Foam and sludge overflowed the homogenisation tank of the wastewater treatment plant (WWTP) of a malthouse. This overflow reached the municipal road and drained into two former tanks now out of use. To contain the overflow, the operator increased the WWTP’s recirculation and the extraction of sludge. Pending WWTP treatment of the effluents contained in the homogenisation tank and in the upstream networks, no new grain soaking operation was started.

The overflow appeared to be due to clogging of the pipeline for evacuation of the sludge to the collection bin, this pipeline being located outdoors. Temperatures below 0°C were apparently the cause of this clogging and alarms were apparently triggered 2 days earlier. Given the pressurisation of the force-feeding press, the latter went into safety configuration. Since sludge was no longer evacuated from the clarifier, the clarifier’s sludge blanket detector (high level sensor) placed in safety configuration the lift pump between the homogenisation tank and the aeration tank. Since water drainage from the soaking process was still operational, the homogenisation tank was filled, causing foam and sludge to flow outside this tank.

In order to manage this emergency situation, the inspectorate requested as follows:

• establish monitoring of treatment by the WWTP, according to an appropriate frequency;
• take daily measurements of the WWTP exit values until the WWTP returned to normal operation;
• organise monitoring at the point of discharge into the Oeuf, in order to detect any aquatic pollution or mortality;
• call on a specialist company to clean the area downstream of the homogeniation tank and pump the effluents contained in the inoperative tanks. Then send the collected effluents to a facility duly authorised to eliminate them;
• preventively close the sectional valve of the retention basin located alongside the 2 former basins affected by the pollution.