on exhaust gas 

sea water washing systems was partly based on results from the dispersion model we have 

described briefly in the previous chapter. The cases that were considered were defined as follows: 

Table 3.1. Vessel dimensions and location of outlet port 

Vessel dimensions: 

Length of vessel 163 m 

Vessel width 26 m 

Vessel draft 11.4 

Outlet port: 

Distance ahead of vessel stern 20 m 

Depth below sea level 8.2 m 

Diameter of outlet 332 mm 

Table 3.2. Description of discharges 

Case 

Quay side 

manoeuvring Port load Transport load 

Discharge rate 130 m3

/h 130 m3/h 440 m3/h 

Vessel speed 0 knots 4 knots 14.5 knots 

As a starting point for the present discussion on scaling laws, we have reproduced the results for 

the port load and transport load in Table 3.3. 

Table 3.3. Results from dispersion calculations of process water for the port load and transport 

load cases. Maximum concentrations are given as parts per thousand (ppt) of process 

water as a function of distance from the stern of the vessel. The corresponding transit 

times are computed from the distance from the discharge point and the given vessel 

speed. 

Distance 50 m 100 m 200 m 500 m 1000 m 

Case: Port load (4 

knots)

Max. conc. (ppt) 0.570 0.300 0.170 0.110 0.090 

Transit time 34 sec 60 sec 110 sec 250 sec 500 sec 

Case: Transport 

load (14.5 knots) 

Max. conc. (ppt) 0.450 0.290 0.200 0.090 0.075 

Transit time 10 sec 16 sec 30 sec 70 sec 140 sec 

 

As pointed out in the EIA-report, it was found that the computed concentrations at a given 

distance from the stern of the vessel were strikingly similar for the two cases. By inspecting the 

discharge conditions for the port load and the transport load (Table 3.2), one will also find that the 

ratio q = Q/U is almost equal for the two cases, i.e. about 18 L/m for port load vs. 16 L/m for 

transport load. 

This significance of the ratio q can be explained by the following deduction: 

 The volume of process water V discharged in a given time period t is V = Q t, where Q is the 

discharge rate. 
6

 The transit time to a certain downstream distance x is t = x/U, where U is the vessel speed. 

 Hence, the volume of process water contained within a certain downstream distance x is given 

by V = Q x/U = q x, where q is the volume of process water contained per unit downstream 

distance. 

This implies that for an arbitrary discharge rate Q and vessel speed U, we may estimate the 

maximum concentrations of process water at a given downstream distance x by scaling the 

concentration values with a factor q q qref r = / , where qref is the ratio Q/U for the reference case. If 

the vessel dimensions (vessel width and draft) also differ for the two cases, the downstream 

distances x must be scaled with the radius R of the hull by a multiplication factor . The 

example described in Table 3.4 is chosen to illustrate the method. 

rx = Rref / R

Figure 3.1 shows the results of the proposed scaling procedure (new case compared with reference 

case). The concentration values are shifted upwards for the new case due to the larger discharge

rate-to-vessel speed ratio (q = Q/U), and the downstream distances are shifted to the left due to the 

larger width of the hull (36 m v.s. 26 m). In total, we find that the concentrations are increased by

a multiplication factor of about 1.7 at a given downstream distance due to the changes in

discharge conditions and vessel dimensions. 

0.01

0.1

1

10

10 100 1000 10000

Distance downstream, m

Max. concentration, ppt

Transport load - reference case

Transport load - new case

Figure 3.1. Example of application of the scaling method described above. See Table 3.4 for 

description of the two cases. 

Table 3.4. Example used to illustrate the use of the scaling laws for dispersion in the wake. 

Case Discharge rate, Q Vessel speed, U q = Q/U Hull width 

Reference case 440 m3

/h 14.5 knot 16.4 L/m 26 m

New case 800 m3/h 12.5 knot 34.6 L/m 36 m
7

However, as mentioned in the previous chapter, the model results are also to a certain extent 

sensitive to the discharge arrangement, particularly the location of the discharge port. As noted, a 

discharge port located in the stern of the vessel rather than in the side of the hull would imply 

stronger turbulent mixing and enhanced dilution of the plume of produced water. Obviously, the 

scaling laws presented above can not account for such effects. 

Finally, it should be noted that for other treatment systems than the one presumed in the EIF 

Report, the recalculated concentration values must be compared with the actual acidity or toxicity 

corresponding to different dilution ratios of the effluent. To illustrate the problem, we will use 

data reported for the treatment system considered in the EIF Report as an example: 

For transport load conditions in seawater the initial pH of the effluent was measured as low as 2.9. 

This value will be raised as a result of dilution with seawater, but the environmentally acceptable 

dilution ratio will depend on the established “no-effect” pH value. In the EIF Report, it is assumed 

that a reduction of 1 pH unit from the average pH of 8 in seawater can be considered acceptable. 

However, it is also mentioned that EPA has recommended a stronger criterion, implying that the 

pH should be reduced not more than 0.2 units below the normal pH in seawater. 

For the transport load case (133 mg SO2/L seawater with 34 ppt salinity), it was found that pH 

could be raised from the initial value of 2.9 to 7 by a dilution of one part of the effluent with 22 

parts of seawater. An increase in pH to 7.8 would on the other hand require a dilution ratio of 300. 

The EIF Report concluded that these “critical” dilution ratios were well below the ratios obtained 

at downstream distances more than 50 meters behind the vessel stern. It should be kept in mind 

that these numbers depend on the makeup of the discharge, and that changes in the treatment 

system might change the picture due to changes in the make up of the effluent. 

The following points may serve as a conclusion: 

 For transport load and port load cases (vessel moving with constant velocity), where the 

dilution process is dominated by turbulent dispersion in the wake behind the vessel, simple 

scaling laws may be used to make estimates of concentrations downstream of the vessel 

(recalculations based on previous model results). 

 These scaling laws are not applicable for quay side manoeuvring (stagnant vessel), where the 

dilution process is dominated by turbulent dispersion in the exit plume model. 

 The results from such recalculations must be assessed against expected acidities of the effluent 

at given dilution ratios, and accepted no-effect values of pH. 

 If such assessments show small margins relative to no-effect values, actual dispersion model 

calculations will be required. If the margins are large, recalculated values may be sufficient. 

Finally, we should add that the EIA Report also included possible regional effects of discharges of 

process water. The possible effects were assessed on the basis of water exchange values for a 

specific region (the Inner Oslo Fjord), and the expected vessel traffic density in the same area. 

The method used in the assessment might be of general value, but it must be pointed out that such 

assessments depends on site-specific water exchange data, traffic densities, and specific 

knowledge of the acidity of the actual effluent at various dilution ratios. 

 
8

4 REFERENCES 

Johansen, Ø, 1993: Exhaust Gas Sea Water Washing System – Dispersion of Process Water. 

OCEANOR Report OCN R-93069, 38 pp. 

Johansen, Ø, K. Tangen, and F. Moy, 1994: Exhaust Gas Sea Water Washing System – 

Environmental Impact Assessment. OCEANOR Report OCN R-94003, 71 pp.