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A 5,000 m long force main (with 200 mm diameter) discharges anaerobic wastewater containing elevated sulphide concentrations into a gravity sewer system. After a few years of operation, the discharge manhole is significantly corroded. As a result, a measurement campaign is initiated and the average total sulphide concentration (H2S(aq) + HS- ) of the wastewater discharged from the force main is found to be 8.6 g S m-3 .

All the sulphide is generated within the force main. The corresponding wastewater pH and temperature is determined at 6.8 and 25 °C, respectively. According to Henrys law, a dissolved hydrogen sulphide concentration (H2S(aq)) of 1 g S m-3 corresponds to an equilibrium gas phase concentration (H2S(g)) of 315 ppm at 25 °C. At pH 6.8, dissolved H2S(aq) represents 61% of the total sulphide concentration.

Q1. Based on the abovementioned information, estimate the equilibrium concentration of gasoues H2S(g) in the sewer headspace.

Q2. Describe how the lower temperatures during winter would affect the following.

A. The sulphide formation rate in the force main.

B. The distribution between dissolved hydrogen sulphide (H2S(aq)) and the bisulfide ion (HS- ).

C. The volatility of the hydrogen sulphide. D. The coefficient for water-air mass transfer of hydrogen sulphide.