Assessment of the Impact of Stormwater Characteristics on Clogging in Stormwater Filters

Hydraulic conductivity of granular filter media and its evolution over time is a key design parameter for stormwater filtration and infiltration systems that are now widely used in management of polluted urban runoff. In fact, clogging of filter media is recognised as the main limiting factor of these stormwater treatment systems. This paper focuses on the effect of stormwater characteristics on the clogging of stormwater filters. Effect of five different operational regimes has been tested in this study of sediment concentration; pollutant concentrations; stormwater sediment size; loading rate and stormwater loading/dosing regime and compared with the Base case. For each operational condition, five column replicates were tested. Results suggest that sediment concentration in stormwater is a significant parameter affecting hydraulic and treatment performance, eventually affecting longevity of these stormwater treatment systems. Further, the size of sediments (and their relation to the size of filter media grains) in stormwater was found to be an important parameter to be considered in design of coarse filters with high infiltration rates that are used for stormwater treatment. As expected, the addition of metals and nutrients had limited or no contribution to changes in hydraulic or sediment removal performance of the studied stormwater filters. Whilst loading rate was found to be an important parameter affecting the hydraulic and treatment performance of these systems, any variation in the stormwater loading regime had a limited effect on their performance. This study therefore develops an understanding of the effect of catchment characteristics on design of filters and hence their longevity and maintenance needs.     

Assessment of Impact of Filter Design Variables on Clogging in Stormwater Filters

Stormwater filters are widely used in stormwater management, sometimes as standalone structures (e.g. stormwater filter beds), or as part of porous pavements, soak ways, infiltration basins and trenches. Due to the high levels of sediment present in stormwater, clogging is the main operational issue for these systems. A laboratory-based study was conducted to investigate the effect of filter bed design variables on the clogging phenomenon in non-vegetated stormwater filters with high infiltration rates. Design parameters studied include: filter media particle sizes (0.5 mm, 2 mm, 5 mm); depth of the filter bed (100 mm, 300 mm and 500 mm); and filter media packing configurations (layered or mixed). The size of filter media particles significantly impact the clogging process, as well as the overall sediment removal performance of the filters; filters with smaller particles had better sediment removal efficiency, but subsequently shorter lifespan. Deeper systems had longer lifespan compared with shallower ones, notwithstanding deeper systems removed more sediment over their life span. Having two layers of distinct sized media in the filter bed improved performance (e.g. volume of water treated; sediment removed) over the single-layered systems. However, the three-layered systems behaved similarly to two-layered systems. Mixed systems also showed improved performance, as compared with single-layered systems, and were similar to the three-layered systems. This study therefore suggests that simple modifications to a stormwater filtration system can help improve sediment removal performance and/or reduce maintenance intervals significantly, while only slightly affecting sediment removal performance.     

Removal of dissolved nitrogen, phosphorus and carbon from stormwater by biofiltration mesocosms.

Biofiltration systems are becoming a popular stormwater treatment device in water sensitive urban design for the removal of fine particulate and dissolved pollutants from stormwater. However, there is limited published data on the effectiveness of these systems for nutrient removal. We constructed biofiltration mesocosms to assess nutrient removal (nitrogen, phosphorus and carbon) under experimental conditions. Different types of media were compared (gravel, sand, and sandy-loam) in vegetated and non-vegetated mesocosms (six treatments in total). Five plant species were used. Vegetated sand and vegetated sandy-loam provided the best overall treatment. Vegetated mesocosms were very effective in removing nitrogen (63-77% removal) and phosphorus (85-94% removal) from synthetic stormwater, and removed substantially more nutrients than the non-vegetated treatments. All treatments removed a substantial portion of the carbon from the stormwater (28-66%). When flushed with tap water, nitrogen and phosphorus were retained by the vegetated mesocosms, but leached from the non-vegetated mesocosms. Plant growth was most vigorous in the sandy-loam media, indicating that this is a good growth media, even without the addition of organic matter.     

Stormwater reuse: designing biofiltration systems for reliable treatment.

Stormwater reuse is increasing in popularity as a technique for overcoming water shortages in urban Australia. However, technology for the reliable treatment of stormwater for reuse is still not fully developed. This paper presents the first steps in refining biofilters for stormwater reuse. Six different filter media were selected, to target specific stormwater pollutants, as well as support plant growth. They were tested in the laboratory, where the filters were dosed three times per week with semi-synthetic stormwater for five weeks. Pollutant removal performance was monitored, and revealed that all soil-based filters performed similarly (while sand filters behaved somewhat differently). All filters removed more than 80% of solids and greater than 90% of lead, copper, and zinc. Three filter types were able to remove some phosphorus (particularly in the top 30 cm of the media). Apart from sand, all filter media were net producers of nitrogen, leading to an important conclusion that non-vegetated, soil-based filters are not suitable for targeting nutrients. However, since heavy metals are the primary pollutant of concern with respect to stormwater reuse for irrigation (the most popular end-use), it was concluded that biofilters may be promising technologies for treatment of stormwater for reuse.     

A pollutant removal prediction tool for stormwater derived diffuse pollution.

This report describes the development of a methodology to theoretically assess the effectiveness of structural BMPs with regard to their treatment of selected stormwater pollutants (metals, polyaromatic hydrocarbons and herbicides). The result is a prioritisation, in terms of pollutant removal efficiency, of 15 different BMPs which can inform stormwater managers and other stakeholders of the best available options for the treatment of urban runoff pollutants of particular environmental concern. Regardless of the selected pollutant, infiltration basins and sub-surface flow constructed wetlands are predicted to perform most efficiently with lagoons, porous asphalt and sedimentation tanks being the least effective systems for the removal of pollutants. The limitations of the approach in terms of the variabilities in BMP designs and applications are considered.     

Preliminary Estimate of Detention Basin Efficiency at Watershed Scale

Urban encroachment in natural floodplain areas and infrastructures interfering with watercourses have caused higher flood risks in lowland areas. In this context, detention basins have become a fundamental instrument for stormwater and environmental management at watershed scale. Numerical methods of flood routing are generally coupled with optimization algorithms to investigate the factors that affect the overall efficiency of detention basins in controlling the peak flows throughout a watershed. To overcome the procedure effort due to numerical integration, a simple innovative approach, based on the linear system theory applied to the solution of hydrologic flood routing, is proposed for a preliminary estimate of overall efficiency. First a numerical analysis is performed to ensure that the schematization of the detention basin as a linear system leads to technically acceptable approximation. Then, a simple analytical equation is provided that allows a preliminary estimate of detention basin efficiency in downstream river reaches. Sensitivity analysis of the above equation provides information about the factors that most contribute to the downstream flow reduction variability. Finally, the proposed methodology, adequately extended to a parallel system of stormwater detention basins within a watershed, can be easily integrated in optimization algorithms.     

Microbial challenge-testing of treatment processes for quantifying stormwater recycling risks and management.

Pathogenic microorganisms have been identified as the main human health risks associated with the reuse of treated urban stormwater (runoff from paved and unpaved urban areas). As part of the Smart Water initiative (Victorian Government, Australia), a collaborative evaluation of three existing integrated stormwater recycling systems, and the risks involved in non-potable reuse of treated urban stormwater is being undertaken. Three stormwater recycling systems were selected at urban locations to provide a range of barriers including biofiltration, storage tanks, UV disinfection, a constructed wetland, and retention ponds. Recycled water from each of the systems is used for open space irrigation. In order to adequately undertake exposure assessments, it was necessary to quantify the efficacy of key barriers in each exposure pathway. Given that none of the selected treatment systems had previously been evaluated for their treatment efficiency, experimental work was carried out comprising dry and wet weather monitoring of each system (for a period of 12 months), as well as challenging the barriers with model microbes (for viruses, bacteria and parasitic protozoa) to provide input data for use in Quantitative Microbial Risk Assessment.     

Biofiltration as pre-treatment to water harvesting and recycling

This paper presents the results of the long term biofilter experiments conducted with raw stormwater collected from a canal at Carlton, in Sydney. Anthracite and granular activated carbon (GAC) were used as a single filter media in biofilter columns. Media heights of 75 and 40 cm were used. The filter columns were operated at filtration velocities of 0.12 and 0.25 m/h. The removal efficiency for turbidity and DOC for the GAC filter media were found to be 75% and almost 100% respectively. The removal efficiency for the anthracite filter was much lower. Molecular weight distribution analysis showed an almost similar trend to the DOC removal. Compared with anthracite filter media, the GAC biofilter removed a much larger range of organic compounds present in the stormwater. The GAC biofilter removes organic matter earlier as compared to anthracite. Based on a limited sample of stormwater, the removal efficiency for phosphorus was upto 74% and that of nitrogen was up to 30%. In general GAC filter shows higher heavy metal removal efficiency than anthracite. The removal of zinc, iron, lead and nickel were good. However the concentration of heavy metal in the raw surface water sample was low.     

A high filtration system with synthetic permeable media for wastewater reclamation.

A novel filtration process with synthetic permeable media was investigated for secondary effluent reclamation. Polyurethane was chosen as the filter medium among three tested media. Compressibility and up-flow velocity were changed to determine the optimum operation for the system. An equation was introduced to express the relationship between the removal efficiency and up-flow velocity. In a pilot study, the synthetic medium filtration with compression showed very stable effluent quality without clogging trouble, though the system operated with three times higher filtration rate and much longer backwashing interval than conventional systems.     

Stormwater pollutant loads modelling: epistemological aspects and case studies on the influence of field data sets on calibration and verification.

In urban drainage, stormwater quality models have been used by researchers and practitioners for more than 15 years. Most of them were initially developed for research purposes, and have been later on implemented in commercial software packages devoted to operational needs. This paper presents some epistemological problems and difficulties with practical consequences in the application of stormwater quality models, such as simplified representation of reality, scaling-up, over-parameterisation, transition from calibration to verification and prediction, etc. Two case studies (one to estimate pollutant loads at the outlet of a catchment, one to design a detention tank to reach a given pollutant interception efficiency), with simple and detailed stormwater quality models, illustrate some of the above problems. It is hard to find, if not impossible, an "optimum" or "best" unique set of parameters values. Model calibration and verification appear to dramatically depend on the data sets used for their calibration and verification. Compared to current practice, collecting more and reliable data is absolutely necessary.     

Hydraulic and treatment performance of pervious pavements under variable drying and wetting regimes

Pervious pavements are an effective stormwater treatment technology. However, their performance under variable drying and wetting conditions have yet to be tested, particularly under a continuous time scale. This paper reports on the clogging behaviour and pollutant removal efficiency of three pervious pavement types over 26 accelerated years. These pavements were monolithic porous asphalt (PA), Permapave (PP) and modular Hydrapave (HP). Over a cycle of 13 days, the period of which was equivalent to the average annual Brisbane, Australia rainfall (1,200 mm), the pavements were randomly dosed with four different flows. Drying events of 3 h duration were simulated during each flow. Inflow and outflow samples were collected and analysed for Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN). To evaluate the rate of clogging, a 1 in 5 year Brisbane storm event was simulated in the 6th, 8th, 12th, 16th, 20th and 24th week. Under normal dosing conditions, none of the pavements showed signs of clogging even after 15 years. However, under storm conditions, both PA and HP started to clog after 12 years, while PP showed no signs of clogging after 26 years. The drying and various flow events showed no effects in TSS removal, with all systems achieving a removal of approximately 100%. The average TP removal was 20% for all flows except for low flow, which had a significant amount of leaching over time. Leaching from TN was also observed during all flows except high flow. The TSS, TP and TN results observed during storm events were similar to that of high flow.     

Elimination and accumulation of polycyclic aromatic hydrocarbons in urban stormwater wet detention ponds

The concentrations of polycyclic aromatic hydrocarbons (PAHS) in water and sediments of seven wet detention ponds receiving urban stormwater were investigated. The ponds comprised traditional wet detention ponds with a permanent wet volume and a storage volume as well as ponds that were expanded with sand filters and other means to improve the removal of micropollutants. The concentrations of sigmaPAH in the sediments varied between 6 +/- 5 and 2,222 +/- 603 ng g(-1) dry weight (mean +/- standard deviation), and were highest in the ponds with lower pond volume per catchment area and did not clearly reflect different activities in the catchments. In general, the concentrations of PAHS in the sediments decreased from inlet to outlet, especially in the systems with good conditions for sedimentation such as systems with flow perpendicular sand dikes and extensive submerged vegetation. High molecular weight PAHs were predominant in the sediments indicating the pyrogenic origin of the PAHS. There was no correlation between PAH species concentrations in water or sediments and their hydrophobicity (log K(ow)). PAH concentrations in water fluctuated in response to intensity and frequency of rain events, whereas concentrations in the sediments integrated the pollutant load over time. Pond systems expanded with sand filters and other technologies to enhance removal of micropollutants consistently had concentrations of PAHS in the effluents below the detection level.     

Mechanical resistance properties of gravel used in subsurface flow constructed wetlands: implications for clogging

Gravel constitutes the filter medium in subsurface flow constructed wetlands (SSF CWs) and its porosity and hydraulic conductivity decrease over time (clogging), limiting the lifespan of the systems. Using gravel of poor quality accelerates clogging in wetlands. In this study, gravel samples from six different wetland systems were compared with regards to their mineral composition and mechanical resistance properties. Results showed that both mineralogy and texture are related to mechanical resistance. Accordingly, gravel with high content of quartz (> 80%) showed a lower percentage of broken particles (0.18-1.03%) than those with lower content of quartz (2.42-4.56% media broken). Although granite is formed by high durability minerals, its non-uniform texture results in a lower resistance to abrasion (ca. 10% less resistance than calcareous gravel). Therefore, it is recommended to use gravels composed mainly of quartz or, when it is not available, limestone gravels (rounded and uniform) are recommended instead. The resistance to abrasion (LAA test) seems to be a good indicator to determine the mechanical properties of gravels used in CWs. It is recommended to use gravels with LAA below 30% in order to avoid a rapid clogging due to gravel crumbling and subsequent mineral solids accumulation.     

Performance of enviss? stormwater filters: results of a laboratory trial

An experimental study was undertaken by Monash University to develop and test enviss? stormwater treatment and harvesting technologies - non-vegetated filtration systems with an extremely low footprint. This paper focuses on the water quality and hydraulic performance of two systems tested over a 'year' of operation in a Melbourne climate: (1) REUSE enviss? filters, designed for stormwater harvesting systems for non-potable supply substitution, and (2) WSUD enviss? filters, developed to treat urban stormwater prior to discharge to downstream systems. The presence of chlorine as a disinfection agent proved to be very efficient for the removal of microorganisms in REUSE enviss? filters. WSUD enviss? filters had the benefit of providing an elevated nutrient treatment performance, due to an extended depth of filter media. However, nutrient outflow concentrations (total nitrogen (TN) in particular) were found to increase during the testing period. Also, extended dry weather periods were found to have a detrimental effect on the treatment performance of almost all pollutants for both filters (nutrients, Escherichia coli and heavy metals). Although hydraulic conductivity results indicated two or three sediment trap replacements per year are required to maintain filtration rates, it is expected that the compressed loading rate schedule overestimated this maintenance frequency.     

Enhancing the removal of arsenic, boron and heavy metals in subsurface flow constructed wetlands using different supporting media

The presence of arsenic and heavy metals in drinking water sources poses a serious health risk due to chronic toxicological effects. Constructed wetlands have the potential to remove arsenic and heavy metals, but little is known about pollutant removal efficiency and reliability of wetlands for this task. This lab-scale study investigated the use of vertical subsurface flow constructed wetlands for removing arsenic, boron, copper, zinc, iron and manganese from synthetic wastewater. Gravel, limestone, zeolite and cocopeat were employed as wetland media. Conventional gravel media only showed limited capability in removing arsenic, iron, copper and zinc; and it showed virtually no capability in removing manganese and boron. In contrast, alternative wetland media: cocopeat, zeolite and limestone, demonstrated significant efficiencies--in terms of percentage removal and mass rate per m3 of wetland volume--for removing arsenic, iron, manganese, copper and zinc; their ability to remove boron, in terms of mass removal rate, was also higher than that of the gravel media. The overall results demonstrated the potential of using vertical flow wetlands to remove arsenic and metals from contaminated water, having cocopeat, zeolite or limestone as supporting media.     

Placement Strategies and Cumulative Effects of Wet-weather Control Practices for Intermunicipal Sewerage Systems

This study investigates the rainfall-runoff process and the pollutant dynamics on theoretical catchments and intermunicipal sewer systems in order to characterize the hydraulic and environmental performance of possible types of sewerage system and to compare the effectiveness of different wet-weather control schemes for intermunicipal sewerage systems. A comprehensive investigation on the placement strategies and the cumulative effects of wet-weather control practices is carried out over a broad watershed as a key preliminary step in addressing surface water safeguard requirements in developing and urban areas. The analysis shows that the different types of intermunicipal sewer system exhibit slightly different performance. Placing wet-weather detention tanks at different spatial levels affects the overall effectiveness of the wet-weather control system. In the examined case study schemes, the insertion of a wet-weather detention tank is better on final than on local flow regulators. The performance control also proves to be easier adopting the solution with intermunicipal tank since, for this scheme, the design criteria have a more important influence than rainfall characteristics on the environmental performance. These results represent an issue of primary focus for the implementation of environmental policies and mitigation strategies against surface water impairment in urban and developing areas.     

O3-BAC工艺对受污染河水中主要污染物去除效果分析

对预臭氧曝气生物活性炭滤池(O3-BAC)工艺处理低碳源北运河通州段原水试验效果进行分析,结果表明:回流比为1∶1时,系统对CODC r、NH3-N的平均去除率分别为42%、94.3%,均高于回流比0.5∶1时;向系统反硝化单元按C/N=3∶1投加外碳源后,TN去除率增长至34.5%,而此时系统对NH3-N和CODC r的去除率变化不大;系统气水比由6∶1降低至4∶1后,TN去除率继续增长至42.5%,CODC r和NH3-N去除率变化幅度不大,此时出水主要污染物指标均达到《城市污水再生利用景观环境用水水质》(GB/T18921-2002)观赏性河道类水质标准。     

High-rate stormwater clarification with polymeric flocculant addition.

Treatment of urban stormwater by clarification, with flocculant addition, was studied in Toronto, Canada using a pilot-scale clarifier with removable lamellar plates. Almost 90 stormwater runoff events were characterised at the study site and found fairly polluted. The previous research phase indicated good treatability of this stormwater by lamellar clarification with flocculant addition (total suspended solids, TSS, removal of 84%, at a surface load of 15 m/h), but there were concerns about cleaning plates after storm events. With the aid of numerical modelling, hydraulic improvements to the clarifier inlet zone were retrofitted in 2004 and permitted the removal of the lamellar pack without a loss in treatment efficiency. In the modified clarifier, a cationic polymeric flocculant dosage of 4 mg/L with conventional clarification provided a TSS removal of 77%, at surface loads up to 43 m/h. The use of the polymer did not increase the acute toxicity of the treated effluent. The clarifier sludge was severely polluted by several heavy metals and would require special disposal. The treatment process tested could be well applied in projects requiring intensive stormwater treatment at compact sites.     

Combined Effects of Parallel and Series Detention Basins for Flood Peak Reduction

A simple analytical method is proposed that allows a preliminary evaluation of the overall efficiency of a detention basin system for flood risk reduction in a specific target section. Solutions are provided both for parallel and series systems, under some simplifying assumptions concerning the linearity of detention basin, river network and watershed responses. Further, for the series configuration a regressive model is proposed for the computation of the overall efficiency, because of the complexity of analytical solution. A case study is also presented, where the overall efficiency of a system of nine detention basins is computed by means of both the analytical and the regressive model. Results are discussed to assess the different influence of detention basins in parallel or in a series system.     

Monitor-based evaluation of pollutant load from urban stormwater runoff in Beijing.

As a major pollutant source to urban receiving waters, the non-point source pollution from urban runoff needs to be well studied and effectively controlled. Based on monitoring data from urban runoff pollutant sources, this article describes a systematic estimation of total pollutant loads from the urban areas of Beijing. A numerical model was developed to quantify main pollutant loads of urban runoff in Beijing. A sub-procedure is involved in this method, in which the flush process influences both the quantity and quality of stormwater runoff. A statistics-based method was applied in computing the annual pollutant load as an output of the runoff. The proportions of pollutant from point-source and non-point sources were compared. This provides a scientific basis for proper environmental input assessment of urban stormwater pollution to receiving waters, improvement of infrastructure performance, implementation of urban stormwater management, and utilization of stormwater.