Odor Reduction

Lime treatment creates a high-pH environment in sludge, thereby eliminating or suppressing the growth of microorganisms that produce odorous gases. Sufficient lime must be added to retard pH decay because odor generation will generally resume once the pH of the sludge falls below 10.

Figure 6.6 Effects of pH on the equilibrium between hydrogen sulfide and ionized sulfides.

Figure 6.6 Effects of pH on the equilibrium between hydrogen sulfide and ionized sulfides.

Hydrogen sulfide gas present in dissolved form in sludge is a major cause of sludge odors. Figure 6.6 shows that as the pH of the sludge is raised, the fraction of total sulfide in the hydrogen sulfide form decreases from about 50% at pH 7 to essentially zero at pH 9 because it is converted to nonvolatile ionized forms. Consequently, above pH 9, there is no longer any hydrogen sulfide odor.

If an air mixing system is used for sludge-lime mixing, initial odors increase as a result of ammonia stripping. After the initial odors are dispersed or treated, odors will be reduced by a factor of 10 (Westphal and Christensen, 1983).

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