Nitrification potential of daily-watered biofiltration designs for high ammonium wastewater treatment

Abstract

The vegetated biofiltration systems (VBS), also known as bioretentions or rain gardens, are well-established technology for treatment of urban stormwater and recently greywater, offering multiple benefits to urban environments. However, the impact of high ammonium strength wastewater (60 mg/L) on the nitrification process in these systems is not well understood. Hence, a laboratory-based column study was conducted to uncover dominant nitrification mechanisms, based on the learnings from similar onsite wastewater treatment systems. The experimental columns tested the effect of contact time (filter media depth, 150 mm, 300 mm and 700 mm), media oxygenation (active and passive) and alkalinity/pH (marble chips 5 % weight), as well as optimal operational conditions (inflow loading, concentrations, and dissolved oxygen (DO)). All nitrogen species (NH₄⁺, NO₃^-, NO₂^-), chemical oxygen demand (COD) and physical parameters (DO, pH, electrical conductivity) were monitored across seven events over thirteen weeks. The results show that dosing with 30 and 60 mg/L of NH₄⁺ resulted in 700 mm sand column depth to perform almost complete nitrification of NH₄⁺ to NO₃^- (< 90 %), while 300 mm designs achieved partial nitrification of NH₄⁺ to NO₂^-, likely due to limited contact time and inefficient nitrite oxidizing bacteria activity. Nitrification potential of all designs further supported that appropriate aerobic contact time is necessary for effective nitrification. Inflow concentration of NH₄⁺ and DO did not significantly impact nitrification performance, while reducing daily volume loading reduced NO₃^- and NO₂^- leaching. Active and passive aeration and alkalinity buffering did not positively affect ammonium removal. While there is a potential to apply both nitrification-denitrification and anammox processes to future VBS design, further understanding of aeration and alkalinity on microbially driven nitrification processes is needed.

Keywords: Bioretention; Decentralised systems; Green infrastructure; Nature-based solutions; Sand filter.