The development of confined water sensitive urban design (WSUD) stormwater filtration/infiltration systems for Australian conditions

Publication Type:
Thesis
Issue Date:
2007
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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- This thesis describes a PhD project that investigated confined water sensitive urban design (WSUD) infiltration/filtration systems and their ability to reduce and retain pollutants present within stormwater runoff. In a confined system, primary, secondary and tertiary stormwater treatment takes place wholly within the system prior to the release of the stormwater into the surrounding soil matrix and to groundwater. Hence, the system effectively functions in isolation to that of the surrounding soil and vegetative landscapes. Systems can be physically and/or biochemically confined. The primary aim of the current study was to develop confined WSUD filtration/infiltration systems that are simple to maintain and suitable for a wide range of Australian conditions. Three field systems formed the basis of the investigations. The systems were designed such that stormwater passed through a series of pre-filtering devices and then entered a porous concrete pipe. The stormwater then exfiltrated through the permeable walls of the pipe into the surrounding substrate media material. The field systems were installed at Hindmarsh Park, Kiama, Mills Park Tennis Centre, .Asquith and the Weathertex industrial site, Heatherbrae. Significantly, the three systems were positioned at different geographic locations and faced different subsoil conditions, pollutant loadings and other constraints. Overall the treatment provided by each of the field systems resulted in the majority of the outlet pollutant levels investigated meeting the ANZECC and ARMCANZ (2000) irrigation and recreational water quality guidelines and the NHMRC and NRMMC (2004) drinking water guidelines. However, using the systems to produce water for drinking purposes is not recommended. Significant pollutant reductions were achieved through the physically confined Kiama system, where the stormwater residence time was the longest. For example, the average zinc concentration reduced by approximately 90%, the average level of faecal coliforms reduced by approximately 98%, the average level of suspended solids reduced by approximately 75% and the average concentration of total phosphorus reduced by approximately 60%. One of the key findings from the study was that the Asquith system had the ability to remove substantial amounts of dissolved copper and zinc from stormwater. This was potentially due to the inclusion of iron oxides in the porous concrete pipes within the Asquith system. Neither the Kiama nor the Weathertex pipes contained iron oxide. For all three field systems there was an increase in the average oxidised nitrogen concentration as stormwater passed through the system. The increases were potentially due to the transformation processes of nitrification and denitrification that are associated with nitrogen. The field investigations indicated that total dissolved solids are potentially released from the porous concrete pipe and pits. They may also be released from the iron oxides in the porous concrete pipe at the Asquith system. The concrete pipes had a neutralising effect on stormwater as the average outlet pH approached 7 for all three systems. The results from the groundwater monitoring program implemented at the Weathertex industrial site indicated that the porous pipe stormwater treatment system did not have an adverse impact on the groundwater at this site. Microbial activity was present within the media of each of the field systems and pollutants were therefore likely to be digested biologically. In addition, there was a build-up of both total phosphorus and zinc on the media. This indicated that the media was effectively removing phosphorus and zinc. The concentration of the influent stormwater, the rainfall depth and the stormwater residence time within the system all influenced the level of microbial activity and pollutant build-up on the media. In addition, there was a build-up of sediment within the pits and on the invert of the porous concrete pipe of each system due to sedimentation and filtration processes. Pollutants were present within the captured sediment and therefore adsorption processes were also likely to have occurred, at least with the finer sediment fractions. There appeared to be a strong correlation between the size of the sediment particles and the location where they were captured. The larger sized particles tended to be collected in the more upstream positioned pits and pipes. In addition, larger sized particles tended to have less associated pollutants than smaller sized particles. Laboratory experiments were undertaken in conjunction with the field investigations to provide a more detailed study into the pollutant removal processes. These included adsorption equilibrium, kinetics and fixed bed column experiments. The influence of the residence time of stormwater within the system; the quantity of media present; and the initial pollutant concentration of stormwater on pollutant removal were investigated. In addition, the influence of wetting and drying on pollutant removal and microbial activity were studied. Mathematical relationships were fitted to these experimental results. The adsorption equilibrium and kinetics experiments were undertaken with the primary purpose of determining an optimum granulated activated carbon (GAC) / sand ratio to be implemented in the Weathertex system. A GAC to sand ratio of approximately 1:25 was chosen based on a cost benefit analysis. The dissolved organic carbon (DOC) concentration of stormwater was expected to be reduced by approximately 91% after it passed through the GAC/sand media section. The Freundlich isotherm equation and the linear driving force approximation (LDFA) model reasonably described the adsorption equilibrium and kinetics experimental data. The predictions based on these experiments, however, were not borne out in the field. The field investigations indicated that the GAC/sand section of the Weathertex system did not perform as well as anticipated. To better understand the factors influencing the performance of the system, fixed bed column experiments were undertaken and these indicated that this was potentially due to the stormwater residence time of the Weathertex system being too short to allow for the GAC to effectively remove organics. The fixed bed column laboratory investigations also showed that increased depths of GAC resulted in greater reductions in the DOC concentration. The mass balance equations provided a reasonable prediction of the fixed bed adsorption data. The wetting and drying fixed bed investigations indicated that the intermittent application of synthetic stormwater and the associated drying times resulted in a greater reduction in the DOC concentration with time. When the synthetic stormwater solution was applied intermittently there was a significant level of microbial activity present, but this was still less than when the synthetic stormwater was applied continuously. This is potentially due to the dislodgement of adsorbed organics and the dislodgement of microbes due to the forces associated with the intermittently applied stormwater and the consumption of adsorbed organics by microbes during dry periods. The mathematical model, which incorporated both adsorption and biodegradation processes, was able to reasonably predict the organics removal efficiency of the fixed bed system when the synthetic stormwater solution was supplied continuously. The main limitations associated with the study were that: 1. The outcomes of the study relate to the three systems investigated and the catchment characteristics associated with these three systems; 2. the field investigations indicated that the GAC/sand media section of the Weathertex system did not perform as well as anticipated and this was potentially due to the limited stormwater residence time at this site; 3. the presence of microbial activity within the systems was measured using heterotrophic plate count (HPC) testing and was not able to identify the type of microbes present; and 4. the laboratory investigations focused on the use of GAC (to reduce DOC) and not on other engineered soils. Overall, the study demonstrated that the complexity of the field system performance depended on an interaction of several variables. This was not best represented by a single model that attempts to take in to consideration all factors that influence the performance of the system. Instead, the treatment performance of the three field systems was compared using linear regression analysis. These linear regression relationships can now be used by designers to estimate the likely range of treatment performance that can be expected from similarly designed confined WSUD systems. In addition, general design recommendations based on the field and laboratory findings have been presented. To achieve optimal system performance these recommendations should be considered when designing a system. The key findings of the current study were as follows. Confined WSUD systems can be designed to bring the quality of stormwater runoff back to acceptable ANZECC and ARMCANZ (2000) standards. Such systems should be designed according to the concentration of the influent stormwater and this concentration is dependent primarily on the land use of the drainage catchment. The inclusion of iron oxides in the porous concrete pipe used in the confined WSUD systems is able to enhance dissolved heavy metals removal from stormwater. Also, the longer the residence time of stormwater within the system the greater the pollutant reduction.
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