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Ammonia (NH3) is produced by breakdown of protein during anaerobic digestion of wastes. Apart from this, ammonia or ammonium (NH4+) is also present as a major component in sludge or wastewaters such as pig or poultry manures, or slaughterhouse by products. Both ammonia and ammonium are considered as the foremost inhibitors, while free ammonia i.e NH3 has been reported as the main inhibitory component to the methanogenesis (Chen et al., 2008; Westerholm et al., 2011). The mechanism behind ammonia inhibition is that higher levels of ammonia cause a change in intracellular pH, more requirement of maintenance energy, reduction in intracellular potassium, and inhibition of specific enzyme catalyzed reactions (Wittmann et al., 1995). The results reported in the literature on the effect of ammonia concentration on methanogenic pathway and community structure are conflicting. Some research indicated that acetoclastic methanogens are inhibited more than hydrogenotrophic methanogens through comparison of methane production and growth rates (Angelidaki and Ahring, 1993; Bhattacharya and Parkin, 1989; Borja et al., 1996; Koster and Lettinga, 1984; Robbins et al., 1989; Sprott and Patel, 1986), while others reported that acetatoclastic methanogens are more tolerant to high total ammonia nitrogen (TAN) concentrations than hydrogenotrophic methanogens (Wiegant and Zeeman, 1986; Zeeman et al., 1985). Strong inhibition effect of ammonia on methanogens was indicated by Jarrel et al. (1987) while investigating the influence of NH4Cl on methanogenesis by using pure acetoclastic and hydrogenotrophic methanogenic cultures. They found that Methanospirillum. hungatei belonging to the order of Methanomicrobiales was sensitive with 50% inhibition... ... middle of paper ... ...e, they concluded that ammonia had a strong impact on the occurrence of syntrophic acetate oxidisers and the partner hydrogenotrophic methanogens. Fotidis et al. (2013) investigated the bioaugmentation of ammonia tolerant SAO co-culture (i.e. Clostridium ultunense sp. nov. in association with Methanoculleus sp.) in a mesophilic UASB reactor under high ammonia loads. However bioaugmentation of SAO co-culture in a UASB reactor was not successful. When, the authors co-cultivated a fast growing hydrogenotrophic methanogen (i.e. Methanoculleus bourgensis) with the SAO co-culture in fed-batch reactors, it was successful. Their results suggested that it is necessary to optimize the growth condition of SAO and the partner hydrogenotrophic methanogens, as the slow growing partner (in this study it was hydrogenotrophic methanogen) can cause the failure of bioaugmentation.

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