The authors review key parameters and engineering variables influencing biological filtration and identify areas requiring further research. Biological filtration, an important process step for the production of microbially safe and aesthetically pleasing drinking water, has attracted increased attention within the water industry. In many cases, the most economical way to implement biological rapid filtration is to achieve biodegradable organic matter (BOM) removal and particle removal within the same filter unit, i.e., single‐stage biological filtration. This requires optimization of the filtration process, keeping in mind both treatment goals: BOM and particle removal. This article presents a critical review of the key parameters and engineering variables influencing the biological performance, as well as the conventional performance, of biologically active filters. Several areas requiring further research have been identified.

Full‐scale biofiltration experiments demonstrated that good removal of biodegradable organic matter (BOM) following ozonation could be achieved without compromising particle removal. BOM removal by granular activated carbon (GAC) filter adsorbers and dual‐media filters was measured using total organic carbon (TOC) and certain BOM components (carboxylic acids). The authors investigated how filter backwashing with water, water and air scour, and water and air scour at collapse‐pulsing conditions affect filter biomass, BOM removal, and particle removal. At 21‐24°C, the media type did not affect BOM removal. At 1‐3°C, GAC provided substantially better removal of oxalate and TOC than did anthracite. For both media types, cold water oxalate removals were significantly impaired, compared with those achieved in warm waters. BOM removal was more resilient than particle removal to changes in backwash protocol. Phospholipid biomass concentration was not directly related to BOM removal by filters.

Physicochemical removal of protozoan pathogens is receiving increased attention because of the difficulty of chemically inactivating these organisms, particularly Cryptosporidium parvum. Most research examining the removal of these and other pathogens by filtration has been conducted under steady-state conditions with optimized pretreatment. This study evaluated the removal of Cryptosporidium and changes in surrogate parameters at various points in the filter cycle and under nonoptimal conditions at two pilot plants with different coagulation regimes.The study found a reproducible 2-log difference in Cryptosporidium removals between the two locations under optimal conditions, with similar low effluent turbidity levels and particle counts. Either suboptimal coagulation or the early stages of breakthrough at the end of a filter run produced substantial deterioration of Cryptosporidium removal capability. Filter ripening or the imposition of a hydraulic step generally had much less effect on removals.A full report of this project, Filter Operation Effects on Pathogen Passage (catalog number 90874), is available from AWWA Customer Service (1-800-926-7337). Reports are free to AWWA Research Foundation subscribers by calling 303-347-6121.

This pilot‐plant study was initiated to evaluate biological filtration for the removal of aldehydes formed during ozonation. An additional objective of this testing was to demonstrate that aldehyde measurements could be used as a surrogate for analysis of assimilable organic carbon (AOC). The use of granular activated carbon (GAC) as an alternative to anthracite coal as the filter medium was also investigated, and it was observed that GAC filters developed biological activity sooner and showed longer‐term stability. Although biological activity was established sooner on slow‐rate filters, the high‐rate filters in time achieved a comparable capability. Data for formaldehyde and glyoxal provide information on removal of readily biodegradable and more recalcitrant ozone by‐products, respectively, and demonstrate trends similar to those for the removal of AOC.

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