Sanguan Wongse Industries (SWI) is one of the major cassava/tapioca starch manufacturers in Thailand, accounting for roughly 12% of Thailand's national production. SWI is based in Korat, and produced 550 tonnes/day with annual costs of US$2.2 million for heavy fuel oil & US$2.3 million for electricity; production volumes were increasing to satisfy the demands of the market.
An open lagoon releasing methane gas to the atmosphere
Inset: Bubble layer on lagoon surface
The by-product from the extraction of starch from the cassava root is a concentrated wastewater stream, which was then discharged into a series of open lagoons. The wastewater was degraded slowly by bacteria & algae present in the wastewater itself. The warm temperature aided degradation by simulating bacteria growth.
Through anaerobic digestion, these bacteria converts organic components in the wastewater into biogas (usually containing 60% methane & 40% carbon dioxide). Therefore these lagoons become active reactors producing methane gas, often seen as a blanket of bubbles on the lagoon surface. Methane is a powerful greenhouse gas, about 21 times more potent than carbon dioxide, hence free emission of methane to the atmosphere is very damaging.
In 2001, Clean Thai Development Company (Thailand) Ltd collaborated with Waste Solutions Ltd from New Zealand to fix the problem. They proposed a plant to use the biogas produced from the wastewater stream for production heating & electricity generation. This proposal also aims to convert methane into the less harmful carbon dioxide, hence reducing the greenhouse effects of methane gas in the atmosphere.
Wastewater is pumped into the CIGAR, which collects the biogas (red) produced by the bacteria (blue)
The main engine of the solution is the Convered In-Ground Anaerobic Reactor (CIGAR) which consists of a constructed in-ground lagoon covered by a plastic membrane. The biogas collected under the membrane is removed by a compressor & pumped through pipelines to the gas burners & electricity generators.
Raw cassava processing wastewater
Inset: After CIGAR treatment
- Environmental: removal of excessive methane gas by converting it into less harmful carbon dioxide reduces greenhouse effects.
- Economic: produces electricity & gas to aid manufacture of starch, reduces costs.
- Land recovery: old treatment sectors can be used for agricultural purposes/town development.
- Process water recovery: the treated process water can be reused for irrigation/industrial water source.
- Biomass recovery: biomass obtained is has high nutrient content & does not contain harmful organisms; can be used as fertilizer, stock food supplement or soil conditioner.
The CIGAR technology is a great example of using bioprocess engineering (I'm studying this elective this sem haha) to create a cleaner & safer environment. This success story is getting attention from various food processing industries around the world.