Chemical Conditioning

Chemical conditioning is the most common conditioning process for sludge thickening and dewatering. Conditioning by adding chemicals can be viewed as coagulation or flocculation by neutralization of colloidal surface charge by oppositely charged organic polymers or inorganic chemicals. Particle size is the most important characteristic of the dewaterability of sludge. By adding chemicals, the particle size increases and the bound water decreases. Different sludges have different dewatering characteristics, and the same sludge source varies from plant to plant. Consequently, the type and dose of chemical addition must be determined on a case-by-case basis.

Tests for Conditioner Selection The most common and recommended procedures used to determine sludge conditioning effectiveness for thickening and dewatering are presented below.

The Buchner funnel test for determining the specific resistance of sludge is obtained by measuring the volume of filtrate collected from a sludge sample and the time it takes to filter. Using different conditioning chemicals and varying the doses of conditioning allow choosing the best reagents and the optimum dose for a particular sludge.

Capillary suction time (CST) is a rapid and simple method for screening dewatering aids. It relies on gravity and capillary suction of a piece of thick filter paper to draw out the water from a small sample of conditioned sludge. The sample is placed in a cylindrical cell on top of a chromatography-grade filter paper. The time in seconds it takes for the water in sludge to travel 10 mm in the paper between two fixed points is recorded electronically as the CST. This test is usually done to determine the optimum dose of polymer in a particular dewatering process. Because CST is a function of solids concentration and a dilute sludge usually has a low CST value, it should be compared for sludges of the same suspended solids concentration. CST for unconditioned sludge is about 200 seconds. A CST of 10 seconds or less is considered a good value for superior dewatering performance.

The jar test is the easiest and most common method used to evaluate chemical conditioning. In this test, 1-L samples of sludge with different con ditioner concentrations are mixed rapidly. The samples are then flocculated by reducing the stir speed for a few minutes and then are allowed to settle. Liquid clarity and the densely settled sludge blanket indicate the ability of the chemical to condition sludge.

Among the foregoing tests, specific resistance using the Buchner funnel test is the best measure for comparing conditioning chemicals and dosages for different sludges. Studies conducted in Russia (Turovskiy, 2000) on the effect of digestion on the dewaterability of sludge have validated this. Other conclusions from the studies include: (1) the same type of sludge from different wastewater treatment plants has different dewatering characteristics; (2) raw sludge has less specific resistance than aerobically or anaerobically digested sludge; and (3) mesophylically digested sludge has less specific resistance than thermophylically digested sludge. The studies and their results are described below in detail.

Sludge Conditioning Studies The degree of sludge conditioning required depends on the sludge source (primary clarifiers, activated sludge treatment system, trickling filter, etc.), sludge quality, subsequent sludge treatment process, and in general, the degree of thickening or dewatering required. Much research has been done in the field of sludge conditioning, thickening, and dewatering. However, it is not well understood how the treatment of sludge affects its dewatering characteristics.

The studies were conducted using anaerobic digester simulators with sludge samples from various treatment plants in Russia, Lithuania, and Poland. Data obtained from treatment plants in Bulgaria, Finland, France, Germany, Hungary, and the United States were also analyzed. The digester simulators worked both in the mesophylic regime (35°C) and the thermophylic regime (55°C). The following parameters of raw and digested sludge samples were measured to determine the influence of the digestion process on the variations of sludge dewatering characteristics:

• Organic content

• Specific resistance

• Variations in the structure of solid particles

• Forms of bound water

• Variations in chemical composition

In addition to the above, samples of activated sludge were collected from clarifiers and thickeners in wastewater treatment plants and also from the thickening and dewatering simulators used in the studies. The following measurements were taken during the thickening process:

• Duration of thickening

• Variations in solids concentration

• Specific resistance

Specific resistance is usually expressed by the formula

where r = specific resistance, m/kg P = pressure of filtration, N/m2 F = filtration area, m2

b = t/V 2 (t = time of filtration in seconds, and V = volume of filtrate in m3)

^ = dynamic viscosity of filtrate, N-s/m2 C = dry solids concentration, kg/m3

In the studies, instead of r the modified specific resistance R = r x 10-11 was used.

To study the forms of bound water with the solid particles, the test method used was thermal drying of sludge. Freezing and thawing, refractometering, and viscosimetering were also investigated; however, the results were not included because of the difficulty in duplicating the test results.

Study Results The structure and sizes of sludge solid particles change due to digestion; a typical example is shown in Figure 3.1. The dewaterability of the same type of sludge varies from one plant to another. Specific resistances

(1) Raw Sludge Solids (2) Digested Sludge Solids Figure 3.1 Variation of sludge solids structure.

TABLE 3.1 Changes in Specific Resistance in Anerobic Digestion

Type of Sludge

Dry Solids

Organics in

Dry Solids

Specific Resistance R (m/kg x 10-11)

Municipal primary sludge Raw Digested Sludge from municipal plus industrial wastewater treatment plant Raw Digested Sludge from municipal plus metallurgical wastewater treatment plant Raw Digested Mixture of municipal primary and thickened activated sludge Raw

Mesophilic digested (35°C) Thermophilic digested (55°C)

62-69 60-63

55-63 51-54

64-690 307-740

118-495 67-940

50-309 172-868

2170-4035 3640-6750 8350-9500

TABLE 3.2 Changes in Specific Resistance (m/kg X 10 n) During Aerobic Digestion

Primary sludge 3.7-4.8 300-410 380-530 2100-4500 3700-6720 1070-1300

Thickened 2.0-2.5 800-1130 1290-4500 5140-6250 4030-5700 970-1160 activated sludge

Mixture of primary 3.0-4.5 602-775 2170-5170 2470-3760 3300-5220 830-1070 and thickened activated sludge of anaerobically digested sludge and aerobically digested sludge are listed in Tables 3.1 and 3.2, respectively. The results in the tables indicate that:

• Raw primary sludge has less specific resistance than a mixture of raw primary sludge and thickened activated sludge.

• Raw sludge has less specific resistance than does digested sludge of the same type.

• A mixture of primary sludge and activated sludge has less specific resistance than that of anaerobically or aerobically digested sludge.

• Sludge digested in a mesophilic condition has less specific resistance than does the same sludge digested in a thermophilic condition.

The studies confirmed the findings of Gosh (1987), Lawler and Chung (1986), Parkin (1986), and Popel (1967) that the loading rate, periodicity of loading, digester mixing, and digestion duration and temperature are important factors that affect the digestion process. All of these factors influence the variation of dewaterability of sludge during and after the digestion process. Disintegration of organics from digestion brings the solids to a homogeneous grain structure; however, particle size decreases and the quantity of colloidal particles increases, both of which influence the dewaterability of digested sludge because of the increased degree of hydration. Experiments showed that the specific resistance increases in greater magnitude during continuous mixing of sludge with mechanical agitators than during periodic slow mixing or mixing by recirculation of gas. Experiments also indicated that aerobic digestion of primary sludge leads to a greater increase in specific resistance than does anaerobic digestion. Therefore, aerobic digestion of primary sludge should not be used if subsequent dewatering of digested sludge is required. An excessively long aerobic digestion time can result in a significant deterioration of sludge dewaterability.

The study of changes of particle size found that thickened activated sludge contains about 90% of particles that are less than 0.15 mm in size, while digested sludge has about 75% of these particles and primary sludge has only about 45% (see Figure 3.2). Specific resistance increases, as particle size dis

Size of fractions, mm Figure 3.2 Solids size distribution in sludge.

Size of fractions, mm Figure 3.2 Solids size distribution in sludge.

tribution favors smaller particles. Reduction of specific resistance can be achieved by removing small particles. Small particles can be removed by elu-triation of digested sludge: that is, washing the sludge with a stream of water. A reduction in specific resistance by elutriation can be expressed by the formula log Rn = log Roe-an (3.2)

where

Rn = specific resistance of washed digested sludge, m/kg

R0 = specific resistance of unwashed digested sludge, m/kg a = coefficient that relates to solids washed out and size fraction removed, usually 0.04 to 0.14 n = quantity of water in sludge, m3/m3

The specific resistance can also be reduced by the coagulation of small particles in sludge. The effects of coagulation by ferric chloride on specific resistance for various types of sludge are shown in Figures 3.3 and 3.4.

Water in sludge exists as either free water or bound water. Bound water is water bound to the sludge particles physically or chemically. The more the bound water exists in sludge, the more the energy that must be spent to remove it. Figure 3.5, developed from the study, presents the effect of bound water on thermal drying of sludge. Straight lines a-b in the figure show the consumption of power for warming up the sludge, lines b-1cr represent the free water that exudes from the sludge, and lines 1cr-2cr represent the bound water that exudes from the sludge. Curved lines 2cr-c show that as the consumption of power is rising, a part of the power is spent to overcome the internal forces that bind the water to the solid particles in sludge.

As can be seen from lines b-1cr, the moisture from the thickening of activated sludge goes down from 98% to 87.5%; for the mixture of digested primary and activated sludge, it goes from 97.5% to 84.6%; and for the primary sludge, it goes from 94.6% to 73%. Despite these differences, 85.6% of the water is removed from activated sludge and 85% is removed from primary sludge. However, the ratio of water to dry solids in the activated sludge is 7 : 1, and in the primary sludge, it is 2.7 : 1. Therefore, the thickened activated sludge contains more bound water than do digested sludge and primary sludge (1cr in Figure 3.5). Experiments showed that there is a close relationship between specific resistance and bound water: The less the moisture in the sludge, the less the specific resistance, which is characterized by the first critical points 1cr.

In the study, during the process of coagulation of sludge with inorganic chemicals or polymers, bound water was separated and the structure of the sludge was altered. Part of the water that was physically and chemically bound with solids exuded from the solids, thereby decreasing the quantity of absorbed water. The position of the first critical point changed accordingly. This change

Polymerization Carbazole With Fecl3
FeCl3 dosage, % of dry solids

3. Raw sludge (w = 93.2 %; R = 350 m/kg) W = Moisture of sludges

R = Specific resistance

Figure 3.3 Effects of coagulation on raw sludge.

is presented in Figure 3.6. This condition allows removing more water by mechanical methods after the coagulation of sludge.

There are certain differences between the specific resistance and critical points. Specific resistance shows the speed of separation of water from dry solids, whereas the first critical point shows the limit of sludge dewatering by mechanical methods. Knowledge of critical point for a sludge allows the most effective conditioning process for the sludge.

Inorganic Chemical Conditioning Until the 1970s the most useful chemical conditioners for sludge dewatering were inorganic compounds such as ferric chloride, ferrous sulfate, and aluminum chloride, one of which is added to sludge first and then usually followed with lime. Alkalinity is an important sludge characteristic that affects inorganic conditioners. Ferric chloride works

FeCl3 dosage, % of dry solids

Figure 3.4 Effects of coagulation on digested sludge.

FeCl3 dosage, % of dry solids

Figure 3.4 Effects of coagulation on digested sludge.

better with the pH of sludge at 6.0 to 6.5 and it reduces the pH to 4.5 to 6.0. Ferrous sulfate or aluminum chloride usually requires a higher dosage than ferric chloride. Lime following iron or aluminum salts increases the pH to 10.5 to 11.5. Generally, the required dosage is an approximate 1 : 3 ratio of ferric chloride (FeCl3) to lime (CaO). The dosage of reagents for dewatering sludge on vacuum filters or on pressure filters depends on the specific resistance of sludge. The higher the specific resistance, the higher the dosage of reagents required. The dosage of chemicals in each case is established experimentally by measuring the specific resistance of the sludge.

The dosage of lime for conditioning sludge for dewatering can be determined by the equation

where

R = adjusted value of the specific resistance (R = r x 10-11, where r is the specific resistance of the sludge, m/kg) B = sludge moisture, %

0 10 20 30 40 50 60 70 80 90 100 Moisture of sludge, %

0 10 20 30 40 50 60 70 80 90 100 Moisture of sludge, %

2 - Digested mixture of primary and activated sludge - 97.5%

1cr - First critical point of moisture in sludges. 2cr - The second critical point of moisture in sludges. Line "b - 1cr." - Removal of free water from sludges. Line "1cr - 2cr - c" - Removal of bound water. "c" - The end of the drying process.

Figure 3.5 Effects of bound water on thermal drying of sludge.

C = concentration of dry solids in sludge, % A = alkalinity of sludge before coagulation, mg/L as CaCO3

The dosage of ferric chloride is usually 30 to 40% of the dosage of CaO computed by equation (3.3).

Typical values of ferric chloride and lime for various types of sludge for vacuum filter and recessed plate pressure filters are shown in Table 3.3. The concentration of ferric chloride in commercial ferric chloride liquid is between 30 and 35% by weight in water. A 30% ferric chloride solution has a specific gravity of 1.39 at 30°C and contains 1.46 kg (3.24 lb) of ferric chloride.

The costs of chemical reagents for conditioning of sludge comprise the principal fraction of the operating costs for dewatering sludge on vacuum or pressure filters. Therefore, the chemical dosage should be minimized but still provide adequate conditioning with satisfactory dewatering results.

Chemical Sludge

Figure 3.6 Changes in critical points.

FeCl3 dosage, % of dry solids

Figure 3.6 Changes in critical points.

TABLE 3.3 Typical Dosages of Ferric Chloride and Lime for Vacuum Filter and Recessed Plate Filter Press Dewatering

Ferric Chloride

Lime

(FeCl3)

(CaO)

Method

Sludge Type

(% dry solids)

(% dry solids)

Vacuum filter

Raw primary

1.6-3.5

4.8-9.4

dewatering

Raw WAS

5-9

16-25

Raw (primary + WAS)

1.6-3.5

4.8-9.4

Anaerobically digested

2-5

6-15

primary

Anaerobically digested

3.7-7.0

8-19

(primary + WAS)

Recessed plate filter

Raw primary

1.8-4.3

5.2-12.0

press dewatering

Raw WAS

6-10

20-30

Raw (primary + WAS)

2-9

8-25

Anaerobically digested

2.8-4.8

9-13

primary

Anaerobically digested

3.5-8.2

10.2-23.6

Source: Adapted in part from U.S. EPA, 1979.

Lime is available in two dry forms: quick lime (CaO), pebble or granular; and powdered hydrated lime [Ca(OH)2]. Lime raises the pH reduced by ferric chloride or aluminum chloride addition and increases sludge porosity. In addition, lime provides a certain degree of odor reduction because sulfides in solution are converted from hydrogen sulfide to the sulfide and bisulfate ions, which are nonvolatile at alkaline pHs. Lime can also be beneficial because of its sludge stabilization effect. Although inorganic chemicals are effective conditioners for control of pH, odor, and specific resistance, they have many disadvantages. Ferric chloride is a very corrosive material. Therefore, special care should be taken in selecting the material for storage tank, piping, and metering pumps. Lime also needs special equipment for storage and feeding. Inorganic chemical conditioning increases sludge mass. Approximately one part of additional solids can be expected for each pound of ferric chloride and lime added.

Design Example 3.1 A wastewater treatment plant produces anaerobically digested sludge that needs to be dewatered using recessed plate pressure filters. At a solids concentration of 3%, the sludge contains 12,000 lb (5443 kg) of solids. The sludge is a mixture of 40% primary and 60% waste activated sludge. The press will operate one shift a day (7 hours effective) five days a week. Determine the amount of ferric chloride and lime required for conditioning, and the total sludge solids after dewatering.

1. Maximum amount of sludge to be dewatered

2. Based on the data in Table 3.3, use a ferric chloride (FeCl3) requirement of 5% of dry solids and a lime (CaO) requirement of 20% of dry solids. These chemical dosages convert to 100 lb/ton (50 kg/ton) of ferric chloride and 400 lb/ton (200 kg/ton) of lime.

H 2000 lb/ ton

Assuming that ferric chloride is available as 35% solution [4.13 lb FeCl3 per gallon (0.5 kg/L) of solution], ferric chloride solution required = 120lb/hr

. „ _ . . (2400lb/hr)(400lb/ton) 4. CaO required = ---——,--'4 2000 lb/ ton

Quicklime is available at 90% CaO; therefore,

5. The amount of extra sludge produced from chemicals is 1 lb (0.45 kg) for each pound of FeCl3 and quicklime added; therefore, total daily solids for disposal = (2400 lb sludge +120 lb ferric chloride

Organic Polyelectrolytes Since the 1960s, organic polyelectrolytes (polymers) as sludge-conditioning agents have become popular. They have many advantages over inorganic chemicals. They are easy to handle, and the feed system takes up less space. To obtain the same degree of reduction in specific resistance, doses of polyelectrolytes are several times less than those of inorganic reagents. All these reduce conditioning costs.

Organic polyelectrolytes are water-soluble large organic molecules repeated in a long chain. Common sludge-conditioning polyelectrolytes can be classified by the polymer compound's charge as anionic, nonionic, or cationic; by molecular weight; and by polymer form as dry, liquid, emulsion, or gel. A combination of molecular weight and polymer molecule's charge is the most useful feature in comparing performance.

Sludge conditioning with polymers is a process of destabilizing small particles and converting them to bigger particles by flocculation. Figure 3.7 illustrates how dry polymer is prepared for use in sludge thickening or dewa-tering processes. Equipment required for preparing a solution of dry polymer includes dry product metering, flocculent dispenser, polymer dissolving tank, storage or day tank, low-speed mixer, and solution metering pumps. Most automatic dry polymer feed systems rely on air to convey the dry polymer to the point where polymer is first wetted with water. Depending on the type and dewaterability of sludge, dosages of polymer can vary from 1 to 10 g/kg (2 to 20 lb/ton) of dry solids. Polymers are diluted by water to a concentration of 0.1 to 0.2%. Polymer conditioning is the common practice for sludge thickening or dewatering with centrifuges and for dewatering with belt filter

Water In

Water In

Dry Polymer Dispenser

Dry Polymer Dispenser

Water n In Te]

Dilution — Water Flow Meter

U Calibration Column r

Process Drain

Polymer Feed Pump

Polymer Flow Meter

TO"

To Point of Application

Static Mixer

Figure 3.7 Typical polymer solution makeup and feed system. (Reprinted with permission from WEF, 1998.)

TABLE 3.4 Typical Dosages of Polymer for Thickening Sludge

Polymer Dosage

Method Sludge Type g/kg Dry Solids lb/ton Dry Solids

TABLE 3.4 Typical Dosages of Polymer for Thickening Sludge

Method Sludge Type g/kg Dry Solids lb/ton Dry Solids

Gravity belt

Raw primary

1-2

2

4

thickening

Raw WAS

2-4

4

8

Raw (primary + WAS)

1-3

2-

6

Digested (primary + WAS)

1.5-3.0

3

6

Rotary drum

Raw WAS

1.0-2.5

2

5

thickening

Digested (primary + WAS)

1.5-3.0

3

6

Dissolved air

Raw WAS

0-2

0

4

flotation

Raw (primary + WAS)

0-3

0

6

thickening

Raw WAS

1-3

2

6

Solid bowl

Raw (primary + WAS)

1-3

2-

6

centrifuge

Aeobically digested WAS

1-4

2

8

thickening

Anaerobically digested

2.5-3.5

5

Source: Adapted in part from U.S. EPA, 1979.

presses. Table 3.4 shows the dosages of polymer for various thickening processes, and Table 3.5 shows the dosages for sludge dewatering. It is important to note that increasing the dosages beyond the optimum values worsens the dewaterability of sludge.

TABLE 3.5 Typical Dosages of Polymer for Dewatering Sludge

Method

Sludge Type

Polymer Dosage g/kg Dry Solids lb/ton Dry So

Belt filter press

Raw primary

1.2-3.3

2.4-6.6

dewatering

Raw WAS

2.6-6.5

5.2-13.0

Raw (primary + WAS)

3-6

6-12

Raw (primary + trickling

3-6

6-12

filter)

Anaerobically digested

2-4

4-8

primary

Anaerobically digested

3-8

6-16

(primary + WAS)

Solid bowl

Raw primary

0.5-2.3

1.0-4.6

centrifuge

Raw WAS

3-8

6-16

dewatering

Raw (primary + WAS)

2-6

4-12

Anaerobically digested

3.6-9.0

7.2-18.0

primary

Anaerobically digested

5.0-9.7

10.0-19.4

WAS

Anaerobically digested

2.5-8.0

5-16

(primary + WAS)

Vacuum filter

Raw primary

2-5

4-10

dewatering

Raw (primary + WAS)

4-7

8-14

Anaerobically digested

3.6-9.0

7.2-18.0

primary

Anaerobically digested

4.4-8.7

8.8-17.4

(primary + WAS)

Recessed plate

Raw (primary + WAS)

2-7

4-14

filter press dewatering filter press dewatering

Source: Adapted in part from U.S. EPA, 1979.

3.2.3 Other Conditioning Methods

Other sludge conditioning methods that have been used include adding non-chemical conditioning aids, thermal conditioning, freeze-thaw conditioning, and elutriation.

Nonchemical Conditioning Aids Power plant or sludge incinerator ash has been used successfully to improve the dewatering performance of vacuum filters and pressure filter presses. The properties of ash that improve dewater-ing of sludge include solubilization of its metallic constituents and its ability to attach to small solid particles of sludge and change their structure. The advantages of ash addition for sludge dewatering include elimination or substantial reduction in other chemical conditioning agents, increase in cake dryness, and significant improvements in filtrate quality. Disadvantages include the addition of a sizable quantity of inerts to the sludge cake and additional material handling. For installations where landfilling of sludge cake follow dewatering, the use of ash to improve the total solids content should be evaluated. The addition of ash to the sludge produces a drier cake, but it does nothing for the fuel value of the cake to be incinerated. Ash has no heating value, and it lowers the percent volatile solids in the cake; therefore, fuel use will increase. Pulverized coal is also a good sludge conditioning aid, especially if incineration is to follow the dewatering.

Other conditioning aids include diatomaceous earth and cement kiln dust. These agents are used primarily as a precoat in pressure filter presses. Sawdust is sometimes used as a conditioning agent, especially for dewatering sludge before composting.

Thermal Conditioning Thermal conditioning of sludge involves heating the sludge in the temperature range 170 to 220°C at a pressure of 1.2 to 2.5 MPa for 15 to 30 minutes. A thermal sludge conditioning system is illustrated in Figure 3.8. The influent sludge is ground before treatment to obtain particle sizes no greater than 4 to 5 mm. Plunger or screw (augur) pumps with working pressure up to 2.5 MPa are used for conveying the sludge to the thermal treatment system. The sludge and air mixture is heated in two stages: first in the heat exchanger by the heat of treated sludge from the reactor, and then by an external heat source in the reactor. The final heating of the sludge in the

1 - Sludge storage, 2 - Grinder, 3 - Transfer pump, 4 - Day tank, 5 - High pressure feed pump, 6 - Heat exchanger, 7 - Furnace, 8 - Steam boiler, 9 - Separator, 10 - Eductor, 11 - Reactor, 12 - Reducer, 13 - Cooler, 14 - Thickener, 15 - Ventilator, 16 - Thickened sludge pump, 17 - Filter press, 18 - Conveyor.

1 - Sludge storage, 2 - Grinder, 3 - Transfer pump, 4 - Day tank, 5 - High pressure feed pump, 6 - Heat exchanger, 7 - Furnace, 8 - Steam boiler, 9 - Separator, 10 - Eductor, 11 - Reactor, 12 - Reducer, 13 - Cooler, 14 - Thickener, 15 - Ventilator, 16 - Thickened sludge pump, 17 - Filter press, 18 - Conveyor.

Figure 3.8 Schematic of a thermal conditioning system.

reactor can be conducted by several methods. The simplest and most effective method is heating with steam, which enters the sludge pipe through an eductor before the reactor. The advantage of this method is in the use of comparatively low-pressure steam at a temperature approaching the sludge treatment temperature. The conditioned sludge is then discharged back through the heat exchanger where it cools to 45 to 55°C. The sludge is then thickened in a gravity thickener before dewatering. Due to the evaporation of water from the surface of the thickener, unpleasant odors are generated. To decrease the degree of evaporation, sludge is additionally cooled in a cooler to a temperature of 30 to 35°C. The thickener may be provided with a cover with forced ventilation to contain the evaporated air. The mechanical dewatering of thermally treated sludge is conducted predominantly in pressure filters. The use of pressure filters permits dewatering sludge to about 50 to 60% solids. The values of the thermal treatment parameters vary depending on the system used and have to be determined experimentally and on the basis of the decrease in the specific resistance of sludge.

In the thermal treatment process, some decomposition of sludge volatile solids occurs. The degree of decomposition depends on the initial properties of sludge. Approximately 75 to 80% of decomposing organics dissolve in the liquid, and 20 to 25% volatilize. Decanted water from the thickener and the filtrate from the filter press contain a high 2000 to 6000 mg/L solids, which can increase the loading to the treatment plant up to 10 to 25%. Some of these solids in sidestreams are difficult to oxidize. Therefore, before discharging to the plant influent, the sidestream may have to be treated by adding chemicals to reduce the organic loads.

Thermal conditioning of wastewater sludge has the following advantages:

• Except for straight waste activated sludge, the process will produce sludge with good dewatering characteristics. Cake solids concentrations of 50 to 60% are typically obtained with mechanical dewatering equipment following thermal conditioning.

• Additional chemical conditioning is normally not required.

• The process sterilizes the sludge, rendering it free of pathogens.

• The process is suitable for many types of sludge that cannot be stabilized biologically because of the presence of toxic materials.

Disadvantages of thermal conditioning include the following:

• The process has a high capital cost, due to the use of corrosion-resistant materials such as stainless steel in heat exchangers. Other support equipment is required for odor control and high-pressure transport.

• The process requires supervision, skilled operators, and a strong preventive maintenance program.

• The process produces an odorous gas stream that must be collected and treated before release.

• The process produces sidestreams with high concentrations of organics, ammonia nitrogen, and color.

• Scale formation in heat exchangers, pipes, and reactor requires acid washing.

Thermal conditioning is being practiced with positive results in the Asher Wastewater Treatment Plant in Paris, France, and the Luberetzkay Wastewa-ter Treatment Plant in Moscow, Russia. At both plants, sludge is dewatered without further chemical conditioning, dewatered cake has a high solids content, biosolids are disinfected effectively, and the stability is retained even after long storage. Several wastewater treatment plants in England, Germany, and the United States have discontinued their thermal conditioning practice because of the disadvantages discussed above.

Freeze-Thaw Conditioning Freezing and subsequent thawing of sludge result in a change in its structure and the conversion of bound water to free water. This has been observed in the natural freezing and thawing of sludge in drying beds in colder climates. This increases the sludge dewaterability significantly. Freezing and thawing of sludge decrease its specific resistance and permit mechanical dewatering of sludge without coagulation or with significant reduction in the quantity of reagents required for coagulation.

In the artificial freezing of sludge, the optimal value of specific heat flux is 230 to 1000 W/m2-h. At higher heat flow values, the specific resistance of sludge decreases insufficiently due to rapid freezing, and at low values it is necessary to increase the surface area of the heat-exchange equipment, with a consequent increase in the capital cost. Effects of artificial freezing depend on temperature and duration of freezing. Slow freezing decreases specific resistance more rapidly.

Artificial freezing of sludge can be conducted in direct-contact freezers using ice generators of the drum or panel type. To decrease power use in the sludge freezing and thawing process, phase-conversion heat recovery should be used in thawing the sludge, that is, using the heat given off during freezing. The electrical energy consumption for artificial freezing of 1 m3 of sludge is about 50 kWh. After thawing, the sludge can be dewatered on pressure filter presses or on sludge drying beds. Filter presses can produce sludge cake as high as 50 to 60% solids. The loading rate on sludge drying beds can be as much as 5 m3/m2-yr.

Elutriation Elutriation is the term commonly used to refer to the washing of anaerobically digested sludge before dewatering. Washing causes a dilution of the bicarbonate alkalinity in the sludge and therefore reduces the demand for acidic metal salt by as much as 50%. Two to four volumes of washwater, typically plant effluent, flow countercurrent to one volume of anaerobically digested sludge. Elutriation tanks are designed to act as gravity thickeners, with a mass solids loading of 8 to 10 lb/ft2-d (39 to 48.8 kg/m2-d).

Presently, the process is not used as extensively as it had been because in addition to reducing alkalinity, it washes out 10 to 15% of the solids from the sludge stream. These solids, when recycled back to the plant influent, can degrade the plant effluent through additional solids and organic loading on the plant if this load has not been accounted for in the design.

3.3 THICKENING

Thickening of sludge is a process to increase its solids concentration and to decrease its volume by removing some of the free water. The resulting material is still fluid. Thickening is employed prior to subsequent sludge-processing steps, such as digestion and dewatering, to reduce the volumetric loading and increase the efficiency of subsequent processes.

The most commonly used thickening processes are gravity thickening, dissolved air floatation thickening, gravity belt thickening, and rotary drum thickening. Table 3.6 presents a comparison of these thickening processes. Selection of a particular thickening process sometimes depends on the size of the wastewater treatment plant and the downstream train chosen. The main design variables of any thickening process are:

• Solids concentration and flow rate of the feed stream

• Chemical demand and cost if chemicals are used for conditioning

• Suspended and dissolved solids concentrations and flow rate of the clarified stream

• Solids concentration and flow rate of the thickened sludge 3.3.1 Gravity Thickening

Gravity thickening is the simplest and most commonly used method for sludge thickening in wastewater treatment plants. Circular concrete tanks are the most common configuration for gravity thickeners, although rectangular concrete tanks have also been used. Figure 3.9 shows a cross-sectional view of a typical circular gravity thickener.

Design Considerations A gravity thickener is similar in design to a conventional sedimentation tank but has a more steeply sloping floor. Tanks usually range from 10 to 24 m (33 to 80 ft) in diameter. Side-water depths vary from 3 to 4 m (10 to 13 ft) and floor slopes vary from 1 : 4 to 1 : 6, both depending on the period of time required to thicken the sludge to the concentration and storage volume required to compensate for fluctuations in the solids loading rate period. The steeper slope also reduces sludge raking problems by allowing gravity to do a greater portion of the work of moving the settled solids to the center of the thickener.

TABLE 3.6

Comparison of Thickening Methods

Method

Advantages

Disadvantages

Gravity

Least operation skill required

Large space required

thickening

Low operating costs

Odor potential

Minimum power consumption

Erratic and poor solids concentration

Ideal for small treatment plants

(2 to 3%) for WAS

Good for rapidly settling sludge

Floating solids

such as WAS and chemical

Conditioning chemicals typically

not required

Dissolved air

Provides better solids concentration

Operating costs higher than for a

flotation

(3.5 to 5%) for WAS than that of

gravity thickener

thickening

gravity thickening

Relatively high power consumption

Require less space than a gravity

Moderate operator attention required

thickener

Odor potential

Will work without chemicals or with

Larger space requirements compared

low dosages of chemicals

to other mechanical methods

Relatively simple equipment

Has very little storage capacity

components

compared to a gravity thickener Not very efficient for primary sludge Requires polymer conditioning for higher solids capture or increased loading

Centrufugal

Effective for thickening WAS to 4

High capital cost

thickening

to 6% solids concentration

High power consumption

Control capability for process

Requires moderate operator attention

performance

Sophisticated maintenance

Least odor potential and

requirements

housekeeping requirements

Requires polymer conditioning for

because of contained process

higher solids capture

Less space required

Gravity belt

Effective for WAS with 0.4 to 6%

Polymer dependent

thickening

solids concentration

Housekeeping requirements

Control capability for process

Odor potential

performance

Moderate operator attention required

High solids capture efficiency

Building commonly required

Relatively low capital cost

Relatively low power consumption

Rotary drum

Effective for WAS with 0.4 to 6%

Polymer dependent and sensitive to

thickening

solids concentration

polymer type

Less space required

Housekeeping requirements

Low power consumption

Odor potential

Moderate operator attention required Building commonly required

Thickener Cross Section
Figure 3.9 Cross-sectional view of a typical circular gravity thickener.

Gravity thickener mechanisms are similar in design to primary clarifiers. The inlet to the thickener is to a center-feed well through a bottom-feed, side-entry (Figure 3.9), or overhead inlet. The rake support truss is often provided with pickets that are thought to help release water from the solids. However, the rake support truss can provide sufficient sludge mixing to make pickets unnecessary. Drive mechanisms are heavier than those required for primary settling tanks, to overcome the problem of island formation of highly viscous solids. Thickeners are equipped with a skimming mechanism and baffling because of the inherent floating of the scum layer associated with sludge.

Design Criteria The most important aspect of thickener design is the establishment of the area required to achieve a desired degree of thickening. If sludge from a particular facility is available for testing, the required surface area can be found using a batch settling column and developing a solids flux for the particular sludge. Solids flux is the mass of solids passing through a unit area per unit time. The required thickener area is calculated using the equation

where

C0 = influent dry solids concentration, kg/m3 (lb/ft3) Qo = influent flow, m3/d (ft3/hr) Gt = solids flux, kg/m2-d (lb/ft2-hr)

The change in sludge volume in a gravity thickener can be determined from the formula where V1 and V2 are initial and final (thickened sludge) volumes, respectively; and C1 and C2 are the sludge concentrations before and after thickening, respectively.

In the prolonged thickening of activated sludge, there is a sharp increase in its specific resistance due to the decomposition of sludge and the increase in the quantity of bound water. Examples of activated sludge specific resistance are shown in Figure 3.10. During the process of gravity thickening waste activated sludge, as the concentration of dry solids increases from 0.2% to 2%, specific resistance increases from 35 m/kg to 1480 m/kg, but the volume of sludge decreases tenfold. However, when concentration increases from 2% to 3.2%, volume decreases only 1.2 times, while specific resistance increases from 1480 m/kg to 7860 m/kg. From Figure 3.10 it can be seen that the specific resistance of activated sludge is closely related to the concentration of dry solids in sludge.

Long-term gravity thickening of activated sludge results in a sharp increase in specific resistance, which significantly worsens the dewaterability of sludge. However, dewatering of nonthickened activated sludge does not make sense

10000

9000

8000

7000

J 6000

Concentration of dry soilds, kg/m3

9000

8000

7000

J 6000

eg 5000

3000

eg 5000

3000

1000

1000

Concentration of dry soilds, kg/m3

1 - Municipal sewage sludge in vertical thickener (20h)

2 - Municipal sewage sludge in circular thickener (16h)

3 - Effluent from synthetic rubber plant (12h)

Figure 3.10 Effects of thickening of activated sludge on specific resistance.

because of the initial high volume and low solids concentration. The kinetics of activated sludge allows determining the optimum concentration of dry solids that corresponds to the best efficiency of the dewatering equipment. The optimum thickened sludge concentration from vertical thickeners is 2.0 to 2.5% and from radial thickeners is 2.9 to 3.4%. The corresponding optimum time for thickening is 10 to 14 hours for vertical thickeners and 9 to 11 hours for radial thickeners equipped with sludge rakes. Mixed liquor from aeration tanks thickens faster than activated sludge from secondary settling tanks.

In most cases the sludge to be thickened is not available for settling testing. In such instances, thickeners are designed based on established solids loading and thickener overflow rates. Table 3.7 provides the typical rates for calculating the required surface area. Based on the type and solids concentration of sludge to be thickened, the thickener can be designed for the thickened sludge underflow concentration required for downstream processing. Recommended hydraulic overflow rates range from 15.5 to 31 m3/m2-d (382 to 760 gpd/ft2-d) for primary sludge, 4 to 8 m3/m2-d (100 to 200 gpd/ft2-d) for waste activated sludge, and 6 to 12 m3/m2-d (150 to 300 gpd/ft2-d) for combined primary and waste activated sludges. A high overflow rate can result in excessive solids carryover. Conversely, a low overflow rate means high thickener detention time, which can produce floating sludge (when methane produced by the anaerobic breakdown of solids buoys the sludge blanket), and odors from septic conditions.

Operational Considerations If thickener is used on a continuous basis without undo storage of sludge, septic or odor problems should be avoided.

TABLE 3.7 Gravity Thickener Design Criteria

Conc. (%)

Thickening Time (h)

Thickened Solids Conc.

(%)

Dry Solids Loading kg/m2 • d lb/ft2-d

Primary (PRI)

3-6

5-8

4-8

100-200

20-40

Trickling filter (TF)

1-4

8-16

3-6

40-50

8-10

Rotating biological

1.0-3.5

8-16

2-5

35-50

7-10

contactor (RBC)

WAS

0.4-1.0

5-15

2.0-3.5

25-80

5-16

PRI + TF

2-6

5-10

5-9

60-100

12-20

PRI + RBC

26

5-12

5-9

60-100

12-20

PRI + WAS

0.6-4.0

5-15

3-7

25-200

5-40

Aerobically digested WAS

0.5-1.0

1.5-12.0

2-5

50-200

10-40

Anaerobically digested PRI

4-7

20-1440

6 -13

Anaerobically digested

2-4

20-1440

8-11

Source: Adapted from U.S. EPA, 1979.

Source: Adapted from U.S. EPA, 1979.

Depending on temperature, primary sludge can be retained in the thickener for two to four days before upset conditions develop. Best practice is to maintain a sludge detention time of one to two days. The activated sludge thickening detention time has to be limited to a maximum of 15 hours.

The development of septic conditions during thickening results in floating solids that passes over into the thickener overflow, foul odors, and reduced underflow concentration. Waste activated sludge with a sludge volume index (SVI) of less than 100 indicates an older, denser, fast-settling sludge. SVI of more than 150 indicates a young, low-density, slow-settling sludge. Typically, a sludge with an SVI greater than 200 is considered to be a bulking sludge. Bulking sludge is characterized by a rapid and obvious rise in the sludge blanket and production of dilute underflow concentration during otherwise normal operation. Generally, sludge bulking is not a function of thickener operations, and the problem must be cured by correcting the basic cause within the plant. Thickener performance problems can also be corrected by the addition of chlorine to the thickener influent.

Provisions for dilution water should be included, especially for primary and secondary sludge mixes, to improve process performance by maintaining proper hydraulic loading. If the sludge temperature never exceeds 15 to 20°C, a dilution-to-sludge volume ratio of up to 4 : 1 is satisfactory. Higher temperature requires more dilution. As an alternative to using a large volume of dilution liquid and recycling it through the plan treatment process, the thickener overflow liquid can be aerated for 15 to 20 minutes and then used as dilution liquid. An added benefit to this is the retention of about 99% of the solids in the thickener because net overflow volume is negligible.

The strength of the thickener overflow or supernatant, as measured by suspended solids, can vary significantly. Reported values vary from 200 to 2500 mg/L. Addition of polymer to gravity thickener feed has been practiced at several plants. Results indicate that the addition of polymers increases solid capture but has very little or no effect in increasing solids concentration.

Design Example 3.2 Design a gravity thickener for the combined primary and waste activated sludge for a plant with average primary sludge flow of 8000 gal/d (30.3 m3/d) at 4% solids, and average waste activated sludge flow of 50,000 gal/d (189.2 m3/d) at 0.8% solids.

1. At average design conditions (neglect specific gravity):

primary sludge solids = (8000gal/d)(8.34lb/gal)(0.04lb/lb) = 2669 lb/ d (1211kg/d)

WAS solids = (50,000gal/d)(8.34lb/gal)(0.008lb/lb) = 3336 lb/d (1513kg/d)

combined solids = (2669+3336) lb/d

combined sludge flow =(8000+50,000) gal/d

2. Combined solids concentration:

3. From Table 3.7, select the lowest solids loading rate of 5 lb/ft2-d (25 kg/m2-d) to compensate for the peak sludge flow conditions:

area required = 60b/flb^d = 1201ft2 (112m2)

4. Diameter of thickener = V(1201ft2 ) (4/n) = 40 ft (12.2 m2 )

5. From Table 3.7, the expected underflow (thickened sludge) concentration is about 5%. Assume a solids-capture efficiency of 90%.

thickened sludge solids = (6005lb/d)(0.90)

thickened sludge flow = 5404.5 lb/d

( 8.34 lb/gal ) (0.05 lb/lb ) = 12,960 gal/d (49 m3 )

solids in supernatant = (6000 - 5404.5) lb/d = 600.5 lb/ d (272 kg/d )

solids concentration in supernatant =--.----,— x 100%

= 0.16% = 1600 mg/L 6. Hydraulic rate = 58,020Q01gfa2^d = 48.3gal/ft2-d (2.0m3/m2 • d)

The recommended minimum hydraulic rate is 380 gal/ft2-d (15.5 m3/m2-d). Therefore, about 330 gal/d (1.25 m3/d) of dilution water should be provided.

3.3.2 Dissolved Air Floatation Thickening

In the dissolved air floatation (DAF) thickening process, air is introduced to the sludge at a pressure in excess of atmospheric pressure. When the pressure is reduced to atmospheric pressure and turbulence is created, air in excess of that required for saturation leaves the solution as fine bubbles 50 to 100 |im in diameter. These bubbles attach to the suspended solids or become enmeshed in the solids matrix. Since the average density of solids-air aggregates is less than that of water (0.6 to 0.7), they rise to the surface. Good solids floatation occurs with a solids-air aggregate specific gravity of 0.6 to 0.7. The floating solids are collected by a skimming mechanism similar to a scum skimming system.

DAF thickening is used most efficiently for waste activated sludge. Although other types of sludge, such as primary sludge and trickling filter sludge, have been floatation thickened, gravity thickening of the sludge is more economical than DAF thickening.

The schematic of a typical DAF thickening system is presented in Figure 3.11. The major components of a DAF system are the pressurization system with an air saturation tank, a recycle pump, an air compressor and pressure release valve, and a DAF tank equipped with surface skimmer and bottom solids removal mechanism. Figure 3.12 illustrates two models of flotation thickeners that are in use in Russia and Ukraine.

There are three ways in which the pressurization system can be operated. In the method called total pressurization, the entire sludge flow is pumped through the pressurization tank and the air-saturated sludge is then passed through a reduction valve before entering the floatation tank. In the second method, called partial pressurization, only a part of the sludge flow is pumped through the pressurization tank. After pressurization, the pressurized and unpressurized streams are combined and mixed before they enter the floatation tank. In the third method, called the recycle pressurization (Figure 3.11), a portion of the subnatant is saturated with air in the pressurization tank and then combined and mixed with the sludge feed before it is discharged into the floatation tank.

The major advantage of the recycle pressurization system is that it minimizes high-shear conditions, an important parameter when dealing with flocculant-type sludge. The recycle pressurization system also eliminates clogging problems with the pressurization pump, air saturation tank, and pressure release valve from the stringy material in the feed sludge. For these reasons, recycle pressurization is the most commonly used. The recycle flow can also be obtained from the secondary effluent, which has the advantage of lower suspended solids and a lower grease content than the subnatant from the DAF tank.

How Refrigeration Systems Work
Saturation Tank

(a) Schematic

Skimmer Chain

Water Level El.

Effluent -

Battle

(a) Schematic

Skimmer Chain

Battle

Cfct

Sludge Scraper Chain

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