A guarantee of continuous biogas production is only as good as the feed it assumes. For a tender in Indonesia asking for 1,500 Nm³/h of biogas from palm oil mill effluent, the technical submission accepted the duty on one condition: the plant must receive at least 66.6 tonnes of biodegradable COD a day. The mill’s reported flow and strength fell short of that, so the guarantee was tied to the load, not to the lagoon.
The challenge: a fixed number with a variable input
The client’s specification asked for a warranty of 1,500 Nm³/h of continuous biogas, at least 55% methane, with 85% COD removal and a retention time of at least 53 days. Those requirements are reasonable individually. The problem is that biogas production is set by how much degradable organic matter arrives, not by how big the reactor is. A larger reactor cannot create gas from COD that is not there.
The approach: work backwards from the gas to the load
The reverse calculation
| Step | Calculation | Result |
|---|---|---|
| Biogas required | 1,500 Nm³/h x 24 h | 36,000 Nm³/day |
| Methane required | 36,000 x 55% | 19,800 Nm³/day |
| COD removed | 19,800 divided by 0.35 Nm³ CH₄ per kg | about 56.6 t/day |
| COD fed | 56.6 divided by 85% removal | about 66.6 t/day |
| Effluent flow | 66.6 t/day divided by 83 kg/m³ (low end of reported COD) | about 800 m³/day |
The mill’s reported effluent flow was about 580 m³/day. At the stated basis that supports a little over 1,000 Nm³/h, not 1,500. The submission said so openly.
Two volumes, one condition
With a 53-day minimum retention, the lagoon volume is flow multiplied by 53 days. At the reported flow of 580 m³/day, that gives about 30,700 m³. At the 800 m³/day needed for the guarantee, it gives about 42,500 m³. The design carried the larger volume as its upper basis for the civil layout, subject to confirmation.
The conditions written into the guarantee
- The feed delivers at least 66.6 t/day of biodegradable COD.
- The reactor is biologically mature and operating within the agreed envelope.
- Gas flow is measured by a calibrated meter at agreed standard conditions, with averaging and test period agreed in advance.
- If biodegradability or methane yield is lower than assumed, the required feed load rises.
Sampling before the design is frozen
The submission made a sampling campaign mandatory. It would measure COD, BOD, solids, oil and grease, pH, temperature, flow variability and biodegradability, including a comparison of untreated and cavitation-treated material. Only then would the guarantee be reconciled with the confirmed daily COD load.
Process choices
The proposal used an enhanced covered-lagoon reactor with hydrodynamic cavitation upstream, to make the organic matter easier to digest, and a lamella separator to settle biomass and return it to the reactor. All concrete tanks exposed to effluent were to be lined with 1.5 mm HDPE, and the project was to finish within 10 months of order.
Why it works
Writing the guarantee as a feed condition is honest, and it protects both sides. The client knows what it must deliver. The supplier is not exposed to a figure that the mill’s own data cannot support.
What to consider for your own project
- Do the reverse calculation before accepting a gas guarantee.
- Tie the guarantee to feed quantity and quality, not only to the plant.
- Agree the metering and test method in advance.
- Sample before design freeze. Reported lab data is not a substitute.
- Treat volume and loading separately. A big lagoon with a weak feed still makes little gas.
For the forward calculation from mill throughput, see our post on covered lagoon sizing for palm oil mill effluent.
FAQ
Can a supplier guarantee a biogas flow?
Yes, if the guarantee states the feed load, methane yield and measurement method it assumes.
What is hydraulic retention time?
The average time the effluent stays in the reactor, found by dividing volume by daily flow.
Why measure biodegradability?
Not all COD converts to methane. The share that does determines the gas yield.
Why is sampling mandatory?
It replaces assumptions with measured data before the reactor is sized.
Reviewing a gas guarantee?
Send us the specification and we will test the guarantee against the feed data.