Constructed wetlands and waste stabilization ponds for municipal wastewater treatment in France: comparison of performance and maintenance operations in terms of durability and reliability

Abstract

In France, vertical flow constructed wetlands and waste stabilisation ponds are both extensive treatment processes well adapted to small rural communities mainly because they are easy to operate and their performance is reliable. Most of the time, the effluent has a better quality than required by legislation; even if WSP performance can be affected by seasonal variations, especially as regards nutrients. For VFCWs, nitrate concentrations remain high due to the fully aerobic conditions prevailing in the 2nd stage filters. In the 1st stage filters only a little denitrification can occur in the drainage layer when water is temporarily stored to batch feed the 2nd stage filters. For both processes the removal of sludge accumulated in the first pond/filters is only necessary every 10 years or more at nominal load. Dry matter content of the sludge removed from VFCWs is obviously higher than that from ponds. For both processes annual routine maintenance operations for a plant designed for 400 p.e. requires approximately 100 hours of presence at the plant by a member of the municipal staff. For a 1000 p.e. WSP plant, less time is needed than for a VFCW plant, because of the annual harvesting of the aerial part of the reeds. Once sludge is removed, these plants are ready to perform a new cycle of operation without having to change any electro-mechanical equipment. This contributes greatly to the reliability of performance.