Using Biofiltration to meet Today’s Water Quality Challenges
- William Becker, Erik Rosenfeldt, James DeWolfe - Hazen and Sawyer
Today’s water utilities are faced with increasingly stringent regulations, a public that is more educated on water quality issues than ever before, and pressure to keep rate increases at bay. At the same time, utilities often have to treat degrading source waters due to climate change and extreme weather events and are pressed to provide water that is not only safe but also aesthetically pleasing. One treatment option that is being implemented with increasing frequency to help meet these challenges is biofiltration.
Biological filtration is not new. It has been practiced in Europe and in warmer climates in the US for many years – often where utilities struggle with DBP control, taste and odor events, and distribution system water quality issues. When properly designed, a biofiltration system can handle varying raw water quality and meet operational and water quality goals. However, biofiltration is not a panacea. While many utilities report great success with biofiltration, several others have experienced difficulties that usually reltate to particle removal and excessive head loss development and short filter runs. Particle removal and headloss development is a function of particle stability and filter media design (depth, effective size, and uniformity coefficient). Particle stability is related to coagulant dose and also related to the application of an oxidant. Microorganisms and their byproducts also influence particle removal in biological filters. While considerable research has been conducted examining optimization of the biological aspects of filtration, surprisingly little has been done on the physical aspects.
Results from two studies that focused on particle removal and plant operations in biological filtration plants will be presented. The first focuses on 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. The second study examined the impact of filter media design (and condition) on particle removal and head loss development in biological filtration plants.
Results from these studies show that filter media design and condition (presence of fines), and oxidant type and dose have significant impacts on the performance and operation of biological filtration plants – even those where nutrient balance has been optimized. Guidance to utilities on these design parameters and operational maintenance strategies for biological filters will be presented.
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