Traditionally waste stabilisation ponds (WSPs) are built as flow-through systems with an anaerobic, a facultative and one or more maturation ponds in series. The anaerobic pond can be substituted with great advantage by an upflow anaerobic sludge blanket (UASB) reactor. The practical advantages are that the required volume for the UASB reactor is much smaller than the anaerobic pond (20 to 30 times) and that the biogas in the UASB reactor is captured, so that odour problems can be eliminated. The UASB reactor is neighbour friendly and can be used in densely populated areas. The UASB reactor is more efficient than the anaerobic pond, so that the residual organic load is small and the subsequent pond configuration can be designed with the specific purpose of eliminating pathogens. By adopting a plug flow regime, the required area for a particular hygienic effluent quality can be greatly reduced. Also, since the UASB effluent has good transparency, photosynthesis in the ponds is intense and pH increases due to biological carbon dioxide consumption, accelerating the death rate of pathogens and opening the possibility of reducing nutrients: Nitrogen by desorption of gaseous NH3 and phosphorus by phosphate precipitation. The fact that an odour free system with a high quality effluent can be produced in relatively small system composed by a UASB reactor + maturation ponds, increases the applicability of this system for sewage treatment in urban areas. As operation of both UASB and ponds is very simple, several small treatment plants instead of one large central unit may be constructed. Thus a very significant of the sewerage network may be achieved. Copyright (C) 1996 IAWQ. Published by Elsevier Science Ltd.
