A mechanistic understanding of the nitrification sand layer performance in a nitrogen removing biofilter (NRB) treating onsite wastewater

Bench-scale columns were used to test the impact of depth, alkalinity, and nitrogen/hydraulic loading on nitrification performance and microbial community abundance in a sand filter treating onsite wastewater. The extent of nitrification was independent of the column depth at the test hydraulic loading rate (20.5 L m⁻² d⁻¹), as full nitrification was observed at 15 cm of the column. The nitrification performance was less sensitive to nitrogen loading increase (0.15 to 0.53 mg N cm⁻² d⁻¹), while increased hydraulic loading (from 20.5 L m⁻² d⁻¹ to 32.8 L m⁻² d⁻¹) and insufficient alkalinity caused reduced nitrification at shallow column depth. Microbial analysis suggested the majority of biomass and functional species were present at the top 15 cm, with several orders of magnitude lower microbial density was observed at 45 cm depth. In addition, the microbial community present in the aged sand matrix could sustain efficient nitrification when treating synthetic wastewater. Collectively, these findings reveal the precise conditions for optimizing for complete nitrification of wastewater by sand filters.