The reasons are
less dilution effect by the effluent mixture from the
pre-anoxic zone and more ammonification effect owing
to higher MLSS content in anaerobic zones of run
3 or run 4 than that of run 1 or run 2 due to further
RAS pre-concentration. In addition, more NH4+ was
released in the pre-anoxic zone of run 3 or run 4 than
in that of run 1 or run 2 because of endogenous digestion.
However, pre-concentration ratios did not affect
nitrification process in the OD bioreactor although 2.7
mg·L−1 effluent NH4+ of run 4 was higher than that of
the others, which were all below 0.5 mg·L−1 because
of the low temperature.