An anaerobic membrane bioreactor (AnMBR) digests palm oil mill effluent in a compact steel tank, keeping the biomass inside with filtration. For a 60 tonne-per-hour mill in Sabah, the design treated 600 m³ a day at 55,000 mg/l COD with 85% removal, giving up to 1.5 MW of electricity. This article explains why it was chosen and what the numbers mean.
The challenge: no history, tight space and a wet climate
The mill had no reliable record of effluent flow or COD, so the design used values from the tender document. The mill already ran steam turbines for about 1 MW and diesel engines for roughly 300 kW. The aim was to replace the diesel generation with biogas. After a client meeting, the supply was widened to engines covering the full 1.5 MW potential.
The design basis
| Parameter | Value | What it means |
|---|---|---|
| Effluent | 0.5 m³ per tonne of fruit bunches; 600 m³/day; peak 36 m³/h | Mill runs 20 hours a day |
| COD in | 55,000 mg/l, 33,000 kg/day | The organic load |
| COD removal | 85% | Most is converted to gas |
| Tank volume | About 6,600 m³ | Small compared with a lagoon |
| Biogas | About 17,900 Nm³/day | 50 to 60% methane |
| Electrical output | Up to 1.5 MW at 37% engine efficiency | Gross |
Holding 33,000 kg of COD a day in a 6,600 m³ tank corresponds to a loading of around 5 kg per m³ per day (our calculation), far higher than a lagoon design, which is why the footprint is much smaller.
Why an AnMBR
- Footprint. A high COD allows a smaller reactor.
- Weather. Civil work is limited to foundations, so construction is less affected by the rainy season, which stops lagoon building.
- Biomass control. It borrows UASB know-how for mixing and biomass management without the UASB weakness with suspended solids.
Materials
The tank is epoxy-lined steel. Digesters are oxygen-free, so general corrosion is low, but POME contains salts that can cause pitting. Bare carbon steel was ruled out to avoid a premature failure.
The gas train
- Bioscrubber taking H₂S from about 2,500 ppm to under 200 ppm.
- Dewatering unit to a 25°C dew point.
- Two boosters in duty-standby, 200 mbar head.
- Standby flare for 750 Nm³/h and 150 to 750 Nm³/h gas range.
- Gas engines, supplied by a local company so service is nearby.
Delivery and exclusions
The scheme was offered as a turnkey package with a local civils partner and 10 to 12 months to commissioning. Import duty was excluded: as a renewable energy project, the owner was expected to seek tax exemption, with technical help from the supplier.
What to consider for your own project
- Get data. Without effluent records, assumptions carry risk. Sample before detailed design.
- Size engines to the gas. Do not size to the mill’s diesel load only.
- Check materials against salts. Coatings matter.
- Plan the sludge. The proposal assumed a disposal pond within 200 m.
For policy background see the UK Anaerobic Digestion Strategy and Action Plan.
FAQ
What is an AnMBR?
A digester that keeps biomass in by membrane filtration, allowing high loading in a small tank.
Why not a UASB?
POME’s suspended solids damage UASB granules.
How long to deliver?
About 10 to 12 months.
Generating from POME?
Talk to us about digester options for your mill.