Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 535 of 707

dewatering equipment (filter press, centrifuge, belt press) and, as a rule of thumb, regardless of the type of dewatering equipment, the thicker the feed sludge, the drier the sludge cake. The objective is to reduce the volume to be dis- posed of by removing as much water as possible. The filter press is most often used in the dewatering of metal finishing sludges because generally it is made to handle smaller volumes of sludge, is simple to operate, and produces a dry, easily disposable filter cake. Sludge from the thickener, or directly from the bottom of the clarifier, is usually pumped via an air diaphragm pump to the filter press. The polypropylene filter media retains the solids while the liquid portion or filtrate flows through the media and discharges. Filtrate usually returns to the collection/equalization tank for retreat- ment. After a certain length of time (2-4 hours), the chambers of the press are com- pletely full and a filter cake of 25-35% solids has formed. The hydraulic pressure that had been holding the plates together is now released and the filter cake is dis- charged. Filter press operation requires little operator attention except at the beginning and end of a press cycle. Presses without an automatic plate shifter often require two people to separate the plates to discharge the cake, one on either side of the press. Cake that has had enough time to sufficiently dewater will literally fall out of the press upon opening. The highest operational cost involved with a filter press is the replacement of the filter cloths. Cloth life is directly dependent on the number of press cycles per year. The metal hydroxide sludges produced from treatment of metal finishing wastes are generally of moderate pH and nonabrasive. Cloth life of 1-2 years is common. Replacement of cloths is labor intensive, especially the caulked, gasketed variety, but all the cloths, even in a large press (10 ft3 ), can be changed in 3-4 hours. Because plates and cloths are usually of polypropylene construction, they can be routinely cleaned by immersion in an acid without damage. SYSTEM OPERATION AND PERFORMANCE The best system design may result in inadequate results unless operators and man- agement devote the necessary resources. These resources include time, talent, and training. Sufficient time is required for normal operation and routine preventive maintenance. The talent of motivated operators is necessary to anticipate prob- lems and take preventive steps to assure continuous compliance. Training is crit- ical for operators to understand how system performance is affected by changes in production, chemicals, or regulatory limits. The operator needs to keep a daily log listing volumes treated, chemicals consumed, sludge produced, and effluent results. Either the operator or man- agement should review these results to evaluate trends so costs can be con- trolled and results improved. For instance, increases in sludge production with- out corresponding increases in production may indicate increased drag-out losses, failure of recovery equipment, or changes in treatment chemistry. Regulatory authorities require timely and accurate analytical data to con- firm compliance with effluent limitations. Operators need daily analytical data to control system performance and to make needed adjustments to treatment chemistry. This is often accomplished using inexpensive troubleshooting analytical tools including pH papers in lieu of a hand-held pH meter, and potassium iodide-starch papers for cyanide oxidation process control. Quick and easy tests 534

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