The use of Nutriox® and ferric chloride to suppress hydrogen sulphide and odour generation in rising mains is evaluatd in a pilot scale study. Without chemical dosing the hydrogen sulphide built up to the commonly predicted levels after six months. Thereafter, addition of both Nutriox® and ferric chloride to sewage entering the rising main totally suppressed the formation of H2S. Nutriox also nearly totally suppressed the odour measured olfactometrically. The use of ferric salt at near stoichiometric levels effectively removed dissolved sulphide. Odour removal was generally good though on occasions reduction of dissolved sulphide levels to below 0.1 mg/l was not guaranteed to be accompanied by near total suppression of odour generation. The use of ferric ions at stoichiometric levels, after H2S and odours have formed in the rising main, also eliminated the dissolved sulphide. When used in this way, a significant residual odour potential, 110,000 ou/m3 remained in the sewage. The simultaneous reductions of odour and hydrogen sulphide observed in this study implied an apparent threshold odour concentration of 0.1 ppb for H2S, significantly lowr than most values quoted in the literature.

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