Does the C/N ratio really affect the bio-methane yield? a three years investigation of buffalo manure digestion

In this paper we study the anaerobic digestion of water buffalo manure collected during a period of three years with the goal of investigating the effect of the substrate C/N ratio on the bio-methane production. The investigated samples show a C/N ratio between 9 and 50, this interval being much wider than that typically considered optimal in the literature for the waste digestion processes. The experimental tests are performed in batch mode, at 37 °C and with a starting pH equal either to 6 or 7, uncontrolled during the digestion process. We observe that, regardless of the C/N ratio of tested samples, the Gompertz productivity, expressed in terms of bio-methane volume fraction, is always larger than 54 %. A slightly larger productivity is recorded for substrates digested starting from an initial pH set to 7. We may then conclude that the C/N ratio of water buffalo manure does not really affect the bio-methane yield.