



Application
of multitrial Monte Carlo design simulations to the design of WASTE STABILIZATION PONDS to produce a final effluent with a required maximum number of faecal coliform bacteria The design equations used for faecal coliform removal in WSP are: (a) Anaerobic ponds
(b) Facultative and maturation ponds
The equation for k_{B(20)} was derived by von Sperling from faecal coliform removal data obtained from 33 facultative and maturation pond systems in tropical and subtropical Brazil (latitude 7–24º S). The retention time in the facultative pond depends on the permissible surface BOD loading at the design temperature T ℃ and on the rate of net evaporation, but it should never be lower than a minimum of 4 days. The retention times in the maturation ponds must be less than that in the facultative pond, but never lower than a minimum of 3 days; additionally the surface BOD loading on the first maturation pond must not be greater than 75% of that on the facultative pond. Details are given in our online Pond Design Manuals. The whole design procedure is now done by Monte Carlo simulation as described by von Sperling (1996) for facultative ponds. The procedure is illustrated by Gawasiri (2003), who compared FC removals in WSP series estimated by this Monte Carlo method (allowing each design parameter to vary by ±20% – but, of course, any percentage could be chosen), with those predicted by the 'classical' Marais design procedure. Further details of both procedures are given by Mara in Domestic Wastewater Treatment in Developing Countries (Earthscan Publications Ltd, London, 2004). A detailed discussion of waste stabilisation pond design is given by Chimwemwe Gawasiri in his MSc thesis Modern Design of Waste Stabilization Ponds in Warm Climates: Comparison with Traditional Design Methods (This is a 1.8Mb PDF document containing 138 pages.) : Email either Professor Duncan Mara or, for questions about the Monte Carlo program, Dr Andrew Sleigh. 

