Strategies for the stable performance and rapid inhibition recovery of a thermophilic digester treating coffee wastes and the synergistic effects of microbes

A 200-day experiment was conducted to evaluate strategies for the stable performance and rapid inhibition recovery of a thermophilic reactor treating coffee wastes, waste activated sludge, and milk waste under a high organic loading rate (OLR) and the synergistic effects of microbes. Excessive accumulation of propionate was determined to be the main reason for the deterioration of the reactor, which resulted from reduced methanogenic activity. Supplementation of inoculum sludge led to the complete inhibition recovery of the reactor in fewer than 35 d by enhancing propionate degradation. The microelement and low concentration of sulfate ensured the stable performance of the reactor at OLR of 4.6 g-TS/L/d. The efficiency of organic matters and propionate degradation was also improved. The microbial community was analyzed by high-throughput sequencing targeting 16S rRNA to determine the relationship between microbial composition and reactor performance. Propionate markedly inhibited Methanosarcina, which dominated the stable state; meanwhile, Methanobacterium and Methanothermobacter were more tolerant and comprised more than 75% of total archaea at the inhibitory stage. The deterioration of the reactor led to a shift in the CH₄ generation pathway from acetoclastic to hydrogenotrophic. Clostridiales was responsible for organic degradation at stable stages; however, almost all of the genera of Clostridiales could not survive at stage II and were replaced by Thermoanaerobacterales and Lactobacillales.