Biological Filtration - Using Pre-oxidants to Avoid Particle Removal Pitfalls
- Erik Rosenfeldt, William Becker, James DeWolfe - Hazen and Sawyer
The use of biological filtration in the United States has increased dramatically in recent years, particularly in moderate to warmer climates where utilities often struggle with disinfection byproduct control (DBP), taste and odor events, and distribution system water quality issues. While biological filtration has many benefits, care must be taken to assure all treatment goals are met and public health is not compromised.
The most important purpose of filtration, biological or conventional, is to remove particles. Optimized filtration from a particle removal perspective is critical for preventing water-borne disease outbreaks from pathogens such as cryptosporidium or giardia. In the push to control DBPs, many utilities have converted to biological filtration by relocating the first point of chlorine addition to downstream of the filters. This can have a dramatic effect on filtered water particle counts.
The purpose of this paper is to present results from pilot and full-scale experiments conducted to evaluate the impact of pre-filter oxidation on filtered water turbidity, particle counts, and head loss development. Chlorine, chlorine dioxide, permanganate, and ozone were evaluated as pre-filter oxidants. The results were quite dramatic – all oxidants were found to significantly reduce filtered water particle counts by as much as an order of magnitude compared to the “no preoxidant” case when applied prior to filtration, even when filtered water turbidity was below 0.1 ntu. This suggests that great care should be taken in designing filters that operate in a biological mode such that particle removal is optimized. The paper will conclude with guidance and recommendations to utilities regarding pre- and intermediate disinfection/oxidation, filter media design, and optimization strategies for biological filters.
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