An Innovative Design for a Separate Sewer System

Abstract

The Environmental Protection Agency (EPA) has stated that what was considered state-of-the-art pollution control a century ago is no longer valid today. Designers of new sewer systems must identify the problems inherited from old design concepts and try to use new technology to revisit and upgrade traditional urban drainage management. Separate sewer systems are currently used in all new developments. They are more prevalent than combined sewer systems, the use of which is limited due to numerous environmental regulations. However, the narrow streets commonly found in the UK, Europe and other densely populated areas, are usually occupied by a complex network of infrastructure services, making providing space to place a traditional separate sewer system is therefore challenging. This research presents an innovative design for manholes, created to overcome the challenges associated with the installation of separate sewer systems in narrow streets. The proposed manhole combines two traditional manholes into one structure with two separate chambers, allowing storm flow and foul flow to pass through the same manhole without mixing. The structural performance and hydraulic properties of the new design have been tested using mathematical modelling, finite element (FE) and computational fluid dynamic modelling (CFD), validated by experimental testing. The results have been compared with the performance of conventional manholes. Testing of the new manhole when buried in soil revealed high stability and resistance under applied traffic loads. With regards to the hydraulic performance of the new manhole, the head loss coefficient and pattern of shockwaves were studied for both manholes (new and conventional), under the same conditions, using independent, dimensionless parameters for each manhole. The new manhole generates higher head losses, about twice the head loss generated in a conventional manhole. Four shockwaves were identified in the storm chamber of the new manhole, the locations and characteristics of these also determined. The new manhole required a new configuration to setup the two pipes in one trench, the storm pipe over the sanitary pipe. The behaviours of these two flexible pipes were tested using a 3D finite element (FE) model, validated against experimental data from a laboratory investigation. A modified Drucker–Prager soil constitutive model was used to simulate elasto-plastic soil behaviour. The results show that this approach, using a large-diameter flexible pipe set above a small-diameter flexible pipe, mitigates the strain on the smaller pipe and decreases the total deflection of both pipes and the soil. These results led to further development of the Iowa Formula so that is can calculate the deflections of two flexible pipes set in one trench. The Improved Iowa formula was tested and compared with the experimental results. The use of the new system promises to reduce construction costs, footprint and construction time. The storage capacity and retention time increased by 280% and 200%, respectively. This new system is an attempt to improve the concept of the design of traditional sewer systems which have been in use for approximately one hundred years. The new system design can be used to install a new separate sewer system or used to separate existing combined sewer systems.