Antimicrobial media for passive removal of pathogen by stormwater biofilters

Stormwater biofilters (designed primarily for nutrient and metal removal) are gaining popularity globally for the treatment of urban stormwater runoff. However, their reported faecal indicator removal efficiency varies from several log to net leaching, and the effluent concentrations rarely meet the requirements for stormwater harvesting. In order to improve the stormwater biofilter design for reliable pathogen treatment, research in three Stages is therefore conducted systematically: identification of key design factors and operational conditions in existing biofilters, development of novel antimicrobial filter media, and new biofilter design incorporated with the antimicrobial media. In Stage 1, best practice stormwater biofilters using sandy loam as filter media were found to remove E. coli by 2 log during typical dosing events (i.e. twice weekly dosing). The absence of vegetation, reduced filter media depth, addition of filter media additives, presence of saturated zone and carbon source (SZ) were of no significance effect on E. coli removal. However, intermittent drying periods reduced E. coli removal in vegetated biofilters, and the presence of a SZ mitigated the negative effect. This led to the conclusion that filter medium, SZ, and intermittent drying periods were the main factors influencing the processes that govern faecal microbial behaviour within stormwater biofilters and thus affecting overall removal. In Stage 2, novel antimicrobial filter media were developed and evaluated by considering the key factors for stormwater treatment identified in Stage 1. Firstly, antimicrobial agents (including copper, zinc, iron, titanium and quaternary ammonium salts)-modified zeolite/granular activated carbon were prepared. It was found that, only the copper-modified media exhibited robust antimicrobial efficiency during five months natural stormwater treatment. Secondly, the copper-modified media were further optimised through heat treatment (calcination) or Cu(OH)2 coating to prepare stable yet more efficient antimicrobial media copper-zeolite. In Stage 3, copper-zeolite biofilters at large-scale were designed and constructed using the optimised antimicrobial media developed in Stage 2. The impact of design factors (vegetation, filter media type and filter media layout) and operational conditions (intermittent drying periods, inflow volume, inflow concentrations and experimental duration) on removal of faecal indicators, reference pathogens, copper and nutrients from synthetic stormwater was examined over 16 months. It was found that the copper-zeolite biofilters (1) exhibited longevity in faecal indicators removal despite operational conditions; (2) effectively removed reference pathogens (81% of effluent concentrations were below limit of detections) and (3) showed effective removal of copper and nutrients. In summary, newly developed copper-zeolite media are able to remove faecal indicators and reference pathogens while not sacrificing the removal of other pollutants subjected to various operational conditions. The main recommendation is that stormwater biofilters could integrate layers of low-cost and stable antimicrobial filter media for improved removal of faecal microbes from urban stormwater. Further research can be recommended in the following two aspects: (1) examine the impact of seasonality on the performance of the copper-zeolite biofilters; and (2) maintain the performance of the antimicrobial media which may lie below clogged surfaces; this creates unfavourable conditions for the functioning of media (unsaturated flow, limited contact time, and reduced oxygen levels).