Effects of Residual Hypochlorite Ion on Methane Production during the Initial Anaerobic Digestion Stage of Pig Slurry

The hypochlorite ion (OCl(-)) is a widely used disinfecting agent in pig rearing in Korea, but its residual effect on CH4 production from pig slurry is unclear. The objective of this study was to investigate the inhibition effects of residual OCl(-) on CH4 production during the initial anaerobic digestion stage of pig slurry. Three organic concentrations (9.9, 26.2 and 43.7 g/L) of volatile solids (VS) were tested with the addition of 52.3 mg/L OCl(-), ten times of the typical concentration used in Korea, or without OCl(-) (Control) in anaerobic batch culture. The culture was run under mesophilic (38°C) conditions for 20 d. At the lowest organic concentration with OCl(-), the VS degradation was 10.3% lower (p<0.05) than Control, while at the higher organic concentration with OCl(-), it did not differ from Control. CH4 yields were higher in the control treatments than their OCl(-) counterpart cultures, and CH4 yields of Control and OCl(-) treatments at the organic concentrations of 9.9, 26.2 and 43.7 g/L differed in the probability level (p) of 0.31, 0.04, and 0.06, respectively. Additionally, CH4 concentration increased steeply and reached 70.0% within 4 d in the absence OCl(-), but a gradual increase up to 60.0% was observed in 6 d in the OCl(-) treated cultures. The Rm (the maximum specific CH4 production rate) and λ (lag phase time) of 9.9 g/L with OCl(-) were 8.1 ml/d and 25.6 d, while the Rm was increased to 15.1 ml/d, and λ was reduced to 11.4 d in PS-III (higher organic concentration) with OCl(-). The results suggest that a prolonged fermentation time was necessary for the methanogens to overcome the initial OCl(-) inhibitory effect, and an anaerobic reactor operated with high organic loadings was more advantageous to mitigate the inhibitory effect of residual hypochlorite ion.