The influence of flush volume and branch drain cross-section on deformable solid transport in attenuating flows

Abstract

The satisfactory removal of faecal and other waste by w.c. flushing and solid transport within a branch drain is a prerequisite of building drainage system design. The increasing importance of water conservation has led to renewed interest in the possibilities for reductions in overall building use through reduced w.c. flush volume operation. Current UK and international legislation has led to the 6 L flush being regarded as the upper acceptable limit, with consideration being given to further reductions to 4 L and below. Maintaining the operational integrity of the drainage system will require an understanding of both w.c. design and transport mechanisms to ensure solid clearance and the avoidance of solid deposition and the consequent disruption to the building user. It is necessary to regard the w.c. and the branch drain as a system. This paper analyses a major data set that includes a wide range of w.c. flush volumes as well as consideration of branch drain cross-sectional area and shape. Data for low flush volume operation with a parabolic cross-section branch drain is presented. Conclusions are drawn that indicate that low flush volume operation is feasible provided drain slope and cross-sectional area and shape are fully considered. The degree of slope enhancement required as flush volume is decreased is demonstrated.