Reducing Chemical Oxygen Demand in Bilge Water with ElectroSAnMBR Technology
Bilge water is a difficult wastewater stream generated by engine rooms, machinery spaces and drainage systems aboard ships. It can contain lubricating oils, fuels, detergents, solvents, suspended solids and biodegradable organic matter. The mixture changes with vessel operations, making treatment more complex than a standard municipal wastewater application.
Chemical oxygen demand (COD) provides a broad measure of the oxygen required to chemically oxidise organic pollutants. A high COD value indicates a heavy pollution load and can signal greater pressure on receiving waters if discharge is poorly managed. ElectroSAnMBR technology addresses this issue by combining anaerobic digestion, electrochemical treatment and membrane separation in a submerged system designed for high-strength bilge water.
Why Bilge Water Produces High COD
Bilge water is rarely a uniform liquid. Oil leaks, cleaning chemicals, hydraulic fluids and food or domestic wastewater can enter the bilge, creating an emulsion that is difficult to separate. Some compounds are readily biodegradable, while others resist biological breakdown or inhibit microbial activity. This variability can cause conventional treatment systems to perform inconsistently.
For Australian shipping, the issue is relevant across busy maritime locations such as Port Botany in Sydney, Fremantle, Brisbane and the Port of Melbourne. Commercial vessels may remain in port for limited periods, so treatment equipment must cope with changing flow rates and pollutant concentrations. Operators also need to align shipboard practices with environmental requirements overseen by bodies such as the Australian Maritime Safety Authority.
COD reduction is therefore more than a matter of achieving a lower laboratory number. Effective treatment should reduce biodegradable organics, retain oil-associated solids, limit toxic impacts on microorganisms and produce a treated stream suitable for responsible management. A reliable process must also fit within restricted shipboard space and energy budgets.
How ElectroSAnMBR Combines Treatment Processes
ElectroSAnMBR uses an anaerobic membrane bioreactor enhanced by electrochemical reactions. In the anaerobic stage, microorganisms convert biodegradable organic matter into simpler compounds, with a portion ultimately transformed into biogas. Because the process operates without continuous aeration, it can require less energy than an aerobic system for comparable organic loading.
The electrochemical component can support pollutant transformation through reactions at electrodes, including the production of oxidising species or hydrogen that assists microbial processes. Depending on the configuration and operating conditions, electrolysis may improve the breakdown of recalcitrant organics, reduce fouling precursors or help maintain suitable conditions inside the reactor. The exact contribution must be established through controlled experimentation rather than assumed for every bilge-water composition.
The ElectroSAnMBR research project investigates this integrated approach for ship-generated wastewater. Its work brings together reactor design, membrane operation, electrochemical treatment and anaerobic biotechnology. The goal is to develop evidence for a process that can reduce organic pollution while managing the practical constraints of marine wastewater treatment.
The Membrane Barrier And COD Control
A membrane separates treated water from biomass and suspended pollutants. In a submerged membrane bioreactor, filtration occurs directly within the reactor, allowing a high concentration of microorganisms to remain in contact with the wastewater. This supports longer solids retention times and gives slow-growing organisms more opportunity to degrade complex compounds.
The membrane does not remove COD through biodegradation by itself. Instead, it retains biomass, colloids and particulate organic matter while treated water passes through the membrane pores. Biological conversion and physical separation work together: microorganisms break down soluble pollutants, while the membrane prevents much of the remaining particulate load from leaving the system.
As explained in research on membrane bioreactors, membrane performance is closely connected with sludge characteristics and fouling control. Oil droplets, extracellular polymeric substances and fine solids can form a resistant layer on the membrane surface. Electrochemical conditioning and suitable hydraulic control may help, but cleaning schedules, flux management and pretreatment remain important.
Operating Conditions That Shape Performance
A treatment system for bilge water must be designed around variability rather than a single representative sample. Salinity, temperature, oil concentration, pH and chemical toxicity can shift rapidly. Australian coastal waters also present seasonal differences in temperature, while long-distance voyages may expose systems to changing feed conditions and storage times.
Important operating variables include:
- Influent COD concentration and biodegradability
- Oil and grease loading entering the reactor
- Hydraulic retention and solids retention times
- Electrical current, voltage and electrode condition
- Membrane flux, transmembrane pressure and cleaning frequency
Pretreatment can protect the biological and membrane stages. Screens, settling, oil separation or equalisation tanks may remove large debris and free oil before the mixed wastewater reaches the reactor. Equalisation is especially useful when galley wash water, machinery drainage and tank-cleaning residues arrive in separate pulses.
The Australian maritime market includes large bulk carriers, container ships, offshore support vessels and smaller coastal operators, each with different available space and maintenance capability. A system suitable for a large vessel may require modification for a compact coastal vessel. Demonstrations should therefore consider practical access, electrical supply, crew workload and the availability of replacement membranes or electrodes in Australian ports.
Monitoring COD And Treatment Stability
COD analysis should be paired with measurements that explain how and why performance changes. Total COD shows the overall organic load, while soluble COD helps distinguish dissolved compounds from particulate material. Oil and grease, total suspended solids, pH, conductivity and volatile fatty acids provide additional insight into biological stability and separation efficiency.
A useful monitoring programme may include:
- Influent and effluent total and soluble COD
- Oil, grease and hydrocarbon fractions
- Membrane permeability and transmembrane pressure
- Biogas production and methane content
- Electrical energy use per volume treated
COD removal percentage alone can be misleading when the influent concentration fluctuates. A high removal rate at low loading may not represent performance during a heavy oil or chemical shock. Reporting mass loading, concentration, retention time and energy consumption gives a clearer picture of treatment efficiency.
Researchers can also use respirometric tests, microbial analysis and electrochemical measurements to identify limiting factors. For example, a sudden fall in COD removal may result from toxic cleaning agents, insufficient alkalinity, membrane fouling or a change in the organic fraction. Diagnosing the cause is essential before altering current density, sludge wasting or hydraulic conditions.
Comparing Integrated Treatment Options
Different treatment technologies address different parts of the bilge-water problem. Gravity separation can remove free oil but is less effective against stable emulsions. Dissolved air flotation can improve oil removal, while aerobic biological systems can provide strong oxidation of biodegradable COD. Anaerobic treatment offers lower aeration demand and potential biogas recovery, but it may need protection from toxic or poorly biodegradable compounds.
ElectroSAnMBR is intended to combine complementary mechanisms rather than rely on one treatment step. Its value should be assessed through pilot testing that compares COD removal, oil separation, membrane stability, energy use and operational complexity under realistic marine conditions.
| Treatment approach | Main COD mechanism | Strength for bilge water | Key limitation |
|---|---|---|---|
| Gravity or coalescing separation | Removes free oil and solids | Simple pretreatment with low energy demand | Limited removal of dissolved and emulsified organics |
| Aerobic biological treatment | Microbial oxidation with oxygen | Well-established for biodegradable COD | Higher aeration energy and sludge production |
| Anaerobic membrane bioreactor | Anaerobic conversion plus membrane retention | Compact, high biomass retention and possible biogas recovery | Sensitive to toxic compounds and fouling |
| ElectroSAnMBR | Anaerobic conversion, electrochemical reactions and membrane separation | Integrated approach for complex, high-strength wastewater | Requires optimisation of electrodes, membranes and operating energy |
Performance claims should be based on representative bilge-water mixtures rather than synthetic wastewater alone. Testing with variable salinity, oil content and cleaning chemicals will help show whether the process remains stable during realistic shipboard conditions.
From Laboratory Research To Marine Application
Moving from a laboratory reactor to shipboard operation requires more than scaling tank volume. Researchers must evaluate safety around electrical equipment, gas handling, corrosion, vibration and confined machinery spaces. The system should also tolerate intermittent operation when a vessel is in port or when bilge production is temporarily low.
The project’s work packages and affiliated laboratories provide a framework for studying these issues systematically. Experimental results can guide membrane selection, electrode configuration, anaerobic microbial management and process control. Collaboration between environmental engineers, electrochemists, microbiologists and maritime operators is especially valuable because each discipline addresses a different operational risk.
A successful system could support more consistent treatment before discharge to an approved reception facility or other authorised management route. It would not remove the need for careful onboard segregation, spill prevention and compliance with Australian and international marine pollution rules. Technology works best when paired with good housekeeping and accurate recordkeeping.
ElectroSAnMBR offers a research pathway for reducing COD while exploring lower-energy treatment of a challenging maritime waste stream. Following the project’s findings and experimental updates can help Australian researchers, ship operators and port stakeholders assess where this integrated technology is technically and economically suitable. Explore the project research and objectives to follow developments in electrochemical anaerobic membrane treatment.