Hydraulic characteristics and performance of stormwater pollutant trap respect to weir's height, flow gradients, pipe diameters and pollutant capture

Abstract

The main focus of urban stormwater runoff disposal has traditionally been to provide structurally-sound drainage systems to carry runoff from many different surfaces without considering water quality at outfall. This has contributed to the decline of water quality in rivers and lakes and other receiving bodies. According to Lord (1987), "stormwater management is primarily concerned with limiting future flood damages and environmental impacts due to development, where as flood control aims at reducing the extent of flooding that occurs under current conditions". Recent developments in stormwater pollutant trap (SPTs), which are generally end-of-the-line devices designed to capture and store gross pollutants, for subsequent removal and disposal.During the last few decades, use of SPTs as a source of collecting and removing pollutants from stormwater (which carries many different types of chemicals and nonchemical pollutants that contaminates our rivers, lakes and other receiving bodies) has increased considerably. Wide-ranging efforts and attempts have been made in both academic and industrial research to improve the quality of stormwater by improving the use of gross pollutant traps (GPTs – known as hydrodynamic separators) by utilising and improving available experimental and modelling techniques. The use of vortex phenomena has always been a challenging problem and available data is rare and complicated in the literature. This research focuses on detailed investigation by experimental means. The generated vortex in this experiment is created in a cylindrical chamber above the level of a cylindrical screening basket. In addition, the research analyses the processes involved in this separation technique.One scale model of a Versa Trap (Type A) was experimentally analysed to investigate and establish the relationship between headloss and flow rate and hydraulic characteristics of a weir in a diversion weir pit. The Versa trap Type A storm pollutant traps are usually used as off-line traps in city and urban areas to capture and store debris – especially those which are captured from surfaces such as rooftops, paved streets, highways, parking lots, lawns, and paved and gravelled roads (Allison et al., 1998). The Versa Trap Type A utilises an upstream diversion weir pit to divert the design treatment flow (DTF) into the treatment chamber. Treated flow returns to the diversion pit downstream of the weir, where it re-enters the drainage system. Peak flow in excess of the DTF bypasses the SPT over the weir into the pipeline downstream.It has been demonstrated that the aggregate of all flows of three months average recurrence interval (ARI) and less represented the majority (up to 97.5%) of the total flow generated by a stormwater drainage catchment (Works, 2006). There is some conjecture as to the veracity of the ‘first flush’ theory, which holds that most of the pollutants in the catchments are transported during the first flush of the storm event (Lee et al., 2007). However, it is generally accepted that SPTs should be sized so as to treat only a portion of the peak flow, with excess flows bypassing the trap. The three month ARI peak flow is commonly taken as appropriate for establishing the minimum DTF required of the SPT.The measurement of headloss across a scale model of a VT Type A storm pollutant trap at a range of flow rates through the SPT, provide data from which a mathematical relationship between flow rate and the headloss cab be established for the device.The resultant relationship then can be used in another part of the experiment to establish the hydraulic characteristics of a weir across a cylindrical chamber, as used for the upstream diversion weir pit in conjunction with the Type A VT range of SPTs. By varying the weir height in a scale model of a diversion weir pit and measuring the flow rates associated with headlosses determined from the previously established relationship, the relationship between weir height and diverted flow can be established. This allows the designer to specify the weir height required to divert the flow rate associated with a specific peak flow or treatment flow of SPT design.Two main characteristics which determine the performance of a gross pollutant trap are trapping efficiency and required maintenance. The trapping efficiency is defined as the portion of the total mass of gross pollutant transported by stormwater that is retained by the trap. A low trapping efficiency means that gross pollutants pass through the trap and reach downstream waters. A poorly-maintained trap will be inefficient at trapping pollutants and is also a potential source of pollutants as trapped materials break down.The experiment parts of this project were tested at Curtin University of Technology’s Hydraulic Laboratory. To replicate typical in-situ conditions, the VT Type A was tested for 0, 22, 33, 44, 55, 66 and 77% simulated blocked screen conditions for trapping efficiency. Data analysis has demonstrated that the headloss increases in proportion to flow rates and screen blockage condition. The results were scaled up to provide data on the full range of unit sizes. This research describes the testing and scaling methodologies in detail, with graphical representation of headloss and other hydraulic parameters at various conditions. The study’s findings have capabilities to optimise any other types of stormwater treatment systems. These types of traps’ are used in commercial and residential environment.This experiment is in continuation of the experiment which was conducted by Muhammad Ismail on industrial gross pollutant traps using double basket to trap the debris for industrial application.Also another good reference for pollutant build up and wash off modelling of impervious surfaces in Perth area, is done by Saadat Ashraf in his PhD thesis. For more information refer to references.