The effect of HRT in the successful bioaugmentation of CSTRs working under ammonia toxicity

High levels of ammonia is a common inhibitory factor in anaerobic digestion (AD) resulting in low methane production and unbalance of the process. In the present study, the adjustment of the hydraulic retention time (HRT) and the bioaugmentation process (BP) are investigated to counter the negative effects of ammonia toxicity. Two lab scale continuously stirred tank reactors (CSTR) were operating with cattle manure at a low total ammonia nitrogen (TAN) concentration (1.8 g TAN/L). The reactors were working with 20 (R1) and 30 (R2) days HRT, respectively and ammonia toxicity conditions (6.1 g TAN/L) were achieved through the direct addition of ammonium chloride. In both reactors after the increase of TAN the average daily methane production was reduced by 37.04% in R1 and 38.52% in R2. The stepwise acclimatization of the microorganisms to high concentrations of TAN (6.5 g TAN/L) for the BP was performed in batch reactors. After the BP there was a recovery of the methane production in both reactors. In R2 the recovery was immediate, however, a delay of 20 days was observed in the recovery of R1. A likely explanation for the R1’s delayed response is the slow reproduction rate of the introduced acclimatized population and the low HRT.