Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 570 of 707

Typical treated wastewater is: • Very low in dissolved metals • Very high in total dissolved solids (TDS) from neutralization and treatment • Consistent pH, typically slightly alkaline from metal precipitation process • At room temperature • Often mixed with residuals such as oils, soaps, or emulsifiers Both money and labor were spent to treat this wastewater and money was spent to purchase the water and send it to the sewer. Therefore, reusing the water in the process is a means of recovering a portion of that cost. A reverse osmosis (RO) system is one means of recovering at least 50% of this treated water and making it very useable as rinse water again. Reverse osmosis is a technology that filters water with a membrane and allows only water molecules and small amounts of sodium, chloride, or potassium to pass through the membrane (0.5 to 3% leak- age of salts is typical). The actual process works by applying pressure to the "dirty" water, which forces the clean water through the membrane and leaves the larg- er molecules behind. ADVANTAGES OF RO FILTRATION: • Removes everything: ions*,bacteria, viruses, solids, dissolved solids • Relatively simple, low maintenance system DISADVANTAGES OF RO FILTRATION: • Low temperature water produces lower pure water yields • Higher TDS water produces lower pure water yields • Tends to leak small amounts of single charge ions (Na+, K+, Cl-) • Membrane can foul rapidly if suspended solids are high (may require pre-filtration with an ultrafilter) • The RO process is relatively slow such that the most economical RO unit will be running during both production and non-production hours (filtering stored treated wastewater and storing filtered water during off hours) Current technologies allow up to about 75% fresh water yields. More typical yields are 50% at optimum conditions of temperature and minimal TDS levels. Even with recovery rates of 50%, typical RO systems have a payback of one to two years with water savings. As an example from a case study, an RO unit rated for 15,000 gallons per day water recovery would cost approximately $20,000 and save approximately 3.2 million gallons per year ($17,000 savings/year). Before purchasing an RO system, it is important to implement other water sav- ings measures first so that the RO system is properly sized for the reduced 569

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