A covered lagoon anaerobic reactor can turn palm oil mill effluent (POME) into boiler fuel with simple operation and low skill demands, provided the site has space and the ground is sound. For a mill processing 60 tonnes of fresh fruit bunches (FFB) per hour, the design in this worked example treats about 660 m³ of POME a day and is expected to yield roughly 1,100 Nm³/h of biogas at about 55% methane for co-firing in the mill boiler.
The challenge: effluent, emissions and a boiler that could use the fuel
A plantation company in South-East Asia wanted to capture methane from a 60 t/h palm oil mill and use it as boiler fuel. The mill runs about 20 hours a day, processing roughly 1,200 tonnes of FFB daily. Using a typical ratio of 0.55 m³ of POME per tonne of FFB, that gives:
- Daily effluent: about 660 m³.
- Peak flow: about 33 m³/h while the mill is running.
- Strength: a COD of about 75,000 mg/L, which is roughly 50 tonnes of COD arriving every day.
COD (chemical oxygen demand) measures how much organic matter the effluent carries. Methane-forming bacteria convert that organic matter into biogas, which is why strong effluent is a good fuel source and a poor thing to leave in open ponds.
The approach: a covered lagoon with a recirculation loop
How the reactor works
Raw POME is first screened to 3 mm to protect the pumps, then enters a combined structure that acts as both buffer and mixing tank. Here it blends with liquid recirculated from the lagoon. The mixture is fed in along pipes laid lengthwise on the lagoon floor, so the flow rises up through the contents instead of settling out at the bottom. That matters because it largely prevents the thick sludge layer that builds up in plain lagoons, so the design does not need daily sludge removal, because solids leave with the effluent discharge. An emergency sludge discharge network is still installed.
The lagoon has an HDPE liner underneath, with a network of drainage below it, and a walk-on HDPE cover above. Sampling ports in the cover let operators check the contents without opening anything.
Design figures, and what they mean
| Parameter | Design value | What it means |
|---|---|---|
| COD removal | 85% | About 42 of the 50 tonnes of COD arriving daily is converted. |
| Wetted volume | about 15,800 m³ | The liquid volume that does the work. |
| Retention time | about 24 days | Each day’s effluent spends roughly three and a half weeks in the reactor. |
| Biogas | about 26,800 Nm³/day | Roughly 1,100 Nm³ per operating hour. |
| Methane content | 55% | Equivalent to about 6 MW of thermal power. |
The gas leaves the lagoon through a duty-and-standby pair of stainless steel centrifugal blowers. These deliver it at 300 mbar total head, which covers the pressure drop in the pipework to the boiler. A standby enclosed flare takes any gas the boiler cannot use, with a turndown of 5:1 so it can burn small flows as well as full output.
A smaller lagoon through higher recirculation
The client’s own technical specification had assumed a larger lagoon, with a wetted volume of about 22,500 m³. The proposal instead increased the recirculation rate, which allowed the lagoon to be about 30% smaller. The proposal states that COD removal and biogas production are not reduced as a result. A smaller lagoon means less excavation, less liner and a smaller cover, which matters on a site where earthworks are a large part of the job.
Why a covered lagoon suits POME
- Stability. Long retention makes the reactor tolerant of spikes in load and of a few days without feed.
- Simple operation. A competent mill technician can be trained in a week or two, rather than needing a process chemist to adjust parameters continually, as high-rate reactors such as UASBs often do.
- Complete conversion. A long retention time gives slow-to-digest COD time to break down.
The trade-off is land. A lagoon of this kind needs roughly 4,000 m² for the reactor alone, and it needs soil that can support embankments. High-rate tank reactors use less space but demand tighter control.
For the wider case for capturing methane from palm oil effluent, see the International Renewable Energy Agency for bioenergy background, and the UNFCCC Clean Development Mechanism for the carbon-credit framework that has applied to many POME projects.
What the site visit changed
A site visit after the first draft produced a list of practical findings that altered the design and budget. They are typical of what to expect on any lagoon project:
- Soil. The soil excavated for the lagoon was not suitable for building the embankment. That means importing fill and, in turn, a soil investigation before the design is final. It also lengthened the programme from about 12 months to about 14.
- Levels. The existing effluent ponds sit several metres above the highest ground on the proposed plant site, so pipework needs gantries and the pumping arrangement has to be thought through carefully.
- Pumps. Extra pumps, priming tanks, a raised platform and a shelter were added at the control tank.
- Monitoring. Two monitoring points, each a sealed HDPE pipe with a small sampling port, were added around the lagoon.
- Construction power. A generator was needed for the build, to supply electricity during construction.
- Connectivity. Poor internet at the site affects remote SCADA monitoring, so the data link needs planning.
What to consider for your own project
- Get a soil investigation early. Lagoon cost and programme depend on it. Here, foundations and lagoon design were priced on assumptions until the investigation could be done.
- Define the interface points. In this scheme the biogas plant stops at the boiler house wall, and the bio-scrubber, dehumidifier, burners and co-firing modifications were excluded. Decide who owns each of those.
- Check your boiler. Co-firing requires burner changes and instrumentation on the boiler side.
- Seed the reactor. At least 30% of the wetted volume must be filled with seed material from an active digester or pond.
- Plan for the flare. The flare handles start-up, boiler outages and surplus gas.
FAQ
What is a covered lagoon anaerobic reactor?
It is a lined earth lagoon, sealed with a gas-tight HDPE cover, in which bacteria digest effluent without oxygen and release methane-rich biogas.
How much biogas does POME produce?
It depends on strength and flow. In this example, 660 m³ a day at about 75,000 mg/L COD gave a design yield of about 26,800 Nm³ of biogas daily.
Can biogas fire a palm oil mill boiler?
Yes, co-firing is common, but the boiler needs suitable burners and controls, and the gas usually needs drying and sometimes hydrogen sulphide removal first.
How long does a project like this take?
The proposal allowed about 12 months from order to commissioning, extended to roughly 14 when a soil test was added.
Planning a POME biogas project?
Tell us about your mill’s throughput and site and we will outline a lagoon design and what it will need from the site.