Effects of predation and ORP conditions on the performance of nitrifiers in activated sludge systems

Effects of changing oxidation–reduction potential (ORP) and grazing of protozoa on nitrifiers in activated sludge systems was investigated. This study used sequencing batch reactors which were acclimated under aerobic and alternating anoxic/aerobic conditions, with and without inhibition of protozoa, the predatory microorganisms. The feed used was a synthetic wastewater containing beef and yeast extracts as a carbon source. It was found that the biomass, determined by mixed liquor volatile suspended solids (MLVSS) in the reactors, was significantly affected by predation while ORP (aerobic and alternating anoxic/aerobic conditions) has no impact on the MLVSS regardless of the presence or absence of predation. However, the nitrification rates in the reactors show completely different trends indicating that the ORP of the system has a significant impact on the rates while predation does not. It was found that nitrification rates in alternating reactors were almost double the rates in the aerobic reactors, both with and without predatory inhibition. The decay rate of autotrophic bacteria (bA) in aerobic reactors was determined by tracking the decrease of the maximum nitrification rate under both anoxic and aerobic starvation conditions. The bA in alternating anoxic/aerobic reactors was also determined under alternating starvation conditions. It was found that, in any case, the alternating anoxic/aerobic autotrophic biomass bA was much smaller that the bA of aerobic biomass determined under aerobic and anoxic starvation conditions. The alternating anoxic/aerobic bA was 62.1% less than the aerobic biomass bA under aerobic starvation and 40.2% less for the aerobic biomass starved under anoxic conditions. No statistically significant differences in bA were observed between reactors with or without the inhibition of predators.