A tank used for storing and mixing biodegradable material before it is sent on to a digester overflowed at an agricultural anaerobic digestion plant. The plant’s employees noticed the overflow when they arrived at 7:30 a.m. They increased the flow rate to the digester in order to lower the level inside the tank. A portion of the tank’s contents flowed down onto the cement, with 10 m3 being collected in buckets and returned to the tank. The remaining traces were left to dry and then cleaned up. The other portion seeped into the soil. The operator had the affected area dug up and the soil was dumped onto a compost pile.

What caused the tank to overflow was foam produced following the addition of 25 tonnes of draff in one go. This type of waste had not ever been added before. The foaming was increased by the high temperature in the tank. At around 4:30 p.m. the day before, the tank’s high-level sensor triggered an alarm. The stirrer override was activated, which temporarily solved the problem. However, later on that night the alarm sounded again although stirring was still taking place. The on-duty technician did not increase the amount of substrate being sent to the digester and did not go to check the tank. The tank overflowed during the night.

Following this incident, the operator decided that draff should be added in small amounts and the reaction inside the tank should be monitored. Operator’s vigilance with regard to alarms was reinforced.