The concentrations of different inorganic nitrogen compounds
in the integrated system were shown in Fig. 5. Since the dominant
inorganic nitrogen compound in coking wastewater was ammonia,
TIN was defined as the sum of NH3-N, NO2
−-N and NO3
−-N
herein. As it could be found from Fig. 5(a), the removal of NH3-N
occurred in Reactor A, O1 and O2 and the concentration of NH3-N
was almost the same between Reactor O2 and ZVI. For the dilution
function of recirculation, the concentration of NH3-N in Reactor A
was decreased to 100 mg l−1 below and the residual NH3-N was
biodegraded by nitrification in Reactor O1 and O2, which could be
proved by the increased NO2
−-N and NO3
−-N shown in Fig. 5(b)
and (c). From Fig. 5(d), it was indicated most of TIN was removed
in Reactor A and O1, which could be explained by denitrification
of NO2
−-N and NO3
−-N. Interestingly, some TIN disappeared in
Reactor O1, which reflected the simultaneous nitrification and denitrification
(SND) existed in the biofilm reactor system. No use was
found for the ZVI process to removal of inorganic nitrogen compounds,
which implied that these compounds were removed by
nitrification and denitrification occurred in the biofilm reactors.
In the 60 days of operation, NH3-N of ZVI effluent was always less
than 10 mg l−1 and the maximal removal efficiency was up to 99.2%,
which was much higher than the performance of single biofilm
reactor system in the previous study [20]. The maximal removal
efficiency of TIN was 92.3% and the effluent from Reactor ZVI was
only composed of low concentrations of NH3-N and NO3
−-N. Some
other residual refractory organic compounds could be removed in
the ZVI process, which in turn enhanced the biodegradation of inorganic
nitrogen compounds.