Metal Finishing Guide Book


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removed from the clarifier by means of a timer on the sludge pump. This reduces the possibility of solids drafting over the clarifier weir(s) because of a high sludge blanket. Finally, sludge stored in a thickener may increase in solids content to as much 3-4%. Increased solids content does two things: it decreases cycle time required by the 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 disposed 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 retreatment. After a certain length of time (2-4 hours), the chambers of the press are completely 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 discharged. 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 management 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 problems and take preventive steps to assure continuous compliance. Training is critical 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 management should review these results to evaluate trends so costs can be controlled and results improved. For instance, increases in sludge production without 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 confirm 593

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