Impact of Design and Operational Parameters on Rapid, Deep Bed Biological Filtration of Drinking Water

Abstract

A series of pilot and full-scale experiments were carried out at the Mannheim Water Treatment Plant in Kitchener, Ontario to examine the impact of backwash technique, filter media characteristics, and combinations thereof on single stage drinking water biological filter performance. The media characteristics investigated were effective size, uniformity coefficient, and media type (GAC and anthracite). Backwash techniques investigated were the collapsed pulse backwash, the extended terminal subfluidization wash (ETSW), and the presence of chlorine in the wash water. Single stage biological filters must serve the dual purpose of biologically mediated removal of biodegradable organic matter (BOM), as well as meeting traditional filter performance criteria such as turbidity removal with minimal head loss accumulation. Accordingly, dissolved organic carbon removal, biodegradable dissolved organic carbon removal, biological respiration potential, turbidity removal, filter ripening time, and head loss accumulation were all quantified as measures of biological filtration performance. The results of this study have several implications for optimized design and operation of biological filters during drinking water treatment. An increase in effective size of media grains from 1.0 mm to 1.3 mm was shown to significantly extend filter run time by minimizing head loss accumulation without compromising turbidity or BOM removal. Uniformity coefficient however, showed no significant effect on biological filter performance; indicating that the performance benefits associated with highly uniform media may not be commensurate with cost. GAC was found to be significantly more resilient to backwashing in collapsed pulse and chlorinated modes, which impaired BOM removal in anthracite filters. This resilience imparts a high degree of operational flexibility to backwashing GAC filters. The significant decrease in BOM removal by anthracite filters can be minimized; however, by using an optimized backwashing technique. Collapsed pulse backwashing was found to have a significant effect on biological filter performance. When chlorinated collapsed pulse was used, filter cycles were significantly shortened by approximately 30 – 50% due to a sudden surge in effluent turbidity. This effect is thought to be the result of biofilm, damaged during the course of backwashing sloughing from the media. Extended terminal subfluidization wash was found to significantly reduce, and often eliminate filter ripening entirely. Additionally, the extended contact time with chlorine associated with chlorinated ETSW did not appear to have a significant effect on filter BOM removal. By eliminating filter ripening without compromising biological performance, ETSW shows promise for significant water and production cost savings by minimizing the filter-to-waste period during filter ripening. The presence of chlorine however, was associated with decreased DOC, 24 hours in to the filter cycle. This factor, combined with the negative interaction between chlorine and collapsed pulse suggests chlorinated wash water should be avoided in biological filtration systems like the ones investigated.