Understanding functional efficiency of a sewer overflow screening device using combined CFD and analytical modeling

Recent developments of urban drainage are more concerned with water quality issues and aesthetic concern in receiving water bodies as sewer overflow can cause detrimental effects on the environment. The impacts of such uncontrolled sewer overflow include environmental, aesthetic, ecological and public health concerns. To overcome these problems different types of screening devices are used. Due to increasing public complaints, scientists and engineers are focusing on retention of the entrained sewer solids within the sewer overflow device. There are a number of different screening systems used in sewer overflow locations. Some of the common drawbacks in the available commercial devices include inadequate screening capacity, external power needs and high cost. Research has been undertaken to develop a new sewer overflow screening device to overcome these existing limitations. Design criteria for the conceptual device needs to have a self-cleansing capacity to work efficiency in unstaffed remote locations, robust start-stop operations, device fail outlet for extreme events, no moving parts, no sophisticated electrical-mechanical signal, low maintenance and establishment cost etc. Establishing an experimental setup involves significant cost and time; moreover it is important to maximize functional efficiency of the device as alteration of the device would be an expensive, troublesome and at times difficult to customize accordingly to existing urban drainage systems. State of art CFD modelling techniques can provide detail on the impacts of velocity, water level, shear stress, wave reflection etc. A self-cleansing sewer overflow screening device with a sewer overflow chamber, a rectangular tank and a slotted ogee weir to capture the gross pollutants was investigated. To design an efficient sewer overflow screening device a 3D fluid dynamics model (CFD) was used. To understand dynamic flow properties such as velocity, water levels, wave refection and shear stress, two different inlet orientations; parallel and perpendicular to the weir direction were tested. The results are compared using simplified analytical model based on well-established physical laws. Numerical results show that the flow is not uniform (across the width of the inclined surface) at the top of the inclined surface; however flow becomes uniform near the bottom. Uniform flow at the bottom of the inclined surface will help to remove pollutants adhered to the perforations. Due to varying water levels (high water level near the right side and low water near the left side), near the top of the weir surface, the self-cleansing property will not be as effective near the top region. The CFD simulated shear stress is less than the analytical model as it is unable to consider flow undulations. Analysis of the shear stress along the flow path was performed to identify efficient self-cleansing screeners. CFD simulation showed that the shear stress increases significantly at the bottom of the inclined surface of the sewer screener device, which suggests that, the location of screen should be towards the bottom. Discussion of the comparison of CFD and analytical results will help to design an effective and efficient sewer overflow screening device.