Inan anaerobic process, the excess protein is degraded to NH3, and an elevated concentration of NH3 may be toxic to anaerobic bacteria. For this reason, cow dung was used as a co-substrate to maintain the C/N ratio. The reactors were operated at C/N ratios of 7.5, 17, 20.88 and 6.99, which were slightly lower than the reference value of 20–30