Waste water treatment costs reduced by 50% with rotary vacuum precoat filter - Case Study
The solids separation system at Lindc's waste water treatment facility had high operating, maintenance and sludge disposal costs. High operating costs were caused by the large pumps and special treatment chemical required to operate the filter, and frequent cleaning, repair and replacement of filter elements resulted in high maintenance costs.
Union Carbide Corporation's Linde Division in Tonawanda, N.Y. fabricates and assembles process equipment. Surface treatment of metal parts, such as cleaning, etching and passivation, produces waste water streams containing acid, chromate, and substrate metals, including traces of heavy metals.
The treatment system for the acidic, heavy metal bearing waste water only needed to handle about 30 gpm, yet it was required to meet all the stringent standards for discharge to the municipal sewer system. Of particular importance is the removal of heavy metals. The total treatment system, which provided pH adjustment, chromate reduction, heavy metal precipitation and solids removal, is shown in Figure 1. The filtration system, though providing excellent removal of metal hydroxide solids from the discharge water, required FeS04 flocculant and a high operating pH (10.5), and suffered frequent downtime for cleaning and repairs. In addition, the resultant sludge contained only 10% solids, which necessitated expensive disposal.
In May 1982, Union Carbide installed a small rotary vacuum precoat filter on a trial basis. Using a precoat of coarse diatomite filter aid, the filter not only removes metal hydroxide from the treated waste water, but also produces a sludge that, at 40% solids, is sufficiently dry to allow disposal in other than expensive chemical landfills.
The 3' diam, 1' wide rotary filter is precoated with 50 lh of diatomite to form a 2' cake. No body feed is required to aid in filtration. During operation, a knife blade scrapes off the filter cake and spent diatomite at a rate proportional to the drum speed. A sensing device automatically adjusts this rate so as to maintain an optimum operating vacuum level (in this instance, 19-20' Hg.) This automatic scraping feature allows the precoat lo last for 24-30 hr before it is all scraped off and must be replaced.
Tests of the rotary vacuum precoat niter also demonstrated that FeSO, was not required to aid in the filtration, nor was a high filtration pH required. Reduction of filtration pH to only that required to precipitate the heavy metals allowed elimination of the post treatment pH adjustment step. Figure 2 shows the final flowchart of the treatment system.
The rotary vacuum precoat filter with diatomite precoat performs the same functions as were previously accomplished with the high pressure filter and a centrifuge, and docs these with less total horsepower, fewer water treatment chemicals, reduced maintenance, and lower water content in the sludge. AH of these improvements have added up to a 50% reduction in the total cost of waste water treatment.
On a trial-basis, the small rotary vacuum precoat filter satisfactorily handled all the waste water treatment required at present plant production rates. Based on full plant operating rates and the installation of an appropriate sized filter, total savings from using this system are estimated at $92,000/yr.