Before sludge undergoes treatment such as dewatering or thickening, it must be stored and pretreated. Sludge storage is an important, integral part of every wastewater sludge treatment and disposal system. Sludge storage provides many benefits including equalization of sludge flow to downstream processes, allowing sludge accumulation during times of non-operation of sludge-processing facilities, and allowing a uniform feed rate that enhances thickening, conditioning, and dewatering operations.
Sludge is stored within wastewater treatment process tankage, sludge treatment process systems, or separately in specially designed tanks. Sludge can be stored on a short-term or a long-term basis. Small treatment plants, where storage time may vary from several to 24 hours, may store sludge in wastewater clarification basins or sludge-thickening tanks. Larger plants often use aerobic digester, facultative lagoons, and other processes with long detention times to store sludge. The pretreatment of sludge is often necessary before dewatering or thickening can take place. It includes degritting and grinding. Sludge degritting involves the installation of grit removal and precessing facilities at the head works where raw wastewater first enters the treatment plant. As a result, there is reduced wear on influent pumping systems and primary sludge pumping, piping and thickening systems. Sludge grinding involves shearing of large sludge solids into smaller particles. This method is used to prevent problems with operation of downstream processes. Inline grinders reduce cleaning and maintenance down time of equipment. The grinders can shear sludge solids to 6 to 13 mm, depending on design requirements.
Sludge-pumping systems play an important part in wastewater treatment plants, particularly those operations experiencing average flows of greater than 1 million gallons per day (mgd). There are different types of pumps within this process. Typical advantages of kinetic pumps for sludge transport include lower purchase cost, lower maintenance cost due to wear, less space used, and availability of both dry-well and submersible pumps. Advantages of positive displacement pumps include improved process control and pumping capability at high pressure and low flow.
Sludge cake storage (where a cake is the dewatered solid part of sludge) provides similar benefits for downstream disposal alternatives, like composting and incineration, to sludge storage which is used for thickening and dewatering. Storage of sludge cakes increases operational reliability, evens out flow fluctuations, and allows accumulation when downstream operations are not in service. Bins or hoppers are used to store sludge cakes. These can be made of any size form several cubic meters to 380 cubic meters capacity. Existing sludge dewatering operations can produce cakes that are 15 to 40% solids. These cakes range in consistency from pudding to damp cardboard. Since they will not flow by gravity in a pipe or channel, sludge cakes must be transported by one of the following methods: mechanical conveyors such as flat or troughed belt, corrugated belt, or Archimedes screw; gravity drop from dewatering equipment into storage hoppers directly below; and pumping by positive displacement pumps.
Before any of the sludge can proceed to dewatering or thickening processes, it must be conditioned. Sludge conditioning involves chemical or thermal treatment to improve the efficiency of the downstream processes. Chemical conditioning involves use of inorganic chemicals or organic polyelectrolytes, or both. The most commonly used inorganic chemicals are ferric chloride and lime. Other chemicals are popular outside of the U.S.. Organic polymers, introduced during the 1960's, are used for both sludge-thickening and dewatering processes. Their advantage over inorganics is that polymers don't greatly increase the amount of sludge production: 1 kg of inorganic chemicals added will produce 1 kg of extra sludge. The disadvantage of polymers is their relatively high cost. There are several important factors that affect conditioning of sludge. They include: sludge characteristics, sludge handling, and sludge coagulation and flocculation. The fundamental purpose of sludge conditioning is to cause the aggregation of fine solids by coagulation with inorganic chemicals, flocculation with organic polymers, or both. A critical design parameter in conditioning is dosage. Selection of the right dosage of a chemical conditioner is critical for good performance. The dosage affects the solids content of sludge cakes as well as solids capture rate and solids disposal cost. Dosage is determined form pilot studies, bench tests, and on-line tests. In the following sections we will cover the basics of sludge stabilization and then conditioning. Our objective is to gain a working knowledge of these operations and to build our vocabulary.
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