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Evaporation-A Wastewater Treatment Alternative Page 3 of 5 <br /> Forced circulation evaporators, or crystallizers, are often used for applications requiring <br /> high solids concentration or crystallizing, or in applications involving large amounts of <br /> suspended solids. Energy costs for forced circulation units can be more than for other <br /> evaporation systems because of their high recirculation rates. <br /> Combined and hybrid systems: Combining different types of evaporators, or combining <br /> them with other processes to reduce capital and operating costs, or meet specific <br /> treatment objectives, often is possible. One fairly common arrangement uses a falling film <br /> evaporator followed by a forced circulation crystallizer. In this scheme, an evaporator <br /> concentrates the wastewater stream to 20 to 30 percent solids, and a crystallizer further <br /> concentrates it to a solid. Energy costs may be reduced by using steam vented from the <br /> evaporator to operate the crystallizer. <br /> Hybrid designs are becoming more common in zero liquid discharge applications. A hybrid <br /> system may consist of an evaporator or evaporator/crystallizer preceded by a reverse <br /> osmosis or electrodialysis preconcentration step. The concentrate, or reject, from the <br /> preconcentrator becomes the feed for the evaporator. <br /> Although a hybrid system adds complexity, it can reduce the size of the evaporator unit, <br /> as well as the system's energy needs. But note that not all wastewaters, especially those <br /> with high scaling tendencies, are candidates for hybrid systems. Table 1 shows the energy <br /> savings obtained by selecting an 85 gpm hybrid system. <br /> Evaporator Applications <br /> Because evaporation is an energy- and capital-intensive process, the selection and design <br /> of an evaporator system must be carefully considered for each application. <br /> Evaporators have been used successfully in many industrial wastewater treatment <br /> applications, e.g., power and chemical plant wastewaters, metal finishing wastes, spent <br /> pulp liquors, emulsified oil streams, high soluble BOD (sugar) streams, and nonvolatile <br /> aqueous organic or inorganic streams containing dyes, acids and bases. <br /> Zero liquid discharge: Federal, state and local regulations governing industrial wastewater <br /> discharges continue to become more stringent. All direct dischargers must obtain a <br /> National Pollutant Discharge Elimination System (NPDES) permit that sets the maximum <br /> permissible limit for regulated pollutants. NPDES permits are subject to renewal, and <br /> permitted discharge levels may be lowered to reflect changes in the receiving water body. <br /> Dischargers therefore must take into account their wastewater production as well as <br /> possible future variations. <br /> In many industrial plants, evaporators can be installed to achieve zero liquid discharge of <br /> wastewater. These systems often consist of a falling film evaporator followed by a <br /> crystallizer and filter press (Fig 7). A rotary spray dryer may take the place of a crystallizer. <br /> However, precautions must be taken to control fugitive dust emissions. Figure 7 illustrates <br /> the water balance for a hybrid zero liquid discharge system used to treat a power plant's <br /> cooling tower blowdown. <br /> Water reuse: In this area evaporation has several advantages over conventional physical- <br /> chemical processes, one of the most significant being the high quality of the distillate. <br /> http://www.estormwater.com/print/4458 11/28/2012 <br />