The effect of different inoculums on anaerobic digestion of swine wastewater

•The nature of inoculums influenced the methane productivity.•Sewage sludge was the best inoculums for methane production in AD of fresh swine wastewater.•For the substrate studied, methanogenic stage seems to be the rate limiting step of the global methane production rate.

Methane production from swine wastewater was evaluated by using three inoculums: rumen (I1), stabilized swine wastewater (I2) and sewage sludge (I3). Experimental design was based on four treatments by duplicate: T0: swine wastewater as substrate (S) without inoculum (I), T1: S + I1, T2: S + I2 and T3: S + I3 all with 90 (S)–10 (I) % vol with a ratio S/I approximately constant (1:0.05). ANOVA test was applied to evaluate the significance of treatments at 95% confidence. After a batch experiment of 140 days, results indicated that the addition of any inoculum improved methane production rate and shortened the start-up of methane exponential growth stage. I2 and I3 promoted the highest percentage of organic matter removal (close to 50% in terms of VS and COD) and, in relation to the control test, a higher methane production achieving 0.25 L CH4/g VS. The use of rumen (I1) did not improve methane production to the same extent as the other inoculums while organic matter removal only achieved 15%. The evolution of VFA and alkalinity show that methanogenic phase could be considered as the rate-limiting step of the global methane production rate.