A covered lagoon digester can turn the slurry from a 2,400-sow pig farm into roughly 550 kW of average electrical power, enough to meet the farm’s own demand. That is the result of a proposal we prepared for a pig producer in South-East Asia. This article walks through how the numbers were built, why a covered lagoon was chosen, and why two engines were specified rather than one.

The challenge: a farm that is not built yet

The client was expanding into pig farming and planned several farms, each producing slurry that could generate energy. The first farm was not yet operating, so there were no manure measurements to work from. Design had to start from herd data and standard manure production rates, with the soil survey and substrate testing still to come.

The farm’s electricity use was substantial. Ventilation, lighting, feeding and slurry handling were extrapolated from data for a larger reference farm, giving:

• Average load: about 445 kW, equal to roughly 3,900 MWh a year.
• Peak load: about 862 kW, driven mainly by ventilation.

A digester that only matches the average would leave the farm buying power at peak times. The design had to cope with that swing.

The approach: from herd numbers to biogas

Estimating the slurry

Each age class of pig produces a different amount of manure per day, from about 0.5 kg for a pre-weaner to roughly 18.6 kg for a sow. Multiplying these by the herd structure gave an estimate of 65,000 to 70,000 tonnes of manure a year. Volatile suspended solids were taken as about 9% of the manure, and a standard conversion of 0.29 kg of biogas per kg of volatile solids was applied.

The resulting design figures were approximately 6,000 Nm³ of biogas a day at 55 to 65% methane, or about 3,600 Nm³ of methane a day. Because these are estimates, the proposal states clearly that substrate testing must confirm them.

Diluting the slurry

Fresh manure was assumed to be about 10% solids. It is diluted to roughly 2% solids in a mixing tank so that it can be pumped and handled. The actual dilution depends on how the farm collects manure, so it is a data point to confirm early.

The treatment train

The facility runs in this order:

1. Mixing and dilution tank.
2. Covered lagoon of about 28,000 m³, with a liner, a gas-tight cover, internal pipework and a lamella separator that controls the solids in recirculation and effluent.
3. Hydrogen sulphide bioscrubber, designed to take gas from about 3,000 ppm H₂S at the inlet to about 200 ppm at the outlet.
4. Gas dewatering, a blower and a final particulate filter.
5. Two gas engines, with a standby flare for surplus gas.

Why a covered lagoon

The covered lagoon suited this farm for four reasons:

• Stability. It tolerates shock loads and does not need constant adjustment, unlike an upflow anaerobic sludge blanket (UASB) reactor.
• Simple operation. A competent technician can be trained to run it in about two weeks, with no need for a specialist biochemist.
• Long retention time. Slowly degrading material still has time to convert to gas.
• Suspended solids. It copes with solids better than most alternatives.

The trade-off is land. A covered lagoon needs space, so it is best where the site can accommodate a large footprint and where a soil survey confirms the ground is suitable.

Gas storage under the cover

The cover doubles as a gas holder. That matters here because the farm’s demand varies so much. Gas produced at quiet times is stored under the cover and used when demand rises, so the engines can follow the load rather than the digester’s instantaneous output.

Why two engines

The proposal specifies two 600 kW engines instead of one larger unit. Together they can deliver up to about 1,100 kW to cover the peak, and the output can be turned down to around 300 kW when gas production is low. A single engine is cheaper but gives less flexibility, and the whole farm loses its generation whenever that engine is down for maintenance. With two, one can be serviced while the other runs.

Engine electrical efficiency was taken as 39% rather than the 40% or more quoted in manufacturers’ data, to reflect realistic site conditions. Because the electricity is for self-use, no grid connection is needed.

Gas conditioning: what protects the engines

Engines are sensitive to moisture and contaminants, so the feed train does the following work:

• The bioscrubber uses bacteria growing on a packed bed to consume hydrogen sulphide, with a gas analysis panel to monitor the outlet.
• The dewatering unit chills the gas to remove moisture. In the base case the dew point is below the minimum ambient temperature. Colder options exist if siloxanes are a concern, in which case a carbon polishing bed follows.
• The blower is a 100% duty-standby stainless steel centrifugal unit, typically with about 300 mbar total head, delivering gas at about 200 mbar.
• The final filter is a 5-micron unit rated EN779 F9.

What to consider for your own project

1. Collect manure data early. The solids content of the slurry changes the dilution and the lagoon volume.
2. Size on load as well as gas. The gap between average and peak demand drives the engine arrangement.
3. Use the cover as storage. It allows generation to follow demand.
4. Check the ground. A soil survey is needed before the lagoon is designed in detail.
5. Plan for sludge. A settling tank and belt press can reduce lagoon sludge from about 97% to 70% moisture, which makes it far easier to handle.

For UK readers, the government’s Anaerobic Digestion Strategy and Action Plan sets out how AD is supported, and NNFCC’s biogas information portal gives further background on feedstocks and technology.

FAQ

How much biogas does pig slurry produce?

It depends on the herd and how the manure is collected. In this design, a 2,400-sow farm was expected to give roughly 6,000 Nm³ of biogas a day, based on standard manure rates and volatile solids content.

Is a covered lagoon better than a tank digester?

For wet slurry where land is available, a covered lagoon is stable and easy to run. Tank digesters take less space but need more careful operation.

Why remove hydrogen sulphide before the engine?

H₂S is corrosive and damages engines and pipework. A bioscrubber reduces it to a level the engine manufacturer can accept.

Can a farm run on its own biogas power?

In this example the average generation of about 550 kW exceeded the average demand of about 445 kW, so yes. Peak demand was met by gas stored under the cover and a second engine.

Planning a farm biogas project?

Send us your herd size and power data and we will give you an outline of the digester, engine size and gas train.