x.Mixing rate

The biogas production plants need an appropriate mixing system to provide favorable conditions for microbial community, for process stability and efficiency in anaerobic digestion [96, 109]. The mixing intensity or shear rate and the length of time that shear rate is applied to a mixer determines the degree of mixing realized. Application of uniform shear rate is an important strategy to achieve stability of bio digestion process. In some studies, higher mixing intensity of 67 rpm for 5 min h−1 led to 15–18% higher production output compared 10 rpm and 30 rpm without generating instability in terms of VFA accumulation and dead zones. A higher mixing rate reduces dead zones to less than 2%. Therefore, mixing significantly affects the biogas production rates but the impeller design is the principal factor. The best results were achieved by using a large diameter impeller running at medium mixing rate in anaerobic reactor [109, 110].

#### xi.Inoculum

The inoculum is used to reduce the start-up time of biodigestion by seeding of the which accelerates biogas formation. By use of inoculum, about 80% biogas can be obtained within the first 10 days of digestion period [101].

## xii.Moisture

The substrate dilution or moisture content affects the process of anaerobic digestion with best results in terms of methane yield achieved under 60–80% humidity [89, 101].
