*4.2.4 Carbon/nitrogen (C/N) ratio*

The concentrations of carbon and nitrogen determine the anaerobic digestion performance. Anaerobic digestion ideally occurs at C/N ratio ranges between 20:1 and 30:1. For the optimal operation, the ratio of the carbon, to, nitrogen should be about 30:1 in the raw material. Methanogenic bacteria use nitrogen to meet their protein requirements. Whereas, carbon constitutes the energy source for the microorganisms, nitrogen serves to enhance microbial growth. If the amount of nitrogen is limiting, microbial populations will remain small and it will take longer to decompose the available carbon [102]. Consequently, in cases of high C/N ratios higher than the optimum ranges, the nitrogen will be promptly consumed by the bacteria and thus will not react on the excess carbon in the feedstock, hence decreasing the biogas yield. For cases of lower ratios than the limited range, the excess nitrogen will result into ammonia (a strong base) formation, thereby increasing the working pH over the required 8.5 inhibiting the microbes and finally dropping gas generation rates [96, 107]. It has been found that the bacteria in the digestion process use up the carbon present 30–35 times faster than the rate at which they convert nitrogen [102]. The high amount of nitrogen content in animal manures reduces its utilization in anaerobic digestion for biogas generation because of its C/N ratio [62, 111]. To solve this problem, nitrogen-free raw material or carbohydrate-rich source is used to raise the carbon content in the animal manure before the AD process proceedings [62].
