For palm oil mill effluent (POME), a covered lagoon anaerobic reactor is usually the safer choice where land is available, because it tolerates shocks, needs little skilled operation and holds the biogas under its cover. A UASB takes far less space but is more sensitive and needs more control. A proposal for a mill processing 30 tonnes of fresh fruit bunches an hour shows how the comparison plays out, and what the numbers mean for biogas and power.
The challenge: turning mill effluent into biogas that runs reliably
A palm oil mill produces a strong, warm effluent. In this case it was about 245 m³ a day, at a chemical oxygen demand (COD) of roughly 72,000 mg/L. That is a lot of energy in a liquid: around 17.5 tonnes of COD a day entering the plant. Anaerobic digestion converts it to methane-rich biogas, which can run a gas engine and cut the mill’s own power costs.
The question for the mill was which reactor type to use. Remote mills often have no on-site biochemist, and a digester that fails after a shock can take weeks to restart.
The approach: compare three reactor types
The proposal set the covered lagoon (CLAR) against two alternatives, the UASB (up-flow anaerobic sludge blanket) and the CSTR or CMAR (continuously stirred or continuous-movement reactor).
| Factor | Covered lagoon (CLAR) | CSTR / CMAR | UASB |
|---|---|---|---|
| Footprint | Large | Moderate | Small |
| Digestion rate | Slow, very stable | Moderate | High-rate, more sensitive |
| Retention time | Longest | Longer, helped by biomass recycle | Shortest |
| Gas yield from a given feed | Highest | Medium | Lowest; some COD is bypassed to avoid solids build-up |
| Operation | Trained mill technician | Trained technician | Twice-daily volatile fatty acid tests, pH control |
| Alkali | None in normal operation | Midway | Often needed, can be a major cost |
| Gas buffer | Six to twelve hours under the cover | Reactor roof | Needs a separate gasometer |
| Bacteria | Grown locally from lagoon seed | Grown locally | Granules must come from an existing plant |
Why the lagoon suits POME
- Stability. The loading is low, so it could be doubled with little loss in performance. As evidence, the proposal cites a waste-to-energy facility in Thailand whose flow rose from 400 to 700 tonnes a day with no change to its covered lagoon.
- Simple operation. A competent mill technician can be trained in a week or two. A lagoon could run for days with minimal supervision.
- Biomass reserve. The large volume of biomass lets the process recover if it is damaged.
- Gas buffering. Storage under the cover smooths supply to the engine and flare.
The proposal concluded that a UASB was not suitable for POME, and that a CSTR/CMAR was the better choice only where footprint is tight or the rainy season would disrupt a lagoon build.
What the numbers mean
| Parameter | Value | Meaning |
|---|---|---|
| Lagoon volume | About 11,600 m³ | Roughly 47 days’ retention at 245 m³/day (our calculation) |
| Loading | 1.3 kg COD/m³/day | Low, which is why the lagoon absorbs peaks |
| COD removal | 85% | About 15 tonnes of COD a day converted to biogas |
| Biogas | About 9,500 Nm³/day | 50 to 60% methane; about 5,200 Nm³ of methane a day |
| Electrical output | 0.8 to 1.2 MW | 0.8 MW if the engine runs 24 hours; up to 1.2 MW if it runs only the mill’s 16 operating hours |
The methane figure is consistent with about 0.35 Nm³ of methane per kilogram of COD converted, a figure we derived from the quoted numbers. At 37% engine efficiency, that methane supports roughly 0.8 MW running continuously. Run for 16 hours instead, the gas stored under the cover is used over a shorter period, which lifts the rating to about 1.2 MW. The proposal suggested diesel generators for the idle hours.
Why it works: the biogas train
Raw biogas needs cleaning before an engine can use it. The proposed feed train was:
- Bio-scrubber. A sealed vessel with packing that grows hydrogen sulphide-consuming bacteria, taking H₂S from about 5,000 ppm to 200 ppm. It needs no chemical scrubbing liquor, only nutrients and air.
- Gas blower. Multi-stage stainless steel centrifugal blower with a 300 mbar head, sized for 150 to 750 Nm³/h.
- Dewatering. A chiller and condenser lower the dew point so water does not condense in the pipework.
- Final filter. A 5-micron filter ahead of the engine.
- Standby flare. An open-type elevated flare, sized for 750 Nm³/h, burns surplus gas.
A carbonic acid control unit manages pH in the lagoon without chemical dosing, and SCADA gives data logging, alarms and remote monitoring. Where siloxanes are a concern, the dewatering stage can be taken to about minus 30°C, followed by activated carbon, to control them. For a deeper look at H₂S removal on a high-sulphide mill gas, see our post on bio-scrubber design.
What to consider for your own project
- Land. A lagoon needs space and suitable ground. Do a topographical and soil survey first.
- Measure before you design. The proposal included a ten-day sampling campaign (flow, COD, solids, temperature, pH) and a feasibility study before the design was fixed.
- Season. Build timing depends on the rainy season. A programme of about 12 months was quoted.
- Prove the performance. Acceptance was based on five consecutive days of meeting the process guarantees, with repeat trials if not.
- Plan for the engine. Decide whether the engine is included, and how you will cover non-operating hours.
For general background on biogas in the UK, see the NNFCC Biogas Info Portal.
FAQ
Why not use a UASB for palm oil mill effluent?
The proposal judged it unsuitable: it is sensitive to shocks and suspended solids, needs frequent testing and pH control, and requires separate gas storage.
How long does a covered lagoon take to start?
It can be seeded with bacteria from existing settlement lagoons, which is quicker. Growing bacteria on site is possible but slower.
How much electricity can POME produce?
For a 30 tonne-per-hour mill, the design gave 0.8 to 1.2 MW depending on engine running hours. Confirmation depends on substrate testing.
Does a covered lagoon need chemicals?
Not for normal pH control, which uses a carbonic acid unit. Nutrients may be needed for the bio-scrubber.
Talk to us about POME biogas
If you run a palm oil mill and want to compare digester options, contact us with your throughput and effluent data.