Effect of Thermal Hydrolysis Pretreatment on Anaerobic Digestion of Protein-Rich Biowaste: Process Performance and Microbial Community Structures Shift

Abstract

To reveal the effects of thermal hydrolysis (TH) pretreatment (THPT) on anaerobic digestion (AD) of protein-rich substrates, discarded tofu was chosen as the object, and its batch AD tests of tofu before and after being subjected to TH at gradually increasing organic loads were carried out and the AD process characteristics were compared; furthermore, its continuous AD tests without and with THPT were also conducted and the difference of the microbial community structures was investigated. The results showed that, during AD of protein-rich tofu with increase in the organic load, inhibition from severe acidification prior to accumulation of ammonia nitrogen (AN) occurred. THPT helped overcome the acidification inhibition present in batch AD of tofu at such a high TS content of 3.6%, to obtain the maximum methane yield rate of 589.39 ml·(gVS)−1. Continuous AD of protein-rich tofu heavily depended on ammonia-tolerant hydrogenotrophic methanogens and bacteria. The continuous AD processes acclimated by HT substrates seemed to be resistant to severe organic loads, by boosting growth of ammonia-tolerant microorganisms, above all methylotrophic methanogens such as the genera RumEnM2 and methanomassiliicoccus. The process response of continuous AD of HT tofu was hysteretic, suggesting that a sufficiently long adaptation period was required for stabilizing the AD system.

Meta-omics analysis revealed structural and metabolic shifts of acidification communities after feeding with thermal hydrolysis pretreated food waste

“…Under the HRT 2 days, the THP reactor exhibited a significantly lowered Shannon diversity index (P = .03, Figure 3) than the non-THP reactors, supporting the hypothesis that THP may reduce indigenous microbes due to potential sterilization effects [12,19,20]. The surviving indigenous organisms after THP were primarily those with high thermal stability and dry resistance such as Clostridium, Thermobrachium, and Bacillus [21].…”

“…This could be attributed to the trade-off effects of THP. By-products released after THP such as the ammonium from protein degradation [8][9][10][11][12], phenols/furfural from hemicellulose degradation [13,14], the formed refractory compounds such as the melanoidin, and other recalcitrant dissolved organic nitrogen [15][16][17] may impact VFA production. Those compounds reduce the feedstock biodegradability and correspondingly impact microbial communities [11,12].…”

“…By-products released after THP such as the ammonium from protein degradation [8][9][10][11][12], phenols/furfural from hemicellulose degradation [13,14], the formed refractory compounds such as the melanoidin, and other recalcitrant dissolved organic nitrogen [15][16][17] may impact VFA production. Those compounds reduce the feedstock biodegradability and correspondingly impact microbial communities [11,12]. It was found that the Shannon diversity of microbial communities was significantly decreased after the inclusion of THP during anaerobic digestion [12,19,20], which was potentially due to the sterilization effect of THP on indigenous microbes [21].…”

“…Those compounds reduce the feedstock biodegradability and correspondingly impact microbial communities [11,12]. It was found that the Shannon diversity of microbial communities was significantly decreased after the inclusion of THP during anaerobic digestion [12,19,20], which was potentially due to the sterilization effect of THP on indigenous microbes [21]. The structure of the methanogens' community also showed adaptation to TH pretreated substrate during biogas production.…”

“…The structure of the methanogens' community also showed adaptation to TH pretreated substrate during biogas production. The most abundant methanogens were shifted from acetoclastic methanogens to less sensitive hydrogenotrophic methanogens [11,12,20,22], potentially due to free ammonia inhibition resulting from protein degradation after THP and anaerobic digestion. Compared to methanogens, limited research specifically investigated the impact of TH pretreated feedstock on the structure and function of acidification communities.…”