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The public health risks associated with pathogens in urban stormwater have been well established, making it necessary to ensure adequate treatment of the stormwater before it is discharged into recreational water bodies or is harvested for reuse. Biofilters, also known as stormwater bioretention systems or raingardens, have shown promising, yet variable, results in reducing indicator bacteria in stormwater. Different biofilter design elements, such as filter media composition and vegetation type, have been found to cause this variable removal performance. Although plants play a key role in the treatment of pollutants, relatively little work has been conducted to understand the importance of interactions between vegetation and the biofilter microbial community on fecal microbial removal. A laboratory-scale biofilter experiment was conducted using Escherichia coli as the test fecal microorganism. Biofilter columns with differing soil media and vegetation types were dosed over a two month span, during which inflow and outflow samples were collected to evaluate system performance. The columns were then decommissioned to collect rhizosphere and bulk soil samples. Root exudates were extracted and used in an E. coli survival study to evaluate their contribution to system performance. The study demonstrated that the antagonistic effects of root exudates/rhizosphere microbes and Cu2+ exchanged zeolite antimicrobial filter media adversely impact the survival of E. coli retained within stormwater biofilters. Furthermore, leaf and flower/seed extracts of L. continentale showed some potential antibacterial activity against E. coli. This work supports the concept that natural processes in biological systems can deliver effective results in the removal of fecal microorganisms, and should be promoted to the extent possible in stormwater green infrastructure.

Knowledge of pathogen removal in stormwater biofilters (also known as stormwater bioretention systems or rain gardens) has predominately been determined using bacterial indicators, and the removal of reference pathogens in these systems has rarely been investigated. Furthermore, current understanding of indicator bacteria removal in these systems is largely built upon laboratory-scale work. This paper examines whether indicator organism removal from urban stormwater using biofilters in laboratory settings are representative of the removal of pathogens in field conditions, by studying the removal of Escherichia coli (a typical indicator microorganism) and Campylobacter spp. (a typical reference pathogen) from urban stormwater by two established field-scale biofilters. It was found that E. coli log reduction was higher than that of Campylobacter spp. in both biofilters, and that there was no correlation between E. coli and Campylobacter spp. log removal performance. This confirms that E. coli behaves significantly differently to this reference pathogen, reinforcing that single organisms should not be employed to understand faecal microorganism removal in urban stormwater treatment systems. The average reduction in E. coli from only one of the tested biofilters was able to meet the log reduction targets suggested in the current Australian stormwater harvesting guidelines for irrigating sports fields and golf courses. The difference in the performance of the two biofilters is likely a result of a number of design and operational factors; the most important being that the biofilter that did not meet the guidelines was tested using extremely high influent volumes and microbial concentrations, and long antecedent dry weather periods. As such, the E. coli removal performances identified in this study confirmed laboratory findings that inflow concentration and antecedent dry period impact overall microbial removal. In general, this paper emphasizes the need for the valida

Biofilters are widely adopted in Australia for stormwater treatment, but the reported removal of common faecal indicators (such as Escherichia coli (E. coli)) varies from net removal to net leaching. Currently, the underlying mechanisms that govern the faecal microbial removal in the biofilters are poorly understood. Therefore, it is important to study retention and subsequent survival of faecal microorganisms in the biofilters under different biofilter designs and operational characteristics. The current study investigates how E. coli survival is influenced by temperature, moisture content, sunlight exposure and presence of other microorganisms in filter media and top surface sediment. Soil samples were taken from two different biofilters to investigate E. coli survival under controlled laboratory conditions. Results revealed that the presence of other microorganisms and temperature are vital stressors which govern the survival of E. coli captured either in the top surface sediment or filter media, while sunlight exposure and moisture content are important for the survival of E. coli captured in the top surface sediment compared to that of the filter media. Moreover, increased survival was found in the filter media compared to the top sediment, and sand filter media was found be more hostile than loamy sand filter media towards E. coli survival. Results also suggest that the contribution from the tested environmental stressors on E. coli survival in biofilters will be greatly affected by the seasonality and may vary from one site to another.

Biofilters, also known as bioretention areas or raingardens, are an effective treatment option for the removal of various pollutants from stormwater. However, they show variable treatment efficiency for the removal of indicator bacteria, and the operational and design factors which impact this variability are largely unknown. This study uses a laboratory scale column set-up to explore how Escherichia coli (the chosen indicator organism) removal in the stormwater biofilters is impacted by: plant presence and species type, the presence of a submerged zone (SZ), and operational conditions (duration of dry periods and changes over the initial stages of the system's life-span). Vegetation selection was found to be important for E. coli removal and the highly performing plant species were associated with lower infiltration rates. Based on the current results, a biofilter planted with Leptospermum continentale, Melaleuca incana or Palmetto buffalo and comprising a SZ can be recommended for improved E. coli removal. Inclusion of SZ was found to generally enhance the removal performance; which may be explained by the contribution of microbial processes that are happening within the SZ (such as predation/competition and natural die-off). Results also suggest that the E. coli removal performance is reduced after a significant dry period, while the overall removal performance improves over time as systems mature.

Stormwater biofilters are not currently optimised for pathogen removal since the behaviour of these pollutants within the stormwater biofilters is poorly understood. Modelling is a common way of optimising these systems, which also provides a better understanding of the major processes that govern the pathogen removal. This paper provides an overview of a laboratory-scale study that investigated how different design and operational conditions impact pathogen removal in the stormwater biofilters. These data were then used to develop a modelling tool that can be used to optimise the design and operation of the stormwater biofilters. The model uses continuous simulations where adsorption and desorption were dominant during wet weather periods and first order die-off kinetics were significant in dry periods between the wet weather events. Relatively high Nash Sutcliffe Efficiencies (>0.5) indicate that the calibrated model is in good agreement with observed data and the optimised model parameters were comparable with values reported in the literature. The model's sensitivity is highest towards the adsorption process parameter followed by the die-off and desorption rate parameters, which implies that adsorption is the governing process of the model. Vegetation is found to have an impact on the wet weather processes since the adsorption and desorption parameters vary significantly with the different plant configurations. The model is yet to be tested against field data and needs to be improved to represent the effect of some other biofilter design configurations, such as the inclusion of the submerged zone.

Knowledge of pathogen removal in stormwater biofilters has predominately been extrapolated from bacterial indicators; indeed, the removal of actual pathogens in these systems has not yet been studied. Furthermore, there is no knowledge on as to how the pathogen removal capacity varies in biofilters with different design parameters (e.g. media composition, inclusion of submerged zone). Hence, this study presents the performance of two field biofilter cells (with different designs) dosed with semi synthetic stormwater in removing ten faecal microorganisms, including: indicators and pathogenic bacteria, protozoa and virus. A net reduction in concentrations was observed for all pathogens and indicators, but the removal performance varied between bacteria, protozoa and viruses. Interestingly, the behaviour of the chosen bacterial pathogens was comparable to the bacterial indicators, whereas protozoan and viral pathogen removal were lower than their corresponding indicators. Biofilters containing a submerged zone generally had higher removal.

Biofilters are often used as treatment systems for urban stormwater runoff and have been shown to efficiently reduce nutrient loads. However, there is limited knowledge on how biofilters perform reduction of micro-pollutants in stormwater. This topic is of high importance if we are to harvest stormwater for human uses and in particular treat it to potable standards. This study investigated the micro-pollutant removal efficiency of two field-scale stormwater biofilters dosed with seminature stormwater, which included some key stormwater micro-pollutants (THMs, PAHs, TPHs, phthalates, halogenated phenols, glyphosate, and triazines). The results show excellent removal of PAHs, TPHs, phthalates and halogenated phenols, while there are variable levels of removal of glyphosate, triazines and chloroform. The outflow concentrations varied with the total pore volume over three tests, with the two cells having very different removal efficiencies. This preliminary study confirmed that in-situ challenge tests could be carried out to validate stormwater harvesting biofilters. It also suggests that design characteristics have a large impact on micropollutant removal, and a targeted lab study is needed to optimise biofilter design for this important function.

Biofilters are common, low energy technologies used for the treatment of urban stormwater. While they have shown promising results for the removal of stormwater microorganisms, certain factors affect their performance. Hence, this study investigated the effects of particle-microbial interaction, inflow concentration, antecedent microbial levels and plant species on microbial removal capacity. A biofilter column study was set up to evaluate removal performance and a sequential filtration procedure was used to estimate microbial partitioning. The columns were dosed with different concentrations of free phase Escherichia coli only and E. coli mixed with stormwater sediment. Results indicate that the microbial removal is significantly affected by inflow concentration and antecedent microbial levels. Leaching was only observed when a relatively low inflow concentration event occurred within a short period after a very high inflow concentration event. Finally, Lomandra longifolia showed better removal compared with Carex appressa.