As ship design has evolved, so, too, have conditions in the bilge. In the days of steam-powered vessels, water from a variety of sources, including valve and pipe leaks, boiler blow down, and steam condensate, drained into the lowest part of the ship's hull, the bilge.
Diesel and gas-turbine-powered ships, however, don't have the boiler and steam cycle components that drain water to the bilge – thus the term "dry bilge." There has also been an increase in the use of solvents and detergents and other substances that emulsify oil in the bilge water.
Gravity-type separators were not designed to remove emulsified oil, very small droplets of oil particulates from oil-saturated bilge water. They are, therefore, unable to achieve a final discharge concentration of 15 parts per million (ppm) oil – the level required by U.S. Coast Guard and International Maritime Organization (IMO) regulations. Some regions have even stricter discharge requirements.
REGULATORY BACKGROUND
Because of the significant disparity in bilge water discharge requirements among various regions, ships are subject to a myriad of regulations. To address this situation, The U.S. Department of Defense (DOD), led by the U.S. Navy, the U.S. Environmental Protection Agency (EPA), and the U.S. Coast Guard are leading an effort, known as the Uniform National Discharge Standards (UNDS) program, to develop national standards for controlling discharges from Armed Forces vessels.
The standards are being developed in batches. Bigle-water/oil-water separator (OWS) discharge from surface vessels is one of the first sources being tackled. The NAVY and EPA are currently finalizing proposed performance standards and preparing the Batch One proposed rule, which is scheduled to be published in the Federal Register (and available on the EPA's website and the UNDS homepage).
Detailed technical analyses were conducted the standard development effort. Among the factors evaluated were the nature of the discharge and its effect on the environment, the practicability, operational impact, and cost associated with installing and using a marine pollution control device, and relevant U.S. law and international standards.
Most commercial ships must comply with USCG and IMO rules concerning how much oil can be discharged overboard with bilge water. MARPOL 73/78, Annex 1 sets a limit of 15 ppm for oily water discharge.
TECHNOLOGY TO MEET THE STANDARD
The problem for the Navy was that the current available technology could not achieve the lower levels of 15 ppm under the bilge conditions that exist on Navy ships. "The certification testing did not test for emulsified oil. Equipment manufacturers were able to design relatively inexpensive devices that could remove bulk but not emulsified oil," says Dan Debusshere, senior engineer at Science Applications International Corporations/Geo-Centers, Inc., Engineering and Manufacturing Department (SAIC EMD), West Mifflin, PA. In 2005, however, the IMO added emulsified oil to its certification testing requirements.
To meet existing requirements, and in anticipation of the UNDS rules, the Navy's Naval Surface Warfare Center, Carderock Division, initiated work on bilge water polishing technology. It contracted Geo-Centers to assist in the development of secondary oil-removal systems or polishers that would be installed downstream of the conventional oil/water separator on naval vessels.
Together, the Navy and Geo-Centers developed a system based on membrane ultrafilter in a recirculation loop. The filter consists of a ceramic substrate roguhly six inches in diameter by three feet long filled with thousands of tiny, extruded square tubes running lengthwise through it. The ceramic portion is merely a somewhat porous substrate; aactual filtering is accomplished by a very thin coating on the surface of the membrane's passages.
System operation is straightforward; a feed pump introduces the dirty bilge water into the filtration loop. A recirculation pump increases the water's velocity as it travels through the loop at a very high velocity. This scours the surface of the membrane to prevent clogging. 
Meanwhile, the membrane's pore size such that the pressure drop across the membrane forces the clean water through the membrane, retaining the oil and other contaminants inside while the clean permeate is bled off and discharged. Some of the dirty fluid in the recirculation loop is periodically discharged to the waster oil tank to maintain a constant loop concentration.
During the development process. Geo-Centers fabricated several pre-production prototypes. Laboratory and shipboard testing by the Navy confirmed this effectiveness and reliability of this technology for bilge water processing.
THE KEY IS FRP
The first production contract for membrane/ultra-filtration polishers on DDG-51 Aegis class destroyers was awarded to Aircraft Appliances and Equipment Limited (AAE) Brampton, Ontario, Canada. The compan's design for a 5-gpm system is functionally similar to the Navy's design. "With ceramic membranes as the filtration media, the upstream components connecting to is must be non-metallic – i.e. fiberglass or some other resin-based material – to protect the filter from corrosion/erosion damage," says Andy Willnecker, executive director of Aircraft Appliances. For example, the membrane housings and the piping are made of fiberglass-reinforced plastic (FRP). This construction has the added advantage of eliminating the possibility of metal particulates being introduced into the discharge water.
The recirculation pump, too is made of FRP, specifically fiberglass-reinforced vinyl ester resin. Bilge water can have a significant seawater component, which is corrosive to metal pumps, and may contain other trace elements as well. This accelerates or exacerbates the problem of metal metal particulate discharge via the bilge pump. "FRP Pumps, since they have no metal content, cannot erode or leach materials into the discharge stream," according to Pete Cerutti, president of Federal Resources, Chester, MD, a distributor of Fybroc pumps, a division of Met-Pro Corp. While worldwide, unified regulations on discharging metals overboard do not exist, the Navy was concerned that some regulations along these lines may be introduced in he future. This was a key factor for requiring FRP pumps in their bilge polishing systems, Debusschere points out.
"Because of the small pore size, a very high-velocity flow must be maintained through the ceramic membrane in order for the polishing system to function properly. If a bronze pump with a Monel impeller were used, a standard choice for many Navy pumps, the combination of seawater corrosivity and high velocity could lead to increased metal particulates in the bilge discharge. Use of an FRP recirculation pump eleiminates this source of pollution. Since the recirculation loo[ components are also made of non-metallic materials, the system cannot discharge any metal particulates," he explains.
THE FRP PUMP
Before equipment could be specified for use on Navy ships, the FRP pumps and the entire bilge water polishing system had to be tested under a variety of stringent conditions to ensure that they met military specifications for resistance to shock, vibration, and electromagnetic interference (EMI), Wilnecker notes.
The pump had to meet Grade B shock requirements (MIL Spec MIL-S-901), which mandate that the pump maintain its structural integrity in case of a catastrophic event as might be encountered in a hostile environment. It did not need to remain operable if the ship hit a mine., but it did not need to maintain its structural integrity to the level that it would not become a danger to the people in the vicinity. The pump also had to be able to withstand the vibrations generated by the propeller shaft, according to MIL-STD-167-1.
Various Fybroc pumps recently received ABS Type approval for use in freshwater and seawater services. These types include horizontal, vertical sump, close-coupled and self-priming models, with capacities up to 5,000 GPM and heads to 400 feet.
Source: "Polishing up Bilge Water" by Gennaro A. D'Alterio, Fybroc Division, Met-Pro Corp. Publsished in the March 2006 Issue of Marine Log magazine.