Supernatants were observed in the clarifier of a new wastewater treatment plant (WWTP) in a dairy facility. A tractor equipped with a slurry tanker was used to pump out the supernatant. The suction from the tanker caused eddy currents to form, and part of the supernatant passed over the toothed blade of the clarifier for about ten minutes. This foam of organic origin was discharged into the SEVRON River. Of the 100 kg of supernatant discharged, 80% was released during the pumping operations. The remaining 20% was probably blocked in the pipes downstream from the clarifier and was removed over the following two days. No consequences for flora or fauna were reported. Two days later, samples were taken upstream and downstream from the discharge point. Measurements (pH 7.7 upstream and pH 7.6 downstream), showing that the water quality had not been affected.

A specialised company was called in to pump out the supernatants three days later. From this moment, the clarifier was once again clean. The 20 m³ of pumped supernatants were stored in a sludge silo at the site for reuse during the next spreading campaign, after checking their compatibility.

Supernatants are the result of filamentous bacteria which disrupt the sludge while it is settling. The use of a slurry tanker is not suitable for pumping the clarifier, and a specialised company must be called in to perform this type of operation.

The operator chlorinates the pond to eliminate these bacteria. This is a lengthy process, and the protocol must be strictly adhered to. The following actions were taken:

• a procedure was prepared concerning the removal of foam from the clarifier;
• staff are now trained on the operation of the WWTP, its possible malfunctions and how to handle them.