Water and Wastewater Engineering

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13-18 WATER AND WASTEWATER ENGINEERING

Disinfection Practice

To be of practical use, disinfectants must possess the following properties:

1 . They must destroy the kinds and numbers of pathogens that may be introduced into

water within a practicable period of time over an expected range in water temperature.

2. They must meet possible fluctuations in com

position, concentration, and condition of the

water to be treated.

3. They must be neither toxic to humans and domestic animals nor unpalatable or otherwise

objectionable in the concentrations required for disinfection.

4. Their strength or concentration in the treated water must be determined easily, quickly,

and, preferably, automatically.

5. Their cost must be reasonable.

I deally, disinfectants should also possess the following characteristics:

1 . They should be safe and easy to store, transport, handle, and apply.

2. They should persi

st in a sufficient concentration to provide reasonable residual protection

against possible recontamination before use, or—because this is not a normally attain-

able property—the disappearance of residuals must be a warning that recontamination

may have taken place.

Effective germicidal dose, mW·s/cm

4

3

2

Log survival, log N/N

1

100 150 200 250 3000

6

5

FIGURE 13-7

Relationship between log survival of MS2 phage and effective germi-

cidal dose as determined in low pressure UV lamp collimated beam

system. (95 percent confidence interval shown.) ( Source: Linden and

Darby, 1997.)

DISINFECTION AND FLUORIDATION 13-19

Regulatory Context. Selection of an appropriate disinfec tion strategy for water treatment re-

quires a balance among three driving forces:

• Providing water free of pathogens. The regulatory focus for pathogen removal is on coli-

form bacteria, heterotrophic plate counts, Cryptosporidium oocysts, Giardia cysts, Legio-

nella, and viruses.

• Avoiding prod

uction of disinfection byproducts (DBPs). Trihalomethanes (THMs), haloace-

tic acids (HAAs), other halogenated organic compounds, ozone DBPs, oxidation byproducts,

and disinfectant residuals present a health risk. They must be limited in drinking water.

• Maintaining a disinfectant resi

dual in the distribution system. Residual disinfectant is pro-

vided to maintain the bacteriological quality, provide a rapid means for detection of system

contamination, and prevent regrowth of microorganisms.

The disinfectant strategy consists of two parts: primary di

sinfection and secondary disinfec-

tion. Primary disinfection refers to the first disinfectant used to achieve microbial inactivation.

Secondary disinfection refers to the second disinfectant used in a treatment system. Its objective

is to provide di

sinfection residual in the distribution system.

The selection of the primary disinfectant is bounded by four factors: (1) preceding treatment,

(2) total organic carbon (TOC) concentration, (3) bromide ion concentration, and (4) ability to

meet microbial inactivation requirements. If the upstream water treat

ment processes do not in-

clude filtration, the U.S. Environmental Protection Agency strongly discourages the use of ozone

or ozone/peroxide systems because of the potential production of ozone byproducts and biode-

gradable organic matter (BOM) that promotes regrowth in the distribution system (U.S. EPA,

1999). The presence of TOC concentrations above 2 m g/L favor

s selection of a primary disin-

fectant that will not produce DBPs. The reactions of ozone and ozone/peroxide with bromide ion

produces hypobromous acid and bromate ion. If the concentration of bromide ion exceeds 0.10

mg/L, ozone and ozone/peroxide may not be used (U.S. EPA, 1999).

U.S. EPA uses the Ct concept to determ

ine the ability of the primary disinfectant to inac-

tivate three target organisms: Cryptosporidium oocysts , Giardia cysts, and viruses. The target

inactivation is based on inactivation expressed in a logarithmic form (“log-inactivation”):

LI ln

kCt

⎛

⎝

⎜

⎞

⎠

⎟

(13-27)

where LI  log inactivation, dimensionless

N  number of surviving microorganisms per unit volume

 original number of organisms per unit volume

k  rate constant for inactivation, min



C  disinfectant concentration, mg/L

t  contact time, min

LI can be converted to percent removal:

% removal

100

100

⎛

⎝

⎜

⎞

⎠

⎟

(13-28)

13-20 WATER AND WASTEWATER ENGINEERING

EPA’s Long Term Enhanced Surface Water Treatment Rules (LT1ESWTR and LT2ESWTR)

target log-inactivation values are  3.0 for Giardia cysts and  4.0 for viruses. The log-inactiva-

tion required for Cryptosporidium oocysts is a function of the raw water Cryptosporidium oocyst

concentration as shown in Table 13-4 .

These credits assume a treated water turbidit

y 0.5 NTU for conventional and direct filtra-

tion and 1 NTU for slow-sand and diatomaceous earth filtration (Lin, 2001).

In some cases, where the degree of contamination is high, greater log removals may be

appropriate. EPA (1991) recommends the overall treatment requirement be adjusted based on the

degree of contamination as shown in Table 13-5

The selection of a secondary disinfectant depends on the selected primary disinfectant. Of

concern are the assimilable organic carbon (AOC) concentration, DBP formation potential

(DBPFP), and distribution system retention time. AOC is produced when a strong oxidant such as

ozone i

s used as a primary disinfectant with a high TOC concentration in the water. Without fur-

ther treatment, the finished water has a high potential to stimulate regrowth of microorganisms in

the distribution system. High AOC is defined as a concentration exceeding 0.10 mg/L after filtra-

tion. DBPFP is an indication that organic byproducts can be expec

ted to form in the distribution

system if chlorine is used. A high DBPFP is defined as a water meeting one of the following:

• THM seven-day formation exceeds the MCL of 0.08 mg/L.

• HAA5 seven-day formation exceeds the MCL of 0.06 mg/L.

TABLE 13-4

Additional Cryptosporidium log-inactivation requirements for filtered water

Raw water Cryptosporidium

oocysts concentration,

oocysts/L

Additional conventional filtration

treatment requirements including

softening

Additional direct filtration

requirements

 0.075 No additional treatment No additional treatment

0.075 and 1.0 1 log treatment 1.5 log treatment

1.0 and 3.0 2 log treatment 2.5 log treatment

3.0 2.5 log treatment 3 log treatment

A dditional treatment requirements reflect a Cryptosporidium removal credit of 3 log for conventional, slow sand, or

diatomaceous earth filtration plants, and a 2.5 log credit for direct filtration plants.

Source: Code of Federal Regulations, 40 CFR 141.711, 2006.

TABLE 13-5

Recommended overall disinfection

as a function of raw water quality

Raw water concentration, microorganisms/100 L

Microorganism

removal/inactivation 1 1 and 10 10

Giardia cyst

3 log 4 log 5 log

Virus 4 log 5 log 6 log

Overall disinfection includes removal credits for treatment as well as inactivation by disinfectants.

DISINFECTION AND FLUORIDATION 13-21

Water that spends a long time in the distribution system allows for the reactions that form

THMs to proceed toward completion. A distribution system retention time is considered high if it

exceeds 48 hours (U.S. EPA, 1999).

Selection of Disinfectant. Although there are other disinfectants and combinations of disinfec-

tants, this discussion is limited to chlorine gas, liquid sodium hypochlorite, chloramines, chlorine

dioxide, ozone, and UV radiation.

A summary of disinfectant properties and consid erations in their selection are summarized

in Tables 13-6 and 13-7 .

TABLE 13-6

Summary of disinfectant properties (based on typical disinfectant application)

Condition Chlorine Ozone Chlorine dioxide Permanganate Chloramine Ozone/peroxide Ultraviolet

Produce THM with TOC y s nny s n

Produce oxidized organicssy ssn y s

Produce halogenated organics y s nny s n

Produce inorganic byproducts n s y nn s n

Produce BOM s y s nn y n

MRDL applies y n y n y nn

Lime softening impa

cts y nn n y n y

Turbidity impacts n s nnns y

Meet giardia - 2.0 log

yy y nn n y

Meet giardia - 2.0 log

n yy nn n y

Meet crypto - 2.0 log

n yy nn n y

Meet crypto - 2.0 log

n y

nn n y

Meet virus - 2.0 log yy y nn n y

Meet virus - 2.0 log yy y nn n y

Secondary disinfectant y n s n y nn

Operator skill (1  low; 5  high) 1 55 12 53

Applicable to large utilities yy y y y y n

Applicable to small utilities yy y y y y y

y  yes, n  no, s  sometimes

Ct values to achieve 2.0 log inactivation are very high at common water temperatures.

TABLE 13-7

Consideration for selecting disinfectant

Consideration Cl

NaOCl O

ClO

Chloramine UV

Residual persistence Low Low None Moderate Very low None

pH dependenceYes Yes SomeSomeYes None

Safety concerns Very high Moderate High Very high Moderate Moderate

Complex equipment Yes No Very Yes Yes Yes

(continued)

13-22 WATER AND WASTEWATER ENGINEERING

Figures 13-8 and 13-9 provide a means for narrowing the choices of primary and secondary

disinfectant, respectively. The selection flow diagrams do not address the role of microbial growth

in settling tanks and filters or the presence of manganese, iron, and sulfides in the raw water.

Where biological growth is a proble

m in settling tanks and filters, some alternative means of

reducing DBPs include reducing NOM, the use of chlorine dioxide as a pretreatment followed by

chlorine and chloramine, and ozonation followed by DBP removal with anthracite biofilters or

granular activated carbon (GAC).

Consideration Cl

NaOCl O

ClO

Chloramine UV

Equipment reliability Good Very good Good Good Good Moderate

Process control Well developed Well developed Developing Developing Well developed Developing

O&M requirements Low Low High High Low Low

TABLE 13-7 (continued)

Consideration for selecting disinfectant

Sources: Haas, 1999; Hesby, 2005; MWH, 2005.

High

TOC?

Start

Yes

Chlorine dioxide

Chlorine

Need bench or pilot study

Chlorine dioxide

Chlorine

Ozone

Interactive disinfectants

Chlorine dioxide

High DBPFP

Ozone / BAC

Chlorine dioxide

Yes

High

bromide

High

bromide

FIGURE 13-8

Flow diagram to narrow selection of a new primary

disinfectant for systems that filter.

( Source: Adapted from U.S. EPA, 1999.)

DISINFECTION AND FLUORIDATION 13-23

Iron, manganese, and sulfides ex ert an oxidant demand and, in the case of iron and man-

ganese, will form precipitates as a result of oxidation. The precipitates, in addition to being an

aesthetic problem, will interfere with the disinfection process.

The use of the flow charts and boundary conditions in sele

cting primary and secondary dis-

infectants is illustrated in Example 13-5 .

Example 13-5. Select the primary and secondary disinfectants for the town of Stillwater which uses

the Noir River for its water supply. The design flow rate is 18,500 m

/ d. The water is treated by con-

ventional coagulation, sedimentation, and filtration. The time for water to reach the most distant cus-

tomer at the minimum demand flow rate is 31 hours. The Noir River water analysis is shown below.

Noir River water analysis

Constituent Concentration

TOC 5 mg/L

Bromide Not detected

Turbidity 10–500 NTU

Giardia cysts

 1/100 L

Virus  1/100 L

Cryptosporidium oocysts

1.1–2.0/L

High

AOC?

High

DBPFP?

Extended

distribution

time?

Start

Yes

Chlorine

Chloramine

∗

Chlorine

Chlorine dioxide

Chloramine

∗

Treatment to

reduce DBPFP

Biological

treatment

BAF or GAC

*Chloramine is a potential problem because NH

promotes regrowth and because of chloramine

reactions that release Pb and Cu from pipes.

FIGURE 13-9

Flow diagram to narrow selection of a

new secondary disinfectant.

( Source: Adapted from U.S. EPA,

1999.)

13-24 WATER AND WASTEWATER ENGINEERING

Solution:

a . The concentration of TOC is greater than 2 mg/L so it is considered high. Bromide was

not detected. From Figure 13-8 , the primary disinfectant alternatives are ozone, chlorine

dioxide, and UV.

b. A comparison of ozone, chlorine dioxide, and UV in Table 13-6 reveals the following

highlights:

• Neither ozone nor ClO

are clearly superior with respect to production of byproducts

(THM, oxidized organic matter, halogenated organic matter, inorganic byproducts ,

and BOM). UV is clearly superior.

• Ozone and UV do not have MRDLs (Maximum Residual Disinfectant Level) while

ClO

does.

• All three can achieve  2.0 log Giardia inactivation and  2.0 virus inactivation.

From Table 13-5 , the required log removal for Giardia cysts and virus when the

concentration is  1/100 L is 3 and 4 logs, respectively. The conventional filtration

credit for Giardia cyst removal is

2.5 and 2 for viruses. Therefore, all can meet the

additional disinfection inactivation required.

• The concentration of ClO

or UV dose to meet a 2.0 log inactivation for Cryptospo-

ridium oocysts that is required for the concentration found in the Noir River is very

high (See Table 13-4 and Ct tables* ). At the required dose, the potential for exceed-

ing both the ClO

MRDL of 0.8 mg/L and chlorite DBP limit of 1.0 mg/L is high.

c. From Table 13-7 , with the exception of safety concerns, ozone and ClO

are very com-

parable. UV has moderate safety concerns and moderate equipment reliability concerns.

d. Based on this analysis, ozone is selected as the primary disinfectant with the understand-

ing that there will be no residual and that AOC will be a problem.

e. From Fig

ure 13-9 , the flow path leads to a requirem ent for biological treatment and a

high DBPFP.

f. The distribution system time is 31 hours. This is less than the 48-hour criterion. This

leads to ClO

or chloramines as the secondary disinfectant choices. Because chloramine

is rated as (yes) and ClO

i s rated s (sometimes) as a secondary disinfectant in Table 13-6 ,

chloramine is selected.

Comments:

1 . Over time, with improvements in reliability and O&M, the UV alternative will become

more attractive than the ozone option selected here.

2. Other strategies may be more appropriate. For example:

• Removal of NOM m

ay permit consideration of alternative disinfectants.

• An improved watershed management program to lower the Cryptosporidium dis-

charges from agricultural runoff may lower the overall log removal/inactivation re-

quirement and thus open consideration of other disinfectants.

*Ct tables may be found at www.mhprofessional.com/wwe or in the Code of Federal Regulations.

DISINFECTION AND FLUORIDATION 13-25

Weight Percent Chlorine. Chlorine in one of its forms is the most common disinfectant used

in the United States. It is available commercially in pressurized vessels that contain both liqui-

fied and gaseous fractions. Sodium hypochlorite (NaOCl), also known as bleach, is a liquid form.

Calcium hypoc

hlorite (Ca(OCl)

· 4H

O), also known as HTH

, is sold as a granule, powder,

and tablet.

The relative amount of chlorine present in these compounds may be expressed as weight

percent chlorine. The weight percent chlorine is a measure of the amount of chlorine being pur-

chased or being supplied. It is used to calculate the feed rate to produce the d

esired dose of chlo-

rine for compounds other than chlorine gas as well as the storage volume required and equivalent

operating cost for chemicals. It is defined as

Weight chlorine

GMW of chlorine in compou

% 

nnd

GMW of compound

()100%

(13-29)

Example 13-6. Estimate the weight percent chlorine in HTH

Solution. Using the molecular weights of Cl

and HTH

, the weight percent chlorine is

Weight chlorine%%

2 35 45

214 90

100 329

()

().

99 33or %

Comment. If HTH

i s selected to supply the required chlorine dose, the mass of HTH

to be

provided must be (1/0.3299) or about three times the mass of chlorine required assuming the

HTH

is 100% pure.

Safety Precautions and Chemical Handling. Gaseous chlorine is most often employed by

larger utilities. It is normally stored in its shipping container. The recommended standards for

chlorine are provided here in detail because of the extreme hazard of the gas and the wide use of

chlorine gas for disinfec

tion (GLUMRB, 2003):

• Chlorine gas feed and storage shall be enclosed and separated from other operating areas.

The chlorine room shall be:

• provided with a shatter resistant inspection window installed in an interior wall,

• constructed in such a manner that all openings between the chlorine room and the

remainder of the plant are sealed, and

• provided with doors eq

uipped with panic hardware, assuring ready means of exit and

opening outward only to the building exterior.

• The room shall be constructed to provide the following:

• e a ch room shall have a ventilating fan with a capacity that provides one complete air

change per minute,

13-26 WATER AND WASTEWATER ENGINEERING

• the ventilating fan shall take suction near the floor as far as practical from the door and

air inlet,

• air inlets should be through louvers near the ceiling,

• separate switches for the fan and lights shall be located outside the chlorine room and at

the inspection window,

• vents from the feeders and storage shall discharge to the outs

ide atmosphere through

chlorine gas collection and neutralization systems,

• floor drains are discouraged. Where provided, the floor drains shall discharge to the out-

sid e of the building and s hall not be connected to other internal or external drainage

systems.

• Chlorinator rooms should be heated to 15 

C and be protected from excessive heat.

• Pressurized chlorine feed lines shall not carry chlorine gas beyond the chlorinator room.

• A continuous chlorine sensor and alarm is recommended.

A scrubber system that is activated in the event of a chlorine leak is recommend

ed. Section

80.303 of Article 80 of the Uniform Fire Code provides design guidance. The scrubber system

uses sodium hydroxide to neutralize the chlorine gas (AWWA, 2006).

Because of safety and security concerns, many utilities have s

witched to hypochlorite. The

decision to switch is a complex management decision because the cost of NaOCl is significantly

higher than the cost of gaseous chlorine.

Sodiu m hypochlorite (als o called liquid bleach) may be stored in the original shipping

containers

or in compatible containers. Fiber-glass reinforced plastic (FRP) tanks, spe-

cific polyethylene fabricated for NaOCL storage, and carbon steel tanks lined with ru bber

or polyvinyl chloride (PVC) are recommended (Hesby, 2005; MWH, 2005). Most small to

medium-sized plants feed hypochlorite with po

sitive displacement diaphragm metering

pumps or peristaltic metering pumps. The peristaltic pumps are preferred because they oper-

ate without vapor lock. Schedule 80 PVC piping provides good service if it is not exposed

to sunlight. When sunlight exposure is unavoidable, chlorinated pol

yvinyl (CPVC) is recom-

mended (MWH, 2005). Ball valves should not be used because they lock down, trap NaOCL

off-gases, and explode.

A m ajor design issue with the storage of NaOCl is its stability. Commercial bleach is gen-

erally shipped as 12 to 15 percent available Cl

at a pH  12. If the pH is held above 11, the

rate of decay is very low (Gordon et al., 1997). Concentration and temperature are important

considerations in storage. For exam ple, if a 15 weight percent solution is diluted to 7.5 percent,

its half-life will increase from 50 to about 140 days if it is stored at 25  C. If it is stored at 7.5

weight percent and 15  C, its half-life will be on the order of 500 days (Gordon et al., 1997;

MWH, 2005). At the other end of the spectrum, the crystallization temperatures are: 22C

for a 15 trade percent (weight per unit volume so that 1 percent corresponds to a weight of

10 g of available chlorine per liter) s olution; 17C for a 12.5 percent solution; 12C for a

10 percent solution. The im

plications for design of a NaOCl storage facility are that it should

be protected from sunlight, kept at an ambient temperature less than 20  C but greater than

12C, and the NaOCl should be diluted on receipt from 15 to 7.5 weight percent. Because

DISINFECTION AND FLUORIDATION 13-27

it is delivered at a pH of about 12, dilution induced low-pH decay is normally not a problem

(MWH, 2005). However, the dilution water should be soft because the high pH of NaOCl will

precipitate hardness from a hard water.

A proprietary system for on-site generation of hypochlorite was introduced in the United

States in the 1980s. It uses electrolyti

c decomposition of NaCl to produce a NaOCl feed solution.

Because the hypochlorite solution is generated on demand, it has the safety advantages of hypo-

chlorite without the problem of degrad ation while the NaOCl solution is in storage. However,

power consumption is significant.

Ozone at concentrations greater than 23 percent is explo

sive. At ambient temperature and

pressure, it rapidly decays. Unlike chlorine, it cannot be stored under pressure but must be gen-

erated on-site. The corona discharge method of generation is commonly used for drinking wa-

ter disinfection. In this method, oxygen is passed through an elec

tric field that is generated by

applying a high voltage across electrodes separated by a dielectric material. As oxygen passes

through the electric field, it is broken down to oxygen singlets (O •). These react with oxygen

to form O

. The source of oxygen is either ambient air or commercially supplied liquid oxygen

(LOX). Air-fed ozone systems were used widely prior to the mid -1990s. The LOX-fed systems

have become the system of choice since about 1995 (Rakness, 2005). Regardless of the sou rce,

the feed gas must be prepared. The system design goals for the feed gas are: temperature 30  C ;

100 percent removal of particulate matter  0.3 micrometers (  m) in diameter, 95 percent re-

moval of particulate matter  0.1  m; hydrocarbons 4 to 5 ppm, dew point in the range

65C to 100

C (Dimitriou, 1990). Typical components include: air compressor, after-cooler,

refrigerative drying, vapor/liquid separator, prefilters, desiccant dryer, particulate after-filter, and

controls. Even with gas transfer efficiencies of 90 to 99 percent, the off-gas from the ozone reac-

tor may have ozone concentrations on the order of 500–1600 ppm (v/v). This exceeds the o

ccupa-

tional exposure limit of 0.10 ppm (v/v). The ozone in the off-gas can be destroyed thermally with

or without a catalyst. Without a catalyst the required temperature is 300C–350  C at a 5 second

residence time. The use of a catalyst lowers the required temperature to between 30C and

70  C

(AWWARF, 1991).

Chlorine dioxide cannot be stored because it is not safe. The pure gas may explode as a result

of high temperatures, exposure to light, changes in pressure, or exposure to organic contaminants

(Hesby , 2005). Therefore, it is generated on-site. Most generation techniques use a

chlorine/

sodium chlorite mixture. GLUMRB (2003) recommends that sodium chlorite be stored by itself

in a separate room and preferably in a separate building detached from the water treatment facil-

ity. The storage structure must be of noncombustible materials. Positive displa

cement feeders are

used.

Chloramine is formed on-site by reacting ammonia with chlorine as shown in Equations 13-5

through 13-7 . Monochloramine is the desired compound for chloramine disinfection. Dichlo-

ramine is a disinfectant, but it also produces

undesirable tastes and odors. In addition to being

poorly soluble, nitrogen chloride is a foul smelling gas. Monochloramine is formed until the mass

ratio of Cl

/NH

e xceeds 4 (Hesby, 2005). The recommended mass ratio is in the range between

4.5:1 and 5:1 because this minimizes the concentration of unreacted ammonia (AWWA, 2006).

The rate of the reaction is s trongly influenc ed by pH with the highest rate at pH 6 or lower.

However, at this pH the amount of dichloramine formed is significant. At pH 8 and above the

amount of dichloramine is not significant. A pH between 7 and 8 appears to be the best com-

promise.