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Pollution Prevention and Control Technologies for Plating Operations


Section 3 - Chemical Recovery

3.7 REVERSE OSMOSIS

3.7.4 Technology/Equipment Description

This subsection contains names and/or descriptions of commercially available reverse osmosis equipment that is manufactured and/or sold by vendor survey respondents or discussed in the literature. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.

Kinetic Recovery Corporation supplies an advanced reverse osmosis ("ARO") packaged system for chemical recovery from rinse waters. The system consists of: membrane modules (usually two, maximum of three); prefiltration (5 to 10 micron); conductivity controls for rinse water and concentrate quality test; miscellaneous controls (flow, pressure, level) for system automation; recycle and boost pumps for liquid transfer; pressure pumps for liquid/solids separation; internal tanks for concentrate storage; and an electrical control system, including PLC and operator's interface unit. A diagram and system specifications for the ARO system is shown in Exhibit 3-56 (33 kb). A diagram showing a typical layout for chromate conversion coating recovery is shown in Exhibit 3-57 (30 kb). The ARO typically has two membrane modules. The first membrane module is dedicated to the initial feed stream (rinse water) and the second is dedicated to increasing the concentration of the concentrate stream through successive passes. From the first module, the permeate is returned to plating rinses while the concentrate is held in the first internal storage tank. The concentrate is then passed through the second module for further concentrating. The permeate from the second module, depending on its conductivity, is returned to the rinse tank or directed to one of the internal storage tanks. The concentrate moves sequentially through the series of tanks to tank 3, where it is most concentrated, and then is pumped to the bath. The ARO's internal microprocessor changes operating parameters for each pass of concentrate through the second membrane. Pressures and process times are controlled in order to achieve higher concentrated solutions and improve membrane life (ref. 157, Kinetic Recovery Corporation file). The ARO operates in a pressure range of 900 to 1,000 psig, whereas most RO chemical recovery systems operate below 700 psig. According to the manufacturer, an ARO system can concentrate dilute solutions to at or near bath strength without any evaporation or additional concentration technology (however, for most applications a concentration of 40% to 70% of bath strength is targeted) (ref. 157). Results of bench scale tests on various plating and finishing solutions are shown in Exhibit 3-58.

The ARO is the only unit that was advertised for recovery of chromate conversion coating (it is also sold for traditional applications such as nickel plating). These conversion coating baths, which contain chromic acid, are usually operated at low temperatures and therefore, there is little surface evaporation to provide head room for recovered solution. According to the vendor, the multi-pass design and higher operating pressure of the ARO permit this application. Because the first membrane module of the ARO unit contains more dilute rinse waters and the second membrane module contains more concentrated solution, the life-spans of the two module sets are very different. The first module has a life-span of 1 to 2 years and the second module, 4 to 6 months.

Memtek Corporation manufactures an RO wastewater recycle system (Exhibit 3-59 ). This system is used to upgrade wastewaters from hydroxide precipitation systems for use as rinse water. There are four stages in the system. The first stage is a pre-RO conditioning step where, as necessary, pH adjustment is performed, an anti-scalant chemical is added to prevent precipitation of salts in the membrane elements, reducing agents are added to destroy residual oxidizers, and carbon filtration (optional) removes oxidizers and organics. The second stage is prefiltration, which is achieved by a cartridge filter unit (5 micron). Optional multimedia filtration can be purchased for wastewaters with high suspended solids loadings, where cartridge filters would require frequent replacement. The third step is reverse osmosis filtration. The wastewater is pumped through the unit at 200 to 600 psig, depending on the selection of RO elements. Cellulose acetate or thin film composite elements are used for rejection of dissolved salts. The choice of a membrane element is determined by the chemical characteristics of the wastestream and by the percent recycle desired. The permeate from the RO unit is collected in a storage tank equipped with water recycle pumping equipment. The RO unit is equipped with conductivity monitors to monitor product quality. Optional membrane cleaning systems are available.


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