Waste stabilisation ponds in France: state of the art and recent trends.

Waste stabilisation ponds represent 20% of the total number of wastewater treatment plants in France. Practical expertise acquired during these last 20 years has led to modification in the design of the first facultative basin of WSP systems. Its active surface area is now dimensioned at 6 m2(p.e.)(-1) in order to limit the risk of malfunctioning. The cumulated surface of the 2nd and 3rd basin is maintained at 5 m2(p.e.)(-1). Another practical point is also that WSPs must receive mainly diluted influents. Globally, the plants are on average far from their nominal loadings, which explains why the first sludge removals took place on average 13 years after being put in operation. Based on a representative sample of plants, i.e. 1 5% of the French WSPs, it has been possible to estimate the time, material means and cost needed for sludge removal as well as the amount of sludge accumulated. The sludge removed at the 1st yields on average 110 L (p.e.)(-1) which represents 12 kg DM (p.e.)(-1). The current trend of increasing the quality levels necessary for discharge into sensitive receiving bodies has led to adaptive solutions of polishing treatments by intermittent sand filter systems with or without the plantation of reeds.