Algae and Algal Toxins: Impacts on Treatment and Control Stategies
- William C. Becker, Erik Rosenfeldt, Mark Bishop - Hazen and Sawyer
The shut-down of the Toledo, Ohio water system this past summer due to detected algal toxins has brought national attention to a water quality issue that has been increasing in recent years and is expected to become an even bigger issue for more utilities as our climate changes and extreme events like droughts and intense rainfalls occur more frequently.
Algal blooms cause both physical and chemical treatability issues. Low density algae can contribute greatly to particle counts and turbidity, and do not settle well. This often causes poor settled water quality and short filter runs which can limit plant production capacity. Algae also produce raw water quality changes, specifically reductions in dissolved oxygen, and fluctuation in pH and TOC, occurring on a diurnal cycle which can result in treatability issues including wide swings in filtered water particle counts and changes in coagulant demand. Some algae (and actinomycetes) produce taste and odor (such as geosmin and MIB) and algae derived toxins.
This paper will report on results from a study of six water treatment plants located on six different water supplies. The utilities were selected as follows: all utilities are located in the eastern US, all have experienced algae related issues, and the utilities utilize a range of raw water supplies.
Results from the study are being synthesized into recommendations to water utilities on how to best respond to algae and algal toxins. The efficacy of conventional water treatment plants and several advanced treatment processes including ozone, activated carbon adsorption, and advanced oxidation products will be presented. Results from the testing of optimization techniques including coagulant and pH control and the use of an oxidant prior to filtration will also be presented. Finally, recommendations to utilities suggesting strategies for handling algal events in treatment plants and toxic algal byproducts will be presented.
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