Acclimation to extremely high ammonia levels during continuous biomethanation process

Anaerobic digestion (AD) is an effective technology to recover energy (CH 4 ) from biomass. However, when protein-rich by-products are used as feedstocks, they could lead to low methane yields and even to complete process failure. Acclimatized anaerobic communities to high ammonia levels can offer a solution to this problem. Nevertheless, the acclimation to high ammonia levels during continuous AD of protein-rich substrates still poses serious challenges (e.g. microbes can be washed out from reactors). In the present study, acclimation strategy followed by a stepwise increase (0.5 g NH 4+ -N L -1 each step) of total ammonia (TAN) concentration from 3.8 up to 10.5 g NH 4+ -N L -1 was investigated using continuous stirred tank reactors (CSTR) under mesophilic conditions (37±1°C). Microalgae Chlorella vulgaris (>50% protein in dry matter), mixed with cattle manure, was used as substrate. Throughout the acclimation period, even though the TAN increased 1.8-fold at the end of the experiment, the methane yield of the CSTR reactor was stable with an average variation less than 10% compared to the yield before ammonia acclimation started. At the same time, VFA and pH were both within the optimal range of AD process throughout the whole experimental period. These results demonstrated that an acclimation of ammonia tolerant methanogenic consortia to extreme ammonia levels using CSTR reactors is possible. Furthermore, microbiological analyses (16s rRNA sequencing) results confirmed a profound decrease in the microbial diversity due to the acclimation process.