Sludge Treatment

Wastewater treatment systems are designed to remove contaminants from the influent waste stream, thereby producing a clean liquid effluent as well as a concentrated, semisolid residual that contains the extracted contaminants plus newly generated biological cells. Rather understandably, the latter concentrated residuals, particularly those in raw form, can have a rather unpleasant, if not altogether foul nature if not handled and managed properly. In fact, one of the most worrisome aspects of these sludge residuals, in terms of human health, is that of their potential contamination with a wide range of pathogenic, disease-causing organisms, ranging from viruses and bacteria and extending upward in scale to protozoans, as shown in Table 16.9.

TABLE 16.9 Principal Pathogens of Concern Associated with Municipal Wastewater Biosolids

Organism Type


Bacteria Salmonella Shigella Yersinia sp. Vibrio cholerae Campylobacter jejuni Escherichia coli Enteric viruses Hepatitus A virus Norwalk and Norwalk-like viruses Rotaviruses

Acute gastroenteritis








Cryptosporidium Entamoeba histolytica Giardia lamblia Balantidium coli Toxoplasma gondii

Helminth worms

Ascaris lumbricoides Toxocara sp. Trichuris sp. Necatur americanus Hymenolepis sp. Taenia sp.

Salmonellosis and typhoid Bacillary dysentery Acute gastroenteritis Cholera Gastroenteritis Gastroenteritis

Infectious hepatitis Epidemic gastroenteritis



Meningitis, encephalitis

Gastroenteritis and respiratory infections

Gastroenteritis and respiratory infections

Epidemic gastroenteritis

Epidemic gastroenteritis

Gastroenteritis Acute enteritis Giardiasis

Diarrhea and dysentery Toxoplasmosis





Taeniasis (tapeworms)

Taeniasis (tapeworms)

Up until the past decade, all forms of these concentrated residuals, whether raw or treated, were commonly described with the same "sludge" label in deference to the typically negative context associated with words whose spelling begins with "sl...'' (e.g., slap, sleet, slum, slam, etc.). However, despite the unwholesome connotations implied by sludge, the nature and makeup of many properly treated residuals (i.e., suitably degraded, stabilized, disinfected, and dewatered) can actually be rather positive, if not altogether even valuable: harkening back, as it were, to the earliest days of wastewater processing as a means of generating nutrient-rich fertilizers.

In fact, many properly treated sludges can be used as beneficial soil amendments on agricultural lands once their negative attributes (i.e., pathogen content, high-level degradability, etc.) have been effectively resolved. A more positive label for these properly treated sludges, biosolids, was consequently chosen in conjunction with a national campaign by a U.S. association representing wastewater operations (i.e., the Water Environment Federation) during the 1990s to promote a more proactive image for a finished product whose prior standing as raw sludge carried a poor reputation among the general public.

The efficient and economical management of these wastewater residuals correspondingly represents an important engineering task, one that usually relies on the alternative use of several different biosolids treatment schemes. Granted, physical (e.g., wet air oxidation, pasteurization, gamma irradiation) and chemical (e.g., lime stabilization, silicate-based solidification) methods are used to stabilize and/or disinfect these residuals, but the majority of sludge processing systems employ biological mechanisms whose goal is similarly focused on degrading, stabilizing, and disinfecting the putrescent (easily degradable) and pathogenic (i.e., bacteria, viruses, parasites, etc.) problems associated with raw sludges.

The putrescible fraction of wastes can produce both nuisance (e.g., odor generation) and hazardous (e.g., hazardous gas generation, such as hydrogen sulfide) conditions, and a major goal of sludge treatment is stabilization with these putrescible materials. Digestion is a biochemical process by which stabilization can be achieved, such that physical reductions will then be realized in both the original solids mass and volume. Both anaerobic and aerobic digestion options are used with liquid-phase sludge processing systems, and aerobic digestion can also be completed in a more concentrated slurry- or solids-type mode using composting procedures.

One additional goal with any of these processes would be that of disinfection, by which another reduction will be secured in the original pathogen content. Of course, extending beyond any of these digestion schemes there is usually a final processing step, called dewatering, which is intended to reduce the water content of the biosolids and produce a semisolid material for subsequent disposal with a total suspended solids concentration above about 25%. The latter dewatering step is usually completed by physical processes such as filtration or centrifugation.

Before addressing specific biological options for sludge processing, further clarification is warranted as to the fact that two versions of raw residuals are produced at most primary and secondary wastewater plants. In either case, these raw residuals are then referred to as sludge, while their biochemical transformation and treatment subsequently generates a finished biosolids product. The primary sludge fraction includes heavy, parti-culate organic solids as well as various combinations of grit and inorganic fines extracted from the raw wastewater during its primary sedimentation. On the other hand, a large fraction of secondary sludge residuals is simply that of bacterial cells that grew within, and were then wasted from, the secondary biological treatment operations. Table 16.10

TABLE 16.10 Raw Primary and Secondary Sludge Characteristics

Organic Grouping

Primary %

Secondary %

Protein content



Grease and fat content



Cellulose content



Volatile solids



aBy weight of dry total sludge solids.

Source: Adapted from Metcalf and Eddy (2003); Crites and Tchobanoglous (1998).

aBy weight of dry total sludge solids.

Source: Adapted from Metcalf and Eddy (2003); Crites and Tchobanoglous (1998).

provides a general characterization of primary and secondary sludge forms. Before using any one of the various biological digestion and stabilization schemes, these primary and secondary streams will typically be blended and concentrated to achieve a desired starting point with a total solids content of 1.5 to 6% solids (by weight). In the following sections we provide additional details regarding strategies for converting sludge into biosolids.

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