Monitoring Hydrogen Sulphide in Sewer Systems

Protecting your assets

Sulphide build up in sewers is a serious issue for water utilities. Aside from the noxious smell associated with the generation of hydrogen sulphide (H2S) which may upset local communities. Lack of control of sulphide can also lead to dangerous levels of hydrogen sulphide gas build up causing a danger to sewer workers and cause corrosion leading to the expensive replacement of the sewer infrastructure.

Despite these issues, historically, monitoring sulphide levels in water in the sewer system has been challenging, leading to difficulties in taking preventative action.  Hydrogen Sulfide gas detectors have their place but once the gas has been detected in the air above the water, it’s too late.

The chemistry of sulphur in sewers

Sulphur can enter the sewer system in a number of ways. Sulphate is reduced in anaerobic conditions to form hydrogen sulphide and carbon dioxide. The presence of sulphur oxidising organisms oxidise the hydrogen sulphide to sulphuric acid which corrodes the concrete infrastructure of the sewer.  Little can be done to prevent sulphur entering the sewers, but action can be taken to prevent it forming hydrogen sulphide which prevents both the formation of the poisonous gas and prevents its oxidation to sulphuric acid.

It primarily enters as sulphate SO 4 2- as a waste product from industry, from sea water ingress and somewhat ironically from the use of aluminium sulphate used in the clarification of raw water in the production of drinking water.

In situ sulphide monitoring

Many studies have demonstrated that a loss in water flow and increased residence times and/or water with a high BOD concentration are favourable conditions for the generation of hydrogen sulphide in water and hence the gas phase.  Hydrogen sulphide can exist in water in either the molecule or the dissociated form, which is dominant is a function of the pH.

Low pH values drive the generation of the gaseous H2S molecule so monitoring the pH of the water is an essential part of controlling hydrogen sulfide generation.

Several studies have now shown that the use of the s::can UV/Vis spectrolyser combined with a pH probe (the s::can pH::lyser) can be an effective tool to monitor ‘hotspots’ in a sewer system where the conditions are favourable to the formation of hydrogen sulfide gas.

Typically, in sewers, hydrogen sulphide and the bisulfide ion are dominant.  The bisulfide ion absorbs strongly in the UV region (~ 230nm) and can, therefore, be determined directly with the fully submersible spectrolyser probe. Using the pH reading from the ph::lyser probe enables hydrogen sulphide concentrations to be calculated with little cross sensitivity to other ions.

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Employing the s::can spectrolyser system enables asset managers to protect the sewer system from acid corrosion, protect sewer workers from exposure to hazardous hydrogen sulphide gas and also enables utilities to prevent production of offensive odours.

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