The effect of biogas sparging on cow manure characteristics and its subsequent anaerobic biodegradation

A study of the influence of biogas sparging on the characteristics of raw liquid cow manure and its subsequent anaerobic digestion process was carried out. Preliminary experiments have demonstrated that a biogas sparging rate of 2 L h−1 for 120 min causes a reduction in the initial pH of the substrate to values close to 7.0, which are appropriate for its posterior anaerobic biodegradation. Two laboratory-scale anaerobic digesters were operated for 40 days, one (D1) which digests cow manure after biogas sparging and the other (D2) which treats untreated manure as the control. After the sparging process the biogas slightly increased its average methane content from 61% to 62%, decreasing the average CO2 concentration from 35.4% to 32.4%. However, after sparging and during the first 35 days of the experimental period, the average H2S concentration in the biogas increased from 231 to 637 ppm, which resulted in a decrease in the H2S concentration in the liquid manure, favoring its subsequent anaerobic digestion. Higher average COD, total solid (TS) and volatile solid (VS) removals were always achieved in digester D1 when compared to D2. For instance, mean VS removal efficiencies of 45.1% and 36.5% were reached in digesters D1 and D2, respectively. An increase of 9.3% and 9.1% in the biogas and methane production rates respectively were also obtained when the influent was previously treated with biogas sparging (D1). Finally, an increase in the methane yield coefficient from 0.13 to 0.20 L CH4 g COD−1 was also achieved after the sparging pre-treatment.