Assessment of drinking water biofilter health under warm and cold temperatures

Conventional filtration studies have long-established operational parameters for drinking water treatment systems; however, these parameters are frequently adopted for biofiltration systems without verifying their suitability. A full-scale monitoring program was conducted over a period of two years across two sets of dual-media biofilters at two water treatment plants (Plant A and Plant B). This study investigates temperature effects, floc retention, unit filter run volume (UFRV), recovery, and backwashing modifications. Findings reveal that seasonal water quality variations significantly influence floc retention, with colder temperatures leading to increased solids accumulation. Backwash modifications, such as reducing air scour duration, maintained acceptable floc retention, while extending air scour and increasing backwash velocity showed minor performance improvements. In this study, UFRV and recovery values confirmed overall filtration efficiency, even when conventional floc retention limits were exceeded. The conventional 60 NTU floc retention threshold for media health may be overly conservative for biofiltration systems, recommending an adjustment to 120 NTU or a site-specific value to better reflect biomass-related variability and to account for the specific needs of biofilters. • Floc retention analysis of biofilters demonstrated material capture by depth. • Hydraulic only backwash had consistently higher floc retention than air-scoured filters. • Air scour duration was optimized utilizing floc retention analysis. • Higher floc retention was present in colder (winter) <5C versus warmer (summer) conditions.