|M.Sc Student||Firer Dan|
|Subject||Investigation of the Use of Iron Salts to Control Odour|
and Corrosion in Urban Sewer Systems
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Eran Friedler|
|Professor Ori Lahav|
Hydrogen sulfide in sewers is very problematic, it may cause odour problems at very low concentrations, it is highly poisonous and accelerates corrosion and dissolution of sewage pipes. Addition of iron salts is one of the most common methods used to solve the hydrogen sulfide problem in gravity sewers. The main two species of iron , Fe3+ and Fe2+, can reduce sulfide concentration in the sewage by two ways: the ferric ion (Fe3+) oxidizes the sulfide, while the ferrous ion (Fe2+) precipitates with the sulfide to form FeS. The chemistry of iron and sulfide is complex and data found in the literature is often insufficient or unclear.
The experimental results show that the removal efficiency of Fe2+ is far lesser than that of Fe3+ and of a combination of Fe3+ and Fe2+ (2:1 ratio). It was further demonstrated that in order to reach low sulfide concentrations by using Fe2+ (precipitation of FeS), the required Fe2+ dose was 40-100% higher than the stochiometric ratio required (1:1 Fe2+:S2- ratio). The removal efficiency of Fe3++Fe2+ mixture was found to be slightly better than Fe3+ alone, although the theoretical removal efficiency of Fe3+ alone is higher than that of Fe3++Fe2+ mixture. It was also shown that it is possible to reach sulfide concentrations as low as 0.1 mg/l when using Fe3+ and Fe3++Fe2+ mixture
Comparison between experimental results and results of the Minqle+ thermodynamic model shows that when using Fe2+ the real sulfide removal efficiency was quite close to the theoretical one, which implies that the main precipitation product is FeS. It was also shown that when using Fe3+ or Fe3++Fe2+ mixture the oxidation product of sulfide was elemental sulfur and not sulfate as predicted by thermodynamic analysis performed with the Mineql+.
Addition of two types of iron salts (Fe2+ and Fe3+) to Haifa's sewer main which is characterized by many sharp turns in an extensive short segment and serious odour problems, resulted in sulfide removal efficiencies lower than the efficiencies obtained in the laboratory. Nevertheless most of the time there was an apparent decrease in sulfide concentration, and at times it reached concentrations as low as 0.1 mg/l.
Reduction of hydrogen sulfide concentrations in the sewer atmosphere was also detected. At times, hydrogen sulfide concentrations decreased to as low as 4 ppm. Further, the frequency of high hydrogen sulfide concentrations in the gaseous phase while iron salts were dosed was lower than without dosing.