Predicting methane production in simple and unheated biogas digesters at low temperatures

Millions of unheated and simple biogas digesters are currently used on small-scale livestock farms in developing countries. These digesters are not equipped to measure methane production and methane production is low during cold periods. The aim of this study was to develop kinetic models for predicting methane production in such conditions by determining specific methane productivity in biogas digesters under psychrophilic (<20°C) and psychrotrophic (20–30°C) conditions. The models developed were based on data obtained in laboratory-scale batch anaerobic digesters in Denmark, where specific methane production from pig manure (3.18% of dry matter; DM) and cow manure (7.72% of DM) was determined at 5, 10, 15, 20, 25 and 30°C. A full-scale field study in four continuous biogas digesters (10 October 2012–29 January 2013), with 40days of hydraulic retention time (HRT), were carried out in Vietnam and the models were then validated using the data. In prediction using the pooled mean data for each week, R2 of predicted versus measured production was 0.79, Root mean square error (RMSE) was 23.3 NL CH4 digester−1day−1 and relative root mean square error (RRMSE) was 18.6%. The precision of the model was higher using the pooled mean data for each month: R2=0.99, RMSE 8.0 NL digester−1 day−1 and RRMSE 6.0%. The model could be used for estimation of methane production in simple, unheated biogas digesters operating in either psychrophilic or psychrotrophic conditions.