Vacuum Filtration

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. 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. The drum with horizontal axis is set in a tank with about one quarter of the drum submerged in conditioned sludge. 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 application of the vacuum is continued as the drum rotates out of the sludge and into the atmosphere. This pulls water away from the sludge, leaving a moist mat or cake on the outer surface. This mat is scraped, blown or lifted away from the drum just before it enters the sludge tank again. 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. 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 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. 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. Prolongation of the life of the material used as the filter may be effected by proper care. 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.

With regard to chemical use -diluted ferric chloride solutions (10% to 20%) usually give better results in the conditioning of the sludge. A high calcium lime is preferable or sludge filtration work. One should avoid excessive use of chemicals. The quantities of chemicals used for conditioning can be frequently reduced by careful control of the

Operational Considerations

• Conditioned sludge should be filtered as quickly as possible after the addition of the chemicals and adequate mixing.

• Continuous feeding is preferable to batch conditioning. In raw sludge filtration, fresh sewage solids and sludge filter more readily than stale or septic sludge. The applies to a raw sludge filtration.

• Completely digested sludge usually filters more readily than partially digested sludge.

• The concentration of sludge to be filtered is critical as sludge with the higher solid content usually filters more readily than that with a lower solid content.

• The presence of mineral oils and wastes from dry cleaning establishments makes sludge filtration difficult. Such wastes should, therefore, be kept out of the sewer system and disposed of separately.

• After every use the vacuum filter should be cleaned, and all sludge drained from the unit. This sludge and wash water should not be returned to the sludge storage tank but to the raw sewage channel or to a digester.

mixing and flocculation equipment. The maintenance of a uniform vacuum is necessary for satisfactory operation. Loss or fluctuations in vacuum usually indicate a break in the filter material, poorly conditioned sludge or uneven distribution of the sludge solids in the filter pan.

We should spend just a few minutes talking about the Rotary Drum Precoat Filter. This machine is used to polish solutions having traces of contaminating insolubles, so it is not a dewatering machine per se, but its use is often integrated into the process. To polish the solution the drum deck is precoated with a medium of a known permeability and particle size that retains the fines and produces a clear filtrate. The following materials are used to form the precoat bed: Diatomaceous Earth (or Diatomite) consisting of silicaceous skeletal remains of tiny aquatic unicellular plants; Perlite consisting of glassy crushed and heat-expanded rock from volcanic origin; Cellulose consisting of fibrous light weight and ash less paper like medium; Special ground wood is becoming popular in recent years since it is combustible and reduces the high cost of disposal. There are nowadays manufacturers that grind, wash and classify special timber to permeabilities which can suit a wide range of applications. These materials when related to precoating are wrongly called filter-aids since they do not aid filtration but serve as a filter medium in an analogy to the filter cloth on a conventional drum filter. The Precoat Filter is similar in appearance to a conventional drum filter but its construction is very different. The scraper blade on conventional drum filters is stationary and serves mainly to deflect the cake while it is back-blown at the point of discharge. The scraper on a precoat filter, which is also called "Doctor Blade", moves slowly towards the drum and shaves-off the blinding layer of the contaminants together with a thin layer of the precoating material. This movement exposes continuously a fresh layer of the precoat surface so that when the drum submerges into the tank it is ready to polish the solution. The blade movement mechanism is equipped with a precision drive having an adjustable advance rate of 1 to 10 mm/hr. The selected rate is determined by the penetration of fines into the precoat bed which, in turn, depends on the permeability of the filter aid. Once the entire precoat is consumed the blade retracts at a fast rate so that the filter is ready for a new precoating cycle. The cake discharges on conventional drum filters by blow-back hence a section of the main valve's bridge setting is allocated for this purpose. On precoat filters the entire drum deck is subjected to vacuum therefore there are two design options:

1. A conventional valve that is piped, including its blow-back section, to be open to vacuum during polishing. When the precoat is consumed its blow-back section is turned on to remove the remaining precoat heel over the doctor blade.

2. A valveless configuration in which there is no bridge setting and the sealing between the rotating drum and the stationary outlet is by circumferential "o" rings rather than by a face seal used on conventional valves.

The flow scheme for a conventional precoat filter station typically looks like that shown in Figure 9. The doctor blade discharge configuration for this machine is illustrated in Figure 10.

Precoat Fiter

Precoat Fiter

Figure 9. Precoat drum filter flow scheme for polishing operations.
Figure 10. Doctor blade discharge for precoat filter.
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