Special Focus

Control of Septicity and Sulfate Reduction in Sewage

Sewage has substantial quantities of organic components and bio-degradation can therefore be expected to occur. When such bio-degradation occurs in situations where the supply of oxygen is limited (for example, in sewers), septic conditions tend to develop. This in turn results in the microbial utilisation of combined forms of oxygen such as sulphates. The reduction of sulphates, as a result of activity by sulphate reducing bacteria (SRB) under anaerobic conditions, yield the sulphide ion and where pH conditions are suitable, hydrogen sulphide could then be released in gaseous form.

The build-up of hydrogen sulphide in a wastewater collection system is a common problem in many countries. Hydrogen sulphide, formed in sewage, is a toxic gas which can subsequently contaminate the atmosphere in manholes, gravity sewers and the wet-wells of pumping stations. In addition, it is corrosive to metals and may be oxidised on moist surfaces by autotrophic thiobacilli to sulphuric acid, causing even greater corrosion of concrete and steelwork. Hydrogen sulphide also has a highly disagreeable odour at levels as low as 0.01 ppm in the atmosphere.

In view of the above-mentioned consequences of septicity, the Ministry of the Environment (ENV) currently has a joint research project with the Wastewater Biotreatment Group (WBG) at the Department of Civil Engineering, NUS, to study the options available for controlling septicity in sewage. This project started in June 1998, with a WBG team consisting of academic staff, a research engineer, research scholar and research technologist. ENV is represented by two senior engineers. The thrust of the project is to develop a methodology that would either inhibit sulphide formation and/or oxidise the sulphide already present.

Since biological activity resulting in septic conditions is largely unavoidable in a sewer, the solution to the problem of odorous emissions may be not to stop the biological activity. Instead, biological activity is allowed to proceed with a sulphate substitute so that sulphides do not form. Such an approach has been attempted in Europe and the United States of America with compounds such as nitrates serving as the substitute and the results, in terms of sulphate reduction, have been shown to be promising. The key to the successful application of the technique lies in knowing the quantity of nitrates and the type of dosing protocol to apply.

However, direct application of the protocol that has been developed in Europe to the Singapore environment may be inappropriate due to reasons such as different organic composition and concentrations, ambient temperatures and sewage retention times in the sewers. From laboratory studies, it has been found that nitrate reactions require an amount of reaction time of 1.5 to 2 h, depending on the concentration of nitrate solution added, to oxidise 0.5 mg/L of dissolved sulphide. This reaction time is generally longer than the sewage retention times in Singapore sewers. As a result, there is a possibility that excess nitrates could reach the Primary Sedimentation Tanks, which would in turn result in the formation of rising sludge in the tanks.

The short sewage retention times in Singapore sewers have prompted the team to focus on ‘faster’ acting oxidising agents. The use of such oxidising agents would require a shorter reaction time. Laboratory studies have shown that such oxidising agents are effective in reducing dissolved sulphide concentration by over 90 % under anaerobic conditions. Moreover, the relatively faster depletion of oxidising agents as compared to nitrate depletion rates implies that the oxidising agents could be dosed at headworks of a sewage treatment plant, rather than into the sewers upstream of the treatment plant. The research team has established a dosing protocol and will conduct a field trial in the near future. This study is being jointly funded by the Ministry of the Environment and the National University of Singapore.

Contact Persons:

Assoc Prof WJ Ng ,Tel: 874 2172/2102, Fax: 779 1635, Email: cvengwj@nus.edu.sg