Clogging of stormwater filters with high filtration rates
thesisposted on 23.02.2017, 01:05 by Kandra, Harpreet Singh
Hydraulic performance of granular filter media and its evolution over time is a key design parameter for stormwater filtration and infiltration systems that are now increasingly used in management of polluted urban runoff. Clogging of filter media is recognised as a limiting factor of these stormwater treatment systems. However, very limited studies have been undertaken to understand clogging processes in the context of stormwater treatment systems. Of particular interest are non-vegetated high-flow rate filtration systems, which have the potential of maintaining high rate treatment at the same time as providing consistent and high pollutant removal. This thesis therefore focuses on the clogging of stormwater filters with high infiltration rates. The impacts of both design and operational variables on clogging have been studied in controlled laboratory environments using a compressed timescale approach. Laboratory investigations have also been made to assess the importance of biological clogging for these stormwater filters. Finally, observations from a field and modelling study of a filtration system located in Melbourne were compared with the findings from the laboratory studies. It was found that while angularity and smoothness of filter media may not be important for design, the flow rate through the stormwater treatment system is a key design aspect that needs to be considered. The infiltration rate of a system should be guided by the objectives of the system - whether to treat more volume of stormwater or to achieve better treatment performance, longevity and maintenance. The size of the filter media particles significantly impacted the clogging process, as well as the overall sediment removal performance of the filters. Deeper systems were found to have longer lifespan compared to shallower ones, even though the deeper systems removed more sediment over their life span. Having two layers of distinct sized media in the filter bed improved performance over the single-layered systems. Results suggest that sediment concentration in stormwater and size of sediments stormwater are important parameters that affect the performance and eventually longevity of these treatment systems. While hydraulic loading rate was found to be a significant parameter affecting the performance of these systems, any variation in the stormwater composition and loading regime had a limited effect. This study therefore developed an understanding of the effect of catchment characteristics on design of filters and hence their longevity and maintenance needs. It was also found that filters with enhanced biological conditions clogged faster as compared to filters with suppressed biological activity. Although the evidence was not overpowering, the variations observed in this study suggest that more attention should be given to biological clogging in stormwater filters, which is mostly ignored at present. Data from a field system using granular filters was collected and observations were compared with findings from laboratory studies. Similarities in evolution of infiltration performance in field based systems were observed. An exponential relationship between decline of infiltration rate and cumulative volume of treated stormwater to predict the system’s hydraulic performance was developed. This research has provided both theoretical and practical insights which will be useful in the application of stormwater infiltration systems that use filters with high infiltration rates for both stormwater harvesting and protection of receiving waters from stormwater pollution.