Biogas technology for cassava wastewater treatment and energy recovery in Thailand
Warinthorn Songkasiri1, Annop Nopharatana2 and Kanchana Saengchan1*
1) National Center for Genetic Engineering and Biotechnology, Excellent Center of Waste Utilization and Management, Bangkhuntien, Bangkok 10150 Thailand
2) Pilot Plant Development and Training Institute (PDTI), King Mongkut’s University of Technology Thonburi (KMUTT), Bangkhuntien, Bangkok 10150 Thailand
*, Registrant ID# 4768
Biogas technology, one type of anaerobic digestion, converts organic compounds from agricultural or domestic waste materials to methane (CH4) and carbon dioxide (CO2). The captured biogas can then be used as a source of renewable energy. Since 2002, 70% of cassava starch factories in Thailand have adopted biogas technology for the following objectives: (1) Reduce energy costs by replacing fossil fuel oil with renewable biogas produced on-site from the factory’s wastewater; (2) Reduce electricity cost by replacing electricity from grid and achieve a profit from selling electricity to national grid; (3) Improve wastewater treatment; (4) Reduce the carbon footprint of cassava starch production by capturing methane, a greenhouse gas 25 times stronger than CO2, which would otherwise escape to the atmosphere.
A typical cassava starch factory produces approximately 16000 - 18000 m3/day of biogas, containing 50-60% methane, from the following inputs:
- Production capacity: 200-240 t starch/day (24-hour operations);
- Wastewater generated: 10-20 m3/t starch;
- Chemical organic demand (COD) of the wastewater: 20000 - 23000 mg/L;
- Organic loading: 55.2 t COD/day or 230 kg COD/t starch;
- Biogas reactor size: 12000 m3;
- Retention time: 5 days;
- Savings: 8270 L/day of crude oil equivalent (106 USD/year).
Three main biogas technologies are used in Thailand: Covered lagoons, Upflow Anaerobic Sludge Blanket (UASB), and Anaerobic Fixed Film Reactor (AFFR). Covered lagoon, a low-rate technology, have lower initial investments costs but are less efficient. UASB and AFFR, high-rate technologies, use concrete reaction tanks with higher initial investment costs, but are more efficient because of better control of the fermentation parameters. UASB and AFFR also use less land area than covered lagoons. For smaller factories with lower production capacity and organic loading, maintaining a stable fermentation reaction and biogas production from only wastewater can be difficult. Most small factories use covered lagoon technology.
The rapid adoption of biogas technologies in Thailand was made possible with the support of several government agencies, which facilitated access to loans covering 30-75% of the investment costs, low-interest loans, and flexibility to extend loan repayments. Tax incentives, Carbon credits (clean development mechanism, CDM) and enforcement of environmental laws also contributed to promote biogas technology.
Keyword: Anaerobic digestion; Biogas technology; Cassava starch; Methane