Wastewater treatment plants commonly experience upset events due to inflow of toxic chemicals resulting mainly from industrial sources. These events may have significant impacts on the biological treatment process, including loss of chemical oxygen demand (COD) removal efficiency, inhibition
of nitrification or deflocculation with corresponding increased effluent solids. This work investigated the source-effect relationships of four categories of toxic chemicals on the activated sludge process, including heavy metal (cadmium, Cd), electrophilic chemical (chlorodinitrobenzene,
CDNB), organic hydrophobic chemical (octanol) and ammonia. These studies were performed using lab scale sequencing batch reactors, operated at two solids retention times − 2 and 10 days (non-nitrifying and nitrifying cultures) − and initially dosed with three different toxin concentrations,
based upon respiratory inhibition of 15%, 25% and 50% of a non-shocked system. Ammonia was dosed at up to 10x the typical influent concentration. Process performance was monitored over time for effluent quality and mixed liquor stability. Results obtained for Cd showed
increasing deflocculation, loss of soluble COD removal efficiency and loss of nitrification with increasing Cd concentrations. Results for octanol showed foaming, minor deflocculation and nitrification inhibition as the main effects. Ammonia shock was found to have no major effect on process
performance, but minor nitrite buildup and minor deflocculation occurred. CDNB had effects similar to those seen for Cd on process performance, including deflocculation, loss of COD removal efficiency and loss of nitrification. It is hoped that these macroscopic effects will serve as a basis
for the determination of molecular causal mechanisms for upset events and possibly lead to the development of mechanistically-based upset early warning devices.
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