Contact Time and pH

The primary objective of lime stabilization is to inhibit bacterial decomposition and inactivate pathogenic organisms. The effective factor in achieving this objective is the pH level; however, as with most disinfection processes, the time of exposure is equally important. This is confirmed by the fact that the addition of lime for conditioning of sludge just before dewatering does not produce sludge cake with adequate bacteriological destruction. The alkaline material added must provide enough residual alkalinity to maintain a high pH until the product is used or discarded. The high pH prevents growth or reactivation of odor-producing and pathogenic organisms.

The drop in pH (referred to as pH decay) of stabilized sludge occurs because of two reactions that occur in succession. Atmospheric carbon dioxide is absorbed and then gradually consumes the residual alkalinity of the stabilized sludge, and the pH gradually decreases. Eventually, a pH is reached (below 11.0) where bacterial action resumes, continuing the drop in pH from the production of organic acids. Therefore, the design objective is to maintain pH above 12 for two hours or more to ensure pathogen destruction and to provide enough residual alkalinity so that the pH does not drop below 11 for several days, allowing sufficient time for disposal or use without the possibility of renewed putrefaction. The procedure recommended to accomplish these objectives is to bring the sludge to pH 12.5 by lime addition and maintain it at that level for 30 minutes, which keeps the pH above 12 for two hours.

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