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the issue. The city had used the tool for other projects, but using the risk- management tool to study water qual- ity in relation to algal toxins was a new application for this tool. Treatment Schemes Recommendations from the study included both short-term and long- term capital upgrades, monitoring practices, and operating strategies. Due to the size of the water body involved, there is no practical way to remove the risk in the source; therefore, a multi- barrier approach is required to transfer and manage the city's risk of algal events. Two areas where control can be provided include: 1. Raw water intake — removal of algae and cyanobacteria from the raw water (intact cells), as well as any algal metabolites (intracel- lular material) present in the raw water; 2. Treatment processes — removal of algae and cyanobacteria (intact cells), as well as any remaining metabolites (such as T&O; com- pounds and cyanotoxins) present in the treatment process, from either the source water or from cells ruptured as a result of treatment. A multibarrier approach is necessary to physically remove intact cells (algae and cyanobacteria) before they rupture in the treatment process and remove dissolved metabolites through oxidation and adsorption. Under the multibarrier treatment approach, the inclusion of an oxida- tion step is implied. To manage the risks and benefits of raw water oxidation of the algae, cyanobacteria and their metabolites, a postfilter disinfection approach must be employed. Taking this approach into consideration, the city has several options available to use and/or modify its existing infrastructure to mitigate the risk of algae and cyanobacteria, including: • Raw water intake prechlorination modifications to the dose • Pretreatment modifications to operate with a higher coagulant dose, apply a polymer as a coagulant aid during algal events, or apply a powered activated carbon seasonally during algal events • Filtration modifications to promote adsorption in the granular activated carbon filters; this may impact the frequency of the GAC (granular activated carbon) replacement. (The WTP had taken proactive steps following a 1999 taste and odor event on Lake Ontario to protect itself from water quality impairments by replac- ing the media in its filters with GAC and adopting a media replacement schedule of four to five years.) • Filtration modifications to reduce chlorine residual in both filter influent and backwash water to pro- mote biological activity • Increase chlorine contact time to provide all primary disinfec- tion needs downstream of the filters; alternatively, install UV downstream of the filters. • Shut down the WTP for 12 to 24 hours during algal events to reduce the volume of water that passes through the treatment process and into the distribution system, and sup- ply water using system storage. Future Risk The above list identifies the short-term solutions that the City of Hamilton can implement to manage risks associated with algae and cyanobacteria. The long-term solutions such as adding new oxidation processes to destroy toxins, new pretreatment processes to remove algae, and extending the intake pipelines deeper into Lake Ontario could not be justified for implementation at this time based on the cur- rent understanding of the risks using the available data. In the future, the City of Hamilton will continue to monitor the source water to confirm the prolonged risk profile to determine if the city should invest in the longer-term solutions for managing algae and cyanobacteria risks. In the near term, by modifying its existing infrastructure, the city is adequately managing related risks. While these solutions could impact the plant's operations, the plant currently has a surplus capac- ity and large reservoir storage capability. To determine the need for the higher-cost long-term solutions, the city will need to collect and ana- lyze more water samples to confirm the raw water quality at the intake. The City of Hamilton is leading the industry by implementing a proactive approach to monitoring algal blooms, ensuring residents continue to have access to a safe drinking water supply. In addition to reviewing and planning for algae risk and control needs, Hamilton is currently undergoing an overall study of its facility, closely examining plant performance and production, which enables the city to be pragmatic with recommended upgrades and take a multipurpose, holistic approach to managing its overall risk. wateronline.com ■ Water Innovations WATERQUALITYANALYSIS 17 Christopher A. Mills, PE, is a senior project manager at Hamilton Water for the City of Hamilton, Ontario. For nearly 20 years, his career has focused on municipal water and wastewater infrastructure, best practices, and system efficiencies. Mills received his B.E.Sc., Chemical and Biochemical Engineering, from The University of Western Ontario. Quirien Muylwyk is CH2M HILL's Technology Manager for Water in Canada. She has more than 20 years of experience in municipal water treatment and process engineering. Over the years, she has worked with owners and stakeholders on the strategic planning of plant upgrades, expansion projects, regulatory compliance needs, and optimization/operation of large plants. Muylwyk is active with the AWWA and serves as Chair of the Water Quality and Technology Division. About The Authors Postfilter disinfection with granular activated carbon (GAC)