where VD, mmanure, and mcs are the digester volume (m3), manure quantity (t/a), and co-substrate quantity (t/a), respectively. Design of a biogas plant is directly linked to its hydraulic retention time (HRT), which may be defined as time period during which mixture of feedstock stays in digester to produce biogas [28]. The required retention time for optimal biogas production depends on the temperature. The mesophilic temperature range requires a retention time of 25e40 days.