A molasses mill discharging 700 m³ a day at around 85,000 mg/l COD can generate roughly 40,000 Nm³ of biogas a day, enough for up to about 3.3 MW of electricity. This article follows the sizing of a covered lagoon for such a mill in South-East Asia, and shows why the gas train, especially hydrogen sulphide removal, is as important as the lagoon itself.
The challenge: very strong, sulphur-rich effluent
Molasses processing produces some of the strongest industrial effluents. Here the COD ranged from 70,000 to 100,000 mg/l, with 85,000 mg/l used as the average. High-sulphate effluent also produces a great deal of hydrogen sulphide (H₂S) in the biogas: the design inlet figure was 18,000 ppm, several times higher than in the other digesters we have seen.
The sizing chain
- COD load. 700 m³/day at 85,000 mg/l is about 59,500 kg of COD a day. With 85 to 90% removal, around 45,000 kg a day is converted to biogas.
- Lagoon volume. The design loading was 1.3 kg COD per m³ per day, which gives a lagoon of about 37,000 m³. Dividing volume by flow gives a hydraulic retention of roughly 50 days, long by any standard and the reason such a lagoon is stable.
- Gas. About 39,800 Nm³ of biogas a day at 50 to 60% methane, around 19,900 Nm³ of methane.
- Power. At an assumed engine efficiency of 39%, up to 3.3 MW gross. This was the figure used for sizing the gas train; the engines themselves were not in the proposal.
The gas train
| Stage | Design |
|---|---|
| Gas flow | 250 to 1,750 Nm³/h |
| H₂S removal | Three bioscrubber vessels, 18,000 ppm in, 300 ppm out |
| Dewatering | Chiller; dew point 20°C, discharge 40°C maximum |
| Booster | One blower, 300 mbar head |
| Final filter | Particle filter |
| Surplus gas | Standby flare |
Compare this with the 2,500 ppm inlet figure for a palm oil digester in the same region: here the scrubbing duty is seven times higher, which is why the vessel count rises from one to three.
How a bioscrubber works
Bacteria growing on a packed bed consume H₂S in the presence of a small amount of air. A recirculation pump sprays liquid over the packing, an auto-drain removes surplus bacteria and gas analysers monitor the outlet.
Why a lagoon suits this effluent
- It tolerates variable strength, which suits a mill that may run seasonally.
- It needs only a trained mill technician.
- Gas stored under the cover buffers production against engine demand.
The trade-off is the land: tens of thousands of cubic metres. A site survey and soil investigation are needed before detailed design.
What to consider for your own project
- Measure the COD over a season. A 70,000 to 100,000 mg/l range means a 40% swing in gas.
- Test for sulphate and H₂S. It drives scrubber size and engine life.
- Decide on engines separately. Engines and boiler burners can be quoted apart.
- Think about start-up. Plan the seeding and the rainy season.
See the UK government’s Anaerobic Digestion Strategy and Action Plan for background on AD policy.
FAQ
Why is H₂S so high from molasses?
The effluent contains sulphur compounds which sulphate-reducing bacteria convert to H₂S.
How long is the retention time?
Roughly 50 days, calculated from lagoon volume and daily flow.
Are engines included?
No, they were priced separately.
Treating strong effluent?
Send us your effluent figures and we will outline a lagoon and gas train.