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.