Natural lagooning in France: state of the art and recent trends

Abstract

Waste stabilisation ponds (WSP) 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 design of the first facultative basin of WSP systems. Its active surface area is now dimensioned at 6m2(p.e.)-1 in order to limit the risk of malfunctioning. The cumulated surface of the 2nd and 3rd basin is maintained at 5m2.(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 loads, which explains that the first sludge removals took place on average 13 years after being put in operation. Based on a representative sample of plants, i.e. 15% 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 removal yields on average 110L.(p.e.)-1 which represents 12 kg DM.(p.e.)-1. The current trend of increasing the levels of quality 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.