At around noon, a leak was found on a pump used to oxidise the sulphides contained in vinasse at a sugar refinery. The settling tank was isolated to prevent odours forming. The leak was transferred to a detention basin for 22 hours until it could be repaired. However, as a valve on the transfer line had a leak, a portion of the sulphide-laden water backflowed from the basin to the settling tank. As they degraded, the sulphides produced hydrogen sulphide (H2S). The leak on the valve was detected and repaired on 28 March. On 30 and 31 March, the operator noticed that the sensor used to measure the hydrogen sulphide content in the ambient air was recording unusual spikes. It seeded sulphate-reducing bacteria into the settling tank on 31 March.

Hydrogen sulphide at a concentration greater than 30 µg/m³ was measured for 20 cumulative hours between 2 and 5 April. An average hourly concentration of more than 100 µg/m³ was measured twice. A number of local residents complained of unpleasant odours. The operator pumped in 50 m³/h of bore water in addition to effluent from the distillery to flush the settling tank more quickly and accelerate the discharge of the sulphides. It informed the local residents on 4 April.

Seeing that the effluent disposal procedure did not work, on 5 April the operator again isolated the settling tank and discharged the effluent to the detention basin. Aeration and stirring of the tank were maintained and a massive supply of potassium nitrate was fed in as a shock treatment. This worked, for the hydrogen sulphide concentration dropped below 30 µg/m³ starting on 6 April.

The inspection authorities for classified facilities found a lack of preventive organisation in terms of maintenance and crisis management.

To avoid a recurrence of this type of accident, the operator plans to:

• define an alert threshold for the level of sulphides in the effluent and oxygen in the settling tank;
• preventively perform shock treatment by seeding the settling tank with sulphate-reducing bacteria as soon as a problem is detected;
• add a post-valve operation check in the settling tank bypass procedure;
• record any precursor event susceptible to produce odours in the incident register;
• secure the connection between the settling tank and the basin by a device such as a non-return valve;
• raise staff awareness of odour risks;
• modify the suction circuit of the pump used to oxidise anaerobically digested vinasse;
• explore using continuous treatment and shock treatment before sending effluent to the basins;
• conduct a risk analysis of the refinery’s fluids (anaerobic digestion, stripping, effluent).