Action Research in Bhutan: Production of Biogas from Rice Cooking Generated Wastewater

Rice is a staple of any South Asian or even greater Asian household diet. Particularly, in Bhutan, at least two meals are based on the main staple of rice. The process of cooking rice in this culture is to boil the rice in water till the rice grain is soft. Then the liquid is completely decanted out the from the rice. Finally, the rice is simmered down in low heat. The decanted liquid is, in most cases, dumped down the sink as grey water to the drain in most urban and rural settings. This wastewater is high in biochemical oxygen demand and contributes to surface water pollution by adding nutrients to the water bodies unless treated. In this research, in a student hostel in the Kingdom of Bhutan, rice cooking wastewater was used to generate biogas using an anaerobic up-flow sludge blanket reactor. The generated biogas was used for cooking lentils, another daily staple in the South Asian diet. This paper reports on the design and efficacy of operation of the reactor, generation potential of the biogas for the rice cooking wastewater with sufficient methane content to sustain combustion and application to thermal cooking on a sustained daily basis. The reactor feed rate was 10 L/hour of rice cooking wastewater producing 1.2 m3 of biogas over a 24-hr period. The reactor was operated at mesophilic operating temperature and a hydraulic retention time of 48 hrs. Mass balance showed that 1.0 L of rice cooking wastewater produces 5.0 L of biogas. This research shows the classic successful conversion of a waste to useful energy, a successful example of the new nexus of operation in a circular economy.