Upset Events At Wastewater Treatment Plants: Implications for Mitigative Strategy Development and Bioreactor Microbial Ecology
This study consists of three research phases. First, we developed corrective action strategies to mitigate the impact of calcium hypochlorite and cadmium pulse shocks for the Plum Island Wastewater Treatment Plant (WWTP) in Charleston, SC. The corrective action strategies were developed in consultation with industrial consultants and operational personnel from the utility. These strategies were tested using a laboratory scale system, which was constructed and operated similar to the parent facility. Two corrective actions were tested for calcium hypochlorite, while only one strategy was tested for the cadmium at the laboratory scale. This study shows that no corrective action strategies are required for an acute hypochlorite stress. This is due to the fact that hypochlorite is highly reactive and dissipates rapidly on contact with the wastewater matrix, thus causing only low level process deterioration. In fact, implementation of corrective action strategies results in greater process deterioration as compared to the non-intervention approach. The corrective action tested for cadmium stress showed potential for reducing the peak impact of the toxin and allowed for faster process recovery as compared to the unstressed control.
For the second phase, the corrective actions were tested at a pilot scale facility operated at the Plum Island wastewater treatment plant. We tested two different corrective action strategies for cadmium, while only one strategy was tested for hypochlorite during the pilot scale study. Similar to the laboratory scale experiments, we conclude that no mitigative approaches are necessary for an acute hypochlorite stress. Additionally, the implementation of mitigative approaches for the pilot scale cadmium stress events resulted in greater process deterioration as compared to the non-intervention approach. In contrast to the laboratory scale experiments, theoretical effluent blending calculations showed that corrective actions may not reduce the impact of the cadmium stress. This was attributed to the lower intensity of process deterioration caused by the simulated cadmium stress. The pilot scale study shows that prior to implementing a corrective action strategy, the operator should determine the probable extent of process deterioration due to the detected chemical contaminant before deciding if a corrective action is needed. The pilot scale study also evaluated the effectiveness of current sensor technologies towards the upstream detection of influent anomalies and reliable monitoring of process performance during an upset event. Multivariate analysis on the rate of change of influent sensor signals was reliably able to detect the presence of both toxins tested during this study.
For the third phase of this research, we investigated the impact of cadmium stress on the structure and function of bioreactor microbial communities. We observed significant increases in post-stress heterotrophic and autotrophic bacterial respiration rates for the bioreactors subjected to cadmium stress. The higher respiration rates were due to an increase in bacterial abundance in the cadmium stressed reactors. We were also able to show that the increase in bacterial abundance was not due to changes in community structure or due to cadmium induced deflocculation. In fact, this study demonstrates that transient cadmium stress reduces predator abundance within the activated sludge community and this reduction in predator grazing was responsible for the increase in bacterial abundance. This research highlights the importance of higher life forms, specifically eukaryotic microorganisms, in regulating bacterial community dynamics in systems undergoing chemical perturbations.