Such care includes washing of the filter material with the spray jets after every period of use, removal of grease and fats with warm soap solution if clogged, treatment with diluted hydrochloric acid for removal of lime encrustations, maintenance of scraper bade in careful adjustment to filter drum to prevent tearing of the filter material. Prolongation of the life of the material used as the filter may be effected by proper care. While operating costs, including conditioning of sludge for vacuum filtration, are usually higher than with sludge beds, filtration has the advantage of requiring much less area, is independent of seasons and weather conditions, and can eliminate the necessity for digestion since raw sludge can be dewatered sufficiently to be incinerated. The moisture content in the sludge cake also varies with the type of sludge from 80 to 84 percent, for raw activated sludge to 60 to 68 percent for well digested primary sludge. For various sludge this rate may vary from a low of 2.5 for activated sludge to a high of 6 to 11 for the best digested primary sludge. The common measure of performance of vacuum filters is the rate in pounds per hour of dry solids filtered per square foot of filter surface. This mat is scraped, blown or lifted away from the drum just before it enters the sludge tank again. This pulls water away from the sludge, leaving a moist mat or cake on the outer surface.
The application of the vacuum is continued as the drum rotates out of the sludge and into the atmosphere. Valves and piping are so arranged that, as a portion of the drum rotates slowly in the sludge, a vacuum is applied on the inner side of the filter medium, drawing out water from the sludge and holding the sludge against it. The drum with horizontal axis is set in a tank with about one quarter of the drum submerged in conditioned sludge. The vacuum filter for dewatering sludge is a drum over which is laid the filtering medium consisting of a cloth of cotton, wool, nylon, dynel, fiber glass or plastic, or a stainless steel mesh, or a double layer of stainless steel coil springs. We have already discussed this important technology in Chapter 5, but a review may be helpful in placing its importance to sludge processing into perspective.