Its constructed wetland-CW and wastewater treatment plant-WTP

Plant components

The plant is fed by a 2-km pressure pipe from La Coujoune, renovated in 2008, and includes (in upstream to downstream direction):

• an electromagnetic flow meter
• a pumping unit feeding the primary filters – PU1
• a set of primary vertical-flow constructed wetlands – VFCW1
• a pumping unit feeding the secondary filters – PU2
• a set of secondary vertical-flow constructed wetlands – VFCW2
• a pumping unit to the ponds – PU3
• a Venturi flow meter at the outlet into the river.

Layout
of Nègrepelisse’s WTP

The electromagnetic flow meter measures the incoming wastewater quantity at the plant inlet and allows, together with a sampler, to measure the input loads.

The inlet pumping unit – PU1 distributes the raw wastewater by pressure surge on the surface of the primary filters. The pumps are powerful enough to be spread the wastewater evenly over the entire filter surface in use.

The primary filters – VFCW1 ensures a preliminary treatment of the raw wastewater by bacteria fixed on the gravel and the reed roots. They are designed for 1.2 m²/PE as usual in France. Effective primary filtration surface: 4000 PE x 1.2 m²/PE = 4800 m², divided into two large filter units of 2400 m² each, the largest in France at the time of construction.

The pumping unit to the secondary filters – PU2 receives the pre-treated wastewater from the VFCW1 unit (from the ponds during summer – cf. l'animation) and spreads it by pressure surge on the secondary filters.

Photograph below (from front to rear) electric box, PU1 – grid covering PU1 – shelter of the swan neck duct and inlet sampler.

The secondary filters of the vertical-flow constructed wetland – VFCW2 complete the treatment process through the action of bacteria fixed on the sand, gravel and reed roots. They are designed for 0.9 m²/PE, i.e. slightly larger than usual. Effective secondary filter surface: 4000 PE x 0.9 m²/PE = 3600 m², divided into two large units of 1800 m² each, the largest in France at the time of construction.

Pumping unit - PU3 only functions in summer. It receives the treated wastewater from VFCW1 and sends it to the ponds for intermediate treatment. The water flows between VFCW1 and VFCW2 via PU2.

The Venturi flow meter measures the wastewater quantity leaving the plant. Together with a sampler, it allows to determine the output loads and plant efficiency (through comparison of the input and output loads) – a useful tool for plant management.