Belt press filtration

Belt filter presses employ single or double moving belts to continuously dewater sludges through one or more stages of dewatering. All belt press filtration processes include three basic operational stages: chemical conditioning of the feed sludge; gravity drainage to a nonfluid consistency; shear and compression dewatering of the drained sludge. When dewater-"/>
Figure 7.5. A schematic diagram of a belt filter for sludge dewatering. (Courtesy of Falke Bruinsma at

ing a 50:50 mixture of anaerobically digested primary and waste activated sludge, a belt filter press will typically produce a cake solids concentration in the 18-23% range.

Fig. 7.5 depicts a simple belt press and shows the location of the three stages. The dewatering process is made effective by the use of two endless belts of synthetic fiber. The belts pass around a system of rollers at constant speed and perform the function of conveying, draining and compressing. Many belt presses also use an initial belt for gravity drainage in addition to the two belts in the pressure zone.


Centrifugal dewatering of sludge is a process that uses the force developed by fast rotation of a cylindrical drum or bowl to separate the sludge solids from the liquid. In the basic process, when sludge slurry is introduced to the centrifuge, it is forced against the bowl's interior walls, forming a pool

Figure 7.6. A schematic illustration of a centrifuge for dewatering of sludge.

Figure 7.6. A schematic illustration of a centrifuge for dewatering of sludge.

of liquid. Density differences cause the sludge solids and the liquid to separate into two distinct layers. The sludge solids cake and the liquid centrate are then separately discharged from the unit. The two types of centrifuges used for municipal sludge dewatering, basket and solid bowl, both operate on these basic principles. They are differentiated by the method of sludge feed, magnitude of applied centrifugal force, method of solids and liquid discharge, cost, and performance.

Basket centrifuge

The imperforate basket centrifuge is a semicontinuous feeding and solids discharging unit that rotates about a vertical axis. A schematic diagram of a basket centrifuge in the sludge feed and sludge plowing cycles is shown in Fig. 7.6. Sludge is fed into the bottom of the basket and sludge solids form a cake on the bowl walls as the unit rotates. The liquid (centrate) is displaced over a baffle or weir at the top of the unit. Sludge feed is either continued for a preset time or until the suspended solids in the centrate reach a preset concentration. The ability to be used either for thickening or dewatering is an advantage of the basket centrifuge. A basket centrifuge will typically dewater a 50:50 blend of anaerobically digested primary and waste activated sludge to 10-15% solids.

Bowl centrifuge

A solid bowl centrifuge is a type of centrifuge with a rotating bowl into which sludge is fed at a constant flow rate; chemicals or polymers can be added to aid the dewatering. The centrate obtained from the separation usually contains fine solids and is returned to the wastewater treatment systems. The cake is 70-80% of moisture and is discharged from the bowl by a screw feeder into a hopper or a convey belt. Sludge cakes with above 25% solids are desirable for disposal by incineration or landfill.

Filter press

The filter press is an intermittent dewatering process. A filter comprises a set of vertical, juxtaposed recessed plates, pressed against each other by hydraulic jacks at one end of the set. The pressure applied to the joint face of each filtering plate must withstand the chamber internal pressure developed by the sludge pumping system.

This vertical plate layout forms watertight filtration chambers, allowing easy mechanization for the discharge of cakes. Filter cloths tightly meshed are applied to the two grooved surfaces in these plates. Orifices feed the sludge to be filtered under pressure in the filtration chamber. They are usually placed in the center of the plates, allowing a proper distribution of flow, right pressure, and better drainage of sludge within the chamber. Solids sludge gradually accumulates in the filtration chamber until the final compacted cake is formed. The filtrate is collected at the back of the filtration support and carried away by internal ducts (Fig. 7.7).

The filter press has a number of advantages over other filtration equipment such as vacuum filters and centrifuges. Filter presses can operate well at variable or low-feed solids conditions. They can also produce a relatively dry cake because of the high-pressure differential they can exert on the sludge. A typical filter press operating at 100 psig will produce sludge with a solids content of 25-60% solids, depending on the chemicals used for precipitation. As a comparison, the basket centrifuge produces sludge with a solids content of 10-25%, and the vacuum filter produces sludge with 15-40% solids.

The disadvantages of the filter press include its batch operating cycle, the labor associated with removing the cakes from the press, and the downtime associated with finding and replacing worn or damaged filter cloths. The original filter press design consisted of alternating plates and frames, and these types of units were referred to as the plate-and-frame filter press. The new and improved design is the recessed plate filter press. The plates (usually constructed of polypropylene) are recessed on each side to form cavities, and they are covered with a filter cloth. The two types of presses work in basically the same manner. At the start of a cycle, a hydraulic pump clamps the plates tightly together and a feed pump

Figure 7.7. A picture of a filter press for sludge dewatering.

forces dilute sludge slurry into the cavities of the plates. The liquid (filtrate) escapes through the filter cloth and grooves that are molded into the plates, and is transported by the pressure of the feed pump to a discharge port. The solids are retained by the cloth and remain in the cavities. This process continues until the cavities are packed with sludge solids. The hydraulic pressure is then released and the plates are separated. The sludge solid or cake is loosened from the cavities and falls into a hopper or drum.

Modern recessed plate filter presses may be equipped with the following design enhancements:

• Lightweight polypropylene plates that exhibit good chemical resistance and provide a long service life.

• Gasketed plates that reduce leakage during the filtration cycle. These replace nongasketed types where the filter cloth extends beyond the plate to form the seal between plates.

• An air blow-down manifold that is employed at the end of the filtration cycle to drain remaining liquid in the system, thereby improving sludge dryness and aiding in the release of the cake.

• Microprocessor control, which permits unattended operation throughout the filtration cycle. Capable of automatically adjusting the feed pressure and deactivating the pump whenever hydraulic pressure falls below preset limits.

• Manual, semiautomatic or automatic plate shifters that are used to separate the plates prior to releasing the sludge cake."/>
Figure 7.8. A freezing-thawing sludge bed. (Courtesy of Falke Bruinsma at

Sludge freezing

Freezing and then thawing sludge will transform the structure of the sludge amenable to draining of interstitial water trapped in the sludge. The expansions of sludge structures due to formation of ice crystals during freezing apparently also help loosen the binding of water molecules to the sludge solids. Freezing and thawing processes of waste-activated sludge have been investigated extensively (e.g., Reed et al., 1986). It is found that surface water can be expelled by freezing. When water in sludge begins to freeze, it creates a thin upper layer that sends needles into the sludge. As ice growth continues, the smaller the solid particles are, the faster the advancing ice front moves them. Some large particles (perhaps greater than 100 ^m) cannot be pushed in front of the ice and are trapped within the frozen mass without being moved. In time, the ice crystals dehydrate captured sludge flocs, pushing the particles into more compact aggregates.

The freezing/thawing process can effectively take place in a "freezing bed" as proposed by Martel (1989) (see Fig.7.8). The freezing bed oper-"/>
Figure 7.9. A sample of freezing-and-thawing treated sludge from anaerobic digester. (Courtesy of Falke Bruinsma at

ates somewhat like a sludge drying bed, except that the sludge is applied in thin layers during the winter months and allowed to freeze. During warmer weather, the sludge thaws and the water drains out, leaving a dry residue (Fig. 7.9).

The freezing/thawing treatment of activated sludge is not economically feasible unless natural freezing and thawing are used. In the United States, the freezing and thawing method for dewatering of sludge is not suitable for regions south of the Mason-Dixon line, California, most of Arizona and New Mexico, and parts of the northwest coastal states. A simple equation that can be used for preliminary assessment of feasibility of the freezing and thawing method for sludge dewatering correlates the total depth of sludge that could be frozen if applied in 8 cm increments with the maximum depth of frost penetration (Equation 7.1) (Reed et al., 1995):

2Y = 1.76 (Fp) - 101 (metric units) = 1.76 (Fp) - 40 (imperial units)


2Y = the total depth of sludge that can be frozen in 8 cm (or 3 in) layers during the warmest design year, cm or in Fp = the maximum depth of frost penetration, cm or in

Sludge drying

Sludge drying is achieved through vaporization of water in sludge. There are two categories of drying sludge. One is the drying bed type; the other is a mechanical device type that requires auxiliary heat to increase vaporization of moisture in sludge. Drying by natural means is possible only during a long period of time. Faster and smaller, but also more cost-intensive, are mechanical processes. Sludge drying beds are used for small communities for dewatering sludge from wastewater treatment. The beds are basically a constructed storage area for holding sludge; dewatering (drying) is achieved by draining the sludge by gravity and vaporization of moisture in the sludge exposed to the atmosphere. Some designs employ air to create a vacuum under the draining system of the bed. After drying, the sludge is destined for landfill. The principal advantage of drying beds is low cost and low maintenance. The disadvantage is the cost of removing sludge and replacing draining bedding (sand); so the drying bed type of dewatering operations is suitable for large communities with populations over 20,000.

Mechanical processes of heating sludge include the flash dryer (Fig.7.10), spray dryer (Fig. 7.11), rotary dryer (Fig. 7.12), and multiple-hearth incinerator (discussed below in the section "Incineration"), and multiple-effect evaporator (Fig. 7.13).

Flash dryers involve pulverizing sludge in a cage mill or in the presence of hot gases. The process is based on exposing fine sludge particles to turbulent hot gases long enough to attain at least 90% solids content. The dried sludge may be used as soil conditioner or it may be incinerated.

Spray dryers typically use centrifugal force to atomize liquid sludge into a spray that is directed into a drying chamber where it contacts with hot air that rapidly dries the sludge mist into powders. The flow directions of hot air and the sludge streams are either concurrent or counter-current.

Rotary dryers function as horizontal cylindrical kilns. The drum rotates and may have plows or louvers that mechanically mix the sludge as the

Figure 7.10. A schematic diagram of a flash dryer for sludge dewatering.

Figure 7.10. A schematic diagram of a flash dryer for sludge dewatering.

drum turns. There are many different rotary kiln designs, utilizing either direct heating or indirect heating systems. Direct heating designs maintain contact between the sewage sludge and the hot gases. Indirect heating separates the two with steel shells.

Multieffect evaporators in sludge drying operations use the proprietary multieffect Carver-Greenfield process in which dewatered sludge is mixed with oil. This mixture, which can be pumped easily, is pumped through a series of evaporators (multieffect evaporation system) that selectively remove the water in the sludge, which has a lower boiling point than the oil. The oil maintains the mixture in a liquid state, even when virtually all the water has been removed. The product of this process, an oil and dry sludge mixture, is put through a centrifuge to separate the dry sewage sludge solids from the oil. The recovered oil can be reused in the process.

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