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Since industrial wastes have a broader range of characteristics than domestic wastes they are treated by a wider variety of processing schemes. Industrial wastes are more likely to contain toxic and nonbiodegradable components that require physicochemical treatment instead of biological one. In some cases industrial wastes are discharged to a municipal plant directly or after limited pretreatment. In other cases they are treated in a separate plant designed for the specific wastes.
Mass transfer is an important consideration in many wastewater treatment systems. In order to carry out chemical or biological reactions, it is necessary to transfer substances into or out of the wastewater, as well as to move them adequately within the water to control concentration differences. The material transferred can be as diverse as gases, liquids, ions, charged colloids, or suspended solids. However the rate at which these substances are transferred is the primary concern of the field of mass transfer.
Sedimentation is the removal of solid particles from suspension by gravitational settling. Sedimentation basins are often referred to as either clarifiers or thickeners. The terms clarifier and thickener are often used interchangeably in describing tanks for effluent streams from activated sludge reactors. Since both clarification and thickening occur in any sedimentation basin, both functions should be considered in the design.
In water treatment plants, sedimentation is used to remove readily settleable particles, flocculated or coagulated impurities, and precipitated impurities from softening operations. In wastewater treatment plants, sedimentation is applied to a variety of organic and inorganic solids from raw or treated wastes. Primary settling tanks are used to remove solids, from the waste stream entering the plant. Secondary settling tanks handle the solids in the effluent from a biological reactor.
The purpose of biological waste treatment is to convert complex molecules into simple products and biomass by .using, a mixture of microorganisms. Since successful waste treatment depends upon suitable biological activity, it is necessary to operate the system to encourage microbial growth.
Since direct land or water disposal of raw wastewater sludges is rarely feasible or acceptable, sludge treatment is usually necessary to reduce its volume and to make it less offensive.
Selection of treatment processes for sludges depends upon the nature of the sludge, environmental factors, and ultimate disposal options. The various alternatives examined to select the major processes are concentration, stabilization, conditioning, and dewatering.
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