Waste stabilisation pond treatment was widely developed during the 1980s in France, where there are now over 3,000 plants. Desludging the ponds has now become essential. In 19 primary facultative ponds, in operation for 12–24 years, the net average sludge accumulation rate was 19 mm/yr. The average per capita accumulation rates ranged from 0.04–0.148 m3/person.year (mean of 0.08 m3/person.year). In primary facultative ponds the volume of sludge represented 15–39% of the total volume of the basin. A filling rate above 30% necessitates desludging. In France, a desludging interval of 15 years is recommended for primary facultative ponds. The cost evaluation of desludging and landspreading showed differences according to the desludging technique used. Desludging after emptying the water had an average cost of 38 €/m3 of sludge with 10% dry solid (range from 20 to 83 €/m3). Under-water desludging was 50% more expensive. Although desludging is carried out only after several years of operation, its cost must be allowed for in the annual operation and maintenance costs of the process. It can be estimated to be 3 €/person.year. Even with this additional cost, waste stabilisation pond treatment remains less expensive than other treatment processes.

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