Application of a system approach and optimisation of different alternatives in the practice of decentralised wastewater reuse

Wastewater reuse systems are discussed, considering both subsystems - the wastewater treatment and conveyance facility, and the effluent reuse structure. Emphasis is given to solutions typical for decentralised wastewater systems at two scales - single house level and at the level of a townhouse complex. A practical case of on-site grey water treatment and reuse has been studied with respect to four reuse alternatives: landscape irrigation (surface and subsurface), toilet flushing (TF) and subsurface constructed wetland, with corresponding different treatment and conveyance facilities. Design and cost estimation of the different alternatives has been performed, based on the current conditions and costs in Gaborone, Botswana. An objective function (model) has been developed, minimising the building cost of the different alternatives at the two scales, and used to find the most cost-effective solution. The optimisation procedure has been executed by the standard evolutionary method in the Premium Solver tool of Excel 2003. Results show that, for cases of limited irrigation area, the wetland is the most cost-effective alternative, but if larger areas are available, the subsurface lawn irrigation should be the preferred choice. The TF alternative becomes cost effective at larger population densities.