Pressure Filter Press

Pressure filter press dewatering is a batch process in which dewatering is achieved by forcing the water from the sludge under high pressure. It produces a cake that is drier than that produced by any other dewatering alternative. Another advantage is that the high solids capture results in good filtrate quality. Disadvantages include high capital cost, relatively high operation and maintenance costs, high chemical costs, and a large area requirement for the equipment in small wastewater treatment plants. The area requirements for large treatment plants are relatively small because presses are available with large plates that dewater several tons of sludge per hour. Additional plates can be added to a press without a significant increase in floor area.

Among the various types of pressure filter presses that have been used for dewatering sludge, the two most commonly used are the fixed-volume recessed plate filter press and the variable-volume recessed plate filter press.

Fixed-Volume Recessed Plate Filter Press The fixed-volume press consists of a series of recessed plates held in a frame and pressed together either hydraulically or electromechanically between a fixed end and a moving end (see Figure 3.22). Volume for the sludge to be dewatered is provided by the recesses on both sides of the plates. A filter cloth is mounted over the two surfaces of each plate. The plates have drainage ports between the filter cloths and the plate surfaces to drain the filtrate.

solids capture solids in feed - solids in filtrate solids in feed x 100

Slurry Inlet (Top or Bottom)

Caki

Caki

Minute Set Bulked Sludge Wastewater

Cake Under Compresson

Cake Under Compresson

Shape of filter chamber during filtration

Shape of filter chamber during cake compression by diaphragm

(c) Schematic of a variable volume press assembly Figure 3.22 Recessed plate filter press.

A typical dewatering cycle begins with the closing of the plates. Chemically conditioned sludge is fed until the flow rate drops to 5 to 7%: usually, 20 to 30 minutes. The pressure at this point is generally the desired maximum, typically 226 to 1550 kN/m2 (40 to 225 psi) and is maintained for 1 to 3 hours. During this time, more filtrate is removed and the desired cake solids concentration is achieved. The plates are then separated mechanically, and the cake is dropped from the chambers onto a truck or conveyor belt for removal. The cake thickness varies from 25 to 38 mm (1 to 1.5 in.) and solids concentration varies from 35 to 50%. Following cake removal, the filter press is washed and ready for the next cycle. Cycle time varies from 1.5 to 4 hours.

Variable-Volume Recessed Plate Filter Press The variable-volume press, commonly called the diaphragm press, is similar to the fixed-volume press except that a diaphragm is placed behind the media as shown in Figure 3.22. A dewatering cycle begins as conditioned sludge is fed into each recessed chamber at a sustained pressure of about 690 to 860 kN/m2 (100 to 125 psi). Once the chambers are filled and the filter cake formation is started, the sludge feed pump is turned off automatically based on a set instantaneous sludge flow rate, filtrate flow rate, or time. Water or air under high pressure (normally, water, because of safety concerns) is then pumped into the space between the diaphragm and the plate, squeezing the already formed cake and releasing additional water from the cake. Typically, 15 to 30 minutes of constant pressure at 1380 to 2070 kN/m2 (200 to 300 psi) are required to dewater the cake to the desired solids content. At the end of the cycle, the water is returned to a reservoir, plates are automatically opened, and sludge cake is discharged.

The diaphragm press differs significantly from the fixed-volume press in that the volumetric capacity is generally less, the cake is much thinner, there is greater flexibility in achieving the desired level of cake dryness, and the press is highly automatic. However, a diaphragm press requires considerable maintenance.

Performance Pressure filter press is an advantageous choice for sludge of poor dewaterability or for cases in which it is desirable to dewater sludge to solids content of greater than 35%. If sludge characteristics are expected to change drastically over a normal operating period, variable-volume units are recommended rather than fixed-volume units.

Pilot testing and scale-up from pilot to full scale is recommended for designing a dewatering system. All major manufacturers have small skid-mounted units that are simple to test and operate. Pilot testing generally results in cake solids 2 to 4% higher than that achieved in a full-scale unit.

Tables 3.17 and 3.18 contain typical performance data for fixed- and variable-volume units, respectively. Table 3.19 shows the performance of filter press dewatering of municipal sludge in Russia and Ukraine. These data indicate significant variations that reflect differences in filtration characteristics of different types of sludge.

TABLE 3.17 Performance Data for a Fixed-Volume Filter Press

Conditioning

TABLE 3.17 Performance Data for a Fixed-Volume Filter Press

Conditioning

Feed

Chemicala

Cake

Cycle

Solids

FeCl3

Lime'

Solidsc

Timed

Type of Sludge

(%)

(%)

(%)

(%)

(h)

Primary (PRI)

5-10

5

10

45

2

Primary + WAS (50 : 50)

3-6

5

10

40-45

2.5

Primary + WAS (50 : 5o)

1-4

6

12

45

2.5

Primary + trickling filter

5-6

6

20

38

2

Primay + ferric chloride (FC)e

4

10

40

1.5

Primary + WAS (FC)e

8

5

10

45

3

WAS

5

7.5

15

45

2.5

Primary + two-stage high lime

7.5

50

1.5

Digested (PRI)

8

6

30

40

2

Digested (pRI + WAS)

6-8

5

10

45

2

Digested (pRI + WAs) (FC)e

6-8

5

10

40

3

Digested (pRI + WAs) (50: 50)

6-10

5

10

45

2

Digested (pRI + WAs) (50: 50)

1-5

7.5

15

45

2.5

Source: Adapted from WEF, 1998. a As % of dry solids. b As CaO.

c Includes conditioning chemicals.

d Length of time from initiation of feed to fed pump termination; excludes cake discharge time.

e Ferric chloride used as a coagulant aid in the secondary process.

Source: Adapted from WEF, 1998. a As % of dry solids. b As CaO.

c Includes conditioning chemicals.

d Length of time from initiation of feed to fed pump termination; excludes cake discharge time.

e Ferric chloride used as a coagulant aid in the secondary process.

TABLE 3.18 Performance Data for Variable-Volume Diaphragm Filter Press

Feed

Conditioning Chemicala

Cake

Total Cycle Time

Solids

FeCl3

Lime

Solids

Type of Sludge

(%)

(%)

(%)

(%)

(min)

Primary sludge(Metropolitan Wastewater Commision

12

2.9

13

48

21

Minneapolis-St. Paul, MN) Heat-treated primary + WAS (25 : 75) (Metropolitan Wastewater Commision Minneapolis-St. Paul, MN)

12-16

0

0

52

22

Source: Adapted from WEF, 1998. a As % of dry solids.

Source: Adapted from WEF, 1998. a As % of dry solids.

TABLE 3.19 Performance of Filter Press Dewatering of Municipal Sludge in Russia and Ukraine

Type of Sludge

Feed

Solids

Conditioning Chemical"

Lime

Filter Dry Solids Capacity (kg/m2 • cycle)

Raw primary (PRI) 3-6

Raw WAS 1-2

Digested primary 3.5-6.0

Digested WAS 2.0-3.5

Mesophilically digested 3-5

(PRI + WAS) Thermophilically digested 3-5

Mesophilically digested 3-4 (PRI + WAS)

12-18

Polymer 0.2-0.4

7-13

7-11

30-38

24-30

b Dewatered with variable diaphragm filter press.

Design Considerations The principal design elements in a filter press dewatering facility include chemical conditioning system, precoat system, sludge feed system, washing system, and cake handling. Standby capacity to overcome excessive downtime for equipment maintenance, adequate clear space around the machine, parts and material-handling equipment such as a bridge crane, adequate ventilation of the building, and a sludge grinder ahead of the conditioning tank are other elements that should be incorporated into the design.

For conditioning, most facilities use ferric chloride and lime added in batches to sludge contained in an agitation tank and the conditioned sludge is pumped from the tank to the filter press. The lime system requires a silo, a slaker, lime slurry tank, and feed pumps. Ferric chloride handling requires a storage tank and feed pumps.

Sludge with high biological content or some industrial sludge often has a tendency to stick to the filtration media. In such instances, a precoat system aids cake release from the filtration media and protects it from premature blinding. Precoat material can be fly ash, incinerator ash, diatomaceous earth, or cement-kiln dust and is pumped in slurry form to the chambers of the filter press before the start of each filtration cycle.

The sludge feed system should be designed to pump 1.9 to 126 L/s (30 to 2000 gpm) of a viscous, abrasive slurry at pressures of 276 to 1550 kN/m2 (40 to 225 psi). Reciprocating ram high-pressure pumps with variable-speed drives are generally used in such a system. An alternative is to use a combination of pumps and pressure vessels.

Because recessed plate pressure filters operate at high pressures and because many use lime for conditioning, the filter cloth will require routine washing with high-pressure water as well as periodic washing with acid to remove the scale buildup from the clothes and from the walls of the plates. Practices vary according to the particular sludge and type of filter cloth. Some facilities wash with water after 20 cycles and wash with acid after 100 cycles.

Providing suitable cake breakers is a function of the structural properties of the cake and the ultimate solids disposal method. Cake dropped from the filter press is usually friable enough that use of breaker wires or bars beneath the press is sufficient, especially when cake is dropped directly onto trucks for hauling away. If further processing of sludge cake, such as incineration, is required, additional cake breaking equipment, such as a cake breaker screw conveyor, needs to be provided.

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Responses

  • sandra
    Does a pressed sludge filtrate add to VOC emissions?
    6 months ago
  • robert
    How much water is removed from sludge in a filter press?
    5 months ago
  • tuomo
    How much air pressure should give for filter press cake drying?
    2 months ago

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