Evaluation of a biogas process co-digesting slaughterhouse waste and swine manure : impact of increased organic loading and addition of iron

Anaerobic digestion by microorganisms generate methane which can be used as a renewable source of energy. A complex biological system with a community of microorganisms that work together can be used for generation of methane from substrates such as household wastes and agricultural wastes. The rest product from the process can in turn be used as a bio fertilizer in agricultural industry. In this project four continuously stirred laboratory scale digesters with a working volume of 5L, a temperature of 52˚C (thermophilic) were used to evaluate biogas production from slaughterhouse waste and swine manure and the importance of organic load and iron addition for performance and stability. The performance was evaluated in two digesters using iron sludge as an additive and these digesters were compared to two reference digesters receiving no addition of iron. Iron was used as an additive to reduce the amount of hydrogen sulphide (H2S) produced by high protein substrates. Differences of the impact of three different iron compounds (iron(III)oxide, iron(II)chloride and iron sludge) were evaluated with samples from an earlier experiment by specifically analyzing the methanogens by qPCR. The specific methane yield for iron treated digesters C2 and D2 were 700-800 ml/gVS*day and 400-500 ml/gVS*day, respectively. The reference digesters C1 and D1, without iron addition, had values between 200-400 ml/gVS*day and 400-500 ml/gVS*day, respectively. The iron treated digesters showed somewhat better methane yield as compared to the references but due to differences between technically identical digesters the results were difficult to evaluate. The content of volatile fatty acids was increasing in the reference digesters without any addition of iron towards the end of the experiment, indicating unstable processes. Hydrogen sulphide levels were lower for the iron treated digesters resulting in better gas quality than for the reference digesters. qPCR analysis shows a slight difference in methanogenic gene abundance during the period where iron(II)chloride were used as an additive, with a comparably lower value during this period. No difference in methanogen gene abundance could be seen for the period where iron(III)oxide and iron sludge were used as additive