NEW TECHNOLOGIES FOR 

CONTROLLING 

OILY BILGE WATER DISCHARGES 

Presented at the Marine Environmental 

Engineers Technology Symposium 

(MEETS) 

May/June 2001 

Author: Hal Alper 

1
Hal Alper 

New Technologies for Controlling 

Oily Bilge Water Discharges

ABSTRACT 

US military vessels are limited to oily bilge 

water discharge concentration of 15 ppm under 

40 CFR part 1700, uniform national discharge 

standards (UNDS). International commercial 

discharge standards appear to be stabilizing at 5 

ppm. In order for these regulations to have the 

desired effect of reducing oily bilge water 

discharge, improvements in treatment 

technology and enforcement must take place. 

This paper reviews developments in treatment of 

oily bilge water and detection of oily discharge 

from ships. New technologies developed to 

bridge the gap between oil/water separator 

(OWS) capability of approximately 250 ppm

and discharge standard of at most 15 ppm are 

covered, with emphasis on cross flow membrane 

filtration technologies and oleophilic polymeric 

surfactant (PS) infused filtration devices. 

Developments in detection of oily discharge 

utilizing synthetic aperture radar (SAR) and 

enhanced tracking procedures are also reviewed. 

INTRODUCTION 

Of the oil released by vessels, 25% is reported to 

come from spills and 75% from operational 

discharges. Oily bilge discharge is second only

to oily ballast tank discharge in its contribution. 

Historically, oily bilge water has been treated 

using oil/water separator (OWS) technology or 

discharged with minimal treatment. OWS 

technology alone is not capable of meeting 

current oily bilge water discharge standards. 

Discharge standard for vessels of the United 

States armed forces under 40 CFR part 1700, is 

15 ppm. International commercial discharge 

standards are as low as 5 ppm. A variety of 

technologies have risen to fill the gap between 

OWS capability and discharge standards. 

Among these are systems based on gravity

separation, size separation, chemical affinity 

separation and flocculation. There has also been 

a gap between regulation and enforcement. 

Unregulated discharges have been problematic. 

There have been significant advances in the 

ability to detect such discharges in the past few 

years; the most significant of which is synthetic 

aperture radar (SAR). We will review OWS 

technology and developments in oily bilge water 

treatment and detection. For purposes of this 

paper we will consider engine wet exhaust 

treatment as similar to bilge water. 

REGULATION

Section 312 of the CWA requires the Secretary

of Defense and the administrator of the U.S. 

Environmental Protection Agency (USEPA) to 

develop uniform national standards to control 

certain discharges from vessels of the armed 

forces. These measures are implemented in 40 

CFR. Chapter VII and Part 1700, entitled 

Uniform National Discharge Standards (UNDS). 

The following discharges require Marine 

Pollution Control Devices (MPCD): 

Aqueous Film-Forming Foam

Catapult Water Brake Tank and Post-Launch 

 Retraction Exhaust 

Chain Locker Effluent 

Clean Ballast 

Compensated Fuel Ballast 

Controllable Pitch Propeller Hydraulic Fluid 

Deck Runoff 

Dirty Ballast 

Distillation and Reverse Osmosis Brine 

Elevator Pit Effluent 

Firemain Systems 

Gas Turbine Water Wash 

Graywater 

Hull coating Leachate 

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Motor Gasoline Compensating Discharge 

Non-oily Machinery Wastewater 

Photographic Laboratory drains 

Seawater Cooling Overboard Discharge 

Seawater Piping Biofouling Prevention 

Small Boat Engine Wet Exhaust 

Sonar Dome Discharge 

Submarine Bilgewater 

Surface Vessel Bilgewater/Oil-Water Separator 

 Discharge 

Underwater Ship Husbandry

Welldeck Discharges 

We will address oily bilge water discharge with 

some discussion of small boat engine wet 

exhaust discharge. Discharge limit for oily bilge 

water under UNDS is 15 ppm but the need to 

have environmentally sound ships extends 

beyond U.S. coastal waters. European countries 

and other countries with large coastlines (i.e. 

Japan, New Zealand) have been very active in 

the pursuit of eliminating operational discharges 

in conjunction with International Maritime 

Organization (IMO) and independently in some

areas. 

Nature of Discharge (NOD) 

Bilge water arises from a variety of sources, 

which are allowed to drain to the lowest part of 

the ships' hull, known as the bilge. Sources 

include valve and piping leaks, boiler blowdown, and condensed steam. On surface vessels, 

bilge water is usually transferred to a holding 

tank where it is stored for disposal or treated in 

an OWS before being discharged. Oily bilge 

water contains bio-accumulative persistent 

organic pollutants (POP's) such as polyaromatic 

hydrocarbons and chlorinated aromatic 

hydrocarbons, aromatic hydrocarbons (BTEX) 

and oil, copper, iron, mercury, zinc and nickel, 

in addition to emulsifying agents such as 

detergents and solvents. 

TREATMENT 

Gravity Separation-OWS Technology 

In the past, OWS have been used exclusively for 

treatment of oily bilge water (OBW) however 

this technology is unable to meet the 15 ppm

requirement. Many OWS systems on cruise and 

naval ships produce an effluent in the average 

range of 100-250 ppm. Ships equipped with oil 

content monitors (OCM's) have the ability to 

return bilge water, not meeting discharge 

standards to the OWS for reprocessing. 

The effluent concentration after OWS is more a 

measure of the degree of emulsification of the 

influent than of the efficiency of the OWS. With 

100% non-aqueous phase pollutants, OWS are 

capable of achieving quite low effluent 

concentrations. All OWS’s work based on 

gravity separation driven by density differences. 

If the suspended particles or droplets have 

effectively neutral buoyancy, OWS ceases to be 

effective. Additionally OWS’s are ineffective in 

removing colloidal metals and soluble 

compounds. By definition, these are close to or 

at neutral buoyancy. We must look for other 

properties to exploit in order to achieve the 

desired reduction of pollutants in bilge water. 

The technologies discussed below can all be 

considered post OWS polishers, as OWS 

technology is optimal as a first step for bulk 

removal of non-aqueous phase components.

Size Separation – Membrane Technology 

Cross Current Flow Membrane technologies 

essentially work as molecular sieves and have 

been used to produce pure water in municipal 

and industrial applications. Cross flow 

operations typically fall into three categories: 

Ultrafiltration (UF), Nanofiltration (NF), 

Hyperfiltration, more commonly known as 

Reverse Osmosis (RO) with the following 

particle size and molecular weight ranges. 

RO – 5 to 15 angstroms 

 100 to 300 MW 

Components Retained: 99% of most ions, 

most organics over 150 MW 

Process Applications: Brackish sea water, 

Desalting, boiler feed purification, 

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blowdown reclamation, pretreatment to ion 

exchange, ultrapure water production. 

NF – 10 to 80 angstroms 

 200 – 10,000 MW 

Components Retained: 95% divalent ions, 

40% monovalent ions, organics greater than 

150-300 MW 

Process Applications: Hardness removal, 

organic and microbiological removal, dye 

desalting, color removal 

UF – 100 to 1000 angstroms 

 1,000 – 100,000 MW 

Components Retained: Most organics over 

1000 MW 

Process Applications: Pre- and posttreatment ion exchange, beverage 

clarification, concentration of industrial 

organics and dilute suspended oils, removal 

of pyrogens, bacteria, viruses, and colloids. 

The most commonly used of these is Reverse 

Osmosis due to its highest capability for removal 

of dissolved impurities. There are four major 

configurations for membrane modules: plate and 

frame units, hollow fiber, tubular and spiral 

wound. Membranes are available in a variety of 

materials. Some common ones are cellulosic and 

polyamide for RO and NF and polysulfone, 

ceramic and fluorinated for UF. UF has 

molecular weight cutoff of 1,000 to 100,000. 

Pressure ranges are UF – 25 to 400 psi, RO and 

NF operate in the 500 to 1000 psi range. 

Crossflow is necessary in membrane systems 

due to the necessity of running continuously in a 

self-cleaning mode. This is due to the fact that 

even a tiny fraction of foulant mass can have a 

severe effect on membrane performance. 

Backwashing is not possible because the 

polymeric membrane is coated onto a support 

layer. Flow reversal causes separation of 

membrane from support layer. All cross flow 

systems separate the influent wastestream into 

two effluent streams. These are the permeate 

(purified water which has passed through the 

membrane) and concentrate (pollutants rejected 

by membrane) which must be continuously

flushed away. 

The inherent tendency of membranes to catch all 

but the smallest particle sizes renders them

susceptible to fouling by organic, inorganic and 

biological materials. Cross current flow does not 

suffice in keeping the membranes clean and so 

they must be periodically cleaned. This can be 

tricky business, as membrane chemical 

compatibility may be similar to those of the 

fouling agent, in which case, the cleaner will 

also dissolve the membrane. Short of 

dissolution, membranes can be denatured by

solvents, high or low pH's and temperature 

extremes. At the very least, membrane / cleaner 

compatibility must be tested. It is unlikely that 

membrane cleaning will ever be totally

eliminated even with the use of low 

concentrations of chemical cleaning agents in 

the process stream (Ning). 

Effective pre-filtration of particulate and 

chemical fouling agents should be considered 

when designing a membrane filtration system. 

Currently, Ultrafiltration (UF) has been the 

primary membrane technology utilized in post 

OWS bilge water polishing. RO and UF 

membranes are susceptible to organic and 

inorganic fouling. UF systems are not fine 

enough to capture ionic pollutants and therefore 

inorganic fouling has not been an issue. UF 

systems are able to capture most organic 

compounds over 1000 mw and are consequently

susceptible to organic fouling. 

Ceramic module ultrafiltration systems have 

been tested and used on naval ships for 

treatment of OBW with generally good results. 

The systems are able to achieve a reduction from

approximately 232 ppm to < 5 ppm at flow of 

about 5 gallons per minute (GPM). Ceramic UF 

systems require periodic cleaning and 

maintenance and can be overwhelmed by

concentrated slugs of influent, however they are 

more robust than most polymeric membranes 

and can withstand wider pH and temperature 

ranges therefore making cleaning easier. Better 

pre-filtration of organics (especially

concentrated slugs) would probably make the 

system more efficient and more cost effective. 

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Chemical Affinity Separation-PS 

Technology 

Chemical Affinity Separation involves the use of 

standard filter materials infused with curable 

polymeric surfactant (PS). Once cured into a 

substrate, the oleophilic properties of PS are 

transferred into a substrate, thereby greatly

enhancing its' ability to attach organic 

compounds to its' matrix. Once attached, these 

compounds become hydrophobic and tend not to 

re-release. 

Regular 5-micron melt-blown polypropylene 

(MBPP) filters treated with curable viscoelastic 

PS results in a filter similar in capability to cross 

flow UF membranes for removal of organic 

compounds and colloidal metals. PS infused 

filters exhibit less than 1 psi pressure drop 

across the filter to saturation. 

(See Figure 1 and Table 1) 

Table 1

Removal of Colloidal Metals

MYCELX

Cadmium, ICP <0.020 

Chromium, ICP 0.026 

Copper, ICP 0.351 

Lead, ICP <0.10 

Nickel, ICP <0.050 

Silver, ICP <0.020 

Zinc, ICP <0.020 

After Flocculation

Cadmium, ICP <0.020 

Chromium, ICP 0.070 

Copper, ICP 0.452 

Lead, ICP <0.10 

Nickel, ICP 0.052 

Silver, ICP <0.020 

Zinc, ICP 0.884 

Holding Tanks 

Cadmium, ICP <0.020 

Chromium, ICP 0.062 

Copper, ICP 1.0 

Lead, ICP 0.12 

Nickel, ICP 0.060 

Silver, ICP <0.020 

Zinc, ICP 3.1 

All Tests EPA Method 200.7 

Pressure Drop Analysis

 

Figure 1 

∆P vs. % Saturation 

This is a critical property that enables PS to 

capture concentrated slugs of oil without clogging 

the system, therefore making them useful as prefilters for more sensitive and therefore easily

fouled filtration methods. 

PS technology works by chemically immobilizing 

the pollutants into the filter matrix. The filters can 

be regenerated and the oil recovered water free by

centrifugation up to 100 times. This technology

has been utilized in multiple oily bilge 

applications in different ways, two of which are 

highlighted below. 

PRIMARY POLISHING TREATMENT 

The pictured unit in figures 2, 3 and 4 is a MX-22 

PS Technology device. It contains (22) 30-inch 

length, 5-micron MBPP filters infused with PS. 

This unit sees an average of 250 ppm post OWS 

effluent and discharges less than 1 ppm effluent 

in a single pass, even when periodically exposed 

to slugs as concentrated as 10,000 ppm. This 

system has been used for one year and has 

required only 1 filter set changeover. Spent filters 

are regenerated by centrifugation. Discarded 

filters are incinerated. (See Figures 2-4) It is an 

interesting property of PS technology that high oil 

loading improves its' ability to capture highly

emulsified oils and colloidal metals. PS filters are 

not capable of capturing metals that are not 

organically bound. 

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Figure 2

Figure 3

Figure 4 

Treatment of Oily Bilge Water on Oil Drilling Ship 

Figure 5 

Cruise Ship Oily Bilge Water 

SECONDARY POLISHING 

This trial was conducted in conjunction with a 

major cruise line on its' flagship vessel. The ship 

is equipped with a clay filter followed by two 

OWS in series and an oil content monitor. 

Effluent is re-circulated until a concentration of 

15 ppm is achieved. 

A PS MX-12 unit (twelve 30” 5 micron filters) 

was installed post OWS to assure a margin of 

safety before discharge. To date, this system has 

processed 99,000K gallons of 15 ppm bilge water 

in 3 weeks (~ 4000K gal/day) and discharge 

concentration has remained below 1 ppm. One 

filter set replacement has been required. 

Approximate quantity of oil removed was 15 lbs. 

PRETREATMENT TO ENHANCE 

MEMBRANE PERFORMANCE 

A third type of use for PS MX units which is 

being field tested for industrial applications but 

is also applicable to oily bilge applications is in 

pre-filtration to enhance the robustness and 

performance of membrane filtration systems. As 

stated earlier these systems are susceptible to 

organic and metallic colloid fouling. The ability

of PS units to absorb concentrated slugs without 

breakthrough or pressure drop makes them ideal 

chemical prefilters to work in conjunction with 

membrane systems. Most of the time the 

membrane will see clean water greatly reducing 

fouling and preserving capacity. The membrane 

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capacity is conserved for NF or RO range 

pollutants. Preliminary results indicate that 

membrane life is extended about 20 fold while 

operating costs are reduced by 50%. 

Figure 6 

Composite Unit 

We feel that similar improvements would be 

realized in UF membrane bilge systems 

currently in use. It is interesting to note, contrary

to our expectations, that OBW we have 

encountered has not been sufficiently emulsified 

to require NF or RO polishing post PS MX 

filters. In most cases post PS effluent is BDL 

(below detectable limits). It appears that 

measures taken to segregate emulsifiers from

OBW are having an effect. 

OTHER METHODS 

Various other methods have been developed for 

bilge water treatment. I will briefly mention 

these. 

- OWS / flocculation – these systems follow 

OWS with flocculate in treatment tanks 

followed by additional filtration of effluent 

to remove the flocculating agent. 

Flocculating agent is then dewatered. 

- Biological – mechanical separator 

 followed by a biological chamber 

consisting of oil eating microbes 

supported in matrix. Microbe systems have 

lacked “robustness” in the past. 

- Emulsifying agents, which render oil 

digestible to microbes. 

- Burning after heating and vapor separation. 

EXHAUST METHODS 

Exhaust gas discharge can contain many of the 

pollutants present in bilge water including 

BTEX, metals, diesel and oils. Exhaust 

discharge directly into the water has become a 

significant waste minimization initiative. PS 

infused filters have been used to polish wet 

exhaust processed through water separation 

devices resulting in effluent less than 1 ppm

concentration. Figure 7 is a schematic of process 

flow for this system. When treated this way, wet 

exhaust polluted water can essentially be looked 

at as another source of bilge water.

250ppm 1-5ppm < 1ppm

OWS PS RO

Figure 7 

Wet Exhaust Treatment System 

DETECTION OF DISCHARGES 

The need to have environmentally sound ships 

extends beyond US coastal waters as European 

countries and other countries with large 

coastlines (i.e. Japan, New Zealand) have been 

very active in the pursuit of eliminating 

operational discharges in conjunction with 

International Maritime Organization (IMO) and 

independently in some areas. 

One of the areas in which European and other 

foreign countries have been very active in is 

detection and enforcement. The development of 

Synthetic Aperture Radar (SAR) has made it 

possible to detect surface or dissolved oil day or 

night under a variety of weather conditions 

discharged above or under the surface. Synthetic 

aperture radar as the name implies does not 

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actually use a lens like conventional systems. 

The aperture is actually the arc described by an 

orbiting satellite in a given period of time. 

Sensors monitor scattered electromagnetic 

reflections in this time period and a computer 

image is generated. One of the interesting 

properties of the SAR is that higher resolutions 

can be achieved at greater distances due to larger 

lens (arc) radius. 

The European space agency has been working 

on oil detection at sea using its SAR, ERS1 and 

ERS2 (European Remote Earth Sensing 

Satellites). These satellites have successfully

been able to detect test and real spills and have 

been used for observing discharge trends. 

A study of 660 SAR images acquired over the 

southern Baltic, the North Sea and the Gulf of 

Lyon in the Mediterranean Sea by ERS2 taken 

since 1996 indicates that seas are most polluted 

along the main shipping routes. SAR images 

acquired during descending (morning) and 

ascending (evening) satellite passes show 

different percentages of oil pollution, because 

most of the pollution occurs during night time 

and is still visible on the SAR images acquired 

in the morning time. The fact that most 

discharges occur at night is a strong indicator of 

the need for effective monitoring. 

Japan has deployed a SAR system since 1991 

consisting of x-band (9.55 GHz) and L-Band 

(1.27 GHz) resulting in high-resolution images. 

The purpose of CRL/NASDA is to monitor 

earth’s environment and disasters. The United 

States hyperspectral NEMO satellite will also 

have the ability to detect oily discharges. In the 

past, other methods have been tested such as the 

use of an infrared line scanner coupled with side 

looking airborne radar (SLAR) to detect oil 

discharges from ships (ISOWAKE experiments), 

with mixed results.

 

Much work has been done up to now to detect 

illegal ocean discharges. It appears that SAR 

systems will be able to provide a higher level of 

detection and enforcement than was possible 

until recently. Besides satellite observation, 

other types of measures are also being 

considered. One idea would be to use a black 

box operating on the basis of probes to indicate 

the level and nature of the liquids ejected. The 

International Maritime Organization (IMO) is 

studying the possibility of equipping passenger 

ships with a black box to record safety-related 

parameters so as to determine the cause of 

accidents, as is the case onboard aircraft. The 

aim is to supplement this type of equipment with 

“environmental indicators”. A second idea 

would be to mark cargoes, or the fuel, with a 

coloring to ensure traceability and to be able to 

work back to the polluting vessel. A computer 

system for keeping tabs on shipping traffic is 

being put through its paces at the regional safety

and emergency center (CROSS) in Jobourg 

(Manche). This system could be used to identify

polluting vessels by spotting the whereabouts of 

oil slicks in relation to the route followed by

ships. This approach could be used to 

complement the two other solutions.

DISCUSSION 

Both enforcement and technology capable of 

meeting discharge standards are necessary to 

reduce oil pollution caused by ships. It appears 

that detection and tracking technology and the 

political desire to enforce regulations will make 

it much more difficult in the near future to 

discharge oily bilge water that is not in 

compliance with local standards. Although we 

primarily discussed the results of SAR imaging, 

there are multiple local efforts under way

ranging from aerial surveillance utilizing 

infrared line scanners to utilization of more 

conventional satellite detection systems. 

Regarding treatment technologies, the ultimate 

oily bilge water treatment technology has 

probably not yet been built. All the technologies 

mentioned in this article have their good points 

and weaknesses. Processes sensitive enough to 

capture highly emulsified or dissolved organic 

compounds are susceptible to fouling, requiring 

maintenance and expense. Conversely, processes 

that are not susceptible to clogging are not 

capable of always capturing the most highly

emulsified or dissolved organic compounds. 

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The optimal system must be sensitive yet robust. 

This probably implies a 2 to 3 stage system, as 

adequate protection of sensitive components is 

probably a critical element in overall system

robustness. It is also possible that OWS could be 

made more efficient by rendering collecting 

surfaces more oleophilic thereby reducing the 

need for pre-filtration. 

The author does not believe that emulsifying 

agents which carboxylate paraffins thus 

rendering them digestible to most microbes is a 

prudent way to go, although probably the most 

facile. Use of these materials will ultimately

alter nutrient balance especially in fresh water, 

thus leading to much faster eutropification. 

CONCLUSION

The importance of treating OBW is illustrated 

by the number of technologies that have 

developed to treat this waste stream. 

The ultimate oily bilge water treatment system

probably has not been built yet and will most 

likely be a composite utilizing multiple 

technologies. Detection of oily discharges and 

the ability to track to specific ships will probably

become a reality regarding enforcement within 

the next 5 years. 

REFERENCES 

Davis, C. O., “The Hyperspectral Remote 

Sensing Technology (HRST) Program”, Naval 

Research Laboratory, Code 7212, Washington, 

D.C. 

Gade, M. and W. Alpers, “Using ERS-2 SAR 

Images for Routine Observation of Marine 

Pollution in European Coastal Waters”, Science 

of the Total Environment, Vol. 237-238, pp. 

441-448, September 1999. 

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Pollution: A Glass Half Full or Half Empty?”, 

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through Process Chemistry”, W&WI, August 

2000. 

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Tompkins, K. T., L. P. Murphy, B. L. Owsenek, 

R. P. Pignataro, A. T. Rodriguez, “Ultrafiltration 

Membrane Polishing System for Shipboard 

Treatment of Oily Wastewater”, Conference 

paper. 

Wahl, T., A. Skoelv, J. H. S. Anderson, 

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9
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ACKNOWLEDGMENTS 

Jackie Pardue, Cortney Hoagland, Dave Hilbig, 

Fikret Kipcakli 

Hal Alper is the author and is President of 

MYCELX Technologies Corporation in 

Gainesville, GA. Mr. Alper received his training 

in chemistry at Cornell University and has 

authored numerous patents in a variety of 

chemistry related fields. 

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