Page 15: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Turbidity
Appendix C: Guidance for achieving turbidity targets
Filtration systems should be designed and operated to reduce turbidity levels as low as possible. Research and field studies support optimizing particle removal in filtration plants to maximize protection of public health from microbiological contamination. Accordingly, filtration systems that use conventional, direct, slow sand or diatomaceous earth technologies should strive to achieve a treated water turbidity target of less than 0.1 NTU at all times.
In order for utilities to achieve a treated water turbidity target of less than 0.1 NTU, the filtration technology needs to be appropriately designed and the process must be optimized. Utilities may achieve plant optimization by following plant optimization programs and setting plant-specific filter performance goals. There are a number of voluntary programs and resources that utilities can use to help achieve filter optimization and ultimately a turbidity target of 0.1 NTU. Some of the programs that are available for conventional and direct filtration plants include the Partnership for Safe Water (PSW), the Programme d'excellence en eau potable (Program for excellence in drinking water) and the U.S. EPA Composite Correction Program (U.S. EPA, 1998b; PSW, 2012a; Réseau Environnement, 2012). In addition, there are a number of comprehensive documents that can aid utilities in optimizing their filtration processes (Renner and Hegg, 1997; U.S. EPA, 1998b; Logsdon et al., 2002; Logsdon, 2008).
Canadian utilities can join the PSW directly through the American Water Works Association (AWWA). The equivalent program is offered for French speaking utilities by Réseau Environnement through participation in the Programme d'excellence en eau potable. In these programs, utilities complete a variety of exercises, including data collection, analysis and a plant self-assessment. Another one of the key components of the program is to conduct filter performance monitoring. Utilities may benefit from considering the filter performance goals set by the PSW when trying to achieve a turbidity target of 0.10 NTU (PSW, 2011). The following selected PSW goals are set for achieving the Excellence in Water Treatment Award level, for plants demonstrating optimized in filter performance:
- Filtered water turbidity less than 0.10 NTU 95 percent of the time based on values recorded at 15 minute time intervals (or more frequent);
- Maximum filtered water turbidity goal equal to or less than 0.30 NTU;
- Maximum backwash recovery period goal of 15 minutes (time turbidity is above 0.10 NTU); and
- Combined filter effluent turbidity of less than 0.10 NTU 95 percent of the time.
For filtration technologies such as slow sand and diatomaceous earth, a similar approach to data collection, filter self-assessment and performance goal setting can be used to help achieve a filtered water target of 0.10 NTU or less. The factors that affect the performance of these types of technologies are discussed in sections 6.2.2 and 6.3.2. This information can be used in the filter self-assessment process to help identify performance-limiting factors, which can then be modified to facilitate filter optimization. In addition, the PSW individual filter performance goals outlined above can be used as the basis for establishing plant-specific goals for slow sand and diatomaceous earth filtration technologies.
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