Biological and electrochemical contributions to COD removal

Ship bilge water can contain fuel oil, hydraulic and lubricating oils, detergents, sewage traces and soluble organic compounds. Chemical oxygen demand (COD) gives operators a broad measure of that oxidisable load, while oil-and-grease testing tracks a different fraction. Australian operators read the two together: oily water that looks clean can still carry high COD, and a low COD result does not make a discharge compliant. Sydney Harbour ferries, Port Botany terminals and offshore supply vessels near Fremantle or Port Hedland face the same separation challenge, although their receiving waters differ.

For ElectroSAnMBR, a submerged anaerobic electrochemical membrane bioreactor (ANMBR) joins controlled electrolysis, anaerobic digestion and membrane separation. Biological and electrochemical contributions to COD removal are separated through a mass balance approach that tracks carbon in liquid, gas and concentrated streams and identifies uncertainty. Membrane capture can lower permeate COD without destroying organics, while biological oxidation can convert COD without producing the electrical response expected from direct electrochemical treatment. Funded through the European Union’s Horizon 2020 programme, the project offers a bridge for Australian ports, universities and treatment suppliers testing vessel-scale technologies.

COD in an Australian maritime setting

COD is an oxygen-equivalent index, not a compound-specific measurement. It captures how much oxidising agent a sample consumes under a defined laboratory method. Bilge streams can contain biodegradable carbon, surfactants, refractory organics and particulates that all influence the result. Australian reports usually spell out “chemical oxygen demand” first, then use “COD load” in routine discussion, but analysts must still distinguish soluble, colloidal and filtered fractions.

Regulation and operations add different constraints. International voyages engage MARPOL Annex I, while AMSA administers Australian biosecurity requirements for domestic commercial vessels. A compliant oil separator is only one part of shipboard control, and COD can remain after visible oil is removed. Around Cairns and the Whitsundays, operators near the Great Barrier Reef also face intense community scrutiny, making traceable treatment performance valuable beyond the statutory minimum.

Australian pilot work commonly links parties that control different parts of the problem: ports provide sites and runoff data, utilities and laboratories provide analytical capability, operators provide vessel access, and universities or vendors develop the technology. In everyday Australian usage, “blackwater” usually means sewage-bearing waste rather than oily bilge, so that distinction prevents a promising COD result being applied to the wrong stream.

Building a defensible mass balance

The balance begins with a fixed system boundary. At steady state, a simplified accounting is: COD in = COD out + COD transferred + biological conversion + electrochemical conversion + unexplained residual. Each term is a mass rate, usually kilograms of COD per day, not a concentration. If permeate is the only liquid leaving, then COD out is the permeate load; if backwash or concentrate is recycled, its COD must still be counted where it leaves the measured system.

Measurement discipline is as important as reactor instrumentation. Analysts need paired flow and COD data for feed, permeate, harvested solids and any concentrate or purge, plus duplicate samples, blanks and recovery checks. Biogas composition and biomass production constrain biological conversion, while current, voltage and charge efficiency constrain electrochemical conversion. A gap between calculated and measured removal should be reported as uncertainty, not assigned to whichever mechanism makes the balance look tidier.

A credible balance must also travel clearly to the people who need to act on it. For broader audiences, Meraki People provides the project’s communication context, helping technical balances reach ports, regulators and industry. Tables of measured loads, assumptions and uncertainty are valuable when Australian stakeholders compare a shipboard reactor with established land-based wastewater processes.

Biological conversion inside the anaerobic reactor

Anaerobic microbes break complex waste through hydrolysis, acidogenesis, acetogenesis and methanogenesis. During methanogenesis, much biodegradable COD becomes methane and carbon dioxide, with another portion incorporated into new cell material. The membrane retains slow-growing biomass and suspended solids, allowing a higher solids-retention time than a washout-only system. This can improve conversion of readily biodegradable material while leaving refractory COD in the liquid.

The biological share should be quantified from carbon flow, not inferred solely from falling COD. Gas flow and methane fraction check mineralisation, while volatile solids, alkalinity, pH and sludge production reveal reactor health. Electrolysis can supply hydrogen for hydrogenotrophic methanogens, but it can also shift pH or release inhibitory compounds; those effects belong in the operating record rather than being credited as COD removal.

Sound experimental design separates mechanism from correlation. Reactors without current establish the biological baseline, while sterile or abiotic controls reveal electrochemical oxidation and physical capture. Paired gas, solids and liquid measurements then show whether extra COD loss follows methane production, electrode charge or membrane capture. Without that separation, a strong result can mask two smaller processes that behave differently when vessel speed or bilge composition changes.

Measuring the electrochemical contribution

Electrolysis can attack dissolved organics directly at anodic surfaces or indirectly through reactive species formed nearby. The cathode may also produce hydrogen, alter local pH and create conditions that support methanogens. Electrocoagulation adds another pathway: electrical charge destabilises emulsions and gathers oil, colloids and some organics into flocs. Floc capture can be valuable, but it is COD transfer unless the collected material is oxidised or disposed of with a measured COD fate.

Charge data make the electrochemical pathway testable. Applied current, cell voltage, operating time and current efficiency determine electron dose, while laboratory COD balances determine how much that dose mineralised. Operators must correct for auxiliary reactions, electrode condition and soluble COD changes caused by pH shifts. A simple proportionality between amperes and kilograms of COD removed is inadequate because water reactions and biomass activity can consume or conceal the same charge.

For attribution, biological contribution can be anchored to methane and biomass formation, electrochemical contribution to verified Faradaic oxidation, and physical contribution to oil, sludge and concentrate streams. The remaining gap is reported with uncertainty. This recognises that the processes can cooperate in one reactor while still giving each a defensible line in the overall COD balance.

Oil, surfactants and membrane separation

Oil complicates every measurement in an ANMBR. Emulsified droplets can contribute to COD, coat membrane fibres and suppress microbial activity. Surfactants keep fine oil dispersed, alter electrocoagulation and change how flocs attach to a membrane. Operators need oil concentration, turbidity, conductivity and COD measured together; a lower soluble COD value may simply reflect organics transferring into material that must be removed from the balance.

The coupled effect is examined in surfactant effects on electrocoagulation. Surfactant type and dose can change both COD partitioning and membrane behaviour. An attractive flux trace may arise after organics have been concentrated into a backwash stream, so permeate data should never be used in isolation.

For trials, the membrane module should be treated as a separator with a measurable COD outlet. Samples of backwash, cleaning liquor or concentrate are needed to close the liquid balance, while transmembrane pressure, conductivity and normalised flux show whether fouling changes the apparent removal rate. Pretreatment and operating adjustments should respond to oil load, surfactant composition and biological stability together, rather than optimising COD reduction at the expense of membrane reliability.

Controlling pH for stable performance

pH links the biological and electrochemical parts of the reactor. Anaerobic digestion needs a buffered environment near its optimum, whereas electrode reactions can consume alkalinity, generate hydrogen ions or promote mineral precipitation. The consequences are described in pH buffering and stability. Alkalinity, volatile fatty acids and dissolved inorganic carbon should therefore be logged alongside COD and current data.

Operational stability is especially important in Australian waters. Warm conditions around northern ports can accelerate biological reactions while encouraging membrane fouling, while ships crossing the Great Australian Bight face changing temperature and salinity. Onboard power is shared with other vessel systems, so the reactor must show useful COD removal without assuming unrestricted electricity. These realities reward compact controls and clear mass balances over one spectacular trial.

The practical control set combines current density, hydraulic retention time, membrane flux, alkalinity and COD loading. Pilot data can identify which variables explain performance across different oils and surfactants. Uncertainty from sampling and unrecovered COD should be carried into scale-up claims, alongside checks on electrode cleaning, biogas safety and concentrate handling. A balanced result treats conversion, transfer and reliability as separate performance indicators.

Explore the ElectroSAnMBR objectives, work packages, methods, research team and affiliated laboratories through the project site. Australian ports, operators, universities and treatment suppliers can engage the team to compare mass-balance data, develop pilot protocols or build a collaboration around vessel-scale bilge-water treatment.