1. Thickener Jae K. (Jim) Park, Professor
Dept. of Civil and Environmental Engineering
University of Wisconsin-Madison

2. Sludge Management Sources of sludge Primary sedimentation tank
Aeration basin or secondary clarifier
Screening and grinder
Filter backwash water (tertiary treatment for TS removal)
Common sludge management processes
Thickening
Stabilization
Dewatering
Disposal

3. Characteristics of Municipal Sludge
Constituents
Primary sludge
WAS
Digested sludge pH
Total dry solids, %
TVS, % dry wt.
Particle specific gravity
Bulk specific gravity
BOD5/TVS
COD/TVS
Alkalinity, mg/L as CaCO3
Cellulose, % dry wt.
Hemicellulose, % dry wt.
Lignin, % dry wt.
Grease and fat, % dry wt.
Protein, % dry wt.
Nitrogen, % dry wt.
Phosphorus, % dry wt.
Heating value, kJ/kg
3~8
60~90
1.3~1.5
1.02~1.03
0.5~1.1
1.2~1.6
500~1,500
8~15
2~4
3~7
6~35
20~30
1.5~4
0.8~2.8
15,000~24,000
6.5~7.5
0.5~1
60~80
1.2~1.4
1.0~1.005 -
2~3
200~500
5~10
5~12
32~41
2.5~7
2~7
12,000~16,000
30~60
1.3~1.6
1.03~1.04
2,500~3,500
5~20
15~20
1.6~6
1.4~4
6,000~14,000
1 kJ/kg = 0.43 Btu/lb

4. Gravity Thickening Accomplished in circular sedimentation basins
Degree of thickening: 2~5 times the incoming solids conc.
Max. achievable solids concentration: < 10%
Chemical and waste activated sludges: difficult to thicken under gravity
Bad design
Alternative

5. Gravity Thickener Design Criteria
Type of sludge
Inf. solids conc., %
Thickened conc., %
Hydraulic loading, m3/m2·d
Solids loading, kg3/m2·d
Solids captured, %
Overflow TSS, mg/L
Primary
1~7
5~10
24~33
90~144
85~98
300~1,000
Trickling filter
1~4
2~6
35~50
80~92
200~1,000
WAS
0.2~1.5
2~4
10~35
60~85
Combined primary + WAS
0.5~2
4~6
4~10
25~80
85~92
300~800
m3/m2·d × = gal/ft2 ·d
kg/m2 ·d × = 1b/ft2·d
Gravity thickener side water depth: 3 m (10 ft)
Detention period: 24 hrs
Hydraulic loading rate: 10~30 m3/m2·d = 250~740 gpd/ft2·d
To achieve the hydraulic loading rate, secondary effluent is often blended with the sludge fed into the thickener.
The sludge-blending tank may utilize mechanical mixing or air mixing.

6. Gravity Thickener Equipment
Generally circular concrete tanks with bottom sloping toward the center.
Equipment
Rotating bottom scraper arm
Vertical pickets
Rotating scum-collection mechanism with scum baffle plates
Overflow weir
Other configurations
Circular steel tank
Generally cheaper because of simplicity of construction, equipment installation, and operation and maintenance
Rectangular concrete and steel tanks
Conventional sludge - collecting mechanisms with deep trusses or vertical pickets are used to stir the sludge gently, thereby opening up channels for water to escape and promoting densification.

7. Gravity Thickener

8. Dissolved Air Flotation (DAF)
Primarily used to thicken the solids in chemical and WAS
Separation of solids is achieved by introducing fine air bubbles created under pressure of several atmosphere into the liquid, attaching to solids to cause flotation of solids
Degree of thickening: 2~8 times the incoming solids concentration
Max. solids concentration: 4~5%
DAF Variations
Pressurize total or only a small portion of the incoming sludge
Pressurize the recycled flow from the flotation thickener – preferred because it eliminates the need for high-pressure sludge pumps.

9. Dissolved Air Flotation (DAF)
The released air bubbles become attached to the suspended particles by one of the following mechanisms:
Condensation
Collision
Entrapment

10. Advantages of DAF Minimal space required
Capability to treat a wide variety of organic and inorganic solids and dissolved waste streams
Low retention time from wastewater stream to effluent ejection
Superior clarification of most waste streams
Easy to clean and maintain
Higher density sludge with low water content
Low installation cost for low flows (For small-scale application, the unit is typically delivered fully prefabricated)

11. Types of Pressurization Flow Schematic
Full Pressurization
The entire wastewater flow is injected with air and pressurized for dissolution of the air in the water. This flow then passes into the flotation cell where pressure is relieved. A pump equalization is used to return water to the pump in order to maintain flooded suction during periods of low flow.
Partial Pressurization
Only part of the wastewater flow is pressurized, and the remainder enters the flotation cell bypassing the pressurization step. A pump equalization return line is also employed in this mode to protect the pressurizing pumps.
Recycle Pressurization
Raw influent is introduced directly into the flotation cell. A portion (generally 50% of raw flow) of the system effluent is pressurized and recycled to the flotation cell where it is blended with the raw wastewater flow.

12. Design Parameters Air/solids ratio, A/S (mL air/mg solids)
sa= solubility of air at the required temperature, mL/L;
Sa = solids in incoming sludges, mg/L;
f = fraction of air dissolved at pressure P, usually 0.5~0.8
P = pressure in atmosphere = (p )/ (SI units)
= (p )/14.7 (US customary units);
p = gauge pressure, kPa (lb/in2);
q = recycle flow or a portion of incoming flow pressurized, m3/d; and
Q = sludge flow to the thickener, m3/day.
Type of sludge
Air/solids ratio
Solids loading rate, kg/m2·d
Hydraulic loading, m3/m2·d
Polymer added (mg/kg)
Solids captured, %
TSS in side stream, mg/L
Primary
0.04~0.07
90~200
90~250
1000~4000
85~95
100~600
Trickling filter
0.02~0.05
50~120
1000~3000
90~98
WAS
0.03~0.05
50~90
60~180
80~95
Combined primary + WAS
60~150
90~95

13. Design Parameters
Hydraulic Loading: Effective design ranges from 1.0 to 2.5 gpm/ft2day, depending upon the application.
Solids Loading: Design points for solid loadings range from 0.5 to 3.5 lbs/hr/ft2, depending on the application and the type of solids involved. It should be noted that any chemical additives used to promote coagulation and flocculation are generally included as solids determining the surface loading since the chemicals used are removed with the float from the system.
Air-to-Solid Ratio: Generally, air is injected in a range of 2 to 8% by volume. Depending upon the type of solids and application, the air-to-solids ratio ranges from to 0.1.

14. Gravity Belt Thickener

15. Belt Filter Press
The first section is a gravity belt thickener and can operate independently by diverting the slurry to discharge with the optional pivoting chute. 
The second stage is a filter press, which applies a mechanical pressure. 

16. Gravity Thickener (1)

17. Gravity Thickener (2)

18. Sludge Blending Tank

19. Sludge Metering System

20. Gravity Thickener(3)
Section view
Layout

21. Centrifuges (1)
Centrifugation is the process of separating solids from liquids by the use of centrifugal force.
The centrifuge is a cylindrical drum that rotates to develop the separating force.
When the slurry enters the interior of a rotating centrifuge, it is thrown out against the bowl wall.
The denser materials are separated first, and hug the interior wall of the rotating machine.

22. Centrifuges (2)
A helical screw conveyor fits inside the bowl. Rotating at a slightly lower rate that the bowl, it conveys solids from the zone of settling to the dewatering beach, where it is discharged to screw conveyors located below.
The main bowl is turned by an electric motor while the screw conveyor inside is controlled or slowed down using a hydraulic backdrive.
Prior to entering the centrifuge, sludge would have been conditioned with polymer. Larger and heavier particles are most easily captured by the centrifuge. Fine particles that cannot be settled separately must be agglomerated by chemicals (polymer) to a size that will settle.

23. Centrifuge - continued

24. Filter Press (1)
Dewatering is accomplished by pumping sludge into chamber (A) surrounded by filter cloths (B). As pumping pressure is increased, the filtrate is forced through the accumulated filter cake (C) and cloth, leaving the chambers full of solid filter cake. The chambers in HEI presses are formed by two recessed plates held together under hydraulic pressure. The hydraulic ram (D) moves the follower (E) against the stack of filter plates (F) closing the press. The ram continues to apply pressure of sufficient force to counteract the high internal compaction pressures. The head stock (G) and tail stock (H) are held in place by specially engineered side rail supports bars (I).
The filtrate passes through the filter cloth and is directed by channels in the plates and drain ports (J) to the head stock for discharge. The filtrate typically contains less than 15 ppm suspended solids. The filter cake is easily removed by simply reversing the hydraulic ram, thus opening the press. The lightweight plates may then be moved apart permitting the compacted cake to fall from the chamber.

25. Filter Press (2)