Startup and stability of thermophilic anaerobic digesters treating organic waste in the absence of an acclimated seed

Abstract

Successful startup and durable stability are key determinants for dissemination and longevity of thermophilic anaerobic digesters treating the source-sorted organic fraction of municipal solid waste (SS-OFMSW). However, startup is often long and problematic because of lack of acclimated inocula, and system stability is challenged by operational disturbances, such as organic shock loading. In this context, the present work attempts to improve startup and system stability by investigating the impact of mixing and addition of hydrogenotroph-rich seeds, namely compost and leachate. In this context, continuous slow stirring induced a stable startup by reducing the average VFA accumulation from 2,890 mg HAc-l to 825 mg HAc-l, propionate content from 2073 mg-l to 488 mg-l, and VFA-to-alkalinity ratio from 0.32 to 0.07. As a result, the startup with slow mixing was faster and smoother accomplishing a higher loading capacity of 2.5 gVS-l-d in comparison to 1.9 gVS-l-d for non-mixing. Mixing equally improved microbial abundance from 6.6 gVSS-l to 10 gVSS-l and enhanced solids and soluble COD removal. In addition, mixing reduced the formation of micro-environments; thus, eliminating the occurrence of niches with high concentrations of inhibiting metabolic intermediates-wastes. As a result, the Archaea in the mixed digester were dominated by hydrogenotrophic Methanobateriaceae (61percent), with 35percent acetoclastic Methanosarcinales, and most of the methane was produced via syntrophic pathways. In contrast, Archeaea in the non-mixed digester were predominated by Halobacteria (65percent), resulting in higher acids levels and lower allowable loading rate. Accordingly, mixing was found beneficial in dissipating potential inhibitors and in improving stability and loading capacity, particularly when a non-aclimated seed, often lacking balanced thermophilic microflora, is used. On the other hand, the addition of compost and leachate induced a surge in the number of hydrogenotrophic (3 times) and acetotrophic (1000 times) methanogens in the initi