Metal Finishing Guide Book

2013

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water and brackish waters. Metal-finishing wastewater requires a relatively high degree of pretreatment and filtration to protect RO membranes from fouling. Pretreatment processes can be designed so that soluble compounds, such as metal silicates and oxides, can be removed as precipitates by a filtration stage to such a high degree that membrane fouling can be significantly avoided; however, because of the wide variety of chemicals used in metal finishing, the water chemistry can be complex, highly variable over time, and difficult to accurately predict. The large commercial scale installations have had mixed results. Success on one plant effluent is not assurance that the next will be workable. In addition the concentration of brine that can be produced is relatively low so that large quantities of low-concentration brine require disposal. Electrodialysis ED has also found extensive commercial applications for desalination of brackish water; however, the efficiency of the process falls off unacceptably if the product water is not in the range of 500 to 600 mg/L TDS or higher. The process can produce a rather high concentration of brine and the water quality limitation can be overcome by using RO or ion exchange for high purity applications within the plant. Since ED is also a membrane process, similar concerns apply as mentioned for RO; however, ED is likely to prove somewhat more tolerant of varying water chemistry. This is due to the ability to frequently reverse the electrical potential across the membrane stack, which helps offset the fouling tendency, albeit at a sacrifice in capacity. Zero Liquid Discharge Systems Some firms, because of their location in small towns with small municipal treatment plants or because of discharge restrictions or other circumstances, have implemented treatment and recovery programs geared to recover all possible process water for recycle and reuse within the plant. Only solid sludge or brine slurry is produced for haul-away and disposal. These firms come as close as practical to having a zero discharge operation. While any of the foregoing methods can be applied individually to condition raw water, the recovery and conditioning of pretreated effluent requires a multistep process. It is not uncommon for a pretreated effluent to still have high TDS, mostly as sodium sulfate or sodium chloride. Some firms have successfully applied all or some of the following process steps to further process pretreated, highTDS effluent to recover clean, reusable process water and to achieve zero liquid discharge: sand filtration, carbon filtration, single- or two-stage RO followed by mixed bed ion exchange (if necessary). The reject from the RO system, which may still represent a considerable volume of dilute brine, can be further processed by vacuum evaporation to achieve a concentration close to the limit of solubility of the brine mixture, which is discharged from the evaporator at an elevated temperature. Upon cooling, salt crystals will separate and settle. The supernatant liquor can be mixed with the RO reject feed stream and circulated back through the evaporator. Meanwhile, the resulting salt slurry can be removed from the settling tank for further dewatering, which is not usually necessary, and readied for haul-away. A process of this nature is probably not economically viable unless the total daily volume of process water used in the plant is in the order of 50,000 gpd or more. 619

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