Use of a Portable Greenhouse for Temperature Control in a Small-scale Biogas Production Unit

Biogas yield in anaerobic digesters is negatively affected by low temperatures during cold seasons and nights, temperature fluctuations and inefficient agitation. In this study a 100 l, agitated portable carbon steel digester housed within a greenhouse, whose operation temperature is automatically maintained at 35 +/- 1 degrees C by means of an electronic circuit for ventilation control through a suitably sized window and insulation offered by an air film between a double layer of polyethylene plastic covering of the greenhouse, was designed. Analysis of biogas produced from cow dung starting from day 6 of the 31-day retention period showed a specific biogas yield of 0.036 m3/kgVS(added) and an improved methane yield of 55% which was higher than 50% achieved by other digester designs. The optimum pH for good buffering capacity and efficient anaerobic digestion was successfully maintained at 7.2 and the chemical oxygen demand (COD) reduction achieved was 61%. The maintenance of a narrow temperature range (34 degrees C-36 degrees C) was successful resulting in solving the challenge of poor temperature control which affects biogas production and methane yield in most small-scale digester designs. This simple, easy to construct, inexpensive and efficient design led to improved biogas yields, quality and faster dissemination of the biogas technology.