A peak concentration of hydrogen sulphide (H2S) exceeding 1000 ppm for several hours was detected in a wastewater lift station. 13 days later, the source was identified by the network manager. It was a fruit processing plant which was located upstream from the manhole. 23 days after the first peak, another peak concentration exceeding 1000 ppm of H2S (saturation of the sensor) was recorded on the lift station during 2 hours. After 3 days, the network manager informed the government services and all the companies in the sector. The latter had to check the functioning of their wastewater drainage plug and postpone work on the drainage network.

The H2S was the result of fermentation of the effluents of the plant’s pretreatment station and was released into the network when emptying the plant’s pretreatment basin. This fermentation occurred in the wake of the following context:

• water restrictions following the signature of a “drought” order. This concentrated the effluents and increased the residence time in the pretreatment buffer tank (refrigeration unit instead of a cooling tower);
• exceptionally high temperatures in the past several weeks;
• addition of sulphuric acid to neutralise the pretreatment pH level.

Production was stopped for 3 days. A prefectural emergency order was signed prohibiting discharges into the wastewater treatment plant via the network. 550m³ of untreated effluents were stored in the site’s retention basin. They would be treated outside the site.

The operator called on a specialist company which took the following corrective measures:

• stoppage of the pretreatment station and locking of the pumps for dispatch to the town station;
• drainage of the pretreatment ponds and the isolated outside retention basin;
• blocking and marking out of manholes.

The operator then took the following additional action:

• treatment of the plant’s basins (upstream/downstream lift station and buffer) with a peracetic acid solution;
• destruction of sulphite-reducing bacteria and inhibition of their growth;
• measurement and monitoring of the H2S level by adding 3 sensors (including one at the level of the pretreatment basin in order to know the H2S concentration before release into the network);
• elimination of the sludge deposits which host and develop bacteria producing H2S;
• elimination of the main source of sulphur in the effluent. The high pH levels of the detergent solutions were neutralised with sulphuric acid which introduces sulphur, an element that can be metabolised into H2S by bacteria;
• neutralisation of the effluents with a nitric acid solution. The dose of neutralising acid is controlled by continuous monitoring of the pH level and automatic actuation of a delivery pump. Moreover, nitric acid increases the water’s nitrate (NO3) content which inhibits the activity of sulphite-reducing bacteria;
• drafting of a procedure for urgent action in the case of detection of H2S.

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