Pollution
Humain
Environnement
Economique

The results of water samples taken at the outlet of a stormwater pond at the wastewater treatment plant (WWTP) of a beverage manufacturer were found to be anomalous. The WWTP has two separate collection tanks, one for wastewater and another for stormwater. Each type of water is then stored in a specific pond, referred to as P1 and P2. The water in P1 is fed into a treatment system and then discharged to the River Loire. Analyses are carried out regularly and at various points on both collection and treatment circuits.

Over the course of 13 days, the results for P1 showed that the pH (greater than 10), chemical oxygen demand (COD), platinum, and nitrogen levels were above the regulatory limits. There were no environmental impacts because the water in the ponds is treated at the biological treatment plant before being discharged to the natural environment.

The pollution came from wastewater produced by the company’s manufacturing processes. Several pipes supply the wastewater tank, but they are not directly connected to it. In particular, the pipe from the hydrogen peroxide vapour neutralisation tower does not dip into the tank. As a result, the alkaline discharges (diluted sodium hydroxide) are dispersed in the space above it. Added to this is wear of the concrete pad separating this space from the one above the stormwater tank. The stormwater tank’s seals gradually deteriorated, allowing alkaline wastewater to flow into the tank and then enter the stormwater network and continue on to pond P2. This diffusion continued on towards P1 when the connection between the two ponds was opened to regulate the levels.

The operator first took the following measures:

  • he identified unsuitable connection points on the WWTP’s various pipes;
  • he temporarily sealed the tanks to make them impervious;
  • he extended the neutralisation tower’s discharge outlet to ensure that the flow enters the network.

The situation returned to normal in the second half of May, when the pH at the ponds’ outlet became neutral again. Only the platinum concentration remained just above the regulatory limit. All the levels were stable by June.