Water and Wastewater Engineering

Подождите немного. Документ загружается.

TERTIARY TREATMENT 26-11

At low alum doses ( 12 mg/L), membrane fouling may be worse than filtration without

coagulant. At high doses ( 25 mg/L) membrane performance may be improved significantly

(Howe and Clark, 2006). Jar testing must be used as an operational control of chemical dose.

The feed water is chlorinated to prevent biofo

uling. A typical dose is 3 to 5 mg/L of NaOCl.

Strainers with a pore size  500  m are used to prevent debris from mechanical abrasion of,

or adhesion to, the membrane fibers.

The membrane systems are hydraulically limited to a peaking factor of about 2.0 to 2.5. If

the upstream processes

do not provide sufficient equalization, it may be required for the efficient

operation of the membrane.

Design Criteria

Table 26-8 provides a summary of range of design values for MF and UF membranes used for

filtration of secondary effluent.

ND  not detected.

Source: Extracted from WEF, 2006.

TABLE 26-7

Typical filtrate water quality for MF/UF treatment of

secondary effluent

Parameter Range of values Comment

Flux

MF 17–90 L/m

· h Prefiltering with cloth media may permit

up to 127 L/m

· h

UF 17–34 L/m

· h

Transmembrane pressure (TMP)

MF 70–170 kPa

UF 100–700 kPa

TABLE 26-8

Range of design values for membrane filters

Upper limit of range achieved in pilot scale testing.

Sources: Gnirss and Dittrich, 2000; Metcalf & Eddy, 2003; Tooker and Darby, 2007; WEF, 2006.

Parameter Range of values

Biochemical oxygen demand (BOD) 2–5 mg/L

Total organic carbon (TOC) 5–25 mg/L as C

Total Kjeldahl nitrogen (TKN) 5–30 mg/L as N

Total phosphorus 0.1–8 mg/ as P

Total suspended solids (TSS)

Turbidity 0.1 NTU

Fecal coliforms 2–10 per 100 mL

Virus 1–300 PFU per 100 mL

26-12 WATER AND WASTEWATER ENGINEERING

Fouling and Cleaning

Biofouling is pervasive in tertiary treatment membranes. It may be described as a combination of

the formation of biofilms and the accumulation of bioorganic material. The bioorganic material is

composed of extracellular polymeric substances such as proteins, carbohydrates, polysacchari

des,

and lipids.

The addition of chlorine and/or chloramination of the feed water is a standard maintenance

procedure. Thus, it is important to consider the effects of continuous addition of chlorine or

chloramines in the selection of the membrane material for the filter. Detailed discussions of

the contributions to m

embrane fouling, and fouling mechanism s, as well as maintenance and

recovery cleaning procedures are discussed in Chapters 9, 12, and 23.

26-5 CARBON ADSORPTION

Even after secondary treatment, coagulation, sedimentation, and filtration, soluble organic mate-

rials that are resistant to biological breakdown will persist in the effluent. The persistent materials

are often referred to as refractory organics. Refractory organic compounds can be detec ted in

the effluent as soluble COD. Second

ary effluent COD values are often 30 to 60 mg/L. The most

practical available method for removing refractory organic compounds is by adsorbing them on

activated carbon (U.S. EPA, 1979).

Process Alternatives

Both powdered activated carbon (PAC) and granular activated carbon (GAC) are used. PAC may

be added to the effluent of a biological treatment process, or it may be applied directly to the

aeration tank. GAC is used in a column. The columns may be fixed bed or moving bed.

GAC columns have been favored in tertiary treat

ment applications. Of the column types, the

fixed-bed is is most common (Metcalf & Eddy, 2003). Downflow columns are favored because

of the advantage of achieving both adsorption and filtration in one step. Backwashing is provided

to limit the headloss due to build up of particulate matter in the column.

Pretreatment

To make efficient use of the carbon and extend ru n times, typical flow charts show the

carbon column prec eded by granular media filtration and either chlorination or break point

chlorination (U.S EPA, 1973). Rules of thumb suggest that total suspended solids applied to

the GAC column not exceed 5 mg/L, and fat

s, oil, and grease (FOG) be less than 10 mg/L.

Chlorination is to inhibit microorganism growth on the carbon. Break point chlorination is to

remove ammonia.

Reaction Vessels

The reaction vessel for the GAC column may be pressurized, or it may use a gravity flow system.

Pressurized columns are fabricated with a flat, conical, or dish-shaped head. They have a carbon

screen and support grid installed in the bottom.

Gravity reaction vessels are similar to rapid sand filters

in configuration. They are designed

using existing sand filter technology (U.S. EPA, 1973).

TERTIARY TREATMENT 26-13

Carbon Selection

The critical element in the design of the carbon column is the selection of a manufacturer’s activated

carbon. The selection proceeds stepwise with the investigation of alternatives by simple laboratory

tests to develop adsorption isotherms for the wastewater. From these data one or more carbon types

are sele

cted, and small scale column tests are used to develop kinetic data for design. The rapid

small-scale column test (RSSCT) is recommended for this evaluation (Crittenden et al., 1991).

Other issues to be considered are resistance to abrasion, ash content, and particle size. Of

these, the partic

le size is particularly relevant to the column design.

Carbon size is specified by “mesh” sizes. For example, an 8  30 mesh carbon is one that

passes a U.S. Number 8 sieve (2.36 mm opening) and is retained on a Number 30 sieve (0.6 mm

opening). Typically, an 8  30 mes

h is used for downflow beds, and a 12  40 mesh is used for

upflow beds.

Column Sizing

Empty Bed Contact Time (EBCT). Early reports of EBCT ranged from 10 to 50 minutes (U.S.

EPA, 1973). More recently, EBCTs have been set based on desired effluent quality. For example,

when effluent quality limits require a COD of 10 to 20 mg/L, the EBCT is typically in the range

15 to 20 minutes. For COD limits of 5 to 15 mg/L, EBCTs will range fro

m 30 to 35 minutes

(WEF, 1998).

Hydraulic Loading. The range of hydraulic loading is from 10 to 20 m

/h · m

(or 10 to 20 m/h)

of cross section for upflow columns. Lower hydraulic loading rates between 5 to 12 m

/h · m

are

used for downflow columns. Pressures seldom exceed 7 kPa (WEF, 1998).

Backwash. For typical mesh sizes, backwash rates vary from 20 to 50 m

/h · m

. The back-

wash duration is 10 to 15 minutes.

Bed Depths. Depending on c ontact time, bed depths vary from 3 to 12 m. A minimum depth

of 3 m is recommended.

Column Dimensions. Shop fabricated pressurized columns are restricted in size by transportation

clearance

s. Prefabricated column diameters range from 0.75 to 3.6 m. Lengths do not exceed

18 m. Columns fabricated onsite may exc eed these dimensions. Columns in series are used to

obtain greater contact time and better use of carbon capacity. The ratio of col

umn height to

diameter is in the range 1.5:1 to 4:1. Tall, thin columns are preferred over short, fat ones.

While carbon bed depths are in the range 3 to 12 m, the column shell must be deep enough to

allow bed expansion during backwash of 10 to 50 percent (U.S. EPA, 1973).

Carbon Regeneration

Although the actual mass of carbon must be determined from column tests, typical usage is sum-

marized in Table 26-9 .

After the adsorption capacity of the carbon has been exhausted, it can be restored by heating

it in a furnace at a temperature sufficiently high to drive off the adsorbed organic

matter. Keeping

oxygen at very low levels in the furnace prevents carbon from burning. The organic matter is

26-14 WATER AND WASTEWATER ENGINEERING

passed through an afterburner to prevent air pollution. In small plants where the cost of an onsite

regeneration furnace cannot be justified, the spent carbon is shipped to a central regeneration

facility for processing. There is a loss of about 5 to 10 percent during each regeneration cycle.

Carbon regeneration is a major consideration in the selection and design of GAC facilities.

An extensive

discussion of the options and operational considerations is given in Clark and

Lykins (1989).

Design Criteria

T ypical design criteria are listed in Table 26-10 .

Prior treatment

Carbon use

rate (CUR), g/m

Coagulated, settled, and

filtered activated sludge

effluent 24–48

Filtered secondary effluent 48–72

TABLE 26-9

Typical carbon usage for wastewater

Source: WEF, 1998.

Parameter ValueComment

Carbon mesh size 8  30 Downflow beds and upflow packed

beds

12  40 Upflow expanded beds

Hydraulic loading rate

Upflow column 10–20 m

/h · m

Downflow column 5–12 m

/h · m

EBCT 10 to 50 min Pilot tests are essential

Carbon use rate (CUR) 24 to 72 g/ m

Lower CURs are associated with

longer EBCTs

Column height  18 m Prefab will be sized to fit on

flatbed trailer

Diameter  3.6 m Restriction to 3.6 m for transport

of prefab units

Height:Diameter 1.5:1 to 4:1 Without liquid redistribution use

 4:1 for proper liquid distribution

Backwash rate 20 to 50 m

/h · m

Backwash expansion 10 to 50%

Backwash duration 10–15 min

TABLE 26-10

Typical design ranges for GAC columns for wastewater treatment

Sources: Clark and Lykins, 1989; Culp et al., 1978; U.S EPA, 1973.

TERTIARY TREATMENT 26-15

26-6 CHAPTER REVIEW

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

the aid of your textbooks or notes:

1. Write theoretical precipitation reactions for removal of phosphorus with ferric chloride,

alum, or lime.

2. Explain why the actual dose of chemicals to precipitate phosphorus is greater than the

stoichiometric dose.

3. Sketch and label a granular filter identifying the following pertinent features

: inlet

main, outlet main, wash water outlet, gullet, support media (graded gravel), graded

filter medium, and backwash troughs.

4. Define effective size and uniformity coefficient and explain their use in designing a

granular filter.

5. From a design point of view, explain the role of filtration rate, grain size distribution,

and poros

ity in controlling headloss through a granular filter.

6. Explain the role of estimating the depth of the expanded bed in designing a granular

filter.

7. Compare the advantages and disadvantages in selecting the type of filter, that is, sand,

dual media, or deep-bed monomedium anthracite.

8. Qualitatively com

pare the effectiveness of bed expansion and surface wash in back-

washing a filter.

9. Explain to a client the circumstances that favor the use of MF/UF membranes.

10. Compare the mechanisms of filtration for granular filters and membranes.

11. Explain the role of the pore size and resistance coefficient in the des

ign flux of a

MF/UF membrane.

12. Draw a sketch of the flux or transmembrane pressure as a function of time that shows

reversible and irreversible membrane fouling, and the effect of chemical cleaning.

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

13. Perform a grain size analysis and determine the effective size and uniformit

coefficient.

14. Calculate the headloss through a clean stratified filter bed and determine if it is excessive.

15. Calculate the depth of an expanded filter bed during back wash and locate the back

wash trough elevation with respect to the top of the filter bed during filtration.

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

and a gallery of photos.

26-16 WATER AND WASTEWATER ENGINEERING

16. Calculate the number and size of filter beds given the maximum day flow rate and the

filtration rate.

17. Calculate the equivalent diameter for a filter media having a specific gravity different

from sand.

18. Design a backwash system including the layout for the placement of backwash troughs,

size of the tro

ughs, trough elevation, and the volume of backwash tank.

19. Determine gullet dimensions (depth and width) given the length, backwash flow rate,

and wash water velocity in the effluent pipe.

20. Calculate the maximum headloss that can be achieved without creating a negative pres-

sure in the filter media.

21. Calculate rejection, log removal, and percent re

moval of a constituent by a membrane

filter.

22. Size a membrane system given the design flow rate and flux or determine the flux

from the transmembrane pressure, water temperature, and membrane resistance

coefficient.

23. Determine the number of MF/UF membrane modules and rack arrangement given the

sign flow rate, design flux, membrane area per module, and backwash cycle.

26-7 PROBLEMS

26 -1. Rework Example 26-1 using alum (Al

(SO

)



14H

O) to remove the phosphorus.

26-2. Rework Example 26-1 using lime (CaO) to remove the phosphorus.

26-3. Given that the Al:P ratio is 1.2: in Table 26-1 , show how the alum:P ratio in the table

is calculated.

26-4. In Example 23-1 (Chapter 23), a correction was made for the allowable effluent

BOD

to account for the BOD of the total suspended solids. Assume that tertiary

filtration has been installed to reduce the TSS to 10 mg/L and recalculate the allow-

able effluent BOD

. Use this new estimate of S to determine the volume of the

aeration tank.

26-5. A dual media denitrification filter is being considered for the effluent from the oxida-

tion ditch being built for the city of Pasveer (Problem 23-28, Chapter 23). Using the

data from Pasveer and a permit requirement that NO

-N not exceed 4.0 mg/L, deter-

mine the following:

a. Filter media volume

b. Filter hydraulic loading

c. Number of filters and their nominal dimensions

d. Methanol dose in mg/L and kg/d

A ssume a typical NO

-N loading rate for the sustained minimum temperature and as-

sume that the filters will be sized using the guidance in Chapter 11.

TERTIARY TREATMENT 26-17

26-6. A dual media denitrification filter is being considered for the effluent from the oxida-

tion ditch being built for the city of Brooklyn (Problem 23-33, Chapter 23). Using

the data from Brooklyn and a permit requirement that NO

-N not exceed 2.0 mg/L,

determine the following:

a. Filter media volume

b. Filter hydraulic loading

c. Number of filters and their nominal dimensions

d. Methanol dose in mg/L and kg/d

Assume a typical NO

-N loading rate for the sustained minimum temperature and

assume that the filters will be sized using the guidance in Chapter 11.

26-7. Estimate MF filter area required to treat effluent from the Pasveer tertiary filter

(Problem 26-5). Assume a conservative hydraulic loading.

26-8. Estimate MF filter area required to treat effluent from the Brooklyn tertiary filter

(Problem

26-6). Assume a conservative hydraulic loading.

26-9. To prepare the wastewater effluent from Pasveer’s MF filters (Problem 26-7) for

groundwater discharge, it will passed through an activated carbon filter. Assuming a

conservative EBCT, estimate the volume of activated carbon required for a column.

26-10. To prepare the wastewater efflu ent from Brooklyn’s MF filter

s (Problem 26-8) for

groundwater discharge, it will passed through an activated carbon filter. Assum ing

a conservative EBCT, estimate the volume of activated carbon required for a

column.

26-8 REFERENCES

Clark, R. M. and B. W. Lykins (1989) Granular Activated Carbon: Design, Operation and Cost, Lewis

Publishers, Chelsea, Michigan.

Cleasby, J. L. and G. S. Logsdon (1999) “Granular Bed and Precoat Filtration,” in R. D. Letterman (ed.)

Water Quality and Treatment, 5th ed., American Water Works Association, McGraw-Hill, New

York, pp. 8.1–8.99.

Crittenden, J. C., P. S. Reddy, H. Arora, J. Trynoski, D. W. Hand, D. L. Perrman, and R. S. Summers

(1991) “Predicting GAC Perfor

mance with Rapid Small-Scale Column Tests,” Journal of American

Water Works Association, vol. 83, no. 1, pp. 77–87.

C ulp, R. L., G. M. Wesner, and G. L. Culp (1978) Handbook of Advanced Wastewater Treatment, 2nd

ed., Van Nostrand Reinhold, New York, p. 95.

GLUMRB (2004) Recommended Standards for Wastewater Facilities, Great Lakes–Upper Mississippi

River Board of State and Provincial Public Health and Environmental Managers, Health Education

Services, Albany, New York, pp. 110-5–110-8.

Gnirss, R. and J. Dittrich (2000) “Microfiltration of Municipal Wastewater for Disinfection and

Advanced Phosphorus Removal: Results from Trials with Different Small-Scale Pilot Plants,” Water

Environment Research, vol. 72, no. 5, pp. 602–609.

Howe, K. J. and Mark M. Clark (2006) “Effect of Coagulation Pretreatment on Membrane Performance,”

Journal of American Water Works Association, vol. 98, no. 4, pp. 133–146.

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

ssachusetts, pp. 1,035–1,162.

26-18 WATER AND WASTEWATER ENGINEERING

Pickard, D. W. et al. (1985) “Six Years of Successful Nitrogen Removal at Tampa, Florida,” presented at

the 58th Annual Conference of the Water Pollution Control Federation, Kansas City, Missouri.

Savage, E. S. (1983) “Biological Denitrification Deep Bed Filters,” presented at the Filtech Conference,

Filtration Society, London.

Tooker, N. B. and J. L. Darby (2007) “Cloth Media Filtration and Membrane Microfiltration: Serial

Operation,” Water Environment Research, vol. 79, no. 2, pp. 125–130.

U.S. EPA (1973) Process Design Manual for Carbon Adsorption, U.S. Environmental Protection Agency,

Technology Transfer, Washington, D.C.

U.S EPA (1975) Process Design Manual for Nitrogen Control, U.S. Environmental Protection Agency,

Technology Transfer, Washington, D.C.

U.S. EPA (1976) Process Design Manual for Phosphorus Removal, U.S. Environmental Protection

Agency, Technology Transfer, Washington, D.C.

U.S. EPA (1979) Environmental Pollution Control Alternatives: Municipal Wastewater, U.S.

Environmental Protection Agency, EPA Pub. No. 62

5/5-79-012, Washington, DC, pp. 52–55.

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

Manual of Practice 8, Alexandria, VA, pp. 15-40–15-47, 16-4–16-18.

WEF (2006) Membrane Systems for Wastewater Treatment, Water Environment Federation, Alexandria,

Virgina, pp. 113–140.

27-1

27-9 ANAEROBIC DIGESTION

27-10 SLUDGE CONDITIONING

27-11 DEWATERING

27-12 ALTERNATIVE DISPOSAL TECHNIQUES

27-13 LAND APPLICATION OF BIOSOLIDS

27-14 CHAPTER REVIEW

27-15 PROBLEMS

27-16 REFERENCES

WASTEWATER PLANT RESIDUALS MANAGEMENT

27-1 SLUDGE HANDLING ALTERNATIVES

27-2 SOURCES AND CHARACTERISTICS OF

SOLIDS AND BIOSOLIDS

27-3 SOLIDS COMPUTATIONS

27-4 GRIT HANDLING AND SLUDGE PUMPING

27-5 MANAGEMENT OF SOLIDS

27-6 STORAGE AND THICKENING OF SLUDGES

27-7 ALKALINE STABILIZATION

27-8 AEROBIC DIGESTION

CHAPTER

27-2 WATER AND WASTEWATER ENGINEERING

27-1 SLUDGE HANDLING ALTERNATIVES

In the process of purifying wastewater, another problem is created: sludge. The higher the degree

of wastewater treatment, the larger the residue of sludge that must be handled. Satisfactory

treatment and disposal of the sludge can be the single most complex and costly operation in

a municipal wastewater treatm

ent system (U.S. EPA, 1979). The sludge consists of materials

settled from the raw wastewater and of solids generated in the wastewater treatment processes.

The quantities of sludge involved are significant. For primary treatment, they may be 0.25 to

0.35 percent by volume of wastewater treated. Activated

sludge processes increase the quantities

to 1.5 to 2.0 percent of the volume of water treated. Us e of chemicals for phosphorus removal

can add another 1.0 percent. The sludges withdrawn from the treatment processes are still largely

water. Sludge treatment processes, then, in large part are concerned with separating the large

amounts

of water from the solid residues. The separated water is returned to the wastewater plant

for processing. In addition, the sludge is treated to reduce the pathogen density and to reduce its

putrescence.

The basic processes for sludge treatment are:

1 . Preliminary operations: S creening, grinding, d

egritting, blending, and storage may be

part of the preliminary operations to protect downstream equipment and/or provide a

homogeneous feed to subsequent process facilities.

2. Thickening: These processes are used to separate water from the solids to reduce the

size of subsequent facilities an

d to improve their efficiency. The water is separated by

gravity, flotation, gravity belt, rotary drum filtration, or centrifugation.

3. Stabilization: Sludge is stabilized to reduce pathogens, elim inate offensive odors, and

inhibit putrefaction. Biosolids are the produc t that results from stabilization. The three

stabilization processes di

scussed in this chapter are alkaline stabilization, aerobic

stabilization ( more commonly called aerobic digestion ), and anaerobi c stabilization

(more commonly called anaerobic digestion ). The latter two processes also provide

additional benefits such as

volume reduction and improved dewatering. In the case of

anaerobic stabilization, methane is produced. The methane can be used as part of the

energy source to run the wastewater treatment plant.

4. Conditioning: These processes treat the sludge with chemicals or heat so that more water

can be readily

separated.

5. Dewatering: These processes are used to further reduce the water to meet disposal regu-

lations, improve handling, reduce transportation costs, prevent leachate from disposal

sites and, in the case of follow-on reduction processes (i.e., incineration), reduce the

energy requirem

ents. The separation processes include centrifugation, filter presses, and

drying beds.

6. Reduction: To achieve the most stable form of residue and to minimize the volume of resi-

due, composting or thermal reduction processes such as drying or incineration are used

Although energy and resource recovery have historically been considered an integral part of

the residuals processing scheme, there will be increased emphasis on these aspects in the near

future. Energy recovery from sludge/biosolids is a well-established te

chnology that will find