increased to its normal operating value after VFAs
metabolism. Ammonia nitrogen concentration and pH was
observed to increase substantially with little variation on
the commencement of the semi-continuous operation,
leading to lower biogas yield. Bujoczek et al., [9] have
reported that, the efficiency of converting the organic
matter in cattle manure to methane decreased as the
organic loading increased. It was earlier seen from the
mixing ratio of 0.5:5 tested data not shown, that the
variation of NH3-N concentration had no obvious variation
and maintained below 500 mg/l. At the end of the
experiment, NH3-N was reduced to 87% which is 20%
higher than that at mixing ratio 1:5 and 1.5:5. This showed
that higher reduction in efficiency of NH3-N was achieved
and this is likely caused by the availability of microbial
population in the POME, which aids the anaerobic
bioconversion. It was reported by Angelidaki and Ahring
[10] that process stability due to NH3 resulted in VFA
accumulation and lowering of pH, thus decreasing the
concentration of free NH3 in the reactor. This explains the
observed ability of the operation to stabilize even with
high ammonia concentration and with lower, but stable
biogas production.