Stormwater Biofilters as Barriers against Campylobacter jejuni, Cryptosporidium Oocysts and Adenoviruses; Results from a Laboratory Trial

Plants release, pathogens decease: Plants with documented antimicrobial activity are associated with Campylobacter and faecal indicator attenuation in stormwater biofilters

Stormwater biofilters demonstrate promising treatment of faecal microorganisms, however performance can vary with design and operational conditions. This study investigated whether plants with significant documented antimicrobial activity could improve faecal bacterial treatment within biofilters. Laboratory-scale biofilters (n = 30) were dosed with synthetic stormwater containing faecal bacteria Escherichia coli, Enterococcus faecalis and Campylobacter jejuni under south-eastern Australian climatic conditions. Systems vegetated with Melaleuca species, renowned for their in vitro antimicrobial activity, consistently enhanced removal of all tested culturable bacteria in total outflow and submerged zone water relative to other plant configurations. Within just 1-2 days of stormwater dosing, M. linariifolia submerged zones demonstrated significantly reduced bacterial concentrations compared to C. appressa (p = 0.023 and <0.001 for C. jejuni and E. coli, respectively), removing ∼1.47 log10 MPN/100 mL E. coli, ∼1.14 log10 MPN/100 mL E. faecalis and ∼0.81 log10 MPN/L C. jejuni from inflow. These trends continued even after all but one M. linariifolia replicate perished during an extended drying period (p = 0.002 and 0.003 for C. jejuni and E. coli, respectively). Through a systematic process of elimination, these observations were attributed to enhanced bacterial attenuation with elevated plant inhibitory activity. Cumulative biofilter age reinforced plant-mediated bacterial treatment (p = 0.023 for E. faecalis), ostensibly due to increased plant size/growth and net biological activity. Notably, E. coli and E. faecalis attenuation improved with prolonged antecedent drying length (14 vs. 4 days; p < 0.0001 for both), while the converse was observed for C. jejuni (not significant). This study addresses significant knowledge gaps around plant-mediated faecal microbe treatment within biofilters, providing key direction for real-world system design to optimise stormwater pathogen treatment.

Highly variable removal of pathogens, antibiotic resistance genes, conventional fecal indicators and human-associated fecal source markers in a pilot-scale stormwater biofilter operated under realistic stormflow conditions

Green stormwater infrastructure systems, such as biofilters, provide many water quality and other environmental benefits, but their ability to remove human pathogens and antibiotic resistance genes (ARGs) from stormwater runoff is not well documented. In this study, a field scale biofilter in Southern California (USA) was simultaneously evaluated for the breakthrough of a conservative tracer (bromide), conventional fecal indicators, bacterial and viral human-associated fecal source markers (HF183, crAssphage, and PMMoV), ARGs, and bacterial and viral pathogens. When challenged with a 50:50 mixture of untreated sewage and stormwater (to mimic highly contaminated storm flow) the biofilter significantly removed (p < 0.05) 14 of 17 microbial markers and ARGsin descending order of concentration reduction: ermB (2.5 log(base 10) reduction) > Salmonella (2.3) > adenovirus (1.9) > coliphage (1.5) > crAssphage (1.2) > E. coli (1.0) ∼ 16S rRNA genes (1.0) ∼ fecal coliform (1.0) ∼ intl1 (1.0) > Enterococcus (0.9) ∼ MRSA (0.9) ∼ sul1 (0.9) > PMMoV (0.7) > Entero1A (0.5). No significant removal was observed for GenBac3, Campylobacter, and HF183. From the bromide data, we infer that 0.5 log-units of attenuation can be attributed to the dilution of incoming stormwater with water stored in the biofilter; removal above this threshold is presumably associated with non-conservative processes, such as physicochemical filtration, die-off, and predation. Our study documents high variability (>100-fold) in the removal of different microbial contaminants and ARGs by a field-scale stormwater biofilter operated under transient flow and raises further questions about the utility of human-associated fecal source markers as surrogates for pathogen removal.

Conventional and amended bioretention soil media for targeted pollutant treatment: A critical review to guide the state of the practice

Bioretention systems are widely used green infrastructure elements that utilize engineered bioretention soil media (BSM) for stormwater capture and treatment. Conventional bioretention soil media, which typically consists of sand, sandy loam, loamy sand or topsoil amended with compost, has limited capacity to remove and may leach some stormwater pollutants. Alternative engineered amendments, both organic and inorganic, have been tested to supplement BSM. Yet, municipalities and regulatory agencies have been slow to adopt these alternative amendments into their design specifications, partly because of a lack of clear guidance on how to select the right amendment to treat a target stormwater contaminant under highly variable climatic conditions. This article aims to provide that guidance by: (1) summarizing the current design BSM specifications adopted by jurisdictions worldwide, (2) comparing the performance of conventional and amended BSM, (3) highlighting advantages and limitations of BSM amendments, and (4) identifying challenges for implementing amendments in field conditions. The analysis not only informs the research community of the barriers faced by stormwater managers in implementing BSM amendments but also provides guidelines for their adoption by interested agencies to comply with existing regulations and meet design needs. This feedback loop could catalyze further innovation in the development of sustainable stormwater treatment technologies.

Seasonal operation of dual-mode biofilters: The influence of plant species on stormwater and greywater treatment

The use of stormwater biofilters (also known as bioretention systems and raingardens), in tropical and semi-arid areas is hindered by seasonal rainfall patterns which cause extended dry periods. These periods can result in plant die-off, long-term damage to system health and leaching of pollutants when stormwater inflows resume. Using an additional polluted water source during dry periods could minimise system stress and eliminate the need to irrigate biofilters with potable water during dry spells. As such, the presented laboratory study tested the seasonal operation of biofilters, by switching from stormwater treatment in wet months to greywater treatment in dry months. Forty-five single planted biofilter columns, incorporating sedges, grasses, understory ornamentals and vines, were subjected to four months of stormwater inflows, followed by three months of greywater inflows. We also investigated the impact of including a carbon source in the saturated zone on treatment performance. The results showed plant species selection to be critical for nitrogen and phosphorus removal, with consistently effective species such as Carex appressa and Canna x generalis able to maintain low outflow concentrations (e.g. total nitrogen of 0.2-0.3 mg/L and 0.3-0.6 mg/L, respectively) across both water sources. Low outflow phosphorus concentrations were associated with plant species that had high filterable reactive phosphorus removal across both water sources. Similarly, higher removal of ammonia and oxidised nitrogen was associated with lower overall nitrogen concentrations. In contrast, high removal of total suspended sediment (>94%), biochemical oxygen demand (>98%) and some heavy metals (e.g. lead >98% and copper >93%) was reported across all designs. The results suggest that with the careful selection of plant species, biofilters can be operated seasonally as a feasible and practical solution to maintaining system health during extended dry periods.

Validation and uncertainty analysis of a stormwater biofilter treatment model for faecal microorganisms

Stormwater biofilters, also known as rain gardens or bioretention systems, are effective stormwater treatment systems. This paper presents the validation, sensitivity and uncertainty analyses of a model for microbial removal in stormwater biofilters. The model, previously developed based on a rather limited laboratory study, was fully validated using the data collected in extensive laboratory experiments and field tests. The lab-scale and field-scale systems used for validation were of various designs (e.g., system size, plant type, media type), and have been operated under a wide range of operational conditions (e.g., length of antecedent dry period, and the inflow volume and concentration). For each tested biofilter design, the predicted E. coli concentrations in biofilters' outflow showed relatively good agreement with the measured ones: e.g., Nash-Sutcliffe Efficiency (E_c) ranged from 0.50 to 0.60 for the laboratory tests, and E_c = 0.55 for the field system. The results from sensitivity analysis confirmed the significance of adsorption and desorption processes, and also revealed the impact of temperature on microbial die-off (which was not fully represented in the model development stage). Finally, parameter transferability from one system to another with similar design was examined, achieving generally promising E_c values (0.04-0.56 with the best-fit parameter set for the other system; maximum value: 0.46-0.63) and acceptable uncertainties (intersection between prediction uncertainty band and observation: 50%-97%). Most importantly, the prediction of E. coli outflow concentrations from the field system was reasonably good when laboratory-determined parameter values were adopted: with the best-fit parameter set for the lab-scale system, E_c = 0.39; maximum E_c = 0.55; intersection between prediction and observation = 83%. These results suggested that the very rare biofilter model for microbial removal could provide reliable prediction for large scale field systems, by simply calibrating parameters with limited laboratory-scale experiments.

Real time control of biofilters delivers stormwater suitable for harvesting and reuse

Stormwater biofilters have great potential to treat stormwater for harvesting and reuse, but their variable performance in pathogen removal requires further optimisation prior to widespread uptake. This paper provides the first evidence that real time control (RTC) of stormwater biofilters can mitigate the impact of operational characteristics that result in poor microbial removal. We developed two RTC strategies and validated them using long-term laboratory experiments, utilising biofilters with a raised outlet pipe that creates a submerged zone. The first RTC strategy focuses on delivering the best water quality for harvesting and reuse or for recreational waterways. It has two components which ensure adequate treatment (microbial die-off): (1) it retains water in the biofilter for at least two days before allowing any further inputs into the system, and (2) the input volume is restricted to the submerged zone's pore volume. This strategy was effective and significantly improved water quality in the biofilter effluent. However, since the system favours bypassing influent to ensure good quality effluent, only 28.4% of the stormwater was treated. This still resulted in a 62.3% reduction in the influent E. coli load because the system was effective at removing E. coli under controlled conditions. The second RTC strategy builds upon the first strategy, and focuses on delivering a balance between good water quality for harvesting and protecting the environment (i.e., lower bypass). Three hours before the next rainfall event begins, the water that has remained in the biofilter's submerged zone for at least two days is drained and collected for harvesting through a bottom pipe. When stormwater inflow begins, the bottom pipe is closed and the biofilter operates without control, with water leaving the biofilter to the environment via the raised outlet pipe. The harvested effluent of this RTC strategy met the Australian stormwater harvesting guideline requirements for dual reticulation with indoor and outdoor use and irrigation of commercial food crops. Although only 5.4% of stormwater was collected for harvesting in this strategy, the environment was better protected because of a significantly reduced bypass volume. Our experiments also showed that the nutrient and sediment removal was high for both RTC strategies. This study presents the first stepping stone toward RTC of stormwater biofilters, demonstrating that these systems can deliver safe stormwater for harvesting and reuse, and for active recreational uses.

New prebiotic chemistry inspired filter media for stormwater/greywater disinfection

Greywater and stormwater have received significant attention due to increasing water scarcity. Passive filtration such as biofiltration has been a popular treatment method with its low energy input and environmental friendliness. However, pathogen removal capacity needs improvement to achieve safe water quality. In this study, a prebiotic chemistry inspired copolymer based on aminomalononitrile and 3,4,5-trihydroxybenzaldehyde (AMNT30) was introduced to develop antimicrobial media for passive filtration. The AMNT30 polymer provided an adhesive coating on zeolite substrates following a spontaneous polymerisation process at room temperature. AMNT30 coated media were investigated for metal loading capacity, surface morphology, E. coli removal and metal leaching after filtration of different water sources (i.e. stormwater, greywater, and deionised water) at low/high conductivity. The coating enhanced metal ion loading on the surface and demonstrated that >8 log reduction of E. coli can be achieved for silver loaded materials compared to a 1 log reduction for copper loaded materials. The coating also increased the stability of the metals on the media irrespective of inflow characteristics. This study provided the first example using AMNT30 to create antimicrobial water purification media. It is expected that this technology will find applications in the water treatment industry.

Dual-mode stormwater-greywater biofilters: The impact of alternating water sources on treatment performance

The intermittent nature of stormwater runoff impacts the treatment performance of biofilters, also known as stormwater biofiltration or bioretention systems and raingardens. During extended dry periods, which are common even in temperate climates, plants can perish, creating unattractive and non-functional systems that might leach pollutants during the next rainfall event. The current solution is to irrigate during long dry spells, which is costly and unsustainable as biofilters become more widespread. This paper presents the development of dual-mode biofilters, where stormwater and greywater are treated within the same system. Fifty columns, utilising eight plant species, including understory and climbing ornamentals, and designs with and without a carbon source in the submerged zone, were subjected to alternating greywater and stormwater inflows over five months. Six sampling events investigated treatment performance across these switching inflows and an extended dry period (atypical event). Good removal of total suspended solids (>83%), biochemical oxygen demand (>86%) and some heavy metals (e.g. lead >96%) were reported irrespective of design. Plant species selection was critical for the removal of nitrogen (2 to 79%) and phosphorus (12 to 75%) under dual-mode operation. However, following the extended dry period, plants with the lowest nutrient outflow concentrations also experienced some of the highest sediment and carbon concentrations, suggesting that a mixture of plant species may be beneficial for withstanding abnormal conditions. Differences between the treatment performance of designs with and without a carbon source were negligible, with potential benefits possibly negated due to the increased root mass that comes with age (systems were approximately two years old) and the release of carbon from root exudates. The results demonstrate the potential for dual-mode stormwater-greywater biofilters as an alternative to single-mode systems as they can provide effective treatment, along with greater volumes of treated water, while maintaining system performance throughout the year.

Stormwater biofilter treatment model for faecal microorganisms

This paper presents a new model to simulate long-term microbial removal in stormwater biofilters. The water flow module uses a 'three-bucket' approach to describe the flow processes in biofilters, while the microbial quality module employs the one-dimensional advection-dispersion equation to represent microbial transport and fate under different design and operational conditions. Three governing processes for microbial removal, adsorption, desorption and die-off, are included; temperature is also incorporated as a key factor for die-off. The model was tested using long term monitoring data collected from laboratory columns in which five different biofilter configurations were studied over a period of 44weeks. A multi-objective calibration with the balance of instantaneous ponding levels and event outflow volumes was implemented on the water flow module, and the Nash-Sutcliffe Efficiency (E) values ranged from 0.82 to 0.95. The microbial quality module was tested using the effluent Escherichia coli concentration data, and the E values obtained for different configurations were between 0.46 and 0.68. The optimized parameter values agreed with those presented in literature. However, sensitivity analyses suggested that the model's prediction was not sensitive to all parameters, the explanation for which was hypothesized to be data paucity rather than model structural uncertainties. Model validation was also conducted by splitting the data into calibration and validation datasets. The results further reinforced the needed for more data for model calibration.