Water and Wastewater Engineering

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SECONDARY TREATMENT BY ATTACHED GROWTH AND HYBRID BIOLOGICAL PROCESSES 24-13

Disks

First stage

Second stage

Disk drive motor

Effluent weir

Effluent

discharge

Sludge

scoop

Sludge

discharge

Scoop drive

Clarifier inlet

Inlet from

primary

Secondary

clarifier

Raw

wastewater

Solids

disposal

Primary

treatment

Secondary

clarification

Effluent

Disinfection

RBC Contactors

FIGURE 24-4

Rotating biological contactor (RBC) and process arrangement. ( Source: U.S. EPA, 1977.)

The history of RBC installations has not been exemplary. Poor mechanical des ign and lack

of understanding of the biological process has resulted in structural failure of shafts, disks, and so

on. Many of the problems with early installations have been resolved and numerous installations

are performing satisfactorily.

The principal advantages of the RBC process are sim plic ity of operation and relatively low

energy costs. They have found application in small communities.

Integrated Fixed-Film Activated Sludge (IFAS)

This category includes any activated sludge system that incorporates a fixed-film media in a sus-

pended growth reactor. The purpose of fixed-film media is to increase the biomass in the reactor.

This offers the potential to reduce the basin size or to increase the capacit

y of an existing basin in

a retrofit application. Various types of suspended growth systems have been used. Examples in-

clude conventional, modified Ludzack-Ettinger (MLE), and step denitrification. These processes

differ from the MBBR described below in that they use a return sludge flow.

A number of proprietary

media types have been used including rope (no longer in use),

sponge, plastic carriers, and a honeycomb polyester fabric called BioWeb™. The media that are

24-14 WATER AND WASTEWATER ENGINEERING

fixed in a frame are preferred because they require fewer appurtenances and are less susceptible

to hydraulic problems that result from free-floating media.

The media frames are placed in conventional aeration tanks above a grid of fine bubble

diffusers. The frames vary in size to fit the aeration tank d imensions. For example, an arrange-

ment of two unit

s with dimens ions of 3.8 m  3.8 m  4 m high is set side by side across the

flow path of the reactor. One reported arrangement used 24 units spaced evenly along a plug flow

reactor for a 17,000 m

/ d plant (Jackson et al., 2007).

T ypical design criteria are as follows:

• MLSS: 1,000 to 3,000 mg/L

• MCRT: 3 to 10 days

• HRT: 3.5 to 4 hours

Moving Bed Biofilm Reactor (MBBR)

This process uses small, plastic elements (on the order of 7 to 22 mm effective diameter) to sup-

port the growth of biofilm in the rector. The suspended growth portion of the hybrid is designed

as a complete mix reactor. It is commonly mixed with aeration but may also be mixed with a

mec

hanical mixer. The process does not use a return sludge flow.

The media (typically polyethylene) is formed in a geometry to provide a high surface area

(250–515 m

/ m

). It has a density near that of water ( 0.96 g/cm

). The reactors are normally

filled from one-third to two-thirds of their empty volume with media. Because of their shape, less

than 15 percent of the water is displaced. A screen across the outlet is used to prevent the media

from leaving the aeration tank. Aeration is typically by coarse bubble diffusers.

T ypic

al design criteria are as follows:

• MLSS: 100 to 250 mg/L

• MCRT: 0.15 days

• HRT: 3 hours

Visit the text website at www.mhprofessional.com/wwe for supplementary materials

and a gallery of additional photos.

24 -6 CHAPTER REVIEW

When you have completed studying this chapter, you should be able to do the following without

the aid of your text or notes:

1 . Describe the difference between attached growth processes and suspended growth

processes.

2 . E xplain why a trickling filter does not really “filter” the wastewater.

3. Describe the two methods used to provide oxygen to a trickling filter.

4. Explain how the function of a biofilter differs from that of a tric

kling filter.

SECONDARY TREATMENT BY ATTACHED GROWTH AND HYBRID BIOLOGICAL PROCESSES 24-15

5 . E xplain why an attached growth biological process is diffusion limited.

6. Explain the function of recirculation in a rock filter and in a synthetic media filter.

7. Describe under what circumstances recirculation may be beneficial and when it may not.

8 . Describe three types of hybri

d processes.

9. Define the following terms: media, biofilter, BAF, RBC, IFAS, MBBR.

W ith the aid of this text, you should be able to do the following:

10. Select an appropriate type of trickling filter for a specified application. For example:

super rate for a BOD

loading of 0.5 kg BOD/m

· d.

11. Use the appropriate trickling filter equation to determine one or more of the following, given

the appropriate data: treatment efficiency, filter volume, filter depth, hydraulic loading rate.

12. Calculate empirical coefficients for use with proprietary media.

24 -7 PROBLEMS

24 -1. Envirotech Systems markets synthetic media for use in the construction of trickling

filters. Envirotech uses the following formula to determine BOD removal efficiency:

exp

⎡

⎣

⎢

⎤

⎦

⎥

where L

 BOD

of efﬂ uent, mg/L

 BOD

of influent, mg/L

K  treatability factor, (m/d)

0.5

/ m

y  temperature correction factor

 (1.035)

T  20

T  wastewater temperature,  C

D  media depth, m

Q  hydraulic loading rate, m/d

n  0.5

U sing the following data for domestic wastewater, d eterm ine the treatability fac tor K.

Wastewater temperature  1 3  C

H ydraulic loading rate 

41.1 m/d

% BOD remaining Media depth, m

100.0 0.00

80.3 1.00

64.5 2.00

41.6 4.00

17.3 8.00

24-16 WATER AND WASTEWATER ENGINEERING

24 -2. Using the Envirotech Systems equation and the treatability factor from Problem 24-1,

estimate the depth of filter required to achieve 82.7% BOD

removal if the wastewater

temperature is 20  C and the hydraulic loading rate is 41.1 m/d.

24 -3. Koon et al. (1976) suggest that recirculation for a synthetic media filter may be con-

sidered by the following formula:





 

exp

⎡

⎣

⎢

⎤

⎦

⎥

⎡

⎣

⎢

⎤

⎦

⎥

()1

where R  recirculation ratio and all other terms are as described in Problem 24-1.

U se this equation to determine the efficiency of a 1.8-m-deep synthetic media filter

loaded at a hydraulic loading rate of 5.00 m/d with a recirculation ratio of 2.00. The

wastewater temperature i

s 16  C and the treatability factor is 1.79 (m/d)

0.5

/m at 20  C.

24-4. Determine the concentration of the effluent BOD

for the two-stage trickling filter

described below. The wastewater temperature is 17  C. Assume the NRC equations apply.

D e sign ﬂ ow  0.0509 m

/ s

Influent BOD

(after primary treatment)  260 mg/L

Diameter of each filter  24.0 m

Depth of each filter  1.83 m

R e circulation ﬂ ow rate for each ﬁ lter  0.0594 m

/ s

24 -5. Using a spreadsheet program you have written, plot a graph of the final effluent BOD

of the two-stage filter described in Problem 24-4 as a function of the influent flow

rate. Assume that the ratio of recirculation flow to influent flow remains constant.

Use flow rates of 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.10 m

/ s for the

influent.

24 -6. Determine the diameter of a single-stage rock media filter to reduce an applied

BOD

of 125 mg/L to 25 mg/L. Use a flow rate of 0.14 m

/ s, a recirculation ratio

of 12.0, and a filter depth of 1.83 m. Assume the NRC equations apply and that the

wastewater temperature is 20  C.

24 -7. Using a spreadsheet program you have written, plot a graph of the final effluent

BOD

of the single-stage filter described in Problem 24-6 as a function of the influ-

ent flow rate. Assume that the ratio of recirculation flow to influent flow remains

constant and that the filter diameter is 35.0 m. Use hydraulic loading rates of 10, 12,

14, 16, 18, and 20 m

/ m

24-8 REFERENCES

Germain, J. E. (1966) “Economical Treatment of Domestic Waste by Plastic Media Trickling Filters,”

Journal Water Pollution Control Federation, vol. 38, p. 192.

Grady, C. P. L., Jr., G. T.Daigger, and H. C. Lin (1999) Biological Wastewater Treatment, 2nd ed.,

Marcel Dekker, New York.

SECONDARY TREATMENT BY ATTACHED GROWTH AND HYBRID BIOLOGICAL PROCESSES 24-17

Hansen, R., T. Thogersen, and F. Rogalla (2007) “Comparing Cost and Process Performance of Activated

Sludge (AS) and Biological Aerated Filters (BAF) Over Ten Years of Full Scale Operation,” Water

Science & Technology, vol. 55, nos. 8–9, pp. 99–106.

Iranpour, R., H. H. J. Cox, M. A. Deshusses, and E. D. Schroeder (2005) “Literature Review of Air

Pollution Control Biofilters and Biotrickling Filters for Odor and Volatile Organic Compou

Removal” Environmental Progress, vol. 24, no. 3, pp. 254–267.

Jackson, D. R., L. E. Ripley, T. Maurina, and S. Hubbell (2007) “Up to the Challenge,” Water

Environment and Technology, November, pp. 51–55.

Koon, J. H., R. F. Curran, C. E. Adams, and W. W. Eckenfelder (1976) Evaluation and Upgrading of a

Multistage Trickling Filter Facility, U.S. Environmental Protection Agency, Publication No. EPA

600/2-76-193, Washington, DC.

Metcalf & Eddy (2003) Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill, Boston,

Massochusetts, pp. 602-607.

NRC (1946) “Sewage Treatment at Military Installations,” Sewage Works Journal, vol. 18, p. 787.

Rittmann, B. E. and P. L. McCarty (2001) Environmental Biotechnology: Principles and Applications,

McGraw-Hill, Boston, Massochusetts.

S chulze, K. L. (1960) “Load and Efficiency of Trickling Filters,” Journal of Water Pollution Control

Federation, vol. 32, p. 245.

U. S. EPA (1977)

Process Design Manual: Wastewater Treatment Facilities for Sewered Small

Communities, U. S. Environmental Proctection Agency, EPA Pub. No. 625/1-77-09, Washington, D. C.

p. 8–12.

U.S. EPA (1984) Needs Survey, U.S. Environmental Protection Agency, Washington, D.C.

Velz, C. J. (1948) “A Basic Law for the Performance of Biological Filters,” Sewage Works, vol. 20,

p. 607.

WEF (1998) Design of Municipal Wastewater Treatment Plants, 4th ed., Water Environment Federation

Manual of Practice 8, Alexandria, Virgina, pp. 12-1–12-187.

Williamson, K. and P. L. McCarty (1976) “A Model of Substrate Utilization by Bacterial Films,” Journal

Water Pollution Control Federation, vol. 48, pp. 9–24.

25-1

25 -5 CHAPTER REVIEW

25 -6 PROBLEMS

25 -7 DISCUSSION QUESTIONS

25 -8 REFERENCES

SECONDARY SETTLING, DISINFECTION,

AND POSTAERATION

25 -1 INTRODUCTION

25 -2 SECONDARY SETTLING

25 -3 DISINFECTION

25 -4 POSTAERATION

CHAPTER

25-2 WATER AND WASTEWATER ENGINEERING

25 -1 INTRODUCTION

The EPA’s requirements for secondary treatment include an effluent BOD

less than or equal to

30 mg/L and an effluent suspended solids less than or equal to 30 mg/L. Most National Pollution

Discharge Elim ination System (NPDES) permits also specify a fecal coliform concentration.

Dissolved oxygen modeling may also reveal the need for postaeration to maintain the receiving

body water quality

. The secondary treatment processes to meet these requirements are the subject

of this chapter.

A s noted in the chapters devoted to biological treatment processes, the conversion of BOD

to mic robial cells does little to reduce the BOD in the effluent if the c ells are discharged.

Likewise, suspended

solids limits cannot be met if the microbial mass grown in the biologic al

processes is discharged . The conventional secondary treatment technology to remove the

biomass is settling.

Disinfection may be carried out by a variety of means. The discussion in this chapter is

limited to chlorine and ultraviolet disinfection.

Postaeration to improve the DO of the discharged wastewater completes the discussion of

secondary treatment.

25 -2 SECONDARY SETTLING

The function of secondary settling tanks that follow trickling filters is to produce a clarified

effluent. Secondary settling tanks that follow activated sludge processes also serve the function

of thickening to provide a higher solids concentration for either return activated sludge or wasting

and subs

equent treatment.

The following discussion is divided into two parts: design principles and design practice.

Design Principles

Trickling Filter. Sedimentation theory is discussed in Chapter 10. Trickling filter solids set-

tling may be classified as Ty pe II flocculant settling. Because the suspended solids loading is

low, the overflow rate governs design.

Activated Sludge. Activated sludge solids settling classification falls into eac

h of the four

types depending on the depth in the c larifier. In the upper, clear water level, discrete floc par-

ticles settle (Type I settling). As the particles sink, they begin to flocculate (Type II settling).

In the lower zones , hindered settling (Type III) and compression settling (Type IV) take

place.

Both clarification and thickening are considered in the des

ign of the secondary settling tank

for activated sludge systems. Clarification is governed by the settling velocity of the light fluffy

particles. Thickening is governed by the mass flux of solids in the zone where hindered settling

takes place.

It is not possible to make estimates of overflow rates for clarification bas

ed on first principles.

The irregular nature of activated sludge floc precludes any rational estimate of settling velocity

that could be used to select an overflow rate.

The surface area required for thickening may be determined by one of two methods: solids

flux analysis or state point analysis. Solid

s flux analysis is discussed in Chapter 15. State point

SECONDARY SETTLING, DISINFECTION, AND POSTAERATION 25-3

analysis is an extension of solids flux analysis to provide a means to assess different MLSS

concentrations and operating conditions relative to the limiting solids concentration.

The state point ( shown in Figure 25-1 ) is the intersection of the clarifier overflow

solids-flux rate and the underflow solids-flux

rate. This analysis accounts for the MLSS

concentration, clarifier hydraulic loading, and RAS. It enables an assessment of these

operating parameters to determine whether or not the conditions are within the clarifier solids-

flux limitations.

The clarifier overflow solids flux is



()()

(25-1)

where SF  overflow solids flux rate, kg/m

· d

Q  clarifier effluent flow rate, m

/ d

X  aeration tank MLSS, kg/m

A  clarifier surface area, m

The aeration tank MLSS at any point along the overflow solids -flux line is found by con-

structing a vertical line to the x -axis. In Figure 25-1 , the MLSS is shown as about 3.8 g/L.

Because the underflow velocity ( U

) can be controlled by controlling the flow rate of return

activated sludge, the underflow operating line is used for process control. U

is defined as



(25-2)

where U

 underflow velocity, m/d

 underflow flow rate, m

/ d  Q

, the return sludge rate or RAS

A  surface area, m

Solids flux, kg/m

ⴢ h

Solids concentration X, g/L

12 140

10642 8

State point

MLSS

State point

MLSS

Overflow rate

operating line

Underflow rate

operating line

FIGURE 25-1

State point analysis for assessing clarifier operating

conditions. The state point is at the intersection of the

overflow rate and underflow rate operating lines.

( Source: Metcalf & Eddy, 2003.)

25-4 WATER AND WASTEWATER ENGINEERING

Other relationships for U

can be used to evaluate the clarifier operation:

SF SF





0

MLSS

(25-3)

QQ X A







()( )

MLSS

(25-4)

Where SF

is the total solids flux.

The underflow operating line is represented as the negative slope of underflow velocity. The

underflow solids flux is

SF U X

ubi



(25-5)

where SF

= solids flux resulting from underflow, kg/m

· d

= solids concentration at a given point, kg/m

To determine if the operation of the clarifier is within its solid flux limitation, a gravity flux

curve i s plotted on the state point graph. The gravity solids flux is calculated using Equation 25-6 :

SF C v

g ii



(25-6)

where SF

 gravity flux, kg/m

· d

 initial MLSS concentration, kg/m

 initial settling velocity, m/d

Batch settling curve data similar to that in Figure 25-2 are used in the computation.

U sing Figure 25-3 , the plots may be interpreted as follows:

• The underflow line at state point A is tangent to the gravity flux curve. This is the limiting

solids flux conditi

on.

If the operation is changed to obtain a higher MLSS concentration and the under-

flow line crosses the lower limb of the gravity flux curve, the limiting solids flux will be

exceeded and the clarifier sludge blanket will rise to the effluent weir.

• The underflow line at state point B represents a lower MLSS concentration. This represents

an underloaded operation relative to the solids loa

ding.

From the state point analysis, it is self evident that the design and behavior of the secondary

clarifier is intimately linked to the activated sludge process. The linkage is through control of

the MLSS concentration by wasting and the return activated sludge flow rate. Table 25-1 and

Figure 25-4 illustrate the use of

state point analysis in design and operation of the secondary

clarifier.

Mathematical models are available to facilitate design iterations using set point analysis

(McCorquodale et al., 2004; McCorquodale et al., 2006).

SECONDARY SETTLING, DISINFECTION, AND POSTAERATION 25-5

0.123 4 10 30 100 200200.20.3 0.4 1.0

0.1

0.2

0.3

0.4

1.0

2.0

3.0

4.0

10.0

20.0

30.0

40.0

100.0

Subsidence rate, m/d

Suspended solids concentration, kg/m

Primary sludge

WAS

Grand

Rapids, MI

Mixed activated

sludge and primary

sludge

Activated

sludge

FIGURE 25-2

Batch settling curves. ( Source: Davis and Cornwell, 2008.)

Solids flux, kg/m

ⴢ h

Solids concentration X, g/L

140

1064

2812

Overflow rate

operating line

Gravity flux curve

Underflow rate operating

line for MLSS  4 g/L

Underflow rate operating

line for MLSS  3.3 g/L

FIGURE 25-3

State point analysis illustrating critically loaded (A) and

underloaded (B) state points relative to the gravity flux

curve. ( Source: Metcalf & Eddy, 2003.)