To size a covered lagoon for palm oil mill effluent (POME), start from the mill’s daily flow and chemical oxygen demand (COD), decide how much COD the lagoon must remove, and divide by a design loading rate to get the working volume. For a mill processing 60 tonnes of fruit bunches an hour, that worked out at about 33,000 m³, roughly 46 days of retention, and about 1,100 Nm³/h of biogas.
The challenge: turning a wastewater into a gas supply
A palm oil mill produces large volumes of strong effluent. Left in open ponds, it releases methane to the air. A covered lagoon captures that methane so it can be burned in the mill boilers in place of other fuel. The designer’s task is to turn the mill’s throughput into a lagoon volume and a gas flow, so that the blowers, flare and burners can be sized on solid numbers.
The approach: seven steps from throughput to gas
The design worked through the following sequence for a mill in Indonesia.
| Step | Calculation | Result |
|---|---|---|
| 1. Daily POME flow | 60 t/h of fruit bunches x 20 h/day x 0.6 m³ per tonne | 720 m³/day (36 m³/h at the mill’s peak discharge) |
| 2. COD load | 720 m³/day x 70 kg/m³ | 50.4 t COD/day |
| 3. COD removed | 85% removal | 42.84 t COD/day |
| 4. Working volume | 42,840 kg/day divided by a loading of 1.286 kg COD removed per m³ per day | about 33,300 m³ |
| 5. Retention time | 33,313 m³ divided by 720 m³/day | about 46 days |
| 6. Methane | 42,840 kg x 0.35 Nm³ CH₄ per kg COD removed | about 15,000 Nm³/day |
| 7. Biogas | 15,000 divided by 55% methane | about 27,300 Nm³/day, or 1,136 Nm³/h |
Two checks follow. The applied loading is 50.4 t/day over 33,313 m³, or about 1.51 kg COD per m³ per day. The client’s specification set a maximum of 1.5, so the design was slightly above and the proposal explained the difference as marginal and to be verified in detailed design. Equipment (blowers and flare) was then rated at 1,250 Nm³/h, about 10% above the calculated gas flow.
The lagoon geometry
The 33,313 m³ working volume came from a lagoon 8.5 m deep, with 0.5 m freeboard, so an effective depth of 8.0 m. The water surface is roughly 80 m by 84 m, about 6,700 m². The total constructed volume is about 35,500 m³.
The ground
A soil investigation found very soft to soft surface soil in the top 0.5 to 2 m, hard strata at shallow depth in places and groundwater about 4 m down. The mitigation was:
- Strip the soft layer and proof-roll before placing engineered fill.
- Build the lagoon partly cut and partly fill, to limit excavation below groundwater.
- Lay a drainage and gas-venting network under the liner to relieve water pressure and gas, and to give an early sign of leaks.
- Dewater during construction until the lagoon holds enough liquid to resist uplift.
- Use a 1.0 mm HDPE liner under the lagoon and a 1.5 mm HDPE cover.
Why it works
Every step uses a figure that can be checked against the mill’s own data: throughput, effluent per tonne, COD and the removal efficiency. A methane yield of 0.35 Nm³ per kg of COD removed is the commonly used theoretical value, and the 55% methane content is the middle of the client’s stated range of 50 to 60%. The result is that gas production is a calculation, not a guess. The key uncertainty is the real COD, which is why a sampling campaign belongs in any project.
For UK readers, the government’s anaerobic digestion guidance gives background on biogas from organic waste.
What to consider for your own project
- Measure COD and flow before you design. A change of 10% in either moves the gas flow by 10%.
- Check the loading rate against the client’s limit. Here it was 1.51 against a stated 1.5.
- Add a margin to the gas equipment. Rating the blowers 10% above the calculated gas flow gives room for variation.
- Investigate the ground. Soft soil and groundwater shape the earthworks and the liner system.
- Plan the buffer. The mill discharges for 20 hours a day, but the lagoon runs for 24, so a buffer tank evens out the flow.
For what the gas is then used for, see our post on biogas co-firing in palm oil mill boilers, and for the gas warranty side, see our post on what a biogas guarantee depends on.
FAQ
What is a covered lagoon digester?
It is an earthen lagoon with a lined base and a gas-tight cover, where the effluent digests anaerobically and the biogas is collected under the cover.
How much biogas does POME produce?
It depends on COD. In this design 50 tonnes of COD per day produced about 27,000 Nm³ of biogas per day.
Why is retention time about 46 days?
It results from dividing the lagoon volume needed to treat the COD load by the daily flow.
Why use a cover?
To capture the methane, which is both a fuel and a greenhouse gas.
Planning a POME biogas project?
Send us your mill throughput and effluent data and we will run the numbers.