Metal Finishing Guide Book

2013

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be sized for a maximum surface loading rate of 500 gal/day/ft2 of tank surface. Most commonly employed clarifiers are of the lamella type or inclined plate variety. These units are sized based on volumetric flow rate per square foot of plate pack area projected on the plate incline, or cosine of the degree of plate angle; typically 60O. Recommended loading rates are 0.2-0.4 gal/min/ft2 of projected plate area, and a total suspended solids (TSS) concentration of 500 ppm or less. Units are manufactured in basic hydraulic flow sizes, i.e., 30 gal/min or 75 gal/min, etc. In those cases of high TSS loads (500 ppm or higher), it is not advisable to size a unit based solely on flow. In these high solids load applications, clarifier selection should be based on 1 lb TSS per hour for each 20 ft2 of projected clarifier settling area. Manufacturers will supply design and operational information for their specific unit. As a general rule, it is important to evacuate sludge as it accumulates to prevent its buildup into the plate pack area. This creates blockages and increases the upflow velocity in the open areas and carries TSS with the high flow. Monthly draining is advisable to minimize ratholing and solids concretion. EFFLUENT POLISHING At times, clean water that overflows from a clarifier will require further removal of suspended solids or polishing to meet more stringent discharge requirements. This may be for water reuse or simply as insurance in case of a system malfunction. Sand filters, devices consisting of one or more layers of various sizes and types of granular media, are typically used. Gravel, sand, anthracite, garnet, and activated carbon are common media. The size and number of filters is, as with a clarifier, dependent on the volume of wastewater to be filtered and the surface area of the filter media. Gravityoperated sand filters usually are loaded at 0.25-0.5 gpm/ft2, whereas pressure sand filters can operate in the 5.0-10.0 gpm/ft2 range, depending on the suspended solids of the effluent. Most sand filters need to be periodically cleaned or "backflushed" to remove the solids that have built up. Clean water, process water, or dilute acid solutions may be used for this back flushing. Backflush waters are generally returned to the collection or equalization tank and returned to the treatment system. Pressure sand filters require less backwash water than larger gravity types. Operationally, care must be taken to ensure that pumps feeding or backflushing the filters are operating at design capacity to ensure proper loading and adequate cleaning of the media. Sand filter media are rarely replaced, except when a severe system upset causes solids to block the water distribution headers. SLUDGE THICKENING AND DEWATERING Sludge (settled solids) produced from treatment of metal finishing wastes generally contains between 1.0 and 2.0% total solids. Disposal of such a watery sludge is very expensive. Most medium and large generators of wastewater choose to thicken and dewater sludge, thus reducing the volume of waste to be disposed. A sludge thickener, although not always necessary prior to dewatering, serves several worthwhile functions. First, it creates storage volume for the sludge in the event that the dewatering equipment is not in operation. Second, it allows for a consistent sludge blanket level in the clarifier. Sludge can be intermittently 592

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