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

J. A. McDougall, R. H M Wakelin

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    8 Citations (Scopus)


    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. Practical application: Water conservation is already a major concern in terms of the provision of future accommodation, whether due to refurbishment or green field expansion of the housing stock. With the projected changes in demography indicating a 50% growth in the percentage of single occupation dwellings it is now essential to limit water usage through good appliance design and careful system specification. It is already accepted that w.c. water usage, historically running in excess of 30% of the average domestic consumption, is a prime candidate to lead the water conservation agenda. This paper illustrates the potential impact on waste solid transport of reductions in flush volume supported by reductions in drain cross-sectional area within the building envelope. The results presented confirm that much reduced flush volumes may be considered provided parallel consideration is given to the design of the system. As such the results presented are therefore relevant to both system designers as well as code definition bodies in the UK and internationally. © The Chartered Institution of Building Services Engineers 2007.

    Original languageEnglish
    Pages (from-to)7-22
    Number of pages16
    JournalBuilding Services Engineering Research and Technology
    Issue number1
    Publication statusPublished - 2007


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