Economic and efficient

Economic

The implementation costs are similar to those of an intensive plant, the operating costs divided by two (10 €/PE.year instead of 20 €/PE. year, including sludge disposal). The low operating costs are mainly due to sludge treatment: a constructed wetland produces after treatment in primary filters (no sludge on secondary filters) a relatively dry (generally 25 % dry content) and light sludge, similar to a compost. It is extracted and spread on agricultural land every 10 to 15 years (daily in an active septage muds plant, every 5 to 10 years with two ponds treatment).

Efficient

The outlet quality is higher than the prescribed standard. It will decrease gradually with an increasing population. However, with current plant configuration, the outlet quality standard will be met for more than 4000 PE.
Transit through the ponds lowers the TKN and Pt contents generally observed in constructed wetlands of "French” design.
Parameter efficiencies are all higher than the minimum requirements of the French and European standards.

Consult the 2009-2010 graphs illustrating
the efficiency of Nègrepelisse’s wastewater treatment plant.

Reliable

Plant reliability is ensured by the simplicity of the treatment process (breakdowns are unlikely and rapidly repaired), replication of the main equipment (several pumps in each pumping unit, several compartments on every filter etc.), dimensioning ratios (each equipment works below its nominal capacity).

In the event of power failure (short and most unlikely on this power line), the raw wastewater is automatically diverted and treated in the ponds before being discharged into the river.
The automatic switch restores regular functioning at the end of the breakdown. Meanwhile, the controls of plant and recording of the main parameters are ensured by an emergency battery. Operation of the gates is ensured by the air stored in the compressor.