Influence of NH3 and NH4+ on anaerobic digestion and microbial population structure at increasing total ammonia nitrogen concentrations

Despite the extensive research dedicated to ammonia inhibition, the effect of NH 3 and NH 4 + on each anaerobic digestion stage and the associated microorganisms is still not completely understood. In the past, the focus was mainly on methanogenesis and either on NH 3 or total ammonia nitrogen (TAN). Here, anaerobic digestion of two defined substrates, namely starch/NH 4 Cl and casein, was investigated particularly regarding the effects of different NH 3 /NH 4 + ratios on the involved microorganisms. TAN affected bacteria, primarily gram-positive ones, whereas archaea responded largely to the NH 3 concentration. These sensitivity differences are attributed to differences in the corresponding cell-membrane structures. A TAN decrease via stripping performed in two full-scale agricultural biogas plants resulted in increased bacterial diversity, with a pronounced increase in the propionate acetogens’ abundance. Based on these data, it is suggested that inhibition can be avoided and processes stabilized in biogas plants by adjusting the NH 3 /NH 4 + ratio, when feeding nitrogen-rich substrates.