Water and Wastewater Engineering

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23-108 WATER AND WASTEWATER ENGINEERING

treatment plant ( Problem 23-46 ). The following estimates have been provided for the

design of one of eight basins:

W a stewater depth  4.5 m

E q uivalent length of pipe  160 m

Air flow rate  140 m

/ min

A mbient air pressure  101.325 kPa = 10.333 m of H

O  1 atmosphere

A mbient air temperature  27  C

Steel pipe diameter  350 mm

Diffuser losses  300 mm

Allowance for clogging  250 mm

Silencer  60 mm

Air filter  150 mm

A ssume a blower efficiency of 70%.

23-51. Your firm has been asked do a preliminary study of the fea

sibility of a new A

/O ™

BPR wastewater treatment plant for Pekin. Begin the design of the complete-mix

BOD removal and nitrification stage for the plant. As a first step determine the SRT

and volume of tank required for nitrification using the following design data:

Influent data after primary settling

D e sign flow rate  64,500 m

/ d

BOD

 250 mg/L

bCOD  400 mg/L

rbCOD  50 mg/L

Total Kjeldahl nitrogen (TKN)  50 mg/L

-N  40 mg/L

Soluble phosphorus  9 g/m

Total suspended solids (TSS)  80 mg/L

Volatile suspended solids (VSS)  70 mg/L

Biodegradable volatile suspended solids  50 mg/L

Minimum sustained temperature  9  C

pH  6.9

Alkalinity  100 mg/L as CaCO

Effluent discharge standards

bCOD  20 mg/L

-N  1.0 mg/L

-N  10 mg/L

TSS  10 mg/L

Total phosphorus  2.0 mg/L

A ssume DO for aeration tank nitrification  2.0 mg/L; MLSS  4,000 mg/L,

MLVSS  (0.8) MLSS, NO

 80% of TKN, and RAS  0.50(Q).

23-52. In continuing the design of the BPR process for Pekin’s new wastewater treatment

plant, determine the volume of the anoxic tank. Use the data provided in Problem 23-51 .

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-109

23-53. Check the alkalinity for the BPR wastewater plant being designed for Pekin. Use the

data from Problems 23-51 and 23-52. Assume an alkalinity of 150 g/m

is needed to

maintain a pH of about 7.

23-54. Estimate the effluent phosphorus concentration for Pekin’s new BPR wastewater

treatment plant. Use the data and assumptions from Problems 23-51 through 23-53.

In addition, assume the following:

• 10.5 g rbCOD/g P is removed by BPR

• rbCOD/nitrate ratio is 6.6 g rbCOD/g NO

-N

• Phosphorus content of heterotrophic bacteria is 0.015 g P/g biomass

• No NO

-N in influent

23-55. As part of a preliminary design, estimate the required blower power for the complete-mix BOD

removal and nitrification stage for Pekin’s new BPR wastewater treatment plant ( Problem

23-51 ). The following estimates have been provided for the design of one of eight basins:

W a stewater depth  4.5 m

E q uivalent length of pipe  150 m

Air flow rate  130 m

/ min

A mbient air pressure  101.325 kPa  10.333 m of H

O 1 atmosphere

A mbient air temperature  24  C

Steel pipe diameter  350 mm

Diffuser losses  300 mm

Allowance for clogging  250 mm

Silencer  60 mm

Air filter  150 mm

A ssume a blower efficiency of 75%.

23-11 DISCUSSION QUESTIONS

23-1. Step aeration activated sludge provides more oxygen to the microorganisms by “step-

ping up” the air supplied as the waste moves through the aeration tank. True or false?

23-2. An extended aeration activated sludge plant will be operated at an F/M ratio that is

higher than a conventional activated sludge plant. True or false?

23-3. A wastewater treatment plant operating at an F/M ratio of 0.

32 d

 1

is having trouble

disposing of its sludge. The treatment plant operator knows that he must alter the

F/M ratio to reduce the amount of sludge but cannot remember in which direction.

Help the operator out by telling him which direction he should change the F/M ratio

(higher or lower) and explain what effect this will have on the power requirements.

23-4. Explain the difference between internal recyc

le and return activated sludge.

23-5. Identify the common aerator for each of the following processes and explain why it is

appropriate:

• O xidation ditch

• SBR

• Completely mixed activated sludge

23-110 WATER AND WASTEWATER ENGINEERING

23-6. Using the influent wastewater characteristics and the effluent limits for the plant

shown schematically below, mark how each treatment step affects the concentration

of carbonaceous BOD on the graph (Muirhead, 2005a).

Screening

Grit

removal

Primary

clarifier Activated sludge

Anoxic

Anaerobic Aerobic

Secondary

clarifier Filters

WA S

Alum

Disinfection

Sodium

bisulfite

Sodium

hypochlorite

MLSS recycle 300 3 Q

RAS 0.5 3 Q

Anaerobic

digester

centrate

Wastewater characteristics and effluent limits

Influent characteristics

Effluent limits

Constituent

Concentration, mg/L Constituent Limit, mg/L

250

225

250

CBOD

TSS

TKN

Alkalinity

–

–2

CBOD

TSS

-N

1.0

250

Influent

FIGURE D-23-6

Carbonaceous BOD.

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-111

23-7. Using the influent wastewater characteristics and the effluent limits for the plant

shown schematically below, mark how each treatment step affects the concentration

of ammonia nitrogen on the graph (Muirhead, 2005b).

Wastewater characteristics and effluent limits

Influent characteristics

Effluent limits

Constituent

Concentration, mg/L Constituent Limit, mg/L

250

225

250

BOD/CBOD

TSS

TKN

Alkalinity

–

–2

CBOD

TSS

-N

1.0

Screening

Grit

removal

Primary

clarifier Activated sludge

Anoxic

Anaerobic Aerobic

Secondary

clarifier Filters

WA S

Alum

Disinfection

Sodium

bisulfite

Sodium

hypochlorite

MLSS recycle 300 3 Q

RAS 0.5 3 Q

Anaerobic

digester

centrate

Influent

FIGURE D-23-7

A mmonia nitrogen.

23-112 WATER AND WASTEWATER ENGINEERING

23-8. Using the influent wastewater characteristics and the effluent limits for the plant

shown schematically below, mark how each treatment step affects the concentration

of alkalinity on the graph (Muirhead, 2006).

FIGURE D-23-8

Alkalinity.

Wastewater characteristics and effluent limits

Influent characteristics

Effluent limits

Constituent

Concentration, mg/L Constituent Limit, mg/L

250

225

250

BOD/CBOD

TSS

TKN

Alkalinity

–

CBOD

TSS

-N

1.0

Screening

Grit

removal

Primary

clarifier Activated sludge

Anoxic

Anaerobic Aerobic

Secondary

clarifier Filters

WA S

Alum

Disinfection

Sodium

bisulfite

Sodium

hypochlorite

MLSS recycle 300 3 Q

RAS 0.5 3 Q

Anaerobic

digester

centrate

Influent

–

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-113

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23-114 WATER AND WASTEWATER ENGINEERING

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24-1

24 -5 HYBRID PROCESSES

24 -6 CHAPTER REVIEW

24 -7 PROBLEMS

24 -8 REFERENCES

SECONDARY TREATMENT BY ATTACHED

GROWTH AND HYBRID BIOLOGICAL PROCESSES

24-1 INTRODUCTION

24 -2 ATTACHED GROWTH PROCESSES

24 -3 ATTACHED GROWTH DESIGN

PRINCIPLES

24 -4 ATTACHED GROWTH DESIGN PRACTICE

CHAPTER

24-2 WATER AND WASTEWATER ENGINEERING

24-1 INTRODUCTION

This chapter focuses on processes that utilize films of microorganisms attached to inert media

( attached growth ) to treat wastewater. In addition, hybrid processes that use a combination of

suspended growth and attached growth microorganisms are discussed. Those processes utilizing

only suspension

s of microorganisms ( suspended growth ) to treat wastewater are discussed in

Chapter 23.

The portion of this chapter that addresses attached growth is organized into three parts:

processes for treatment, design principles, and des ign practice.

24 -2 ATTACHED GROWTH PROCESSES

In attached growth processes, the microorganisms form a film on a bed, disk, or other support

material, such as stones, slats, or plastic materials ( media ), over which wastewater is applied.

Because the microorganisms that biodegrade the waste form a film on the media, this process is

known as an attached growth process.

Air, provided by either natural draft or blowers, circulates in the void space between

the media elements. Excess growths of microorganisms slough from the media. This would

cause high levels of BOD and suspended solids in the plant effluent if not removed. Thus

, the

flow from the attached growth process is passed through a sedimentation basin to allow these

solids to settle out. As in the case of the activated sludge process, this sedimentation basin

is referred to as a secondary clarifier. Unlike the activated sludge process, the solids are not

retu

rned to the attached growth process. They are collected and removed for processing and

disposal.

The attached growth process to be c onsidered in this discussion is the trickling filter. In

addition, the application of attached growth for odor control in the form of a biofilter will also be

described

Trickling Filters

Historically, trickling filters have been a popular biologic treatment process. The mos t widely

used design for many years was simply a bed of stones from 1 to 3 m deep through which the

wastewater passed. The wastewater is typically distributed over the surface of the roc

ks by a

rotating arm ( Figure 24-1a) . Rock filter diameters may range up to 60 m.

Trickling filters are not primarily a filtering or straining process as the name implies. The

rocks in a rock filter are 25 to 100 mm in diameter, and hence have openings too large to strain

out solids. The rocks are a means of providing large amounts of surfa

ce area where the microor-

ganisms can cling and grow in a slime on the rocks as they feed on the organic matter.

Although rock trickling filters have performed well for years, they have c ertain limitations.

Under high organic loadings , the slime growths can be so prolific that they plug the void spaces

between the rocks, caus

ing flooding and failure of the s ystem. Also, the volume of void spaces

is limited in a rock filter. This restricts the circulation of air and the am ount of oxygen avail-

able for the microbes. This lim itation, in turn, restricts the amount of wastewater that can be

processed .

To overcome these limitations

, other materials have become popular for filling the trickling

filter. These materials include modules of corrugated plastic sheets and plastic rings ( Figure 24-1b) .