For palm oil mill effluent (POME), a covered lagoon is the most stable and simplest digester to run, a UASB is the most compact but the most sensitive, and a CSTR sits between them. The right choice depends mainly on available land and operator skill. This article uses a proposal for a 60 tonne-per-hour mill in Malaysia, where roughly 600 kW of electricity was wanted from the effluent, to compare the three.
The challenge: strong effluent, a mill to keep running
- Effluent volume: about 0.7 m³ per tonne of fruit bunches, or roughly 756 m³ a day.
- Strength: influent COD of about 70,000 mg/l.
- Biogas: around 33,600 Nm³ a day at 50 to 60% methane.
- Gas system: up to 1,400 Nm³/h, a duty-standby booster at 300 mbar and a standby flare.
COD (chemical oxygen demand) measures how much oxygen the effluent would consume, so it indicates how much organic matter is available to make gas.
The comparison
| Covered lagoon | CSTR | UASB | |
|---|---|---|---|
| Footprint | Large | Much smaller | Smallest |
| Digestion rate | Slow, great inertia | Moderate, helped by biomass return | High-rate |
| Bacteria | Grown from local methanogens | Same | Needs granular sludge, imported if lost |
| Gas yield | Highest, longest retention | Medium | Lowest, some COD bypassed |
| Operator skill | Trained mill technician | Trained mill technician | Higher: twice-daily acid tests, pH control |
| Peak loads | Loading can double with little loss | Less gas if overloaded | Sized close to its maximum |
| Gas storage | Several hours under the cover | In the roof | Separate gasholder needed |
Why UASB struggles with POME
POME carries suspended solids, which can break up UASB granules and destroy the bacterial community. UASBs also usually need alkali to correct acidic feed, a major running cost, and a high recycle rate. A lagoon produces and consumes acid inside the same body of liquid, so no alkali is normally needed.
Where a lagoon is best
Resilience. The proposal gives an example of a Thai facility whose flow rose from 400 to 700 tonnes a day with no change to the lagoon. If part of a lagoon is damaged, the large biomass inventory allows recovery. The cost is land and construction, which stops during heavy rain.
Starting up a covered lagoon
The supplier’s method statements show how much care goes into commissioning:
- Find the seed. Active sludge is taken from the mill’s existing open ponds. Candidates are chosen by pH sampling and by observing gas production, because visible bubbling alone can mislead. Samples are taken from the shore, not from boats, because the ponds emit toxic gas and may be hot.
- Fill and cover together. The basin is filled with treated effluent, the cover is fitted and seed sludge is added to at least 1 to 1.5 m above the feed nozzles, all at the same time so gas does not build up under the cover. No one may stand under the cover membrane.
- Control ignition risk. Any welding needs fans blowing across the area to prevent methane reaching its explosive range.
- Test the pipework. Polyethylene gas lines are pressure tested with air, with relief devices isolated and at least two gauges fitted.
What to consider for your own project
- Start with land. If space is limited, a CSTR is the realistic alternative.
- Test the effluent. Suspended solids and COD decide the reactor type.
- Plan for seeding. Existing ponds are a free source of bacteria.
- Match engines to gas. Allow for gas storage and a flare for surplus.
The UK government’s Anaerobic Digestion Strategy and Action Plan gives background on how AD is supported.
FAQ
What is POME?
Palm oil mill effluent, the strong liquid waste left after processing fruit bunches.
How long does a lagoon project take?
About 12 months from order to commissioning, depending on the rainy season.
Can bacteria be reused from old ponds?
Yes. Active sludge from existing ponds is the usual seed.
Choosing a digester?
Send us your effluent data and we will compare options.