A Unified Approach to Modelling Urban Stormwater Treatment

The mechanisms promoted in the removal of stormwater pollutants encompass physical, chemical and biological processes. Owing to the intermittent nature of stormwater inflow, physical processes associated with detention for sedimentation and filtration (either through vegetated systems or through an infiltration medium) are the principal mechanisms by which stormwater contaminants are first intercepted. Subsequent chemical and biological processes can influence the transformation of these contaminants. In this paper, it is asserted by the authors that the various stormwater treatment measures by which contaminants are first intercepted and detained can be described using a unified model. Grass swales, wetlands, ponds and infiltration systems are considered to be a single continuum of treatment based around flow attenuation and detention, and particle sedimentation and filtration. Hydraulic loading, vegetation density and areal coverage, hydraulic efficiency and the characteristics of the target pollutants (eg. particle size distribution and contaminant speciation) largely influence their differences in performance. In this context, infiltration systems are simply vertical filtration systems compared to the horizontal filtration systems of grass swales and wetlands, reliant on enhanced sedimentation and surface adhesion (promoted by biofilm growth) for removal of fine particles. The validity of this unified conceptual approach to simulating the operation of stormwater treatment measures is demonstrated by empirical analysis of observed water quality (predominantly TSS) improvements in swales, wetlands, ponds and infiltration basins and also by fitting observed water quality data from these treatment systems to a unified stormwater treatment model (USTM) developed by the authors. The USTM provides an efficient mechanism by which urban catchment and waterway managers can predict and assess the performance of stormwater treatment measures.