Choice of the sand for sand filters used for secondary treatment of wastewater

In a range from 100 to about 1,000 People Equivalent (PE), Secondary Wastew ater Sand Filters (SWSF) are used by a lot of rural communities in France. A series of case studies however point out that several criteria concerning global and detailed design and implementation of these systems have to be approached scientifically, in order to obtain the expected results on a lon g-term basis. The choice of the sand constituting the infiltration bed, core of the biolo gical reactor, is of course one of the key elements and is the main subject of this article. It must have a sufficient initial permeability in order t o ensure an adapted infiltration speed, after colonisation by the purifying biomass. The d10 fines content and degree of uniformity mainly control thi s permeability. The quarry or the aggregate extractor, who masters his prod uction this way, usually gives these elements, based on granulometric analy sis. However the adjustment of an infiltration test with clear water is ess ential to check on site the conformity of the deliveries.