Anaerobic vs. aerobic: reading the numbers on high-COD effluent

Every biological treatment decision on a strong industrial effluent comes down to the same trade. Aerobic processes are robust, well understood and produce excellent effluent — and they consume a great deal of electricity blowing air into a tank, then produce a large volume of sludge you have to pay to remove. Anaerobic processes use almost no energy and produce biogas — and conventional ones are fragile in exactly the conditions industrial effluent presents.

Membrane-based anaerobic treatment changes that trade, and it is worth understanding why rather than taking the claim at face value.

The constraint in conventional anaerobic systems

UASB and EGSB reactors depend on the biomass forming granules that settle and stay in the reactor. That single dependency is what limits them. Granulation requires a long start-up with seed sludge. It is disrupted by fats, oils and grease. It struggles with salinity and toxic compounds. And it caps the organic loading the reactor can accept.

The practical consequence is a ceiling. Conventional anaerobic systems handle COD loads below about 15 g/L. Above that, performance degrades and the reactor becomes unstable — which rules them out for many of the streams where anaerobic treatment would be most valuable.

What membranes change

An anaerobic membrane bioreactor retains biomass with an ultrafiltration membrane instead of relying on settling or granulation. That decouples biomass retention from the settling characteristics of the sludge, and the consequences cascade.

The reactor can hold a dense, highly active biomass, so it accepts COD loads above 250 g/L — more than an order of magnitude beyond UASB or EGSB. Start-up is short because no granular seed is required. Pre-treatment needs are minimal. And because the membrane, not gravity, defines the effluent quality, the permeate is consistently clear rather than variable.

The energy line is the one that decides projects

Aerobic MBR reaches up to 99% COD removal, which is excellent, and it does so by running blowers continuously. On a high-strength stream that is a substantial and permanent electrical load.

AnaeroMBR reaches up to 98–99% removal with no aeration at all, no flocculants and no gas scouring. Net energy consumption is 1.5–2.5 kWh per cubic metre of net permeate. Against that, the process converts organic load into biogas with high methane yields — which frequently makes the installation net energy-positive rather than merely cheap to run.

That is the difference between a treatment plant that consumes energy and one that supplies it.

Sludge: the cost nobody quotes

Aerobic treatment produces a lot of biological sludge, and sludge disposal is often a larger recurring cost than the treatment itself. AnaeroMBR cuts sludge production by up to 75% compared with an aerobic process.

This deserves more weight in evaluations than it usually gets. A plant comparison that models electricity but treats sludge haulage as a footnote will systematically favour the aerobic option, and it will be wrong about the total cost of ownership.

Maintenance, and where the membranes sit

A fair objection to any membrane process is maintenance. Submerged membranes in an aerobic MBR require work in the tank. Conventional anaerobic reactors require internal access to a live anaerobic vessel, which is neither quick nor pleasant.

Our AnaeroMBR uses external, out-of-tank tubular ultrafiltration units. Access is safe and dry, membrane durability runs to around eight years, and the modular arrangement means capacity can be expanded without rebuilding the reactor. The design is also compact, which matters on sites where there is no room for another basin.

Where each one still wins

None of this makes aerobic treatment obsolete. On dilute municipal wastewater, where COD is low and biogas yield would be marginal, aerobic processes remain the right answer, and MBR gives excellent effluent in a small footprint.

The case for anaerobic membrane treatment is specific: high COD, high TSS, FOG, salinity or toxic compounds — food and beverage, dairy, distillery, brewery, pharmaceutical, chemical, pulp and paper, palm oil, starch and sugar. In those streams it tolerates conditions that defeat the alternatives, and it turns the organic load from a cost into an energy stream.