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filters normally operate at a rate of 100 to 200 m /d m . A section of a typical gravity filter is shown in Figure 7.2.

The operation of a gravity filter is as follows. Referring to Figure 7.2, drain valves C and E are closed and influent value A and effluent valve B are opened. This allows the influent water to pass through valve A, into the filter and out of the filter through valve B, after passing though the filter bed.

For effective operation of the filter, the voids between filter grains should serve as tiny sedimentation basins. Thus, the water is not just allowed to swiftly pass through the filter. For this to happen, the effluent valve is slightly closed so that the level of water in the filter rises to the point indicated, enabling the formation of tiny sedimentation basins in the pores of the filter. As this level is reached, influent and effluent flows are balanced. It is also this level that causes a pressure differential pushing the water through the bed. The filter operates at this pressure differential until it is clogged and ready to be backwashed (in the case of the rapid-sand filter). Backwashing will be discussed later in this chapter. In the case of the slow-sand filter,

Graded gravel

Concrete subfill

Coarse sand

Fine sand

FIGURE 7.3 Cutaway view of a pressure sand filter. (Courtesy of Permutit Co.)

•Adjustable jack legs

- Header lateral strainer system with expansible strainer heads

Graded gravel

Concrete subfill

Coarse sand

Fine sand

FIGURE 7.3 Cutaway view of a pressure sand filter. (Courtesy of Permutit Co.)

it is not backwashed once it is clogged. Instead, the layer of dirt that collects on top of the filter (called smutzdecke) is scraped for cleaning.

As shown, the construction of the bed is such that the layers are supported by an underdrain mechanism. This support may simply be a perforated plate or septum. The perforations allow the filtered water to pass through. The support may also be made of blocks equipped with holes. The condition of the bed is such that the coarser heavy grains are at the bottom. Thus, the size of these holes and the size of the perforations of the septum must not allow the largest grains of the bed to pass through.

Figure 7.3 shows a cutaway view of a pressure filter. The construction of this filter is very similar to that of the gravity filter. Take note of the underdrain construction in that the filtered water is passed through perforated pipes into the filtered water outlet. As opposed to that of the gravity filter above, the filtered water does not fall through a bottom and into the underdrain, because it had already been collected by the perforated pipes. The coarse sand and graded gravel rest on the concrete subfill.

Using a pump or any means of increasing pressure, the raw water is introduced to the unit through the raw water inlet. It passes through the bed and out into the outlet. The unit is operated under pressure, so the filter media must be enclosed in a shell. As the filter becomes clogged, it is cleaned by backwashing.

Thickened and digested sludges may be further reduced in volume by dewater-ing. Various dewatering operations are used including vacuum filtration, centrifuga-tion, pressure filtration, belt filters, and bed drying. In all these units, cakes are formed. We therefore call these types of filtration cake-forming filtration or simply cake filtration. Figure 7.4a shows a sectional drawing of a plate-and-frame press. In pressure filtration, which operates in a cycle, the sludge is pumped through the unit, forcing its way into filter plates. These plates are wrapped in filter cloths. With the filter cloths wrapped over them, the plates are held in place by filter frames in alternate plates-then-frames arrangement. This arrangement creates a cavity in the frame between two adjacent plates.

Clear filtrate outlet

Solids collect in frames Movable head Fixed head jfV-^ Plate .Framed

Solids collect in frames Movable head Fixed head jfV-^ Plate .Framed

Clear filtrate outlet

FIGURE 7.4 (a) Sectional drawing of a plate-and-frame press (from T. Shriver and Co.); (b) an installation of a plate-and-frame press (courtesy of Xingyuan Filtration Products, China).

Material enters under pressure '-rfzn

FIGURE 7.4 (a) Sectional drawing of a plate-and-frame press (from T. Shriver and Co.); (b) an installation of a plate-and-frame press (courtesy of Xingyuan Filtration Products, China).

Two channels are provided at the bottom and top of the assembly. The bottom channel serves as a conduit for the introduction of the sludge into the press, while the top channel serves as the conduit for collecting the filtrate. The bottom channel has connections to the cavity formed between adjacent plates in the frame. The top channel also has connections to small drainage paths provided in each of the plates. These paths are where the filtrate passing through cloth are collected.

As the sludge is forced through the unit at the bottom part of the assembly (at a pressure of 270 to 1,000 kPa), the filtrate passes through the filter cloth into the drainage paths, leaving the solids on the cloth to accumulate in the cavities of the frames. As determined by the cycle, the press is opened to remove the accumulated and dewatered sludge. Figure 7.4b shows an installation of a plate-and-frame press unit.

Figures 7.5 to Figure 7.7 pertain to the use of rotary vacuum filters in vacuum filtration. In vacuum filtration, a drum wrapped in filter cloth rotates slowly while the lower portion is submerged in a sludge tank (Figure 7.7a). A vacuum applied in the underside of the drum sucks the sludge onto the filter cloth, separating the filtrate and, thus, dewatering the sludge.

A rotary vacuum filter is actually a drum over which the filtration medium is wrapped. This medium is made of a woven material such as canvas. This medium is also called a filter cloth. The drum is made of an outer shell and an inner shell. These two shells form an annulus. The annulus is then divided into segments, which are normally 30 cm in width and length extending across the entire length of the drum. Figure 7.7a shows that there are twelve segments in this vacuum filter. The outer shell has perforations or slots in it, as shown in the cutaway view of Figure 7.6. Thus, each segment has a direct connection to the filter cloth. The purpose of the segments is to provide the means for sucking the sludge through the cloth while it is still submerged in the tank.

Each of the segments are connected to the rotary valve through individual pipings. As shown in Figure 7.7a, segments 1 to 5 are immersed in the sludge, while

FIGURE 7.5 A rotary vacuum filter in operation. (Courtesy of Oliver United Filters.)
FIGURE 7.6 Cutaway view of a rotary vacuum filter. (Courtesy of Swenson Evaporator Co.)

Filter

Filter

saturated, .„„ ^--K Stimng with filtrate °afceformra devicf

■Air connection Continuous rotary filter Moisture Cake 'rap ;

■Air connection Continuous rotary filter Moisture Cake 'rap ;

Vacuums receivers | Filtrate Hn Filtrate t

Filtrate^ ^Fih

Pump

Dry vacuum pump

Barometric

FIGURE 7.7 (a) Cross section of a rotary vacuum filter; (b) flow sheet for continuous vacuum filtration.

segments 6 to 12 are not. Pipes V1 and V2 of the rotary valve are connected to an external vacuum pump, as indicated in Figure 7.7b. The design of the rotary valve is such that when segments are submerged in the sludge such as segments 1 to 5, they are connected to pipe V1 through their individual connecting pipes. When segments are not submerged such as segments 6 to 12, the design is also such that these segments are connected to pipe V2. This arrangement allows for suction of sludge into the filter cloth over the segment when it is submerged (Vj) and drying of the sludge when the segment is not submerged (V2).

We can finalize the description of the operation of the vacuum filter this way. As the segments that had been sucking sludge while they were still submerged in the tank emerge from the surface, their connections are immediately switched from V to V2. The connection V2 completes the removal of removable water from the sludges, whereupon the suction switches to sucking air into the segments promoting the drying of the sludges. The "dry" sludge then goes to the scraper (also called doctor blade) and the sludge removed for further processing or disposal.

Figure 7.7b shows the fate of the filtrate as it is sucked from the filter cloth. Two tanks called vacuum receivers are provided for the two types of filtrates: the filtrate removed while the segments are still submerged in the tank and the residual filtrate removed when the segments are already out of the tank. Vacuum receivers are provided to trap the filtrate so that the filtrate will not flood the vacuum pump. Also note the barometric seal. As shown, this is in parallel connection with the suction vacuum of the filter. The vacuum pressure is normally set up to a value of 66 cm Hg below atmospheric. Any vacuum set for the filter will correspondingly exert an equal vacuum to the barometric seal, on account of the parallel connection. Hence, the length of this seal should be set equivalent to the maximum vacuum expected to be utilized in the operation of the filter. If, for example, the filter is to be operated at 51 cm,

51( 13.6) = (1 )AhH2c; AhH2c = 6.94m where 13.6 is the mass density of mercury in gm/cc, and 1 is the density of water also in gm/cc. Thus, from this result, the length of the barometric seal should be 6.94 m if the operational vacuum is 51 cm Hg. The design in Figure 7.7b shows the length as 9.1 m.

Figure 7.8a shows another type of filter that operates similar to a rotary vacuum filter in that it uses a vacuum pressure to suck sludge into the filter medium. This type of filter is called a leaf filter. A leaf filter is a filter that operates by immersing a component called a leaf into a bath of sludge or slurry and using a vacuum to suck the sludge onto the leaf. An example of a leaf filter is shown in Figure 7.8b. As indicated, it consists of two perforated plates parallel to each other, with a separator screen providing the spacing between them. A filter is wrapped over the plate assembly, just like in the plate-and-frame press. Each of the leaves are then attached into a hub through a clamping ring. The hub has a drainage space that connects into the central pipe through a small opening. As indicated in the cutaway view on the right of Figure 7.8a, several of these leaves are attached to the central pipe. Each of the leaves then has a connection to the central pipe through the small opening from the drainage space. The central pipe collects all the filtrates coming from each of the leaves.

In operation, a vacuum pressure is applied to each of the leaves. The feed sludge is then introduced at the feed inlet as indicated in the drawing. The sludge creates a slurry pool inside the unit immersing the leaves. Through the action of the vacuum, the sludge is sucked into the filter cloth. As the name implies, this is a rotary leaf filter. The leaves are actually in the form of a disk. The disks are rotated, immersing part of it in the slurry, just as part of the drum is immersed in the case of the rotary vacuum filter. As the immersed part of the disks emerge from the slurry pool into the air, the filtrate are continuously sucked by the vacuum resulting in a dry cake.

Leaf

Leaf

Central pipe:

Filtrate outlet

Perforated plates

Filter cloth-

Clamping rings

Filter leaf hub

Drainage space

Central pipe:

Filtrate outlet

Screw conveyor Discharge connection

Filter leaf hub Clamping rings

Filter cloth Perforated plates

Separator screen

Drainage space

Drainage space

FIGURE 7.8 A rotary leaf filter showing cutaway view at right end (courtesy of Swenson Evaporator Co.); (b) section of a leaf.

A mechanism is provided for the cake to drop into a screw conveyor below for continuous removal. This mechanism does not require opening of the case for removal of the cake.

It may be noticed that some of the filters discussed are operated continuously and some are not. For example, the rapid sand filter, the slow sand filter, the pressure filter, and the rotary vacuum filter are all operated continuously. The plate-and-frame press is operated as a batch. Thus, filters may also be classified as continuous and discontinuous. Only the plate-and-frame press is discussed in this chapter as a representation of the discontinuous type, but others are used, such as the shell-and-leaf filters and the cartridge filters. The first operates in a mode that a leaf assembly is inserted into a shell while operating and retracted out from the shell when it is time to remove the cake. The second looks like a "cartridge" in outward appearance with the filter medium inside it. The medium could be thin circular plates or disks stacked on top of each other. The clearance between disks serves to filter out the solids.

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