Modelling water reduction effects: Method and implications for horizontal drainage

Waste solids in building drainage systems may still be considered as discrete, subject to a system of forces including hydrostatic, mass, buoyancy and frictional components. Recent water conservation proposals, including legislation, have led to the reduction in the quantities of water available to transport waste into the sewer, and have had a major impact on the hydraulics associated with the transportation of discrete solids in near horizontal drains in buildings. Current design practice, based on statistical methods, leads to oversized systems that present serious implications for system performance as reduced flows from appliances increase the risk of blockages occurring. The need for predictive techniques to inform appropriate stakeholders has never been greater. The development of a numerical model (DRAINET) to predict single solid or non-interacting solid transport has informed regulatory policy on pipe sizing and maintenance strategies. Further refinements of this modelling technique to include the interaction of solids, particularly in the deposition region under water conservation criteria, confirms that the complex interactions in drainage systems cannot be assessed using 'rule of thumb or statistical approaches. Previous research in this area has shown that, in general, above-ground drainage systems benefit from a reduction in pipe diameter to improve drain self-cleansing. The methods outlined in this paper confirm this recommendation and it is postulated that the process of solid transport is enhanced by the interaction of multiple solids, particularly in the region of deposition.