One Sunday at around 9:15 a.m., an employee at a wastewater treatment plant noticed yellowish-orange smoke floating above storage tanks used to reduce hydrogen sulphide (H2S) production. The workers at the plant were ordered to shelter in place and the on-call managers and emergency services were called. The plant’s central road as well as a road running alongside the site were closed to traffic. Judging from the colour of the cloud, the operator and emergency services considered it to be nitrogen dioxide. The cloud’s composition was determined at around 2:20 p.m. It was formed by the mixture of two substances inside a tank and escaped after a hatch in the tank had been opened to collect a sample. Toxicity tests ruled out any risk to the population and the environment. Emergency services drained the tank. This proved difficult due to the gas present.
The operation ended when a bottom drain point was reached on the next day at 11:00 a.m. The operator then prepared the facilities for treating the tank’s headspace, consisting of nitrogen dioxide (NO2) and chlorine fumes. The vapour phase was fed into an absorption tower packed with activated carbon. The remaining liquid phase (3 cm) was rinsed and pumped out five days after the start of the event. It was disposed of by an outside company. The tank was neutralised. An inspection of two nearby tanks showed the presence of iron in the presence of an acidic pH. The operator drained and cleaned both tanks. The residues were disposed of at an approved facility. The damage caused by the event was analysed so that the tanks could be returned to service.
On March the 15th, the operations underway were halted after hydrogen cyanide (HCN) was detected inside the tank involved in the incident. The headspace was purified by passing it through activated carbon. Measurements taken showed that HCN was no longer present inside or above the two nearby tanks. The operations resumed though to completion.
An unexpected reaction:
In 2013, the operator decided to use a second injection product (intermediate iron) as an alternative to the calcium nitrate solution used to neutralise H2S in the effluent until then.
The mixture of these two substances produced a viscous black precipitate without producing a gas, a reaction previously unknown in the literature. This mixture, consisting of 44 m³ of intermediate iron (66% FeCl2, 33% FeCl3, 1% HCl) and 22 m³ of calcium nitrate [Ca(NO3)2] was kept for several days in a tank not designed to hold it. Laboratory tests conducted by the operator to understand the reaction showed that an exothermic reaction would begin after 72 hours and generate nitrous vapours.
Following the event, the operator reviewed its management of emergencies and audited potential areas in which two substances are liable to come into contact.
Prior to the event:
Problems with pumps and pipe-clogging were reported at the injection station about 10 days before the event. Several maintenance and repair operations were required to make the pumps operational again. An incorrectly positioned network connection valve was moved back to its correct position and locked. The pipes were cleaned, and a tar-like substance was removed from them. The yellow-orange smoke was noticed a few days later.
Three organisational causes can be identified:
- performance on mixing tests with these two substances in too short a time;
- failure to look for causes when the pipes were unclogged;
- faults on lines.
