Production of biogas from the anaerobic co-digestion of tannery fleshing and cow-dung enhanced by sodium and calcium alginates

Thesis Abstract

The subjects of interest in this work were the production of biogas via anaerobic co-digestion of tannery fleshing and cow-dung; and the reduction of sulphide concentration by direct inclusion of sodium and calcium alginate. Comparison of the efficacy of various tannery beam house effluents as substrate diluents indicated that the soaking liquor was most favourable. Use of soaking liquor gave highest methane concentration of 11.8%v/v as against 8.8%v/v, 1.2%v/v, and 4.1%v/v when deliming liquor, liming liquor and a combination of the entire beam house liquors were used respectively. Effect of various fleshing to cow-dung ratio of 10, 10.5, 11 and 12 were investigated. The optimum was established to be ratio 12 as indicated by the highest methane concentration of 45%v/v as against 1.9%v/v, 5.1%v/v, 7.3%v/v for 10, 10.5 and 11 respectively. The ability of minute concentration of sodium and calcium alginates (0.01% wt/v) introduced from the beginning of the anaerobic digestion cycle to increase methane concentration and reduce hydrogen sulphide concentration was ascertained. The alginates acted as chelating ligands thereby boosting methane production and reducing sulphide concentration. Methane concentration was significantly boosted to 70.1%v/v and 63.8%v/v with the addition of sodium alginate and calcium alginate beads respectively, as against 45%v/v for a similar digestion sample without alginate. Generally, sodium alginate performed better than calcium alginate beads both of 0.01% wt/v. However, an increase in concentration of calcium alginate beads to 0.03% wt/v performed better than sodium alginate of 0.01% wt/v. In addition, inclusion of the alginates to the digestion system shortens the retention time for biogas production and hydrogen sulphide evolution. This work thus recommends soaking liquor as diluent in the anaerobic digestion of tannery fleshing, use of tannery fleshing and cow-dung in the ratio of 12 for anaerobic co-digestion and the use of sodium and calcium alginates (0.01% wt/v) acting as chelating ligands to remove hydrogen sulphide and boost methane production above 20%.