Wastewater treatment plants often need to control the "rotten egg" odours resulting from their waste and sewage treatment processes. The unpleasant odours in and around wastewater plants are mainly caused by H2S, but other odorous compounds can include mercaptans, amines, aldehydes, and ketones.

Organic formulations can contain ingredients which effectively control odours having an organic origin. To the extent that the detection of odours is understood, it appears that physical, chemical, and biological mechanisms contribute to the odour eliminating properties of the products.

In order to eliminate malodor, bio-organic catalysts including enzyme like amphiphilic substances, which have a high affinity for physically combining with organic molecules and inorganic malodorous substances such as hydrogen sulphide. Formation of initial physical complexes is required for the subsequent chemical reactions that result in the elimination of odour causing molecules. These physical complexes, alone, can reduce odour, at least temporarily, by altering the physical shape of malodorous molecules so that they are eliminated or not recognised by the same odour receptors.

After the formation of physical complexes, chemical reactions are initiated which permanently eliminate the malodorous characteristics or organic molecules and hydrogen sulphide. Based on the known oxidative properties of organic catalyst, it is assumed that malodorous substances, such as mines and mercaptans, are eliminated primarily through oxidative reactions, although other types are chemical reactions and are possible contributors to odour elimination.

There are two ways by which odours can be eliminated. The above physical and chemical mechanisms are for eliminating odours after they have been formed by the anaerobic decomposition of waste. A second way by which we can prevent odour is to biologically prevent the formation of malodors from organic wastes by the application of organic catalysts.

Organic catalysts treatment prevents the formation of malodors by providing aerobic microorganisms with a favourable environment for the rapid biological oxidation of organic wastes.

Organic catalyst products enhance aerobic biological waste treatment by:

• Directly oxidising biosolid organic wastes;
• Providing additional oxygen for aerobic growth; and
• Providing nutritional factors required for rapid microbial growth.

It should be noted that despite the best efforts in measuring the odour strength (at the source) and in predicting the impact areas, conditions may be such that unacceptable odours can occur beyond the predicted impact area. This can occur because of extremely poor dispersion conditions, however facility managers should be able to demonstrate that appropriate checks have been performed to ensure correct operation prior to assuming that meteorological conditions have resulted in poor dispersion. These checks could include assessment of meteorological data for stability patterns.