For palm oil mill effluent (POME), a covered lagoon anaerobic reactor is usually the most forgiving choice where land is available. It tolerates shocks, needs little skilled operation and gives the longest retention time. A UASB suits tight sites but is easily upset, and a CSTR sits in between. This comparison comes from a turnkey proposal for a 60 tonnes per hour mill in Indonesia.
The challenge: choosing a digester for a strong, variable waste
POME is hot, acidic and high in COD. The design basis for this mill was:
- Mill capacity 60 tonnes of fresh fruit bunches per hour, 18 operating hours a day.
- About 0.7 m³ of effluent per tonne of fruit, or roughly 750 m³/day, with a peak of 42 m³/h for pump sizing.
- Influent COD of about 60,000 mg/l, which is around 45 tonnes of COD a day.
- 85% COD removal, giving about 34,000 Nm³/day of biogas at 50 to 60% methane.
The comparison
| Factor | UASB | CSTR / CMAR | Covered lagoon |
|---|---|---|---|
| Footprint | Small | Moderate | Large |
| Retention time | Shortest | Longer, with biomass recycle | Longest |
| Response to shocks | Sensitive; can stop quickly | Moderate | Very tolerant |
| Skill needed | High; twice-daily fatty acid tests | Trained technician | Trained technician |
| Alkali demand | Often needed | Moderate | None in normal operation |
| Gas storage | Separate gasometer needed | In the roof | Six to twelve hours under the cover |
| Biomass | Granules that must be imported if lost | Grown locally | Grown locally; seeded from existing ponds |
Why the UASB struggles with POME
Suspended solids can break up the granules, and the process relies on a steady pH and a uniform first-stage hydrolysis. If a temperature swing or a pH error kills the bacteria on a remote site, new granules must be brought in. The reactor also stops producing gas almost as soon as the feed stops.
Why the lagoon copes
Its loading rate is low, so it can take double the load with little loss. The example given was an Asian waste-to-energy plant whose flow rose from 400 to 700 tonnes per day with no change to the lagoon. Its large mass of biomass also means it recovers if damaged. The compromise is the footprint, and the build time of 10 to 12 months, which depends on when the rainy season falls.
The package around the lagoon
- Feed pumps and filtration, sludge recycle, a carbonic acid unit that controls pH without chemicals.
- Liner, cover, bottom drainage, and gas collection pipework.
- A biological H₂S scrubber taking gas from about 5,000 ppm to 300 ppm.
- Dewatering, a blower with 300 mbar head and a standby flare.
- SCADA with data-logging, remote monitoring and alarms.
What to consider for your own project
- Check the land. A lagoon of tens of thousands of cubic metres is needed at this scale.
- Do a feasibility study first. Sampling, a topographic survey and a soil test come before final design.
- Time the build around the rainy season.
- Decide about the gas engines. They were excluded here and can be added.
UK background on digestion is available from the NNFCC Biogas Information Portal.
FAQ
How long is the retention time?
The proposal says a covered lagoon has the longest and most stable retention of the three, but gives no single figure.
Does a lagoon need chemicals for pH?
Not in normal operation. Acid produced is consumed by methanogens.
Who can operate it?
A mill technician trained for a week or two.
Is the biogas stored?
Six to twelve hours of gas sits under the cover.