An adaptive process
Advantages
Constructed Wetlands change the practice of sanitation for small and medium size authorities because of their numerous advantages:
- extension possibilities: by simple addition of filters
- simplified running: the equipment are simple and run by non qualified staff
- less expensive running
- less trouble to the neighbourhood: very little noise and smells
- better integration into the landscape
- reliability: simple equipment, very few industrial products, organization in several lines with by-passing possibilities.
Constraints
The septage and wastewater plants of Nègrepelisse address a number of conditions: high sensitivity of the receiving rivers, high quality of the surrounding landscape, running by un-experienced staff, limitation of implementation and maintenance budgets, etc.
The creeks of the Courounets area have small river catchments, low discharges and a water quality already degraded by excess fertilizers and pesticides applications. At full extension, when the wastewater treatment plant will receive 6000 PE, the Montrosies creek will receive 11.6 l/sec (litres per second) of treated wastewater for a summer flow of 0.3 l/sec only. In other words, the contribution of the plant to the Montrosies flow will be 30 to 40 times higher than the contribution of the “natural” catchment basin, 3 to 4 times higher for the Courounets.
The plain of the Aveyron river displays in the Nègrepelisse area a landscape quality that the project must respect. The initial ponds are located on a soft slope descending towards the creek. The location of the filters follows the same logic.
The running simplicity is essential as much as CCTVA chose to maintain and manage the plant with his own staff as opposed to contracting the operation to a specialized firm.
The economic objectives of the project, especially the CCTVA decision to limit the price of water in its territory, imposes to limit the implementation and running costs.
