Ammonia oxidizing bacteria (AOB) release 2 moles of protons per
mole of ammonia that is oxidized to nitrite. If the buffer capacity in
the bulk solution is low, biological ammonia oxidation causes a
substantial pH drop, which in turn affects the rate of ammonia
oxidation. In wastewater treatment, ammonia oxidation decreases
with pH and usually stops when the pH value drops below pH 6
(Painter, 1986). However, the reasons for the rate decrease, but
especially for the cessation of activity are not very well understood.
There are also some exceptions in wastewater treatment where AOB
have adapted to low pH environments and grew at pH values as low
as 4 (Gieseke et al., 2006) or even below (Udert et al., 2005).
The pH dependence and the pH limit of AOB is usually