Electrolytic Treatment Systems Surpass International
Standards for Marine Wastewater Effluent

As of January 1, 2010, the MEPC.159(55) effluent standards adopted by the International Maritime Organization's (IMO) Marine Environment Protection Committee are in effect. The standards apply to all sewage treatment systems installed on board maritime vessels, and they require more robust treatment effectiveness than the previous IMO standard.

Over the years a number of marine sanitation methodologies have been developed – from physical/chemical separation to biological treatment to electrolytic treatment systems.

Physical/chemical separation systems
Physical/chemical separation systems involve a flow-through device that treats liquid waste with a chemical (e.g., chlorine or sodium hypochlorite) or non-chemical disinfection process (e.g., ultraviolet disinfection or ozone) and then pumps treated sewage overboard as permitted. Larger solids are separated from liquids by a screen at the raw influent point, then macerated and transferred to storage for dumping in non-restricted zones. This process involves the transportation, storage and handling of hazardous chemicals, resulting in increased operational costs, and the off-gassing of chlorine. Physical/chemical separation typically is not able to handle large influent surges, often requiring additional tank capacity. As a result, the equipment requires a larger footprint then other marine sanitation systems. The equipment could have up to four times the amount of rotating equipment as electrolytic or biological systems, too. From a maintenance standpoint, physical/chemical separation systems require the periodic manual cleaning of separation screens which could be considered a biological hazard.

Biological treatment
A common land-based method of treatment, biological treatment involves the use of microorganisms (bacteria colonies) to feed on the waste in the presence of oxygen and naturally digested waste. Large collection tanks receive and aerate the wastewater, and excess/dead microorganisms with inert sludge are separated by settling. The clarified liquid from the process is disinfected, typically with tablet-form chlorine, and discharged as permitted. The disadvantages of this process are numerous. The process can take up to 30 hours to complete, depending on the hydraulic capacity of the particular unit, and requires a large footprint. Biological treatment requires a lengthy waste storage time, too, necessitating the use of several large tanks. The equipment is extremely heavy, often exceeding physical/chemical or electrolytic system weights by a larger factor. In periods of low flow or shutdown, the bacterial colonies can be destroyed by the introduction of certain influents (e.g., cleaning fluids), and the destruction of bacteria colonies can cause the sewage to go septic, creating a toxic gas such as hydrogen sulfide and a flammable gas such as methane. The ship's motion also has the potential to affect the settling process, creating an erratic flow that produces an uneven treatment process. These biological processes produce odor problems, too. One major maintenance requirement is the removal of the sludge deposits from the bottom of the tanks, which can be time consuming and require a complete shutdown along with manual removal of the sludge. Lastly, the cleaning fluids and caustics introduced into plumbing lines during the process can shut down the entire system.

In order for a biological treatment system to meet the new IMO standards, ultra filtration membranes are used in combination with the biological process. As a result, another level of complexity is introduced to the treatment process since both the membranes and the live bacteria must be properly maintained. Routine maintenance is undertaken to ensure the membranes are not fouled or damaged. Additionally, the membranes will require periodic cleaning, which creates a solution that must be properly disposed of.

Electrolytic treatment systems
Electrolytic wastewater treatment systems mix sewage with available seawater, which flows through an electrolytic treatment cell. A DC electrical current electrolyzes the seawater creating an oxidant (typically sodium hypochlorite) that oxidizes the organic material and kills off the disease-carrying pathogens within the process stream. The lightweight process is self contained and has a smaller footprint than physical/chemical separation and biological treatment systems. The treatment time is much shorter than with biological treatment systems, too – approximately two hours compared to biological treatment systems' range of eight to 30 hours. Cleaning fluids and caustics will not hamper the system's operation, and the electrolytic process creates no odor.

An example of a technology in which on-site generated sodium hypochlorite is produced from seawater is the novel OMNIPURE™ Series 55 system from Severn Trent De Nora. The OMNIPURE system is the only marine sewage treatment system that oxidizes sewage in an electrochemical cell as well as generates sodium hypochlorite from natural seawater for the disinfection of sewage streams. The system provides effective electrolytic treatment of both black and gray water, meeting personnel treatment capacities ranging from one to more than 500 people (65 m³/day or 17,000 gallons/day). These systems have received Type Certification Approval from Bureau Veritas for the MEPC.159(55) regulation.

The electrolytic treatment process
The OMNIPURE Series 55 units feature an adaptive algorithm wastewater management system to facilitate varying inlet wastewater conditions commonly found in offshore applications. Based on the baseline compliance and criteria of the MEPC.159(55) requirement, the units are factory calibrated to effectively oxidize and treat normal inlet wastewater streams of 500 mg/l (ppm) of TSS and BOD to at, or below, the discharge effluent requirements. For those installations where higher-than-normal sewage influent is expected, the appropriate inlet waste algorithm can be pre-selected prior to system shipment.

The automatic three-stage treatment process is simple and proven effective, utilizing continuous collection and treatment of the accumulated black and gray water. In the first stage, sewage is macerated while BOD and fecal coliform are significantly reduced as the wastewater stream flows through the patented electrolytic cell.

In stage two, an electrocoagulation cell enhances the sewage stream with dissolved iron particles that serve as nucleation sites, forming a flocculation with the fibrous organic matter that readily drops out of solution. A defoaming agent is injected into the waste stream between the electrolytic and electrocoagulation cells to enhance gas-liquid separation of the stream exiting the cell. Gas generated in both cells is removed in a degassing column mounted on top of the primary settler. A proprietary design encourages disengagement of the entrained process gases from the stream, which percolates gently into the primary settler tank. Finally, an internal standpipe provides significant reaction time for coagulated solids to fall to the bottom of the settler, and liquid with minimal solids is routed through the internal standpipe to the clarifier.

Stage three comprises an additional settling or clarification-concentration step. The process stream enters the unique clarifier tank where velocity is kept at a minimum to further encourage settling of any remaining flocculants to the bottom of the clarifier. This results in a clear effluent from the discharge of the unit that meets and/or exceeds the effluent composition requirements for MEPC.159(55). The solids from the settler and concentrator, which are classified as Class B sludge requiring no special transportation criteria or disposal restrictions, can be handled by the operator as a concentrated waste stream or further processed using Severn Trent De Nora's patent-pending bag collection unit.

With the new MEPC.159(55) effluent standards now in force, maritime vessel operators seeking an effective, low-maintenance and space-efficient sewage treatment system are increasingly considering electrolytic treatment for their long-term needs.

For more information, e-mail info@severntrentservices.com.

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