Lawrence K Wang Daniel Guss and Milos Krofta

CONTENTS

27.1 Introduction 1155

27.1.1 Summary 1155

27.1.2 Problems of an Existing Activated Sludge

Wastewater Treatment System 1156

27.1.3 Application of Secondary Flotation Clarification as an

Engineering Solution 1156

27.1.4 Objectives of the Engineering Research and Documentations 1157

27.2 DAF and DAFF Clarifiers 1159

27.2.1 Commercially Available DAF and DAFF

Clarifiers for Biological Wastewater Treatment Systems 1159

27.2.2 General Operational Description of DAF and DAFF Clarifiers 1161

27.3 Improved Biological Treatment System 1166

27.3.1 General Principles and Process Description 1166

27.3.2 General Kinetics of Activated Sludge Wastewater

Treatment System 1168

27.3.3 Process-Specific Kinetics of Conventional Activated

Sludge Process Systems with Sludge Recycle 1168

27.3.4 Specific Kinetics of the Improved Activated

Sludge Process Using Secondary Flotation 1171

27.4 Case History: A Petrochemical Corporation in Texas 1175

27.5 Case History: A Municipal Effluent Treatment Plant in Haltern, Germany 1177

27.6 Case History: A Paper Company in Houston, Texas 1178

27.7 Discussions 1183

27.8 Summary and Conclusions 1185

Acknowledgments 1186

Abbreviations 1186

Nomenclature 1186

References 1187

27.1 I NTRODUCTION 27.1.1 Summary

A description is given of a unique solution for the problems of overloaded conventional activated sludge systems. A high-rate dissolved air flotation (DAF) clarifier is applied in series between the aeration basin and secondary sedimentation in an activated sludge process to separate the living microorganisms before secondary sedimentation. This results in the following improvements in the existing wastewater treatment system:

a. Solids and hydraulic loadings on an overloaded secondary sedimentation are reduced for preventing sludge rising, increasing clarification efficiency, and saving construction cost on expansion of secondary sedimentation facilities.

b. Hydraulic loading on an aeration basin is reduced, thus increasing the retention time without increasing the aeration basin size. This is accomplished by reduced recycle sludge volume due to higher solids content in the recycled sludge.

c. Higher solids content of the wasted sludge represents cost saving and improved operation of sludge thickening, dewatering, and disposal.

d. The living microorganisms, separated by the DAF, are returned to the aeration basin quickly (in less than 15 min). The microorganisms stay in aerobic conditions at all times and are more active than comparable settled microorganisms. Microscopic examinations of settled and floated sludge are made to demonstrate this fact. The oxygen requirement for the mixed liquor suspended solids (MLSS) is also significantly reduced.

e. The problems of sludge rising and sludge bulking can be totally solved when using secondary flotation clarification.

This new concept is applied for improving the operation of an existing overloaded activated sludge plant, or for the expansion of the hydraulic capacity to handle additional wastewater flow.

A pioneering installation of a DAF clarifier (49 ft inside diameter) is described in detail. Pilot-scale results and full-scale design considerations are presented. The newly improved activated sludge process system with a secondary flotation clarifier allows a 50% increase in hydraulic loading to an existing waste treatment plant. Typical applications in treatment of industrial and combined waste-waters for solving sludge rising and bulking problems are introduced and critically discussed.

27.1.2 Problems of an Existing Activated Sludge Wastewater Treatment System

Activated sludge consists of biological flocs that are matrices of microorganisms, nonliving organ-ics, and inorganic substances. The microorganisms include bacteria, fungi, protozoa (Sarcodina, Mastigophora, Sporozoa, Ciliata, and Suctoria), rotifers, viruses, and higher forms of animals such as insect larvae, worms, and crustaceans. The activated sludge process is one of the most common biological wastewater treatment processes, and can be defined as a suspended-growth system in which biological flocs are continuously circulated to come into contact and to oxidize the organic waste substances in the presence of oxygen and nutrients. The waste organic matter is aerobically converted to gaseous carbon dioxide, cell tissue of microorganisms (C5H7NO2), and other simple soluble end products. Part of the microorganisms (i.e., activated sludge) are returned to the aeration basin in order to maintain a constant microbial population (i.e., constant MLSS). The wastewater is considered to be adequately treated when the excess microorganisms (i.e., excess waste sludge) and residual suspended solids are separated from the aqueous phase by clarification, and the clarified effluent meets the Federal and State Effluent Standards. The most common clarification used today is sedimentation clarification, which frequently has sludge bulking and sludge rising problems. Hydraulic overloading is another problem of an existing activated sludge wastewater treatment plant, which must serve a growing community.

27.1.3 Application of Secondary Flotation Clarification as an Engineering Solution

The recent and accelerating emphasis on water pollution control has necessitated the rapid development of improved biological waste treatment systems to aid in cost and energy savings. The use of secondary flotation clarification in place of or in assisting secondary sedimentation clarification in the activated sludge process system is one recent advancement in this basic process. The potential of this development, in terms of higher suspended solids and 5-day biochemical oxygen demand (BOD5) removals from existing plants and expansion of hydraulic capacity at a significantly reduced cost, is expected to result in extremely rapid acceptance by municipalities and industries.

The primary distinguishing feature of the improved activated sludge treatment system is that high-rate DAF is the secondary clarifier for separation of suspended solids from the activated sludge aeration basin effluent, as opposed to secondary sedimentation alone in a conventional activated sludge system.

The concept of using flotation for water-solid separation is not new at all; many engineers have applied the flotation technology in sludge separation since the early 1920s. The major deterrent to flotation use in the municipal and industrial processes envisaged by these early practicing engineers was economics, with objections centering mainly around the cost of gas bubble generation and retention. Wang has reported the evolution of DAF clarifiers during the last 50 years.1 The following progress has been made: (a) specific clarification load increased from 1.5 gpm (gallons per minute)/ ft2 (60 Lpm/m2) to 3.5 gpm/ft2 (140 Lpm/m2) and for triple stacked unit to 10 gpm/ft2 (420 Lpm/ m2); (b) the retention time of water in the flotation clarifier decreased from 30 to 3 min; (c) the largest unit size increased from 260 gpm (1000 Lpm) to 7900 gpm (30,000 Lpm) and for triple stacked units to 23,700 gpm (90,000 Lpm); (d) the size of modern DAF units is much smaller when treating the same hydraulic flow. It allows construction predominantly in stainless steel prefabricated for easy erection; (e) the smaller size and weight 120 lb/ft2 (60 kg/m2) allows installation on posts leaving free passage under the unit; therefore, it is easier to find available space for indoor installation and to construct inexpensive housing; (f) air dissolving is improved and now requires only 10 s retention time in the air dissolving tube instead of the previous 60 s, accordingly, this reduction in retention time results in smaller air dissolving tubes that are predominantly built from stainless steel; and (g) availability of excellent flocculating chemicals gives a high stability of operation and a high clarification degree.

In summary, modern DAF units with only 3 min of retention time can treat water and wastewa-ter at an overflow rate of 3.5 gpm/ft2 for a single unit, and up to 10.5 gpm/ft2 for triple stacked units. Of course, the actual retention time used for DAF design will be higher when an engineering safety factor is applied. Figure 27.1 shows a typical DAF clarifier that will be explained in detail later.

The comparison between a DAF clarifier and a conventional sedimentation clarifier shows that (a) DAF floor space requirement is only 15% of the settler; (b) DAF volume requirement is only 5% of the settler; (c) the degrees of clarification of the both clarifiers are the same with the same flocculating chemical addition; (d) the operational cost of the DAF clarifier is slightly higher than that for the settler, but this is offset by considerably lower cost of the installation financing; and (e) DAF clarifiers are mainly prefabricated in stainless steel for erection cost reduction, corrosion control, better construction flexibility, and possible future changes, contrary to in situ constructed heavy large concrete sedimentation tanks. Ideally, for the design and construction of a new activated sludge wastewater treatment plant, it will be more cost-effective if secondary flotation is used instead of conventional secondary sedimentation.

27.1.4 Objectives of the Engineering Research and Documentations

The primary objective of this research, however, is to introduce the secondary flotation clarification concept that can be applied for improving treatment efficiency of an existing overloaded activated sludge plant, or for expansion of the existing plant's hydraulic capacity to handle additional wastewater flow. A commercially available high-rate DAF clarifier can be applied in series between the aeration basin and secondary sedimentation in a conventional activated sludge process to separate the living microorganisms before settling in the existing secondary sedimentation basins. This results in the following improvements in the existing plant: (a) solids and hydraulic loadings on an overloaded secondary sedimentation are reduced, resulting in increased clarification efficiency and saving of

Milos Krofta

A Diameter of supracell B Depth of supracell tank C Depth of supracell tank with bottom support D Minimum overall height of supracell

FIGURE 27.1 Single-cell high-rate DAF system (Krofta Supracell).

1 Rotating center section

2 Clarified water outlet

3 Settled sludge sump

4 Settled sludge outlet

5 (krofta) rotary contact

6 (krofta) spiral scoop

7 Floated sludge outlet

8 Unclarified water inlet

9 Clarified water extraction pipes

10 Gear motor

11 Distribution duct

Ui 05

Type

Dimensions

Flow

A ft

mm

B in

B mm

C in

C mm

D in

D mm

o m /min

US gpm

m3/h

8

2400

23.5

600

33

850

45

1150

0.56

148

34

10

3200

23.5

600

33

850

49

1250

1.00

263

60

12

3900

25.5

650

35

900

51

1300

1.50

394

90

15

4500

25.5

650

37

950

57

1450

2.00

525

120

18

5500

25.5

650

37

950

58

1480

3.00

789

180

20

6100

25.5

650

37

950

61

1560

3.65

961

219

22

6700

25.5

650

37

950

62

1580

4.40

1160

264

24

7200

25.5

650

37

950

63

1600

5.08

1340

305

27

8100

25.5

650

37

950

67

1700

6.44

1695

386

30

9000

25.5

650

37

950

71

1820

7.95

2090

477

33

10,000

25.5

650

37

950

72

1840

9.80

2580

588

36

11,000

25.5

650

37

950

73

1860

11.87

3125

712

40

12,200

26

660

38

960

76

1920

14.60

3840

876

44

13,400

27

685

39

985

78

1980

17.60

4630

1056

49

14,800

27

685

39

985

82

2070

21.50

5650

1290

55

16,800

27

685

39

985

87

2200

27.70

7290

construction cost on expansion of secondary sedimentation facilities; (b) a reduction in recycle sludge volume due to higher solids content in the recycled sludge reduces the hydraulic loading on an aeration basin, thus increasing the retention time without increasing the aeration basin size; (c) higher solids content in the waste sludge represents cost saving and improved operation of sludge thickening, dewatering, and disposal; (d) the living microorganisms, separated by the DAF, are returned to the aeration basin quickly (in less than 15 min) in aerobic condition and are more active than comparable settled microorganisms, and the oxygen requirement for the MLSS is also significantly reduced; and (e) the problems of sludge rising and sludge bulking, and/or hydraulic overloading, can be totally solved when using the secondary flotation clarification.

In Section 27.2, the principles of a DAF unit and the entire improved activated sludge wastewater treatment system are disclosed in detail. The economic use of secondary flotation in the improved system requires only a relatively inexpensive high-rate DAF cell that is commercially available. The consulting engineers should understand such principles for the selection of an appropriate DAF unit and for the optimization of the entire improved wastewater treatment system.

Section 27.3 introduces the improved activated sludge systems involving the use of either a DAF clarifier or a dissolved air flotation-filtration (DAFF) clarifier as the secondary flotation clarification unit.

Sections 27.4 through 27.6 describe the case history and operating experience. Both pilot-scale results and full-scale design considerations are presented. The new design involving simple addition of a secondary flotation clarifier prior to an existing secondary sedimentation clarifier allows a 50% increase in hydraulic loading to an existing waste treatment plant.

Section 27.7 discloses the common causes of sludge rising and sludge bulking and discusses other possible alternatives for biological process optimization.

Section 27.8 summarizes the feasibility and advantages of an improved biological wastewater treatment system involving the use of secondary flotation clarification.

27.2 DAF AND DAFF CLARIFIERS

27.2.1 Commercially Available DAF and DAFF Clarifiers for Biological Wastewater Treatment Systems

DAF is mainly used to float suspended and colloidal solids by decreasing their apparent density. The influent feed liquid can be raw water, wastewater, liquid sludge, or industrial process water.1-37 A DAF clarifier can be either a continuous reactor or a sequencing batch reactor1,28-30,37 when used in a biological wastewater treatment plant for primary clarification, secondary clarification, or sludge thickening. A combined DAFF clarifier is commonly called a DAFF clarifier. A DAFF clarifier is suitable for secondary clarification or tertiary clarification in a biological wastewater treatment system.1 The shape of a DAF clarifier or a DAFF clarifier can be either circular or rectangular.

The flotation system consists of eight major components: a influent feed pump, air supply, a pressurizing pump, an air dissolving tube (retention tank), a friction valve, a flotation chamber, a spiral scoop, and an effluent extraction pipe. Figures 27.1 and 27.2 show a single cell and a double cell, respectively, of a high-rate DAF clarifier, which is commercially available from Krofta Engineering Corporation (KEC), Lenox, Massachusetts. It should be noted that there are many DAF/DAFF manufacturers and patented DAF/DAFF processes around the world, which are equally effective for either primary clarification or secondary in biological wastewater treatment plants (including an activated sludge wastewater plant).1,28-43 A few selected major DAF and DAFF manufacturers are listed below:

a. Dongshin Engineering Corporation, Seoul, Korea b. KEC, Massachusetts

1 Raw water collection tank

2 Supracell feed pump

3 Supracell inlet pipe (imbedded in ground)

4 Inlet compartment

5 Settled sludge sump

6 Settled sludge discharge

7 Clarified water outlet

8 Clarified water return for level control (1)

9 Level control in supracell with pneumatic sensor and clarified water discharge regulating valve

10 Pressure pump for recycling of clarified water to the air dissolving tubes

11 Air dissolving tubes

12 Supracell main tank

13 Spiral scoop for collection of the floated sludge

14 Floated sludge

15 Steel legs

16 Second elevated supracell in steel construction

Milos Krofta

ffi O

FIGURE 27.2 Double-cell high-rate DAF system (Krofta Supracell).

c. Austep Srl (Extant Environmental Solutions), Milan, Italy d. MarTint Inc., Lexington, South Carolina e. Siemens AG International, Schuhstrasse, Germany f. Komline-Sanderson, Peapack, New Jersey g. WesTech Engineering Inc., Salt Lake City, Utah h. Praxair, Inc., Burr Ridge, Illinois i. Degremont Technologies-Infilco, Richmond, Virginia j. EIMCO Water Technologies, West Valley City, Utah k. HI-Tech Environmental, Hoover, Alabama l. Noram Engineering and Construction, Vancouver, BC, Canada m. KWI North America Corporation, Massachusetts n. Krofta Technologies, Massachusetts.

All DAFs are similar to each other in terms of theory, principles, design, operation, and secondary flotation performance. The authors select the circular DAF process equipment manufactured by KEC for the purpose of feasibility studies. The users should contact more than one major DAF and DAFF manufacturers for appropriate pilot plant demonstrations and cost comparisons. Rectangular DAF and DAFF clarifiers are as good as circular DAF and DAFF clarifiers.

It is seen from Figure 27.1 that the single circular DAF (Krofta Supracell) unit can be as large as 55 ft in diameter, handling a maximum flow of 7290 gpm (or 10.5 MGD, or 37.85 ML/d). However, for doubling the capacity vertically in order to save some land space, a second DAF can be installed on 4 legs over the bottom one, as shown in Figure 27.2. This second DAF is built in steel with steel supports. Three DAFs installed one over the other have also been built, and are all incorporated in lightweight housing.

A commercial DAF unit, such as Krofta Supracell, can be delivered fully prefabricated. Larger units are delivered in parts that are flanged together. Construction materials are painted or stainless steel. A tile or concrete tank is optional. Generally, no heavy foundation or support structure is needed for a single-cell unit as the total load factor when filled with water weighs <150 lb/ft2, which is less than the load for a parking lot. A flat concrete ground pad is usually sufficient.

The following sections describe the operational procedures and the principles and special features of some selected major flotation components.

27.2.2 General Operational Description of DAF and DAFF Clarifiers 27.2.2.1 DAF and DAFF Clarifier Systems

The inlet, outlet, and sludge removal mechanisms are contained in the central rotating section. This central rotating section and the spiral scoop rotate around the tank at a speed synchronized with the flow (Figures 27.1 and 27.2).

Unclarified water, first passing through an air dissolving tube (Figures 27.3 and 27.4) and a friction valve (Figure 27.5), is released through a rotary joint in the center of the tank. It then passes into the distribution duct that moves backward with the same velocity as the forward incoming water. The settling and the flotation processes take place in the quiescent state in the flotation chamber.

The spiral scoop that is shown in Figures 27.6 and 27.7 as a part of the patented structure takes up the floated sludge, pouring it into the stationary center section where it is discharged by gravity for either recycling or disposal.

Clarified water is removed by effluent extraction pipes that are attached to the moving center section. The clarified water that normally contains <30 mg/L of suspended solids can be recycled in the process and/or sewered.

1162

Dimensions of air dissolving tubes

Type

500

1000

1500

2000

2500

3000

Flow US gpm

132

263

395

526

658

789

A diam. inches

12"

12"

18"

18"

18"

18"

B feet inches

3' 10"

6' 8"

10' 2"

6' 8"

8' 5"

10' 2"

C diam. inches

6"

6"

6"

6"

6"

D inches

4"

6"

6"

6"

6"

6"

FIGURE 27.3 Dimensions of air dissolving tubes (air saturation tanks).

Wiper blades attached to the moving distribution duct scrape the bottom and the sides of the tank and discharge settled sludge into the built-in sump, for periodic purging. The variable speed gear motor drives the rotating elements and the scoop. Electrical current for the gear motor feeds from a rotary contact mounted on the central shaft.

According to Henry's law, the solubility of gas (such as air) in aqueous solution increases with increasing the pressure. The influent feed stream can be saturated at several times atmospheric pressure (45-85 psig) by a pressurizing pump. The pressurized feed stream is held at this high pressure for about 10 s in an air dissolving tube designed to provide efficient dissolution of air into the water or wastewater stream to be treated. The pressurized stream usually enters the air dissolving tube tangentially at one end and is discharged at the opposite end. During the short passage the water cycles inside the tube and passes repeatedly by an insert, fed by compressed air. Very thorough mixing under pressure then dissolves the air in the water. Because of the small diameter, the improved air dissolving tube does not require official testing and coding. The small dimensions allow an

FIGURE 27.4 Typical models of air dissolving tubes (air saturation tanks).
FIGURE 27.5 Friction valve (pressure release valve or pressure reduction valve) for pressure reduction and pressurized water release.
FIGURE 27.6 Spiral scoop (top) and effluent extraction pipes.

27.7 Spiral scoop operations.

FIGURE

27.7 Spiral scoop operations.

economical construction in stainless steel. Figure 27.3 introduces the dimensions of air dissolving tubes, and Figure 27.5 illustrates a typical model.

The pressurized water is decompressed in a friction valve (see Figure 27.3), where the liquid is forced through a narrow slot in a coil spring. High shear is produced and dissolved air is forced out of the solution.

From the air dissolving tube (through a friction valve), the water stream is released back to atmospheric pressure in the flotation chamber, as shown in Figures 27.8 through 27.10. Most of the pressure drop occurs after the friction valve (note: the friction valve is located in the transfer line between the air dissolving tube and flotation chamber shown in Figures 27.8 through 27.10), so that the turbulent effects of depressurization can be minimized. The sudden reduction in pressure in the flotation chamber results in the release of microscopic air bubbles (average diameter 80 mm or smaller), which attach themselves to suspended or colloidal particles in the process water in the flotation chamber. This results in agglomeration that, due to the entrained air, gives a net combined specific gravity less than that of water, and causes the flotation phenomenon. The vertical rising rate of air bubbles ranges between 0.5 and 2.0 ft/min. The floated materials rise to the surface of the flotation chamber to form a floated layer, which is carried away by a spiral scoop shown in Figure 27.6. Clarified water (effluent) is usually drawn off from the bottom of the flotation chamber through effluent extraction pipes (see Figure 27.6) and either recovered for reuse or discharged.

The unique, compact and efficient design of a circular DAF cell is made possible by using the principle of "zero velocity." As mentioned earlier, the influent distribution duct moves backward with the same velocity as the forward incoming water. The "zero velocity" quiescent state in the flotation chamber is thus created ideally for flotation.

The retention time in the flotation chambers is usually about 2.5-4 min depending on the characteristics of process water and the performance of a flotation unit. The process effectiveness depends on the attachment of air bubbles to the particles to be removed from the process water. The attraction between the air bubbles and particles is primarily a result of the particle surface charges and bubble-size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation unit can be. Generally, the depth of effective modern flotation units is only between 16 in. and 2 ft.

A DAFF clarifier's top portion is similar to a DAF clarifier, but its bottom portion is an automatic backwash filtration unit. The readers are referred to elsewhere for a detailed description of a DAFF clarifier.4143

27.2.2.2 Spiral Scoops

Specially designed spiral scoops (see Figures 27.6 and 27.7) continuously remove the floated material and subsequently pour it into the stationary center section of a flotation chamber, where it is discharged by gravity for either recycling or disposal.

Pressure release

Flotation Float

Air saturation tank

Pressure release

Effluent

Flotation Float

Pump

Air saturation tank

Sludge

FIGURE 27.8 The DAF full-flow pressurization (total pressurization) system. [From WEF, Sludge Thickening, Manual of Practice No. FD-1, Water Environment Federation (formerly Water Pollution Control Federation), Washington, DC, 1980, pp. 33-66. With permission.]

FIGURE 27.9 The DAF partial flow pressurization (partial pressurization) system. [From WEF, Sludge Thickening, Manual of Practice No. FD-1, Water Environment Federation (formerly Water Pollution Control Federation), Washington, DC, 1980, pp. 33-66. With permission.]

The surface sludge layer can in certain cases attain a thickness of many inches and can be relatively stable for a short period. The layer thickens with time, but undue delays in removal will cause a release of particulates back to the liquid.

27.2.2.3 Flotation System Configurations

There are three common flotation system configurations: (a) full-flow pressurization; (b) partial flow pressurization without effluent recycle; and (c) recycle flow pressurization, which have been graphically illustrated in Figures 27.8 through 27.10, respectively.

In the full-flow pressurization system (Figure 27.8), the entire influent feed stream is pressurized by a pressurizing pump and held in the air dissolving tube. The system is usually applicable to the feed stream with suspended solids exceeding 800 mg/L in concentration, and is not susceptible to the shearing effects caused by the pressurizing pump and the high pressure drop at the friction valve. It is occasionally used for separating some discrete fibers and particles that require a high volume of air bubbles. It is particularly feasible for solid-water separation where suspended solids will flocculate rapidly with the addition of chemical coagulants in the inlet compartment in the presence of the released air. The air bubbles may become entrapped within the floc particles, resulting in a strong air-to-solids bond and thus in a highly efficient separation process.

In the partial flow pressurization without effluent recycle system (Figure 27.9), only about 30-50% of the influent feed stream is pressurized by a high-pressure pump and held in the air dissolving tube. The remaining portion of the influent stream is fed by gravity or a low-pressure pump to the inlet compartment of the flotation chamber where it mixes with the pressurized portion of the influent stream. Materials with low specific gravity can be removed with the partial flow pressuriza-tion system. This system is again not recommended to be used when the suspended solids are susceptible to the shearing effects of the pressurizing pump and the high pressure drop at the friction valve. It is generally employed in applications where the suspended solids concentrations are low, resulting in lower air requirement and, in turn, lower operation and maintenance costs.

In the recycle flow pressurization system (Figure 27.10), a portion (15-50%) of the clarified effluent from the flotation chamber is recycled, pressurized, and semisaturated with air in the air dissolving tube. The recycled flow is mixed with the unpressurized main influent stream just before admission to the flotation chamber, with the result that the air bubbles come out of aqueous phase in contact with suspended particulate matter at the inlet compartment of the flotation chamber. The system is usually employed in applications where preliminary chemical addition and flocculation are necessary and ahead of flotation. It eliminates the problems with shearing the flocculated particles since only the clarified effluent passes through the pressurizing pump and the friction valve. It should be noted, however, that the increased hydraulic flow on the flotation chamber due to the flow recirculation must be taken into account in the flotation chamber design.

While all the aforementioned three system configurations can be used for sludge (or fiber) separation, only the recycle flow pressurization system is recommended for water purification or waste-water treatment.

27.3 THE IMPROVED BIOLOGICAL TREATMENT SYSTEM 27.3.1 General Principles and Process Description

Activated sludge is a continuous flow, biological treatment process characterized by a suspension of aerobic microorganisms, maintained in a relatively homogeneous state by the mixing and turbulence induced by aeration. The microorganisms are used to oxidize soluble and colloidal organics to CO2 and H2O in the presence of molecular oxygen. The process is generally but not always preceded by a primary sedimentation clarifier. The mixture of microorganisms and wastewater formed in the aeration basins, called mixed liquor, is transferred to gravity clarifiers for liquid-solid separation. The major portion of the microorganisms settling out in the clarifiers can be recycled to the aeration basins to be mixed with incoming wastewater, while the excess, which constitutes the waste sludge, is sent to the sludge-handling facilities. The rate and concentration of activated sludge returned to the

Air saturation

Air saturation

Polymer Scoop Krofta

Air saturation tank

FIGURE 27.10 (a) DAF recycled flow pressurization (recycle pressurization) system, (b) flow schematic, and (c) control valves. Note: B = ball valve; P = pressure gauge; R = rotameter; C = check valve; H = high pressure reducer; G = gate valve; T = temperature gauge; S = selenoid valve [From WEF, Sludge Thickening, Manual of Practice No. FD-1, Water Environment Federation (formerly Water Pollution Control Federation), Washington, DC, 1980, pp. 33-66. With permission.]

Air saturation tank

FIGURE 27.10 (a) DAF recycled flow pressurization (recycle pressurization) system, (b) flow schematic, and (c) control valves. Note: B = ball valve; P = pressure gauge; R = rotameter; C = check valve; H = high pressure reducer; G = gate valve; T = temperature gauge; S = selenoid valve [From WEF, Sludge Thickening, Manual of Practice No. FD-1, Water Environment Federation (formerly Water Pollution Control Federation), Washington, DC, 1980, pp. 33-66. With permission.]

Krofta Daf
FIGURE 27.10 Continued.

aeration basins determine the MLSS level developed and maintained in the basins. During the oxidation process, a certain amount of the organic material is synthesized into new cells, some of which then undergoes auto-oxidation (self-oxidation or endogenous respiration) in the aeration basins, the remainder forming net growth or excess sludge. Oxygen is required in the process to support the oxidation and synthesis reactions. Volatile compounds are driven off to a certain extent in the aeration process. Metals will also be partially removed, with accumulation in the sludge. Activated sludge systems are classified as high rate, conventional, or extended aeration (low rate) based on the organic loading. In the conventional activated sludge plant, the wastewater is commonly aerated for a period of 4-8 h (based on average daily flow) in a plug-flow hydraulic mode. Either surface or submerged aeration systems can be employed to transfer oxygen from air to wastewater.

A partial listing of design criteria for the conventional activated sludge process is summarized as follows:

a. Volumetric loading = 25-50 lb BOD5/day/1000 ft3.

b. Aeration detention time = 4-8 h (based on average daily flow).

d. Food-to-microorganisms ratio, F/M = 0.25-0.5 lb BOD5/day/lb MLVSS, where MLVSS = mixed liquor volatile suspended solids.

e. Air requirement = 800-1500 standard ft3/lb BOD5 removed.

f. Mean cell residence time = 5-10 days.

27.3.2 General Kinetics of the Activated Sludge Wastewater Treatment System

The success of an activated sludge process in producing a high-quality effluent depends on the continuous growth of biological flocs having a good separating characteristic.44-46 The growth of biological flocs is accompanied by the organic substrate removal. The rate of microbial growth and the rate of substrate utilization are interrelated. If one assumes that the Michaelis-Menten enzymatic kinetics can be applied to the substrate utilization by microorganisms in the process, then

km S

in which U is the specific substrate (i.e., soluble organics) utilization rate, change of soluble substrate concentration per unit time per unit microbial concentration; S is the the substrate concentration in solutions, mass per unit volume; X is the microbial concentration (VSS) in the reactor, mass per unit volume; km is the maximum rate of specific substrate utilization, time-1; Ks is the MichaelisMenten constant, or half velocity coefficient being numerically equal to the substrate concentration when U = km/2, mass per unit volume; S0 is the initial substrate concentration, mass per unit volume (mg/L); Q is the volumetric wastewater flow rate, volume per unit time; V is the reactor volume; F/M is the food-to-microorganism ratio = S0/TX; T is the hydraulic detention time of the reactor V/Q; and E is the process efficiency = l00(S0 - S)/S0.

The readers are referred to the Nomenclature section for details about the units.

Biological growth is the result of the coupled synthesis-endogenous respiration reactions. The net result can be expressed as u = dX/dt u = X ,

u = YU - b, in which u is the net specific growth rate, the change of microbial concentration per unit time per unit microbial concentration, time-1; Y is the growth yield coefficient, mass microbial growth per unit mass substrate utilized; and b is the endogenous or decay coefficient, time-1.

27.3.3 Process-Specific Kinetics of the Conventional Activated Sludge Process Systems with Sludge Recycle

There are four conventional activated sludge process schemes: (a) complete-mix reactor with sludge recycle; (b) complete-mix reactor without sludge recycle; (c) plug-flow reactor with sludge recycle; and (d) plug-flow reactor without sludge recycle. These process schemes are described elsewhere in detail.38 This report introduces only the conventional system using complete-mix reactor with sludge recycle for the purpose of comparison between a conventional system and an improved system using secondary flotation clarification.

In the conventional activated sludge process with biological sludge recycled from the final sedimentation clarifier, shown in Figure 27.11, the mean cell residence time or sludge retention time is

Influent (primary effluent) Flow = Q

= 5.212 TPD SS = 27.7 ppm = 0.578 TPD VSS = 0 ppm = 0 TPD

Carbon dioxide gas released

Aeration basin effluent

Treatment plant effluent

Aeration basin Retention = 3.5 h Volume = 1.13 MG

BOD:

Q+ Qr 7.7 MGD 6 ppm 0.193 TPD 4473 ppm 143.629 TPD 3578 ppm 114.9 TPD

Return sludge

BOD;

2.7 MGD 6 ppm 0.068 TPD 12,500 ppm 140.738 TPD 10,000 ppm 112.59 TPD

BOD:

Waste sludge

BOD:

Q-Qw

4.960 MGD 6 ppm 0.124 TPD 40 ppm 0.827 TPD 32 ppm 0.662 TPD

Waste sludge

= 0.001 TPD = 12,500 ppm = 2.064 TPD = 10,000 ppm = 1.651 TPD

FIGURE 27.11 The conventional activated sludge process system before the installation of a secondary flotation clarifier (example). Note: 1 MGD = 0.0438 m3/s; 1 ppm = 1 mg/L; 1 TPD = 37.8kg/h.

longer than the hydraulic retention time. When sludge wasting is accomplished from the recycle line, the sludge retention time is calculated as

in which Tc is the sludge retention time, day; Qw is the wasted sludge flow rate, volume per unit time; Xr is the return sludge concentration, mass per unit volume; and Xe is the sludge concentration in the treated effluent from the final sedimentation clarifier.

Assuming that Xe is very small, Equation 27.3 can be rewritten as

By writing the mass balance equation for "sludge" in the entire system, as shown in Figure 27.11, and assuming X0 is in negligible amounts (X0 is the sludge concentration in the primary effluent), one can obtain

where V(dX/di) is the rate of change of microorganism concentration in the bioreactor; (YUX - bX) V is the net rate of microorganism growth in the bioreactor; and [QwXr + (Q - Qw)Xe] is the rate of microorganism outflow from the reactor.

Making use of Equation 27.3 and considering steady-state conditions, Equation 27.5 can be simplified and rearranged to yield

T c in which both 1/T and u are termed the net specific growth rate. The following are the working equations of substrate (S), MLSS concentration (X), and aeration volume (V) for the sludge recycle model:

It is important to know from Equation 27.7 that the performance of a complete mix with recycle system does not depend on hydraulic retention time. For a specific wastewater, a biological culture, and a particular set of environmental conditions, all coefficients Ks, b, Y, and km become constant. It is apparent from Equation 27.7 that the system performance is a function of mean cell residence time.

A typical overloaded complex-mix activated sludge treatment plant is graphically illustrated in Figure 27.11 in detail. The treatment plant treats 5.0 MGD (million gallons per day) of settled sewage having a BOD5 of 250 mg/L. The plant effluent consistently contains over 40 mg/L of total suspended solids (TSS) and about 6 mg/L of soluble BOD5. The effluent TSS violates the effluent standard because of the overloaded existing secondary sedimentation clarifier. Assume that the following field conditions are applicable:

a. Wastewater temperature = 200°C.

b. Return sludge concentration = 12,500 mg/L TSS.

c. Volatile suspended solids (VSS) = 0.8 TSS.

d. Mean cell residence time Tc = 10 days.

e. Growth yield coefficient Y = 0.65 lb cells per lb of BOD5 utilized.

f. Endogenous or decay coefficient b = 0.1 day-1.

g. Waste contains adequate nitrogen and phosphorus and other necessary trace nutrients for biological growth.

h. Aeration basin volume V = 1,130,000 gallons.

i. Sedimentation clarifier volume = 300,000 gallons.

The process conditions of the existing system will be a. MLSS X = 4375 mg/L.

b. Hydraulic detention time of aeration basin = 3.5 h.

c. Hydraulic detention time of the secondary sedimentation clarifier = 0.935 h.

e. Sludge production rate (dX/dt) = 3300 lb VSS/day = 4125 lb TSS/day.

g. Specific substrate (soluble BOD5) utilization rate U = 0.31 day-1.

27.3.4 Specific Kinetics of the Improved Activated Sludge Process Using Secondary Flotation

Figure 27.12 shows the improved activated sludge process in which a new secondary flotation is applied in series between the aeration basin and the final sedimentation clarifier for increasing the overall treatment performance and hydraulic capacity of an originally overloaded existing plant.

A microbial mass balance equation can be established for the improved system as shown in Figure 27.12:

V= (YUX - bX)V- QX + QwiXwi + (Q - Qw1 - Qw)XJ, (27.10)

where V(dX/dt) is the rate of change of microorganism concentration in the reactor; (YUX - bX) V is the net rate of microorganism growth in the bioreactor; [Qw1Xr + Qw2Xw2 + (Q - Qw1 - Qw2)Xe] is the rate of microorganism outflow from the bioreactor; Qw1 is the flow rate of waste sludge from secondary flotation, volume per unit time; Xw1 = Xr is the concentration of waste sludge (float) from secondary flotation, mass per unit volume; Qw2 is the flow rate of waste sludge from the existing final sedimentation clarifier, volume per unit time; Xw2 is the concentration of waste sludge from the existing final sedimentation clarifier, mass per unit volume. The sludge retention time (Tc) can be calculated as

Assuming that the sludge concentration in the treated plant effluent (Xe) is very low, Equation 27.11 can be rewritten as

Again making use of Equations 27.1, 27.2, and 27.11 and considering steady-state conditions, Equation 27.11 can also be simplified and rearranged to yield Equation 27.6. It is, therefore, concluded that the design equation of the net specific growth rate (u or 1/Tc) for the conventional activated sludge system is identical to that for the improved activated sludge system. The numerical hO

Aeration basin effluent

Influent (primary effluent)

BOD;

5.0 MGD 250 ppm 5.212 TPD 27.7 ppm 0.578 TPD 0 ppm 0 TPD

Carbon dioxide gas released

BOD:

Q+Qr 5.888 MGD 4 ppm 0.0982 TPD 4473 ppm 109.825 TPD 3578 ppm 87.86 TPD

Supracell clarified effluent

= 0.083 TPD SS = 16.84 ppm = 0.35 TPD VSS = 13.47 ppm = 0.28 TPD

Treatment plant effluent

Aeration basin Retention = 4.6 h Volume = 1.13 MG

Return sludge

BOD:

0.888 MGD 4 ppm 0.0148 TPD 29,009 ppm 107.419 TPD 23,207 ppm 85.935 TPD

Aeration basin Retention = 4.6 h Volume = 1.13 MG

Return sludge

Settled sludge

BOD:

BOD:

Q-QW1"QW2 4.981 MGD 4 ppm 0.083 TPD 10 ppm 0.208 TPD 8 ppm 0.166 TPD

Settled sludge

0.017 MGD 4 ppm 0.0003 TPD 29,009 ppm 2.056 TPD 23,207 ppm 1.645 TPD

BOD:

0.0019 MGD 4 ppm

0.00003 TPD 17,986 ppm 0.142 TPD 14,388 ppm 0.114 TPD

FIGURE 27.12 Improved activated sludge system with the installation of a secondary flotation clarifier (example). Note: 1MGD = 0.0438 m3/s; lppm-lmg/L; 1 TPD = 37.8 kg/h.

values of the two net specific growth rates, however, are different. Figure 27.13 shows the specific substrate utilization rate versus the limiting substrate concentration for the two activated sludge systems considered. Both systems use identical biological flocs; naturally the maximum specific substrate utilization rates (km) of the two systems are the same. The living biological flocs, separated by a secondary flotation clarifier, are returned to the aeration basin quickly (in less than 15 min), and thus stay in aerobic conditions at all times. Accordingly, the returned sludge (i.e., biological flocs) from secondary flotation (Figure 27.12) is more active (in terms of lower Ks value in Figure 27.13) than the comparable settled sludge from conventional secondary sedimentation (Figure 27.11). According to Equation 27.1, the improved activated sludge system (Figure 27.12) having a relatively lower Ks value (Figure 27.13) will definitely have a higher specific substrate utilization rate (U), signifying a higher biological treatment efficiency.

Microscopic examinations of floated sludge from secondary flotation and settled sludge from secondary sedimentation have been made to further demonstrate the aforementioned facts. Unstained samples of floated and settled sludge showed a marked difference in the number and viability of free-swimming and stalked ciliates (protozoa). Settled sludge contained only a few stationary cells (noted in 100 microscopic fields); floated sludge contained about 200 times more motile protozoan cells. Since protozoa are an integral and very important segment of the biological community, flotation is a desirable follow-up to the provision of dissolved oxygen (DO) within an aeration basin.

The equation of the net specific growth rate (Equation 27.6) holds true for both conventional and improved systems. The latter, having a comparatively higher specific substrate utilization rate (U), has a higher net specific growth rate (u) and requires less mean cell residence time (Tc) provided that the growth yield coefficient (F) and the decay coefficient (b) of the floated sludge and the settled sludge are assumed to be the same.

The mean hydraulic retention time (T) can be determined by Equation 27.13, regardless of the types of treatment system used:

For example, the mean hydraulic retention time for the entire improved activated sludge system can be expressed as

Maximum utilization rate of specific soluble organics (km)

Maximum utilization rate of specific soluble organics (km)

Hydraulic Retention Time Formula

ks ks Soluble organics in aeration basin (mg/L)

FIGURE 27.13 Specific soluble organics utilization rate versus the limiting soluble organic concentration.

ks ks Soluble organics in aeration basin (mg/L)

FIGURE 27.13 Specific soluble organics utilization rate versus the limiting soluble organic concentration.

where Vp is the volume of the primary clarifier, V is the volume of the aeration basin, Vf is the volume of secondary flotation, Vs is the volume of final sedimentation, and Q is the total wastewater flow to the wastewater treatment system.

For the secondary sedimentation alone, the mean hydraulic retention time is expressed as

For the aeration basin alone, the mean hydraulic retention time is expressed as

A typical overloaded conventional complete-mix activated sludge treatment plant (shown in Figure 27.11) has been described in Section 27.3.3. The same conventional treatment plant can be improved by the addition of a secondary flotation clarifier (shown in Figure 27.12). Some advantages of the improved activated sludge system are mathematically presented below.

The hydraulic detention time of the secondary sedimentation of the original overloaded conventional system can be calculated as

(see Figure 27.11) in comparison with Equation 27.15 for the improved activated sludge system. It is seen that the hydraulic loading of the original overloaded secondary sedimentation can be significantly reduced by a parameter of Qw1; thus saving of construction cost on expansion of any secondary sedimentation facilities is expected. With the addition of a small secondary flotation clarifier (such as a Krofta Supracell with a detention time of 3 min), the detention time of the sedimentation clarifier can be increased by 55% (i.e., from 0.935 to 1.45 h), as shown in Figures 27.11 and 27.12.

The hydraulic detention time of the conventional system's aeration basin is also expressed by Equation 27.16. However, the return sludge flow (Qr) of the improved activated sludge system is only about 33% (0.888/2.7 = 0.33) of the conventional activated sludge system, assuming the TSS concentrations (i.e., consistencies) of floated sludge and settled sludge are 2.8% and 1.25%, respectively. Accordingly, the hydraulic loading of an aeration basin can be reduced significantly by a secondary flotation addition (Krofta Supracell or an equivalent DAF from another manufacturer) to increase the hydraulic retention time (from 3.5 to 4.6 h or a 31% increase) without actually increasing the size of the aeration tank.

The higher solids content (Xw1) of the waste sludge produced from the improved activated sludge system, shown in Figure 27.12, represents another cost saving and the improved operation of sludge thickening, dewatering, and disposal. The waste sludge produced from the improved system will be

0.0170 MGD at 29,009 mg/L, and 0.0019 MGD at 17,986 mg/L, or a combined 0.0189 MGD at 27,900 mg/L.

The comparable conventional activated sludge system (Figure 27.11), on the other hand, generates 0.0396 MGD of waste sludge with a concentration of 12,500 mg/L. The sludge treatment cost of an improved system will, therefore, be reduced to one half due to a reduction in sludge flow and an increase in sludge consistency.

The most important fact is that both the effluent TSS (Xe) and effluent soluble BOD5 (S) of the improved wastewater treatment system will meet the governmental effluent standards.

27.4 CASE HISTORY: A PETROCHEMICAL CORPORATION IN TEXAS

Many pilot-scale and full-scale trials involving the use of secondary flotation (Krofta Supracell) in activated sludge treatment plants were conducted by KEC and Lenox Institute of Water Technology (LIWT) (formerly Lenox Institute for Research). Only partial operational data are selected for presentation in this chapter. The readers are encouraged to contact the authors for details. In the illustration of each case history, the true names of the company and its resident engineer involved are omitted for the protection of the Company's privacy.

The first example plant is a petrochemical manufacturing facility with an existing conventional activated sludge process. The waste loading was projected to increase when plant production increases in late 1981. In facing the loading increases the plant needed to meet the present State discharge limits and more stringent limits expected in the future. Mechanical breakdown of the secondary sedimentation clarifier or extremely high hydraulic loads due to sudden rainstorms in the past caused severe problems with high sludge loading, rising sludge, bulking sludge, and poor water quality. Figure 27.11 shows the conventional treatment plant before using a Krofta Supracell as an intermediate secondary flotation unit.

Full-scale trials with the secondary flotation clarifier were conducted in summer 1980, with excellent pilot results in solids removal, sludge consistency, and water clarity. Table 27.1 documents partial operational data. The following are the conclusions drawn from the investigation:

a. TSS loading: TSS loadings of 111-175 lb/day/ft2 were maintained while still maintaining an acceptable quality of clarified water (125-475 mg/L of SS or 94-98% of TSS removal). Here 1 mg/L = 1 ppm. Improvement in water quality could be obtained with lower TSS loadings (60-77 lb/day/ft2). At a loading of 60 lb/day/ft2, and with chemical addition, very clean water (with <20 mg/L of TSS) was obtained.

b. Chemical treatment: Acceptable operation was obtained without chemical aids. It is recommended, however, that the aids be available for full-scale operation. A small chemical addition dosage of cationic polymer (Pearl River Chemical 560 or equivalent) in the 10-20 mg/L range may be desirable for good sludge compaction and improvement in the overall clarification. Larger doses in the range up to 100 mg/L of cationic polymer gave exceptionally clear water. This chemical dose would be used in those cases when the cleanest possible water must be obtained (i.e., breakdown of the existing settling unit).

c. Aeration system: The demonstration plant was operated in the "full-flow pressurization" mode (Figure 27.8) in which all of the incoming water plus dilution water is pumped through the air dissolving tube. For power savings in the large installations, only the recycled water would be aerated (i.e., the recycle flow pressurization system would be used; see Figure 27.10). This will not change the amount of air available for flotation, as the clarified water is a more effective absorption medium than the incoming water. If the raw incoming water is pumped into the unit with a low-shearing-type pump, or by gravity flow, less shearing and breakup of the flocs would be expected in the larger unit than was experienced in the demonstration plant.

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