the yields of methane and biodigestate produced by a biodigestor could be determined in two laboratory assays, They are, Biomethantion potencial (BMP) and anaerobic biodegradability (AB). they consist of input together the substrate (sewash sludges in your case) with an anaerobic sludge with a known methanogenic activity incubated in optimal conditions for a several time (35 days) and 35°C. in that period of time you must to measure the biogas produced periodically. that way you can obtain the fraction of volatil solids biologically degraded and the amount of methane produced. on the other hand you can use standard values to meake a aproximated calcule. to stimating the biodegradability of the substrate a conservative value is 65% and the rate of methane produced could use 0,35 lCH4/Kg DQO