Evaluating expected microcystin removal at three Ontario drinking water treatment plants

Abstract

Cyanotoxins are a group of toxins produced by cyanobacteria that can be harmful to human health. Drinking water is a major pathway to exposure and therefore the presence of cyanobacteria and cyanotoxins in drinking water is a concern for drinking water utilities. Microcystins are a commonly occurring group of cyanotoxins in North America. Microcystin-LR is currently the only regulated cyanotoxin in Canada, with a maximum acceptable concentration of 1.5 µg/L total microcystin-LR in treated drinking water. Cyanobacterial blooms have occurred in the Great Lakes, a major drinking water source in Ontario. Climate change and rising temperatures bring a greater risk of cyanobacteria occurrences. This makes cyanobacteria and cyanotoxins a growing concern for drinking water treatment plants in Ontario. Conventional drinking water treatment processes have the ability to remove microcystins. Removals vary based on plant configuration, operating conditions and water quality characteristics. Understanding how well individual treatment processes are performing can assist utilities in developing a response plan for the event of a cyanobacteria bloom. The aim of this research was to assess microcystin removal at three Ontario drinking water treatment plants under different treatment scenarios. Extracellular (dissolved) microcystin removal, as well as cyanobacterial cell removal (intracellular microcystin removal) were assessed. Cell lysis and the resulting increase in dissolved microcystin concentration are highly variable and difficult to predict; however information was provided on cell lysis and microcystin accumulation from the published literature.

This study evaluated microcystin removal by drinking water treatment processes at three Ontario drinking water treatment plants: Woodward Avenue Water Treatment Plant (City of Hamilton), Elgin Area Water Treatment Plant (City of London), and DeCew Falls Water Treatment Plant (Niagara Region). This study did not involve any sampling. Data on microcystin removal were collected from existing studies and literature. Data on plant operations and water quality were collected from each treatment plant. This information was used to assess extracellular microcystin and cyanobacterial cell removal for each treatment process. The Hazen-Adams Cyanotoxin Tool for Oxidation Kinetics (CyanoTOX®) was used to predict extracellular microcystin removal with chlorination processes.

The three water treatment plants assessed in this study utilize chlorination, coagulation, flocculation, sedimentation, and filtration. One plant also employs chloramination for secondary disinfection, another plant employs powdered activated carbon (PAC) seasonally, and two plants employ UV disinfection. Chloramine and UV disinfection are not effective in treating microcystins. Chlorination is a key mechanism for microcystin removal, but can cause cell lysis and toxin release. Because of this, chlorination can reduce the total microcystin concentration but may increase the extracellular microcystin concentration. Extracellular microcystin removal increases with increasing CT (product of the oxidant concentration and the contact time with water), decreasing pH, and increasing temperature. Treatment scenarios were developed based on CT, pH, and temperature, and evaluated using CyanoTOX®. Cell lysis and dissolved microcystin increase seen in the literature at similar CT values were summarized. PAC can remove extracellular microcystins through adsorption. Treatment scenarios for PAC were developed based on dose and contact time, and assessed using data from existing studies. Limited information on factors affecting cyanobacterial cell removal is available for coagulation, flocculation, sedimentation, and filtration processes. Therefore, a best-case, worst-case, and average scenario for cell removal were estimated based on the literature. Coagulation, flocculation, sedimentation and filtration processes are not effective in treating extracellular cyanotoxins.

This research shows that a scenario-based approach may be used to predict microcystin removals. The results of this study may assist utilities in predicting the risk of microcystin breakthrough in treated water, making treatment decisions, and in developing a cyanotoxin management plan. Overall, under average conditions, the three drinking water treatment plants could expect high (>90%) intra- and extracellular microcystin removals. Chlorination is the primary treatment barrier for dissolved microcystin removal. Coagulation, flocculation, sedimentation and filtration are the primary treatment barrier for cell removal. Chlorination at the intakes may hinder cyanotoxin removal: cell lysis would result in fewer intact cells being removed by coagulation, flocculation, sedimentation and filtration, and the amount of microcystin released may be too much for the current chlorination processes to sufficiently remove. This study is limited by the availability of information available in the literature. In particular, little information was available on cell removal with coagulation, flocculation, sedimentation and filtration processes. For PAC processes, removals vary with different PACs and waters. For more accurate microcystin removal estimates, bench-scale or pilot-scale studies are warranted.