Use of Online Monitoring Instrumentation for Coagulation Optimization

Abstract

Maintenance of optimal water treatment plant (WTP) performance in the presence of rapidly changing raw water quality (e.g. rainfall events and snowmelts) has been shown to be challenging for coagulation/flocculation based plants. During these times, rapid increases in raw water turbidity and colour can be noticed. The purpose of this study was to assess the viability of using online monitoring instruments to provide coagulation process control in response to these changes. Three WTPs from around the Atlantic Provinces were involved in the study. Each WTP was sourced from surface waters that are low in alkalinity and highly variable in terms of natural organic matter (NOM) and turbidity. Furthermore, the NOM in these source waters was primarily in dissolved form. Plant audits, bench-scale experimentation and full-scale set up of online instrumentation, including a streaming current monitor (SCM) and UV254 monitor, were completed to evaluate current operating and coagulation process control approaches as well as to assess the capabilities of each of the monitors under variable source water quality operating periods. Bench-scale experimentation using a jar tester was completed to evaluate the turbidity and NOM removal capabilities of three coagulants at varying pH. At the pH of minimum solubility, optimal dose for each coagulant was based on the point of diminishing returns for turbidity, dissolved organic carbon (DOC) and UV254. Zeta potential ranged from -15 to 5 mV for each optimally dosed coagulant which showed that when pH was held constant, charge analysis could be used to determine coagulant dose. Results from the full-scale online instrumentation set up highlighted the necessity of a stable coagulation pH as well as maintenance of the equipment. When a stable pH was not achieved, streaming current results could not be used. Furthermore, when the process lines into the monitors were not regularly drained to remove build up, samples unrepresentative of the source water quality were sent to the monitor for analysis, giving inaccurate readouts. With proper maintenance and a stable pH, SCM and UV254 monitors detected changes in raw water quality. The UV254 monitor detected increases in raw water UV254 that were independent of turbidity. Raw water UV254 also rapidly increased with turbidity during a rain event. However, turbidity returned to its pre-event level faster than UV254. Streaming current was able to detect chemical inadequacies during a rain event. At the beginning of a rain event, streaming current was shown to drop below set-point, indicating insufficient coagulant addition. The main finding from the study showed that both SCM and UV254 could detect changes in source water quality under variable conditions. Streaming current, however, was shown to be highly affected by changes in pH. Jar tests must also be frequently run to determine the set-point, as it may shift with changing source water quality due to seasonal changes. When maintained, the UV254 monitor can give an instantaneous measurement of organic concentrations in the source water. Preliminary regression analysis demonstrated promise for basing coagulant dose on raw water UV254.