A covered lagoon digester captures methane from palm oil mill effluent (POME) under a gas-tight membrane. A contract for a plant at a palm oil mill in Indonesia set the guarantees at more than 85% COD removal, an organic loading rate below 2.2 kg COD per cubic metre per day, and biogas with 55 to 60% methane. This post looks at what that specification contains and what it means for buyers.
The challenge: capturing mill methane reliably
POME leaves the mill hot and rich in organic matter. Open ponds release methane to the atmosphere. A covered lagoon captures it for use in a boiler or engine, or burns it in an enclosed flare, and reduces the strength of the effluent.
The approach: pre-treatment, covered lagoon, gas handling
Process guarantees
| Parameter | Guarantee | What it means |
|---|---|---|
| COD removal | More than 85% | Most of the organic load is digested |
| Organic loading rate | Below 2.2 kg COD/m³/day | Sets the lagoon volume for the feed strength |
| Methane yield | 0.315 to 0.335 Nm³ per kg COD removed | Close to the theoretical yield, so gas forecasts rest on COD removed |
| Methane in biogas | 55 to 60% | Sets the heating value of the gas |
Pre-treatment and feed
- POME is pumped from the mill’s cooling pond through a 10 mesh screen to a hot basin.
- Two cooling towers reduce the temperature from about 80°C to about 42°C, with a cold basin, so the lagoon is fed at a temperature the bacteria tolerate.
- A mixing tank of at least 160 m³ blends fresh feed with recirculated liquid and sludge. A blower strips CO₂ in the mixing tank.
- Feed pumps are duty and standby pairs, with a larger pump moving the mixed liquid to the lagoon.
- Treated liquid goes to a settling pond and back to the mill’s existing effluent system.
The lagoon
The digester is an enhanced covered lagoon, with a 1.5 mm black HDPE geomembrane cover and a 1.0 mm HDPE liner with a full skirt and drainage. The cover is anchored in a trench, with gas release and four vents, and tested by an air pressure test. A diaphragm pump removes rainwater from above the cover.
Gas handling
- A condenser system, flame arresters and gas train.
- Two multi-stage centrifugal blowers, each 1,500 Nm³/h, in explosion-proof design with variable speed drives, taking gas to the bioscrubber, and two more taking it on to the flare or boiler burner.
- A CDM-compatible enclosed flare with biogas train, flame arresters and pilot.
- Gas analysers for methane, CO₂, H₂S and O₂, and ultrasonic flow meters before the flare and before the burner.
Electrical and control scope covers a motor control centre, a PLC with SCADA and 30-minute UPS backup, lightning protection with defined earth resistances, and explosion-proof lighting.
Why it works: the contract structure
The contract is a design-and-build with commissioning and a 12-month defect liability period. Payment was in three stages: an advance against a surety bond, progress payments against measured work, and a 5% retention released after the defect period. Commissioning included seeding the lagoon, feeding POME in stages, a flaring test and a month of operator training. Late completion attracted a daily penalty up to a cap, which is typical of fixed-price work.
For UK context on anaerobic digestion plant, see the gov.uk anaerobic digestion guidance.
What to consider for your own project
- Base the gas forecast on COD removed, not COD in.
- Cool the feed. Hot effluent can harm the biology.
- Plan for rain on the cover.
- Specify gas analysis and flow metering so guarantees can be checked.
- Define the defect liability and acceptance tests in the contract.
See also biological H₂S removal from biogas.
FAQ
What is a covered lagoon digester?
A lined earth lagoon with a gas-tight membrane cover that collects biogas from digesting effluent.
Why cool POME first?
It leaves the mill near 80°C; the digester biology needs a much lower temperature.
What does organic loading rate mean?
The mass of COD fed per cubic metre of lagoon per day. A lower figure means a larger or gentler system.
What happens to the biogas?
It is used in a burner or boiler, or burned in the enclosed flare.
Treating high-strength effluent?
Send us your effluent flow and COD and we will discuss a digester design.