For high-strength distillery effluent, a continuously mixed anaerobic digester (CMAD) is usually the safer choice. It gives a smaller footprint than a covered lagoon and is far more forgiving than an upflow anaerobic sludge blanket (UASB) reactor when the liquid carries suspended solids. In a proposal for a South-East Asian distillery making about 30 m³ of ethanol a day, that comparison decided the technology, and the biogas was earmarked as boiler fuel.
The challenge: a strong effluent and an expensive boiler
The distillery produced roughly 360 m³ of effluent a day at 30 to 35°C. Its chemical oxygen demand (COD) load was about 36 tonnes a day, which works out at around 100 kg of COD in every cubic metre. That is a very strong waste, and it is also a very good fuel source.
The site ran a 25-tonne steam boiler with a heat input of about 15 MW. The effluent could yield roughly 12,000 Nm³ of methane a day. The biogas would carry about 5 MW of heat, so it could supplement the boiler fuel and cut fuel purchases. As an alternative, the same gas could drive reciprocating engines for around 1.8 MW of electricity, depending on the engine supplier.
The approach: three technologies compared
The proposal compared three ways of digesting the effluent. The comparison is useful for any high-strength industrial wastewater.
| UASB | CMAD | Covered lagoon | |
|---|---|---|---|
| Footprint | Smallest | Small | Large |
| Speed of digestion | Fastest, most sensitive to shock | Moderate | Slowest, very stable |
| Biomass | Granules, which may need to be imported | Grown on site from local seed | Grown on site from local seed |
| Gas yield | Lowest, short retention time | Higher, biomass recycled | Highest, longest retention |
| Operator skill | High, with twice-daily fatty acid tests | A trained factory technician | A trained technician |
| Alkali demand | Often significant | Midway | None in normal operation |
| Gas buffer | Needs a separate gasholder | Roof of the reactor | Six to twelve hours under the cover |
Why the UASB was ruled out
A UASB depends on granular sludge, and suspended solids can break the granules apart. It also needs the feed to be consistently acidified and pH-controlled. If a temperature swing or a pH slip kills the granules at a remote single-digester site, replacement granules have to be brought in by the container load. For distillery effluent, the proposal concluded the UASB was not suitable.
Why a CMAD rather than a lagoon
A covered lagoon is simple and stable, but it needs a large area and its construction stops in the rainy season. The CMAD confines civil work to tank foundations and the gas train. It also borrows from UASB practice, with improved mixing and biomass management, without the sensitivity to solids.
The design: what was specified
- Digesters: two steel tanks, each about 12.5 m high and 22 m in diameter, with an HDPE liner. Even though the inside of a digester is oxygen-free, the liner was kept to protect the steel in the long term. The proposal states plainly that bare carbon steel would not be supplied.
- Biomass management: a sludge separation filter returns biomass to the reactor, which lengthens effective retention time.
- Gas storage: a double-membrane roof gives about 1,000 m³ of balancing volume, so the boiler sees a steadier supply than the digester produces.
- Gas train: a bioscrubber takes H₂S from about 2,000 ppm down to 100 ppm, followed by an optional dewatering unit, a blower with about 300 mbar of head and a 500 Nm³/h standby flare. Peak gas flow was around 900 Nm³/h and the minimum about 100 Nm³/h.
- Controls: a PLC with data logging, remote monitoring and alarm notification.
The H₂S step matters because hydrogen sulphide and moisture damage boilers and engines. For the sulphide step in more detail, see our article on packed-bed bio-scrubber design.
Why it works, and where the limits are
Anaerobic digestion of this type can remove up to about 85% of the effluent COD, and more after pre-acidification. That is a large reduction, but the remaining liquid will still not meet a strict discharge standard. The proposal was candid about this. It offered a sequencing batch reactor (SBR) as an aerobic polishing step, but warned that the discharge limit in force locally could not be met by cost-effective means. Its recommendation was to discuss the consent with the authorities rather than build very high-technology treatment for little environmental gain.
The delivery period was around ten months from order to commissioning. The proposal also offered to split out locally procured items, such as the tanks, so that local fabricators could bid for them.
What to consider for your own project
- Test the effluent first. A front-end test of the raw effluent confirms COD, solids and temperature before a technology is fixed.
- Match the digester to your solids. If the liquid carries suspended solids, avoid granule-based reactors.
- Decide the gas use early. Boiler fuel needs less gas cleaning than an engine, which usually needs a drier, lower-sulphide gas.
- Plan for the rainy season. A lagoon build can stall for months. Tank-based designs are less exposed.
- Negotiate the consent. Digestion alone rarely reaches final discharge standards, so check what is required before you commit to polishing.
FAQ
What is a CMAD?
A continuously mixed anaerobic digester is a mixed tank in which bacteria digest the organic load and release biogas. Here it was combined with biomass recycle to hold more active bacteria.
How much COD can anaerobic digestion remove from distillery effluent?
The proposal states up to about 85% for this effluent, and 90 to 95% when pre-acidification is used.
Why is a gas holder needed?
A UASB stops producing gas almost as soon as the feed stops. A holder, or a membrane roof on the digester, smooths the supply so the boiler or engine is not starved and less gas is flared.
Can biogas replace some boiler fuel?
Yes. In this case about 5 MW of biogas heat could supplement a boiler needing about 15 MW, which is roughly a third of its input.
Planning a distillery or food-processing digester?
Send us your effluent flow and COD and we will compare digester options for your site.