Remedial Intervention Strategies for Wastewater Treatment Plants Exposed To Toxic Chemical Stress: Pilot Scale Evaluations

The detection and mitigation of toxic shock events is crucial for wastewater treatment facilities, especially nutrient removal plants. There is a lack of information on appropriate corrective action strategies to mitigate the process impact of toxins. We present a study that evaluates the effectiveness of a wastewater treatment plant specific corrective action plan matrix in the mitigation of calcium hypochlorite and cadmium shock events. The high reactivity of hypochlorite and the resulting rapid dissipation does not require the implementation of any remedial action. The presence of a primary clarifier provides sufficient hydraulic barrier to allow for complete hypochlorite dissipation. Both corrective action strategies tested in this study were able to mitigate the impact of the Cd stress event; however their efficiency was largely dependent on the influent flow levels. This study also examines the signal reliability of an array of sensors used for detection of toxin upstream of the treatment system and for monitoring the process upset and recovery. The sensors used in this study include pH, ORP, conductivity, dissolved oxygen, and ultra-violet (UV) absorption based sensor for carbon and nitrate monitoring. The pH, conductivity, dissolved oxygen sensor signals were robust to process condition and showed minimal loss of signal accuracy. The ORP sensor was the most sensitive in measuring small changes in water chemistry and exhibited a short response time. However, it was very susceptible to signal drift in the influent line as well as the aeration basin. The UV based sensor lost its accuracy to carbon measurements over time, while still maintaining its precision and sensitivity.