A covered lagoon is sized from the daily COD load and the organic loading rate. In a worked example for a 60 tonnes per hour mill, 42 tonnes of COD per day at 1.3 kg COD per cubic metre per day required about 26,000 m³ of wet volume and produced enough biogas for roughly 1.8 MW of electricity.
Step 1: the effluent flow
- The mill processes about 1,200 tonnes of fruit bunches per day over 20 hours, an average of 60 tonnes per hour.
- At 0.6 m³ of effluent per tonne, the flow is about 649 m³/day, with a peak of 36 m³/h.
- The effluent is cooled to about 40°C before the lagoon.
Step 2: the COD load
At 65,000 mg/l (65 kg/m³), 649 m³/day gives roughly 42 tonnes of COD per day. With 80% removal, about 34 tonnes are converted.
Step 3: the lagoon volume
Dividing the COD load by the loading rate: 42,000 kg ÷ 1.3 kg/m³/day gives about 32,000 m³. The proposal specifies a wetted volume of about 26,242 m³, from the removed COD (34,000 kg ÷ 1.3). The quoted geometry is 8.5 m deep, about 47 m wide and 129 m long with 0.5 m freeboard, a total volume of 28,141 m³ on 6,079 m² of land.
Note that the loading rate can be applied to either figure. Always state which COD load your loading rate refers to.
Step 4: gas and power
| Item | Figure |
|---|---|
| Methane | About 11,800 Nm³/day |
| Biogas at 50 to 55% methane | About 23,600 Nm³/day, or 985 Nm³/h over operating hours |
| Thermal output | About 4.9 MW |
| Electrical output at 37% engine efficiency | About 1.8 MW |
Step 5: the gas train
Conditioning is sized at 200 to 1,000 Nm³/h. The biological scrubber takes H₂S from 3,000 ppm down to the engine-friendly range, followed by dewatering, boosting and a standby flare.
Design features that matter
- Bottom feeding. Influent is pushed up from the lagoon floor, which almost eliminates sludge settlement.
- Continuous recirculation. Digestion continues for days without fresh feed.
- Walk-on cover with sampling ports, resistant to wind.
- Seeding. At least 60% of the wetted volume should be filled with seed material, supplied by the mill.
What to consider for your own project
- Measure real effluent flow and COD, not only the design figures.
- Confirm the soil and groundwater before fixing the depth.
- Check land: this lagoon needs over 6,000 m².
- Decide whether the gas goes to engines or biomethane.
See also our digester comparison for POME.
FAQ
What is the organic loading rate?
The mass of COD fed per cubic metre of reactor per day.
Why 80% COD removal?
It is the design assumption for this lagoon.
How long does it take to build?
About twelve months to commissioning.
Are the gas engines included?
No, but a powerhouse building was.