Introduction to the USTM
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 the development of music, the various stormwater treatment measures by which contaminants are first intercepted and detained are described using a unified model. In the 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. Grass swales are viewed as simply ephemeral vegetated systems operating at a higher hydraulic loading than constructed wetlands. Constructed wetlands are simply shallow densely vegetated systems compared to ponds, which are characterised by deeper open water and fringing vegetation. 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.
Hydraulic loading, vegetation density and areal coverage, hydraulic efficiency and the characteristics of the target pollutants (eg. particle size distribution and contaminant speciation) are considered to largely influence the differences in performance between the various treatment processes modelled under storm operating conditions. It is acknowledged that other processes associated with dry weather flow conditions (ie. inter-event periods) may not be completely described by such a unified approach. This is the subject of ongoing research by the CRCCH.
Two basic modelling procedures are adopted in the unified model - hydrologic routing to simulate the movement of water through the treatment system, and a first order kinetic model to simulate the removal of pollutants within the treatment system.