Belt Filter Press

A belt filter press (BFP) is a continuous-feed sludge dewatering machine with two porous moving belts that has a gravity drainage zone and mechanically applied pressure zones. Belt filter presses have been used in Europe first for dewatering paper pulp and later modified to dewater wastewater sludge. It was introduced in North America in the mid-1970s, mainly because of its ability to dewater secondary sludge economically, and its reduced energy requirements compared to centrifuges and vacuum filters. It is presently the most widely used dewatering equipment in the world.

Figure 3.20 shows schematics of belt filter presses commonly used in the United States and Europe. There are four basic processes in a BFP: conditioning, gravity drainage, low pressure compression, and high pressure compression. Polymer is injected into a sludge-polymer mixer placed in the feed line to the press. Flocculation occurs in a vertical column, a flocculation tank, or in a long stretch of pipe between the pipe mixer and the press. The conditioned sludge is then fed onto the gravity section of the belt via a feed chute. The solids-water mixture sits on the moving porous belt, allowing the water to drain through it. The water is collected in drain pans and routed to a sump. As the slurry moves on the belt, it is turned by plow blades. The plows greatly increase the gravity drainage process by clearing places for water to drain and by turning the solid mass on the belt. The slurry is stopped from running off the sides of the belt by restrainers and rubber seals. At the end of this gravity drainage section, the solids are usually a loosely structured cake.

Disk Belt Filter Press
Shear Zone 1 Pressure Zone Figure 3.20 Schematic of a belt filter press. (From Ashbrook Corporation, 1992.)

From the gravity drainage section, the cake falls onto the bottom belt and begins to be compressed between the belts, forming a wedge. The belts compress the solids, thereby applying a low pressure. The sandwiched solid material then passes over a series of rollers applying pressure to the solids at a gradual rate to expel nearly all the free water from the slurry. At the last roller, the belts separate and the cake is removed from the belts by scrapers. The belts are washed, normally at two different locations, to remove the solids that have been forced into the pores of the weave. The filtrate and washwater are piped to the base of the machine for discharging into the sump.

A BFP can be used to dewater a variety of sludge of initial concentration as low as 1%. A solids capture efficiency greater than 95% can be achieved, and as high as 99% has been reported. Other advantages of the BFP include low capital and operating costs, low power requirements, and ease of maintenance. However, belt filter press dewatering is polymer dependent. Polymer addition of 1 to 10 g/kg (2 to 20 lb/ton) on a dry weight basis is required. Disadvantages of the BFP include being sensitive to incoming feed characteristics, hydraulically limited in throughput, and higher odor potential. Machines are presently available with covers to contain odors and ventilation through odor control equipment.

The most common size of BFPs is 2 m in effective belt width, although machines as small as 0.5 m and lately as large as 3.5 m are available. Support systems for a BFP include sludge feed pumps, polymer feed equipment, sludge flocculation tank, sludge cake conveyor, and cake hopper. Figure 3.21 is a schematic of a belt filter press dewatering system.

Belt Conveyor System For Biosolids

I - Sludge Feed, 2 - Sludge day tank, 3 - Sludge feed pump, 4 - Polymer dilution tank, 5 - Polymer dilution water,

6 - Polymer feed pump, 7 - Flocculation tank, 8 - Belt filter press, 9 - Air to odor control system, 10 - Cake conveyor,

II - Cake dumpster, 12 - Air filter, 13 - Air compressor,

14 - Washwater pump, 15 - Filtrate tand, 16 - Filtrate pump.

I - Sludge Feed, 2 - Sludge day tank, 3 - Sludge feed pump, 4 - Polymer dilution tank, 5 - Polymer dilution water,

6 - Polymer feed pump, 7 - Flocculation tank, 8 - Belt filter press, 9 - Air to odor control system, 10 - Cake conveyor,

II - Cake dumpster, 12 - Air filter, 13 - Air compressor,

14 - Washwater pump, 15 - Filtrate tand, 16 - Filtrate pump.

Figure 3.21 Schematic of a belt filter press dewatering system.

Sludge loading rates for BFPs generally vary from 45 to 550 kg/h-m (100 to 1200 lb/hr-m), depending on the sludge type and feed concentrations. Hydraulic throughput generally ranges from 80 to 380 L/min-m (20 to 100 gpm/ m). Polymer use of 1 to 10 g/kg (2 to 20 lb/ton) on a dry weight basis is required. Table 3.15 presents typical performance data for various type of sludge, and Table 3.16 presents the results of testing a BFP manufactured by a company in Russia for dewatering various types of sludge. The installation for this testing is shown in Figure 3.21.

Design Example 3.4 A wastewater treatment plant produces 50,000 gpd (189,000 L/d) of anaerobically digested combined primary and waste activated sludge. The digested sludge contains 2.8% solids. Design a belt filter press dewatering system for a normal 7-h/d, 5-d/wk operation. Assume the following operating parameters:

cake solids concentration: 22% solids capture: 96%

wash water flow rate: 20 gpm (75.6 L/min)/m of belt width specific gravity of feed sludge: 1.02 specific gravity of sludge cake: 1.05 specific gravity of filtrate: 1.00

TABLE 3.15 Typical Performance Data for a Belt Filter Press

Solids (%)

Loading per Meter Belt Width

Polymer Dosage

Cake Solids (%)

L/min

kg7hr

gpm

lb7hr

g/kga

lb/tonfl

Typical

Range

Primary (PRI)

3-7

110-190

360-550

30-50

800-1200

1-4

2-8

28

26-32

WAS

1-4

40-150

45-180

10-40

100-400

3-10

6-20

15

12-20

PRI + WAS (50:50)

3-6

80-190

180-320

20-50

400-700

2-8

4-16

23

20-28

PRI + WAS (40:60)

3-6

80-190

180-320

20-50

400-700

2-10

4-20

20

18-25

PRI + Trickling filter

3-6

80-190

180-320

20-50

400-700

2-8

4-16

25

23-30

Anaerobically digested

PRI

3-7

80-190

360-550

20-50

800-1200

2-5

4-10

28

24-30

WAS

3-4

40-150

45-135

10-40

100-300

4-10

8-20

15

12-20

PRI + WAS

3-6

80-190

180-320

20-50

400-700

3-8

6-16

22

20-25

Aerobically digested

PRI + WAS

1-3

40-190

135-225

10-50

300-500

2-8

4-16

16

12-20

unthickened

PRI + WAS (50:50)

4-8

40-190

135-225

10-50

300-500

2-8

4-16

18

12-25

thickened

Oxygen-activated

1-3

40-150

90-180

10-50

200-400

4-10

8-20

18

15-23

WAS

Source-. Adapted from WEF, 1998. " Dry solids.

Source-. Adapted from WEF, 1998. " Dry solids.

TABLE 3.16 Results of Dewatering on a Belt Filter Press in Russia

Feed

Polymer Dosage

Belt

Cake

Solids

Solids

g/kg

lb/ton

Speed

Solids

Capture

Type of Sludge

(%)

dry solids

dry solids

(m/min)

(%)

(%)

Primary (PRI)

4-7

2.0-7.0

4.4-14.0

1.7-2.2

25-30

99-99.6

WAS, thickened

2-3

8-12

16-24

2

18-24

92-96

PRI + WAS (50 : 50)

2.6-6.0

4-8

8-16

1.7-2.8

18-27

91-98

Aerobically

2.0-4.4

3-8

6-16

2.6-3.0

18-20

96-97.5

digested WAS

Aerobically

4-6

3.8-6.4

7.6-12.8

2.0-2.6

20-22

96-99.5

digested

PRI + WAS

1. Weekly dry solids production rate

= (50,000gpd)(8.34 lb/gal)(7d/wk )(0.28)(1.02) = 83,376 lb/wk (37,825kg/wk)

2. HourlysoHdsprocessingrate =(5^wk)(7h/d)

Note: The 7-hour operation is one 8-hour shift less 1 hour for startup and cleanup.

3. BFP size required = 2382lb/hr/600lb/m.h = 3.97m. Provide a total of three 2-m BFPs (two for duty, one for standby).

Note: If space is limited, and also to save capital cost, two 2-m BFPs will be adequate if operation can be extended to two shifts a day if one of the two units is out of service. Operating time can also be extended as required for sustained peak sludge loads.

4. Check hydraulic loading rate

(5d/wk)(7hr/d)(60min/h)(4m total belt width) = 42gpm (159 L/min)/m of belt width

5. Sludge cake volume produced based on solids capture rate

= (2382lb/hr)(7hr/d )(0.96) = 16,007lb/d (7263kg/d)

16,007lb/d sludge cake flow rate =

The specific weight of the sludge cake is about 50 lb/ft3. Therefore, the volume of sludge cake for storage or truck loading purposes is

6. Filtrate flow:

daily sludge flow rate = (50,000gal/d)(7d/wk)( 5d/wk) = 70,000 gal/d (264,978 L/d)

filtrate from sludge = (70,000 - 8309) gal/d

washwater flow = ( 20gpm/m)(4m )(60min/hr)(7hr/d) = 33,600 gal/d (127,176L/d)

total of filtrate and wash water flow = (61,696 + 33,600) gal/d

7. TSS in filtrate = (2382 lb/hr) (7 hr/d) - 16,007 lb/d = 667 lb/d (303 kg/d)

8. Filtrate flow can be computed assuming a suspended solids concentration in filtrate (normally, 500 to 1200 mg/L) instead of the solids capture rate, and then developing solids balance and flow rate equations, and solving the equations simultaneously. The following computations illustrate this. Assume the filtrate solids concentration to be 800 mg/L.

Solids balance equation:

solids in sludge feed = solids in cake + solids in filtrate

(2382 lb/hr)(7 h/d) = (C-gal/d)( 8.34lb/gal)(1.05 x 0.22) + (F-gal/d )(8.34lb/gal)(0.0008)

where C is the cake flow rate (gal/d) and F is the filtrate flow rate (gal/d). Flow rate equation:

sludge feed rate + washwater flow rate = C + F

sludge feed rate = (50,000gal/d)(7/5) = 70,000gal/d (264,978L/d) wash water flow rate = (20gpm/m )(4m)(60min/hr)(7hr/d) = 33,600gal/d (127,176L/d) 70,000 + 3600 = 103,600 = C + F

Solving the solids balance and flow rate equations simultaneously yields,

F = 95,276 gal/d C = 8324gal/d solids in filtrate =(95,276 gal/d)(8.34lb/gal)(0.0008) = 636lto/d (288kg/d)

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