Anaerobic digestion of whey permeate: Impact of feedstock ratio and organic loading rate in batch and semi-continuous systems
Aldyon Restu Azkarahman, Denise Cysneiros, Afroditi Chatzifragkou, Kimon Andreas G. Karatzas
Abstract
<h2>Abstract</h2> Anaerobic digestion (AD) plants have been facing significant challenges in maintaining a stable long-time operation when utilizing whey permeate as feedstock. In this study, we investigated the AD performance of whey permeate under batch and semi-continuous stirred tank reactor (s-CSTR) systems to optimize the process. 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 assessed to be used as guideline for s-CSTR operational set ups. The highest methane yield of 653.64 ± 12.16 NL<sub>CH4</sub>kg<sub>VS</sub><sup>−1</sup> 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 g<sub>VS</sub>L<sup>−1</sup>d<sup>−1</sup> with 30 d HRT for 150 d to assess the effect of different feeding regimes towards the overall AD performance. The CH<sub>4</sub> production rate declined for 3 HRTs in all reactors before stabilizing for the rest of the experiment. The decline of CH<sub>4</sub> rate was observed to be correlated with volatile solids degradation, volatile fatty acids and microbial composition. Initially, acetogenic bacteria (e.g., <i>Trichococcus</i> and <i>Sedimentibacter</i>) dominated the digestate which shifted to propionic acid producing bacteria (e.g., <i>Actinomyces</i> and <i>Acidipropionibacterium</i>) over the course of 150 d. A change in archaeal abundance was also observed where abundant <i>Methanosarcina</i> declined and finally substituted by <i>Methanobacterium</i>. 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.