Conditioning of Sludge

The aim of sludge conditioning is improving dewatering characteristics of sludge. Dewaterabilities of sludges are varied; some, such as activated sludge, are difficult to dewater. The difficulty in activated sludge dewater-ing is mainly attributed to the presence of extracellular polymer (ECP). ECP is present in varying quantities in sewage sludge, occurring as either a highly hydrated capsule surrounding the bacterial cell wall or loose in solution as slime polymers. ECP is thought to aid the survival of the bacterial cell by preventing dessication and acting as an ion exchange resin, controlling the ionic movement from solution into the cell. Polysaccharide, protein, and DNA, which entrap the water and cause high viscosity, are the main components of ECP, but humic-like substances, lipids, and heteropolymers such as glycoproteins are also present. Wang et al. (2004) noted that surface properties, like the concentration of ECP, were related to zeta-potential measurements with particle electrophoresis as well as to water contact angle measurements on filter cakes prior to and after the ox-idative conditioning: The sludges with high zeta-potentials and low contact angles were sludges with high amounts of ECP. These sludges with a high surface charge density as well as a high hydrophilicity (low contact angle) prevent efficient flocculation. The experimental results of Neyens et al. (2004) indicate that peroxidation of sludge enhances the flocculation and dewaterability. The responsible mechanism is not fully understood, but the oxidative conditioning might be based on partial oxidation and rearrangement of the surface components (extracellular polymers) of the sludge flocs. The effects of temperature, hydrogen peroxide concentration, pH, presence of Fe2+, and reaction time on the dewaterability of the sludges were tested by Neyens et al. (2002, 2004). They found that peroxidation gave the best results with respect to improving sludge dewaterabil-ity and product quality of the residual filter cake among all advanced sludge treatment processes (Neyens et al. 2002).

The general approaches to conditioning sludge are employment of chemicals (similar to coagulants used in primary sedimentation) and heat treatment to improve the dewaterability of sludge. Other less-common methods—such as freezing, irradiation, solvent extraction, as mentioned previously, and advanced oxidation—are still in the laboratory stages.

Chemical conditioning can reduce 90-99% moisture content of an incoming sludge to 65-85%. As described previously, chemicals such as coagulants (e.g., alum, lime, iron salts, and polymers) disrupt the structure and composition of the sludge by forming aggregates that are more compact in structure and hold less water. The addition of chemicals may contribute to an increase in solid content of the sludge as much as 30%. The dosage and types of chemicals depend on several factors; properties of the sludge are by far the most crucial factor in determining which and how much chemical needs to be added to the sludge. The source of the sludge, solid concentration, age of the sludge, pH, and alkalinity are important properties that affect the selection and dosage of chemicals added to the sludge prior to mechanical dewatering operations. It is generally observed that the difficult-to-dewater sludge requires a larger quantity of chemicals; the more biologically processed the sludge is, the larger the dose of chemicals required. In this vein, aerobically digested sludge that is treated with the most efficient biological process requires the highest dose of chemical conditioners and untreated raw primary sludge needs the least.

Heat treatment can serve both as a stabilization process and as a conditioning process. Heat can facilitate aggregation of particles, break down the gel structure of the sludge, and reduce water affinity to the sludge particles. Heat can also sterilize or inhibit pathogens, bacteria, and enzymes; and heat-treated sludge, if done properly and followed with vacuum filtration or belt presses, seldom causes putrefaction. The other bright spot for the heat-processed sludge is that it has a heating value of 12,000 to 13,000 Btu/lb (28 to 30 kJ/g) of volatile solids (volatile solids are part of the dried sludge that undergoes complete combustion in a muffle furnace at 550°C); this can be valuable in sludge incineration. The disadvantages of heat treatment of sludge are also noticeable: the costs, both capital and operating, are high due to mechanical complexity; operations of heat treatment require skilled operators and elaborate operating procedures; significant odor and gases, which need to be managed, emit from the process; and scale formation in heat exchangers are extensive and difficult to prevent or clean up.

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