Sludge Treatment and Disposal

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Sludge treatment and disposal

The proper treatment and disposal of sludge require knowledge of the origin of the solids to be handled, as well as their characteristics and quantities. The type of treatment employed and the method of operation determine the origin of sludge. In sewage treatment plants, sludge is produced by primary settling, which is used to remove readily settleable solids from raw wastewater . Biological sludges are produced by treatment processes such as activated sludge , trickling filter, and rotating biological contractors. Chemical sludges result from the use of chemicals to remove constituents through precipitation; examples of precipitates that are produced by this process include phosphate precipitates, carbonate precipitates, hydroxide precipitates, and polymer solids.

Sludge is characterized by the presence or absence of organic matter, nutrients, pathogens, metals and toxic organics. These characteristics are an important consideration for determining both the type of treatment to be used and the method of disposal after processing. According to the Environmental Protection Agency (EPA), the typical chemical compositions of untreated and digested primary sludge include solids, grease and fats, protein, nitrogen , phosphorus , potash, iron, silica, and pH .

Municipal sludge is a high-volume waste. Typical volumes for raw primary sludge range from 2,950 to 3,530 gallons per million gallons of wastewater treated. Volumes for trickling filter humus range from 530 to 750 gallons per million, while the volumes for activated sludge are much higher, from 14,600 to 19,400 gallons per million. It is possible to calculate the quantities of sludge theoretically, but the EPA recommends that treatment operations use pilot plant equipment to make these calculations whenever possible. In developing a treatment and disposal system, the EPA also recommends that large wastewater treatment plants adopt a methodical approach "to prevent cursory dismissal of options." For small plants, with a capacity of less than a million gallons a day, the task of determining an operating procedure is often shorter and less complex.

Sludge treatment and disposal generally include several unit processes and operations, which fall under the following classifications: thickening, stabilization, disinfection, conditioning, dewatering, drying, thermal reduction, miscellaneous processes, ultimate disposal, or reuse .

Thickening is a volume-reducing process in which the sludge solids are concentrated to increase the efficiency of further treatment. It has recently been reported that sludge with 0.8% solids thickened to a content of 4% solids yields a five-fold decrease in sludge volume. Thickening methods commonly employed are gravity thickening, flotation thickening, and centrifuge.

Sludges are stabilized to eliminate offensive odors and reduce toxicity. A stable sludge has been defined as "one that can be disposed of without damage to the environment , and without creating nuisance conditions." In sludges, toxicity is characterized by high concentrations of metals and toxic organics, as well as by high oxygen demand, abnormally high or low pH levels, and unsafe levels of pathogenic microorganisms. There are a variety of technologies available for stabilizing toxic sludge; these include lime stabilization, heat treatment, and biological stabilization, which consists of aerobic or anaerobic digestion and composting .

Sludge that has been stabilized may also be disinfected in order to further reduce the level of pathogens. There are several methods of sludge disinfection: thermal treatment such as pasteurization, chemical treatment, and irradiation. This process is important for the reuse and application of sludge on land.

Dewatering is used to achieve further reductions in moisture content. It is a process designed to reduce moisture to the point where the sludge behaves like a solid; at the end of this process, the concentration of solids in sludge is often greater than 15%. Dewatering can include a number of unit operations: Sludge can be dried in drying beds or lagoons, filtered through a vacuum filter, a filter press, or a strainer, and separated in machines such as a solid-bowl centrifuge. It can be determined whether a sludge will settle in a centrifuge by testing it in a test-tube centrifuge, where the concentration of cake solids are determined as a function of centrifugal acceleration. The Capillary Suction Time (CST) and the specific resistance to filtration are important parameters for the filterability of sludge.

Sludge is conditioned to prepare it for other treatment processes; the purpose of sludge conditioning is to improve the effectiveness of dewater and thickening. The methods most commonly employed are the addition of organic materials such as polymers, or the addition of inorganic materials such as aluminum compounds. Heat treatment is also used; in this treatment, temperatures range from 356–392°F (180–200°C) for a period of 20–30 minutes.

After thickening and dewatering, further reduction of moisture is necessary if the sludge is going to be incinerated or processed into, starved air combustion , co-disposal, and wet oxidation. These processes have a number of advantages. They can achieve maximum volume reduction; they can destroy toxic organic compounds, and they also produce heat energy which can be utilized. But sludge combustion cannot be considered an ultimate or long-term disposal option because of the residuals it produces, such ash and air emissions, which may have detrimental effects on the environment. Landfills are another disposal option, but limitations on space as well as regulatory constraints restrict their long-term feasibility. Reuse is probably the best solution for the long-term management of sludge; the most feasible beneficial use option will probably be either land application, land reclamation , or raw material recovery. Examples include the conversion of sludge into commercial fertilizer , fuel, and building products.

Whatever the selected array of sludge treatment and disposal measures are, the main factors that influence the choice will always be cost-effectiveness, public health, and environmental protection.

See also Activated sludge; Municipal solid waste composting; Solid waste; Waste management

[James W. Patterson ]