Belinda E. Hatt

Pollutant removal performance of field-scale stormwater biofiltration systems

The pollutant removal performance of three separate stormwater biofiltration systems in two different climates was assessed. At one of the sites, rain events were simulated, while actual runoff events were monitored at the other two sites. In all cases, concentrations of total suspended solids (TSS), copper, lead and zinc were effectively and reliably reduced, despite variations in inflow concentrations. Two biofiltration systems also effectively reduced phosphorus concentrations, however the third system discharged elevated phosphorus concentrations relative to inflow; this is attributed to poor specification of filter media properties. Effluent nitrogen concentrations were more variable at all sites and ranged from being substantially lower to considerably higher than inflow concentrations. Flow was also measured at two sites, where it was determined that volumetric reductions in runoff further improved pollutant removal. TSS and heavy metals will be reliably removed by a wide range of soil-based filter media, as will phopshorus, as long as the phosphorus content of the filter media is low. However, nitrogen removal remains a challenge because it is easily transformed to soluble forms and is influenced by wetting and drying. These results are essentially consistent with related laboratory studies.

Biofilters for Stormwater Harvesting: Understanding the Treatment Performance of Key Metals That Pose a Risk for Water Use

A large-scale stormwater biofilter column study was conducted to evaluate the impact of design configurations and operating conditions on metal removal for stormwater harvesting and protection of aquatic ecosystems. The following factors were tested over 8 months of operation: vegetation selection (plant species), filter media type, filter media depth, inflow volume (loading rate), and inflow pollutant concentrations. Operational time was also integrated to evaluate treatment performance over time. Vegetation and filter type were found to be significant factors for treatment of metals. A larger filter media depth resulted in increased outflow concentrations of iron, aluminum, chromium, zinc, and lead, likely due to leaching and mobilization of metals within the media. Treatment of all metals except aluminum and iron was generally satisfactory with respect to drinking water quality standards, while all metals met standards for irrigation. However, it was shown that biofilters could be optimized for removal of iron to meet the required drinking water standards. Biofilters were generally shown to be resilient to variations in operating conditions and demonstrated satisfactory removal of metals for stormwater-harvesting purposes.

Temporary storage or permanent removal? The division of nitrogen between biotic assimilation and denitrification in stormwater biofiltration systems

The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a ‘black-box’ approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer 15NO3 − (nitrate) over the course of one inflow event. The immediate partitioning of 15NO3 − between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO3 − under typical stormwater concentrations (∼1–2 mg N/L), contributing an average 89–99% of 15NO3 − processing in biofilter columns containing the most effective plant species, while only 0–3% was denitrified and 0–8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of 15NO3 −, and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems.

Processes and drivers of nitrogen removal in stormwater biofiltration

Biofiltration systems harness the treatment capabilities of plants, microorganisms, and soil to mitigate impacts of polluted stormwater. However, their effectiveness for nitrogen removal can vary, from concentration reductions exceeding 70% to net leaching. Performance is particularly sensitive to plant species selection, presence of a saturated zone with carbon source, and the frequency of inflows. The authors review controls on nitrogen cycling in natural and modified environments to identify important processes and influences within biofilters. Key factors include plant-microbial interactions, root architecture, plant strategy, and moisture heterogeneity. They note a critical lack of studies comparing nitrogen removal through denitrification and plant assimilation.

Retention of heavy metals by stormwater filtration systems: breakthrough analysis

Biofiltration systems are widely used to mitigate the impacts of stormwater on receiving waters, however their long-term capacity to retain heavy metals has not previously been assessed. Accelerated-dosing laboratory experiments were used to assess the likelihood of breakthrough occurring for three different types of soil-based filter media that are commonly used in stormwater biofilters. In all cases, breakthrough of zinc (Zn) was observed, but not of cadmium (Cd), copper (Cu) and lead (Pb). If biofiltration systems are sized so that they are large relative to their catchment (at least 2-3% of its area) or have a deep filter layer (at least 0.5 m deep), then breakthrough will not occur for at least ten years and probably longer. However, after the equivalent of 12-15 years of operation, Cd, Cu and Zn had accumulated in the filter media to levels that exceeded human health and/or ecological guidelines. Further, depending on the design, it is possible that spent filter media may be classified as contaminated soil and thus require special disposal.

Flow-regime management at the urban land-parcel scale: Test of feasibility

AbstractOvercoming the hydrologic shortcomings of conventional approaches to stormwater management requires the protection or restoration of flow regimes at small scales. A better understanding of how stormwater management strategies can achieve this aim is needed. This study modeled 28,800 design configurations of a typical stormwater management strategy at the scale of urban land parcels across a range of urban densities and climatic conditions. Realistic design configurations that achieved three hydrologic response targets were identified as part of this modeling. It was found that meeting the targets required a combination of stormwater harvesting (using tanks) and infiltration (using rain gardens). This was possible primarily because the amount of harvested impervious roof runoff made a large contribution to a hydrologic target, which measured the ability to restore volumetric losses. Management of flow regimes at small scales will require policy mechanisms that necessitate both stormwater harvesting...

Designing living walls for greywater treatment

Greywater is being increasingly used as an alternative water source to reduce potable water demand and to alleviate pressure on sewerage systems. This paper presents the development of a low energy and low maintenance greywater treatment technology: a living wall system, employing ornamental plants (including vines) grown in a sand filter on a side of a building to treat shower, bath, and washing basin wastewaters. The system can, at the same time, provide critical amenity and micro-climate benefits to our cities. A large scale column study was conducted in Melbourne, Australia, to investigate the following design and operational factors of the proposed system: plant species, saturated zone design, rest period, hydraulic loading rate and pollutant inflow concentration. The results indicate that the use of ornamental species (e.g. Canna lilies, Lonicera japonica, ornamental grape vine) can contribute to pollutant removal. Vegetation selection was found to be particularly important for nutrient removal. While a wider range of tested plant species was effective for nitrogen removal (>80%), phosphorus removal was more variable (-13% to 99%) over the study period, with only a few tested plants being effective - Carex appressa and Canna lilies were the best performers. It was also found that phosphorus removal can be compromised over the longer term as a result of leaching. Excellent suspended solids and organics removal efficiencies can be generally achieved in these systems (>80% for TSS and >90% for BOD) with plants having a relatively small impact. Columns had an acceptable infiltration capacity after one year of operation. When planted with effective species (e.g. Carex appressa and Canna lilies), it is expected that performance will not be significantly affected by longer rest periods and higher pollutant concentrations in the early years of system operation. The results of this study, thus, demonstrate that innovative and aesthetically pleasing living walls can be designed for treatment of greywater at the household scale.

Evaluation of sustainable electron donors for nitrate removal in different water media

An external electron donor is usually included in wastewater and groundwater treatment systems to enhance nitrate removal through denitrification. The choice of electron donor is critical for both satisfactory denitrification rates and sustainable long-term performance. Electron donors that are waste products are preferred to pure organic chemicals. Different electron donors have been used to treat different water types and little is known as to whether there are any electron donors that are suitable for multiple applications. Seven different carbon rich waste products, including liquid and solid electron donors, were studied in comparison to pure acetate. Batch-scale tests were used to measure their ability to reduce nitrate concentrations in a pure nutrient solution, light greywater, secondary-treated wastewater and tertiary-treated wastewater. The tested electron donors removed oxidised nitrogen (NOx) at varying rates, ranging from 48 mg N/L/d (acetate) to 0.3 mg N/L/d (hardwood). The concentrations of transient nitrite accumulation also varied across the electron donors. The different water types had an influence on NOx removal rates, the extent of which was dependent on the type of electron donor. Overall, the highest rates were recorded in light greywater, followed by the pure nutrient solution and the two partially treated wastewaters. Cotton wool and rice hulls were found to be promising electron donors with good NOx removal rates, lower leachable nutrients and had the least variation in performance across water types.

Co-optimisation of phosphorus and nitrogen removal in stormwater biofilters: the role of filter media, vegetation and saturated zone.

Biofilters have been shown to effectively treat stormwater and achieve nutrient load reduction targets. However, effluent concentrations of nitrogen and phosphorus typically exceed environmental targets for receiving water protection. This study investigates the role of filter media, vegetation and a saturated zone (SZ) in achieving co-optimised nitrogen and phosphorus removal in biofilters. Twenty biofilter columns were monitored over a 12-month period of dosing with semi-synthetic stormwater. The frequency of dosing was altered seasonally to examine the impact of hydrologic variability. Very good nutrient removal (90% total phosphorus, 89% total nitrogen) could be achieved by incorporating vegetation, an SZ and Skye sand, a naturally occurring iron-rich filter medium. This design maintained nutrient removal at or below water quality guideline concentrations throughout the experiment, demonstrating resilience to wetting-drying fluctuations. The results also highlighted the benefit of including an SZ to maintain treatment performance over extended dry periods. These findings represent progress towards designing biofilters which co-optimise nitrogen and phosphorus removal and comply with water quality guidelines.

Phosphorus Fate and Dynamics in Greywater Biofiltration Systems

Phosphorus, a critical environmental pollutant, is effectively removed from stormwater by biofiltration systems, mainly via sedimentation and straining. However, the fate of dissolved inflow phosphorus concentrations in these systems is unknown. Given the growing interest in using biofiltration systems to treat other polluted waters, for example greywater, such an understanding is imperative to optimize designs for successful long-term performance. A mass balance method and a radiotracer, 32P (as H3PO4), were used to investigate the partitioning of phosphorus (concentrations of 2.5-3.5 mg/L, >80% was in dissolved inorganic form) between the various biofilter components at the laboratory scale. Planted columns maintained a phosphorus removal efficiency of >95% over the 15-week study period. Plant storage was found to be the dominant phosphorus sink (64% on average). Approximately 60% of the phosphorus retained in the filter media was recovered in the top 0-6 cm. The 32P tracer results indicate that adsorption is the immediate primary fate of dissolved phosphorus in the system (up to 57% of input P). Plant assimilation occurs at other times, potentially liberating sorption sites for processing of subsequent incoming phosphorus. Plants with high nutrient uptake capacities and the ability to efficiently extract soil phosphorus, for example Carex appressa, are, thus, recommended for use in greywater biofilters.