The anaerobic decomposition of urea and undigested proteins
in the chicken manure produced large amounts of unionised
ammonia (NH3) and ammonium ions (NH4+) (Chen et al., 2008;
Salminen and Rintala, 2002). Unionised ammonia has been suggested as the actual toxic agent; an increase in pH may therefore
result in increased toxicity (Borja et al., 1996) because the unionised ammonia to ionised ammonia ratio increases at higher pH
(Chen et al., 2008). In Experiment 1, the gradual increase of the
influent pH from 7.1 to 8.0–8.2 increased the amount of un-ionised
ammonia in the reactor by a factor of 10 (He, 1998); thus, the
toxicity of ammonia nitrogen was greatly enhanced.