With respect to thermal hydrolysis, there is debate on how ammonia influences performance. A wide body of literature suggests that acetoclastic archaea are more sensitive to free ammonia inhibition than either acetate-oxidizing or hydrogenotrophic methanogens (Angelidaki et?al., 1993, Fotidis et?al., 2013, De Vrieze et?al., 2015, Zhang et?al., 2014, Koster and Koomen, 1988, Wilson et?al., 2008 and Wang et?al., 2015). Consequently, it is expected that digesters fed thermally hydrolyzed sludge will provide conditions to selectively favour those trophic groups (De Vrieze et?al., 2015 and Ho et?al., 2013). Subsequently, various attempts have been made to account for a shift in bacterial population from acetophiles to hydrogen SRT1720 and acetoclasts in response to increasing ammonia concentrations in predictive modelling (Haile et?al., 2015, Wett et?al., 2014 and Ho et?al., 2013) although none have been used to predict future performance. However, in contrast, a thorough study by Wiegant and Zeeman (1986) showed toxicity inhibited hydrogen utilizers more than their acetogenic counterparts up to at least 4500 mg/l TAN. In one study, Wilson et al. (2012) investigated the influence of FA on Ks for unionized acetic acid to explain why volatile fatty acids were observed in digester effluents. Although they looked at conditions with and without thermal hydrolysis, closer analysis of the data shows that increasing ammonia concentration increased Ks irrespective of presence or absence of thermal hydrolysis. The influence of FA on Ks derived from data presented by Wilson et al., is shown viz.equation(6)Ks[HAc]=−2.077Ln(FA)+12.637Ks[HAc]=−2.077Ln(FA)+12.637where Ks[HAc] = Half saturation constant for unionized acetic acid (mg/l), FA = free ammonia concentration (mg/l). This compares with previous work showing Ks of 0.128 mg/l (Kus and Wiesmann, 1995) for a system without thermal hydrolysis – equivalent to a FA of about 410 mg/l HAc.
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