Factors affecting recirculating biofilters (RBFs) for treating municipal wastewater

Abstract

Recirculating biofilters (RBFs) were studied as an option for treating domestic wastewater. In particular, the objective of this investigation was to examine the hydraulic (hydraulic loading rates or HLRs), operational (dosing frequency and recycle ratio), and media characteristics that significantly impact treatment performance. Four types of filter media were examined in this study: sand, crushed glass, peat, and geotextiles. Laboratory controlled experiments demonstrated that dosing frequency impacted treatment performance significantly. A dosing frequency of 96 times per day resulted in significantly higher BOD5 removal than a low dosing frequency of 48 times/d. The average BOD5 concentration in effluent for 96 times/d was 6.2 mg/L, where it was less than half for a dose frequency of 48 times/d (13.3 mg/L). Crushed glass was found to perform similarly as silica sand; which represents an alternative for biofiltration media. Peat filter resulted in the lowest NH4+-N (84.5%) removals and sand filter provided the highest NH4+-N removal (98.0%). Geotextile provided the highest total phosphorus removal (73.8%). Scanning electronic microscope (SEM) images of the biofilm around particles at different depth of filter suggested that filter depth should be considered as a design criterion as well. From a practical perspective this study provides a greater understanding of the critical design factors for RBFs and also demonstrated the feasibility and limitations of possible filter media alternatives (i.e., crushed glass, peat, and geotextile).Key words: recirculating sand filters, sand, crushed glass, peat, geotextiles.