A covered lagoon biogas plant for a palm oil mill is an earth-lined pond sealed under a gas-tight HDPE membrane, fed with palm oil mill effluent (POME) and held for at least about 35 days. A tender for a mill processing 45 tonnes of fresh fruit bunches an hour set out what the contractor had to deliver: a lagoon, pumps, gas handling, a flare, a gas engine of roughly 600 kW, controls and guaranteed performance.

The challenge: a strong effluent and a lot of gas to handle

A mill of this size processes roughly 200,000 tonnes of fruit bunches a year, running about 20 hours a day for 300 days. Its effluent stream is about 600 m³ a day with a chemical oxygen demand (COD) of about 60,000 mg/L. Multiplying the two gives a COD load of roughly 36,000 kg a day, which is the organic load the lagoon has to digest.

That load is both a pollution problem and an energy resource. Left in open ponds it releases methane to the atmosphere. Under a cover, the methane can be collected, burnt in a flare and used to generate electricity.

The approach: what the specification asked for

A new, modified covered lagoon

The tender called for a new lagoon on about 6.5 hectares rather than the conversion of an existing pond. It was sized to the mill’s capacity, with a minimum hydraulic retention time of 35 days. At 600 m³ a day, that implies an effective volume of at least roughly 21,000 m³. The contractor was to confirm the final dimensions from its own design.

The soil investigation required a topographic survey with contours and cut-and-fill volumes, at least three boreholes and three soundings to about 30 m, laboratory consolidation tests and a slope stability analysis. The foundation of a lagoon is earthworks, and the ground conditions decide whether the design is safe.

Liner and cover

• Cover: 1.5 mm HDPE geomembrane, with operating pressure under the membrane of a few millibar.
• Liner: 1.0 mm HDPE geomembrane over a 250 gsm non-woven geotextile, with baffles, floating accessories and gas collectors.
• Quality checks: every weld tested by water pressure, vacuum and tension tests, with reports, certified welders and a manufacturer’s material warranty.
• Safety: automatic gas venting on the membrane and fire extinguishers on the berm.

The cover is the part of the lagoon most likely to fail if poorly built, which is why the specification asked for certified welders and paperwork for every joint.

Feeding, mixing and sludge

Effluent is pumped from the mill’s cooling pond to a homogenisation pond, then fed to the lagoon. The specification called for duty and standby pumps for each service: feed, circulation, sludge and effluent pumps, plus recirculation pumps within the lagoon. Concrete weir, sludge, mixing and effluent tanks were lined for protection. Pipework was HDPE, with stainless steel fasteners. The contractor was also responsible for seeding the lagoon with active biomass before start-up.

Gas handling, flare and engine

Biogas flows from the cover through HDPE pipework to a gas house containing:

• Two blowers rated at about 1,000 Nm³/h, with explosion-proof motors.
• A chiller and heat exchanger to cool and dry the gas.
• Flame arresters on the lines.
• An open flare rated at about 1,000 Nm³/h with an LPG pilot.
• A gas engine of roughly 600 kW, 400 V, 50 Hz.

The open flare takes any gas that the engine cannot. It is a simpler and cheaper design than an enclosed flare. Compare it with the enclosed designs in our ground flare versus elevated flare post if your permit sets a combustion temperature.

Controls and instruments

A PLC with a SCADA system sits in a control room, backed by a UPS that must hold for 30 minutes. A fixed gas analyser reads methane, carbon dioxide, hydrogen sulphide and oxygen. The instruments include an ultrasonic biogas flow meter with totaliser, an effluent flow meter, temperature and pressure transmitters and level switches. The earthing system targeted below 1 ohm for instruments and below 5 ohms for power panels, with lightning protection.

Why it works: a guarantee that can be measured

Perhaps the most useful part of the specification is the list of guaranteed parameters, because it turns a design into a measurable result. The contractor was asked to guarantee:

• Biogas production, as a flow and as a volume.
• COD removal of at least about 85% to 90%.
• Methane content of at least about 55% to 58%, and carbon dioxide below about 40%.
• Gas pressure under the cover.
• Influent COD and degraded COD, so the removal can be checked against real effluent.

The contractor also provides a performance guarantee for 12 months after a take-over test. The specification sheet required the contractor to fill in the design data: reactor volume, hydraulic retention time, organic loading rate, solids retention time and methane yield per kilogram of COD. Asking each bidder to fill in the same sheet makes tenders comparable.

One caution: the document quoted slightly different guarantee figures in different places, for example biogas flow of 600 and 950 Nm³/h. When you write your own specification, keep one set of numbers and check them against the effluent data.

What to consider for your own project

1. Measure the effluent. Volume and COD, over a full season, set the size of everything else.
2. Fix the retention time. The 35-day minimum drives the lagoon volume and land area.
3. Specify the membrane testing. Weld tests, certificates and warranties are cheap compared with a cover failure.
4. Plan for engine downtime. The flare must handle the whole gas flow when the engine is off.
5. Make the guarantee testable. Name the parameters, the analysers and the conditions of the test.
6. Treat gas areas as hazardous. In the UK, equipment in explosive atmospheres falls under DSEAR, and similar zoning is good practice elsewhere.

FAQ

What is a covered lagoon digester?

It is an earth-lined pond covered by a gas-tight membrane. Effluent digests anaerobically beneath the cover and the biogas is collected.

How long should POME stay in a covered lagoon?

This specification set a minimum of 35 days, with the contractor’s design to confirm it.

What happens to the gas when the engine stops?

It is burnt in the flare, which is sized for the full gas flow.

Who is responsible for seeding the lagoon?

In this specification, the contractor, as part of commissioning and the performance guarantee.

Planning a biogas plant for mill effluent?

Send us your effluent flow and COD and we will outline a covered lagoon design.