Introduction

Thickening is a process to increase the solids concentration of sludge and decrease its volume by removing a portion of the water. The thickened sludge remains in the fluid state and is capable of being pumped without difficulty. The purpose of reducing the volume by thickening is to increase the efficiency and decrease the costs of subsequent sludge-processing steps. Thickening of waste activated sludge is important because of its high volume and low solids concentration. Thickening from 1% solids concentration to 2%, for example, reduces the sludge volume by one-half. If it is concentrated to 5% solids, the volume is reduced by one-fifth of its original volume.

Dewatering is the removal of water from sludge to achieve volume reduction greater than that achieved by thickening. Dewatering the sludge results in a solid-semisolid material that is easier to handle. It can be shoveled, moved about with tractors fitted with buckets and blades, and transported by belt and screw conveyors. Dewatering of sludge is required before composting to improve airflow and texture, and before thermal drying or incineration to reduce fuel demand to evaporate excess moisture. Dewatering is also required prior to disposing of sludge in landfills to reduce leachate production at the landfill site.

Prior to dewatering, sludge requires conditioning by biological, chemical, and/or physical treatment to enhance water removal. Chemical conditioning to improve separation of solid and liquid phases is often required prior to thickening sludge using mechanical thickening equipment. Therefore, sludge conditioning is described below before thickening and dewatering are discussed.

3.2 CONDITIONING

Sludge conditioning refers to the process of improving solid-liquid separation. Conditioning is an important part of mechanical thickening and dewa-tering of sludge. Conditioning of sludge can be performed by inorganic or organic chemicals, power plant or sludge incinerator ash, or by physical processes such as heating, elutriation, and freezing and thawing. However, not all conditioning processes are equal. Although heating, elutriation, and freezing and thawing improve the dewaterability of sludge, it has to be augmented by chemical conditioning: at lower dosages, however. In addition to enhancing the separation of water from solids, some conditioning processes also disinfect sludge, affect sludge odors, alter the wastewater solids physically, provide limited solids destruction or addition, or improve solids recovery.

3.2.1 Factors Affecting Conditioning

Wastewater sludge consists of primary, secondary, and/or chemical solids with various organic and inorganic particles of mixed sizes. Depending on the sources, they have various internal water contents, degree of hydration, and surface chemistry. Sludge characteristics that affect thickening or dewatering and for which conditioning is employed include the following:

• Solids concentration

• Particle size and distribution

• pH and alkalinity

• Surface charge and degree of hydration

• Other physical factors

Source Sources such as primary sludge, waste activated sludge, chemical sludge, and digested biosolids are good indicators of the source of conditioner doses required for thickening or dewatering. Based on published data about chemical conditioning requirements, primary sludge, as a general rule, requires lower doses than those required by biological sludge. Among the varieties of secondary biological sludge, attached growth sludge requires lower doses than does suspended growth biological sludge. Conditioning requirements for aero-bically and anaerobically digested sludge generally are the same as for second ary digested biological sludge. It should be noted that these are general rules and that the conditioning requirements for the same source of sludge may vary from plant to plant. Chemical sludge is difficult to classify as a single source for conditioning requirements because different types of chemical sludge require vastly different doses and types of conditioners.

Solids Concentration Municipal wastewater sludge contains a large number of colloidal and agglomerated particles, which have large specific surface areas. If the sludge has a low concentration of solids, these particles behave in a discrete manner with little interaction. In many applications, conditioning is the neutralization of the surface charge of sludge particles by the adsorption of oppositely charged organic polyelectrolyes or inorganic chemical complexes. With low interaction from the low concentration of solids, more coagulants are needed to overcome the surface charge. As the concentration of solids is increased, interaction increases. Therefore, on a mass of coagulant per mass of dry solids basis, the dosage can be reduced. For this reason, coagulant dose is usually expressed as a percentage of dry solids or as kilograms of coagulant per ton of dry solids. A higher suspended solids concentration produces effective conditioning over a wide range of dosage when organic polymers are used, which means that the higher the solids concentration, the less susceptible the process is to overdosing.

Particle Size and Distribution Particle size is considered the single most important factor influencing the dewaterability of sludge. For the same concentration of solids in sludge, the greater the number of small particles, the greater the surface area/volume ratio. Increased surface area means greater hydration, higher chemical demand, and increased resistance to dewatering. One of the objectives of conditioning is to increase particle size by combining the small particles into large aggregates.

pH and Alkalinity pH and alkalinity affect primarily the performance of inorganic conditioners. When added to water, inorganic conditioners reduce the water's pH. Therefore, the dosage of inorganic conditioners such as iron or aluminum, and the alkalinity or buffering of the sludge, determine the pH of the conditioning process. The resulting pH, in turn, determines the predominant coagulant species that will be present and the nature of the charged colloidal surface. The higher alkalinity of anaerobically digested biosolids is one of the reasons for the associated higher coagulant doses.

Surface Charge and Degree of Hydration For the most part, sludge solids repel rather than attract one another. This repulsion may be due to hydration or electrical effects. With hydration, a layer of water binds to the surface of the solid. This provides a buffer that prevents close approach between solids. In addition, sludge solids are negatively charged and thus tend to be mutually repulsive. Conditioning is used to overcome these effects of hydration and electrical repulsion.

Physical Factors Physical factors such as storage, pumping, mixing, and sludge treatment processes, including the types of thickening and dewatering devices to be used, also affect the thickening and dewatering characteristics of sludge. Sludge that has been stored for a long period of time requires more conditioning chemicals than fresh sludge does because of an increase in the degree of hydration and the fines content of the solids. Because of the fragile structure of sludge particles, some reduction in particle size typically results from the shear forces associated with the pumping process. Proper mixing and flocculation to evenly disperse the conditioning chemicals required also depend on the processing to which the sludge has been subjected and on the mechanics of thickening or dewatering process available.

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