Merging Anaerobic Digestion and Electrochemistry for Ship Wastewater

Commercial shipping remains the backbone of Australian trade, moving more than 99% of the nation's exports across the Pacific and Indian Oceans. Yet for every tonne of cargo, vessels generate streams of oily, chemically-laden wastewater that must be managed before discharge. A new research project is exploring how submerged anaerobic bioreactors fitted with electrochemical cells can scrub bilge water far more effectively than legacy systems, potentially transforming portside treatment in cities from Sydney to Fremantle.

The work combines microbial digestion with direct electrical oxidation inside a single membrane module. By aligning these mechanisms, the team aims to remove hydrocarbons, surfactants, and dissolved organics while producing very little residual sludge. For regulators, port authorities, and vessel operators watching tightening discharge rules around places like the Great Barrier Reef Marine Park, this kind of integrated approach could be a significant step forward.

Why Bilge Water Remains a Headache for Australian Ports

Bilge water accumulates in the lowest compartments of every ship, picking up lubricating oils, fuel residues, cleaning solvents, and seawater salts along the way. When pumped overboard untreated, even small concentrations can damage fish stocks and smother sensitive habitats. Australia's coastline includes World Heritage-listed waters and commercial fisheries worth billions, which makes the stakes for clean discharge unusually high.

Port authorities in Sydney, Melbourne, and Gladstone already enforce strict reception facility standards under the Navigation Act and state-level pollution controls. However, many small operators, fishing fleets, and Defence Auxiliary vessels still rely on oil-water separators that struggle when emulsions are stabilised by modern detergents. The result is grey water that passes through conventional skimmers but still carries invisible contaminants into the marina basin.

Climate change is intensifying the pressure. Warmer waters around the Coral Sea hold less dissolved oxygen, so any organic load dumped from a passing vessel takes longer to break down naturally. Researchers and harbour masters alike are looking for compact, energy-efficient treatment units that can be retrofitted to existing tugs, ferries, and harbour cruise boats without taking up precious deck space.

The Microbial Side of the Equation

Anaerobic digestion relies on communities of bacteria that thrive without oxygen, breaking down complex organics into methane, carbon dioxide, and simpler molecules. In the context of bilge water, specialised microbes can crack long-chain hydrocarbons and fatty acids that would otherwise resist conventional aerated treatment. The process is slow on its own, but it is remarkably thorough when given the right retention time and nutrient balance.

Within the submerged membrane bioreactor configuration, the microbes form dense granules attached to membrane surfaces. These biofilms tolerate the salinity swings common in shipboard systems, where freshwater flushing alternates with seawater ingress. Because the biomass stays in the reactor, operators do not need to constantly waste and replace activated sludge, which is a hidden cost in many land-based sewage plants servicing remote mining camps and island resorts.

The ElectroSAnMBR consortium has tuned its digester chemistry to encourage methanogenic archaea while suppressing sulphate-reducing bacteria that would otherwise generate foul hydrogen sulphide. That balance matters in busy commercial berths such as Port Botany, where odours can quickly draw complaints from neighbouring residential suburbs and cruise terminal passengers.

What the Electrochemistry Adds

Applying a low-voltage current across the membrane module changes the game. As wastewater flows past the electrodes, direct oxidation breaks down stubborn compounds that microbes cannot easily reach, including certain surfactants, phenolic compounds, and trace pharmaceuticals flushed from on-board medical cabinets. At the cathode, hydrogen gas bubbles form, gently scouring the membrane surface and slowing the fouling that plagues conventional bioreactors.

Electrocoagulation is another side benefit. Iron or aluminium ions released from sacrificial electrodes clump fine oil droplets and suspended solids into larger flocs that settle quickly or get captured by the membrane. This reduces the load on downstream polishing units and means the system can handle sudden spikes, such as when a tanker flushes its cargo tanks in port.

Australian researchers have been particularly interested in how electrolysis interacts with brackish water typical of estuarine harbours. Conductivity is naturally higher than in freshwater rivers, so less external power is needed to drive reactions. Trials coordinated through the project platform have logged energy consumption figures that look competitive with purely biological systems, an important consideration for vessels that run on auxiliary generators burning costly marine diesel.

Putting the Two Systems Together in One Tank

The real innovation is not anaerobic digestion or electrochemistry alone, but their integration within a submerged membrane module. Wastewater enters the bottom of the reactor, rises through the active biomass, and exits through ultrafiltration hollow fibres that physically retain solids and microbes. Electrodes are threaded vertically through the same chamber, applying a controlled potential that drives both oxidation and gas scrubbing.

This single-tank architecture simplifies maintenance, a priority for ship crews with limited engineering training. There are no separate flotation cells, chemical dosing pumps, or sludge recirculation loops to babysit. For the Royal Australian Navy's patrol boats and the growing fleet of autonomous survey vessels operating out of HMAS Stirling, such simplicity is a significant advantage on long deployments.

Sensor arrays monitor pH, oxidation-reduction potential, and dissolved methane in real time, feeding data to an onboard control unit. The team behind the architecture, including the research team, has emphasised modular design so that ports or vessel operators can scale capacity by adding parallel cassettes rather than replacing entire systems. That flexibility suits everything from a Sydney Harbour ferry to a large livestock carrier berthing at Wyndham.

From Pilot Trials to Harbour-Side Deployment

Field testing is moving forward in stages, beginning with bench-scale reactors in laboratory settings and progressing toward containerised units that can be plugged into existing pump-out stations. Early results suggest removal rates above 95% for total petroleum hydrocarbons and significant cuts in chemical oxygen demand, even when the feed contains surfactants from engine degreasers and cargo hold cleaners.

Funding from the European Union Horizon 2020 programme has allowed the consortium to partner with Australian universities and CSIRO groups studying tropical marine pollution. Joint publications are already feeding into discussions with the Australian Maritime Safety Authority about updating guidance on bilge water management for vessels operating in sensitive zones.

Commercial uptake will depend on whether shipbuilders can fit the units within standard engine room footprints and on whether classification societies accept the electrical safety case. Conversations are under way with several shipyards in Henderson and Osborne, where the local supply chain has experience integrating modular environmental systems into naval and offshore support vessels.

If you are a port engineer, naval architect, or environmental officer interested in piloting this technology at your facility, the consortium welcomes expressions of interest and technical questions through the project website.