3.2 Stabilization

After thickening, liquid sludge may be transferred directly to conditioning and dewatering operations, after which it is stabilized, or it may be stabilized in liquid form. Stabilization refers to the treatment of sludge to achieve several process objectives, including (1) reduction or elimination of vector attraction, (2) reduction of pathogen concentrations, (3) elimination of offensive odors, and (4) reduction or elimination of the potential for putrefaction [38,74]. Achievement of these objectives normally requires a reduction in the volatile (i.e., organic) fraction of sludge and/or an alteration of the physical/chemical environment to reduce or eliminate biological activity.

To meet the vector attraction reduction criteria as well as the pathogen concentrations specified in the 40 CFR Part 503 rule, sludge is required to be treated by a stabilization process prior to land application. (Note that once sludge is of a quality to be applied or disposed on land, it is called biosolids.) The principal processes used in sludge stabilization include (1) anaerobic digestion, (2) aerobic digestion, (3) lime treatment, (4) chlorine oxidation, and (5) composting. Other stabilization processes such as wet-air oxidation and heat treatment are considered both stabilization and conditioning processes and are covered under sludge conditioning operations later in this chapter.

Depending on the method used, stabilization processes can either reduce or increase the volume of biosolids or sludge [38,74]. The actual amount of stabilized sludge produced in a given stabilization process depends on operational parameters (e.g., chemical addition, temperature, aeration, mixing, pH, detention time,...