Evaluation of Maximum Growth and Decay Rates of Autotrophs Under Different Physical and Environmental Conditions

This study examined the effect of grazing by predatory microorganisms and different ORP conditions on growth and decay rates of autotrophic bacteria in an activated sludge system. Sequencing batch reactors (SBRs) were acclimated to a synthetic wastewater under aerobic and alternating anoxic/aerobic conditions, with and without inhibition of predatory microorganisms. Secondary batch reactors were created from the main SBRs in order to determine decay rates by tracking the decrease in maximum nitrification rates over time while subjected to substrate starvation conditions. Biomass from main aerobic reactors was starved under aerobic and anoxic conditions, while that from alternating ORP-regime SBRs were starved under alternating anoxic/aerobic conditions. Maximum nitrification rates of the main SBRs were not affected by predatory activity, but the alternating anoxic/aerobic reactors showed rates up to 83% greater than aerobic-only reactors. The average decay rate coefficient (bA) of the aerobic, anoxic, and alternating anoxic/aerobic decay reactors without predator inhibition were 0.153, 0.097, and 0.058 d−1 respectively. Aerobic, anoxic, and alternating anoxic/aerobic decay reactors created from inhibited SBRs showed average bA values of 0.152, 0.133, and 0.063 d−1 respectively. Statistical analysis of the data showed there were no significant differences in average bA values with or without predator inhibition. In all cases alternating anoxic /aerobic bA values were found to be significantly lower than in the other ORP conditions examined. Recalculation of the decay rates with consideration of regeneration showed that the impact of re-growth of autotrophs during the starvation period on the resultant bA value obtained can be significant and should be evaluated.