Pro-poor sewerage: solids modelling for design optimization.

More than 2.8 billion people still live without adequate sanitation globally. Increasing urbanisation, and the attendant increase in population density in many 'world cities' increases the urgency for appropriate low cost sanitation options. 'Simplified sewerage' offers the best possible solution, facilitating the removal of human waste away from habitable spaces in an efficient way, offering benefits way beyond the cost of installation and management. Simplified Sewerage systems design is based on fundamental fluid mechanics principles and has been well developed over the past 30 years. Application of small bore building drainage system models are wholly appropriate for simplified sewerage, however considerable modifications are required to account for the shallow gradients and the accumulation of solids due to the low water usage. The importance of local water flow depth on drain self cleansing where large accumulated solids are present has been identified and the solid movement
threshold has been quantified for a range of expected gross accumulated solids. This threshold, together with modifications to multiple solid interaction and deposition predictions have contributed to the aim of a robust model suitable for application to simplified sewerage systems in order to improve efficiency and optimize design.