Mesophilic anaerobic co-digestion of cow manure and biogas crops in full scale German biogas plants: A model for calculating the effect of hydraulic retention time and VS crop proportion in the mixture on methane yield from digester and from digestate storage at different temperatures

Data from 24 full scale biogas plants in Germany digesting cow manure and crops were evaluated. Special emphasis was given to the effect of hydraulic retention time HRT and proportion of crops in the mixture (VS basis) pVS,CropsInp on the methane yield from the digester yCH4D,VSInp and the storage tank yCH4,TS,VSInp at 37 and 22°C. The evaluation has shown model parameters for maximal methane yield of manure and crops ymaxCH4eff,VSInp at 270 and 420 Lkg^-1, respectively. For example, at HRT of 60days, maximum methane yield result to 249 and 388 Lkg^-1 for a crop proportion in the input of 0.0 and 1.0, respectively. The calculation of yCH4,TS,VSInp considers first order reaction rates and a temperature term f_T. Hence, at any arbitrary temperature in the range of 12°C<T<37°C the values of yCH4,TS,VSInp in the course of time can be calculated, which correspond to methane emissions for uncovered storage tanks.