Sludge is rather complex material, both in the sense of its composition and in the characteristics of its fluid dynamics, which are shaped by its moisture content and interactions between water molecules and other solid particles. In general, when the moisture content of the sludge exceeds 90% by mass or weight, the sludge behaves as a Newtonian fluid, whereas below 90% it behaves as a non-Newtonian fluid in a way similar to those of some polymers (both certain biopolymers such as foods and petroleum-based polymers). The water in the sludge is held as either free water or bound water. It is estimated that in sludge with 95% moisture content, approximately 70% of it is free water and the remaining is bound; the free water molecules are easier to separate from the sludge. Among the 30% bound water, 20% is present as a part of flocs or aggregates, 8% is chemically bound, and 2% is capillary water. As a rule, the intimate bound water, such as capillary water and chemically bound water, requires higher energy than free water. Free water in sludge can be removed by gravity thickening; however, this moisture reduction in sludge may not be enough for sludge handling and further processing. Chemicals and mechanical means are used to "condition" the sludge amendable to dewatering. Mechanical devices break up the structure of the sludge to release trapped water from flocs, whereas chemicals such as coagulants FeCl3, alum, or polyelectrolytes are used to break up and alter the structure and composition of the sludge resulting in improved dewaterability.
The application of basic sludge treatment techniques, whether by stabilization or thickening, depends on the sludge quality and characteristics. Different industries produce different types of sludge that affect sludge treatment options and ultimate disposal. Sludge with gross organic solids such as offal from meat-processing operations; pathogens; and heavy metal and other toxic materials from pharmaceutical, chemical, and metal-processing industries may have few disposal options. This issue needs to be carefully considered by sludge management and treatment personnel and designers during the planning phase of their work. The characteristics of solids and sludges from wastewater treatment processes also vary with the processes or unit operations of wastewater treatment plants. The retained solids from screens and grit are large-sized inorganic or organic materials, such as vegetable leaves or other food debris, that are large enough to be removed on bar racks. Depending on the nature of the food processing and the season of the year, scum from the flotation process of primary and secondary settling tanks could consist of grease, vegetable, fruit skim, animal fat, and floatable food wastes. Primary sludge from the bottom of the settling tank appears to be gray and slimy and emits offensive odor; sludge from chemical precipitation with metal salts may be dark in color. Sludge from the secondary settling varies with the nature of the biological process employed; sludge from the activated sludge process has a brownish, floc-culant appearance and smells far less offensive than that of the primary sludge if the process is well oxygenated. Sludge sloshed down from a trickling filter is brownish, flocculant, and relatively inoffensive. Digested sludges, regardless of whether they are aerobic or anaerobic, are dark brown to black and do not have an offensive odor if treated thoroughly.
Because most wastewater sludges are composed of water, the properties of sludges are dependent on water content; for example, once the percentage of solids and their specific gravity in the sludges are known, the volumes of the sludges can be estimated. The mass balances used to describe the basic unit operations of the sludge treatment can be established based on the solids in the sludges.
The characteristics of stabilized or raw sludge that affect its suitability for land application and beneficial use include organic content, nutrients, pathogens, metals, and toxic organics. The fertilizer value of sludge, if evaluated and found to be suitable, is based mainly on the contents of nitrogen, phosphorus, and potassium. In food and agricultural wastewater treatment, sludge may not have sufficient phosphorus and potassium contents to provide good plant growth. Trace amounts of inorganic compounds in the sludge may spur or stunt growth of plants. Heavy metals in sludge, a perennial problem of municipal wastewater treatment plants, is less severe a problem in most food wastewater treatment processes. Detailed information regarding typical wastewater characteristics, including heavy metals, can be found in Metcalf and Eddy, Inc. (1991).
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