Abstract/Summary

There is strong evidence that why permeate anaerobic digestion plants face significant challenges in maintaining a stable operation for long time and this study aimed to optimize this process.We investigated the anaerobic digestion (AD) performance of whey permeate under batch and semi-continuous stirred tank reactor (s-CSTR) systems. Biochemical methane potential (BMP) tests were initially performed in batch reactors to assess whey permeate potential as AD substrate operating at different inoculum to substrate ratios (ISRs) and pH values under mesophilic temperatures. The kinetic parameters from the best AD performance under batch system were also assessed to be used as the guideline for s-CSTR operational set ups. The highest methane yield of 653.64  12.16 NLCH4kgVS-1 was observed at ISR 2, initial pH 7.5 at 37C incubation under batch system. However, when the kinetic parameters from this condition were applied to determine the organic loading rate (OLR) and hydraulic retention times (HRT) in the s-CSTRs, resulted in operational failure. The s-CSTRs were then operated at OLR 2.5 and 4 gVSL-1d-1 with 30 d HRT for 150 d to assess the effect of different feeding regimes towards the overall AD performance. The CH4 production rate declined for 3 HRTs in all reactors before stabilizing for the rest of the experiment. The decline of CH4 rate was observed to be correlated with volatile solids degradation, volatile fatty acids and microbial composition. Initially, acetogenic bacteria (e.g., Trichococcus and Sedimentibacter) dominated the digestate which shifted to propionic acid producing bacteria (e.g., Actinomyces and Acidipropionibacterium) over the course of 150 d. A change in archaeal abundance was also observed where abundant Methanosarcina declined and finally substituted by Methanobacterium. The change in microbial population of whey permeate AD under s-CSTR system in our study, suggests a shift in methanogenesis pathway, which directly affects the AD performance.