Lin S.D. Water and Wastewater Calculations Manual

Подождите немного. Документ загружается.

line can be drawn. However, in practice, most cases do not yield a

straight line relationship. Hom (1972) reﬁned the Chick–Watson model

and developed a ﬂexible, but highly empirical kinetic formation as fol-

lows:

⫺dN/dt ⫽ k′Nt

(5.182)

where m ⫽ empirical constant

⫽ 2 for E. coli

Other variables are as previous stated

For changing concentrations of the disinfectant, the disinfection efﬁ-

ciency is of the following form (Fair et al., 1968):

⫽ constant (5.183)

where t

is the time required to produce a constant percent of kill or die-

off. This approach has evolved into a “CT” (concentration ⫻ contact time)

regulation to ensure a certain percentage kills of Crypotosporidium,

Giardia, and viruses. The percentage kills are expressed in terms of log

removals.

18.4

CT values

On June 29, 1989, the US Environmental Protection Agency (1989b)

published the ﬁnal Surface Water Treatment Regulations (SWTR),

which speciﬁes that water treatment plants using surface water as a

source or groundwater under the inﬂuence of surface water, and with-

out ﬁltration, must calculate CT values daily. The CT value refers to the

value of disinfectant content or disinfectant residual, C (mg/L), multi-

plied by the contact time, T (minutes, or min). Disinfectants include chlo-

rine, chloramines, ozone, and chlorine dioxide.

Regulations. The US EPA reafﬁrms its commitment to the current Safe

Drinking Water Act, which includes regulations related to disinfection

and pathogenic organism control for water supplies. Public water supply

utilities must continue to comply with current rules while new rules for

microorganisms and disinfectants/disinfection by-products are being

developed.

The Surface Water Treatment Rule requires treatment for Giardia

and viruses of all surface water and groundwater under the direct inﬂu-

ence of surface water. Public water systems are required to comply with

a new operating parameter, the so-called CT value, an indicator of the

effectiveness of the disinfection process. This parameter depends on pH

and temperature to remove or inactivate Giardia lamblia (a protozoan)

and viruses that could pass through water treatment unit processes.

Public Water Supply 475

Requirements (US EPA, 1999) for treatment (using the best available

technology) such as disinfection and ﬁltration are established in place

of a maximum contaminant level (MCL) for Giardia, Legionella, het-

erotrophic plate count (HPC), and turbidity. Total treatment of the

system must achieve at least 99.9% (3-log) removal or inactivation of

G. lamblia cysts and 99.99% (4-log) removal of inactivation of viruses.

To avoid nitration, a system must demonstrate that its disinfection con-

ditions can achieve the above efﬁciencies every day of operation except

during any one day each month. Filtered water turbidity must at no time

exceed 5 NTU (nephelo-metric turbidity units), and 95 percent of the tur-

bidity measurements must meet 0.5 NTU for conventional and direct ﬁl-

tration and 1.0 NTU for slow sand ﬁltration and diatomaceous earth

ﬁltration. Turbidity must be measured every 4 h by grab sampling or

continuous monitoring. Cryptosporidium was not regulated by the

SWTR in 1989 because sufﬁcient data about the organism were lacking

at the time.

The residual disinfectant in ﬁnished water entering the distribution

system should not be less than 0.2 mg/L for at least 4 h. The residual

disinfectant at any distribution point should be detectable in 95% of the

samples collected in a month for any two consecutive months. A water

plant may use HPC in lieu of residual disinfectant. If HPC is less than

500 per 100 mL, the site is considered to have a detectable residual for

compliance purposes (US EPA, 1989a).

There are some criteria for avoiding ﬁltration. The water plant must

calculate the CT value daily. The water plant using ﬁltration may or may

not need to calculate CT values, depending on state primary require-

ments. The primary agency credits the slow sand ﬁlter, which produces

water with turbidity of 0.6 to 0.8 NTU, with 2-log Giardia and virus

removal. Disinfection must achieve an additional 1-log Giardia and

2-log virus removal/inactivation to meet the overall treatment objective.

The Interim Enhanced Surface Water Treatment Rule (US EPA,

1999a) deﬁnes a disinfection proﬁle as a compilation of daily Giardia

and/or virus log inactivations over a period of a year or more (may be

3 years). Inactivation of pathogens is typically reported in orders of

magnitude inactivation of organisms on a logarithmic scale.

The disinfection proﬁle is a graphical representation of the magnitude

of daily Giardia or virus log inactivation that is developed, in part,

based on daily measurements of operational data (disinfectant residual

concentration, contact time, pH, and temperature). A plot of daily log

inactivation values versus time provides a visual representation of the

log inactivation the treatment system achieved over time. Changes in

log inactivation due to disinfectant residual concentrations, tempera-

ture, ﬂow rate, or other changes can be seen from this plot (US EPA,

1999b).

476 Chapter 5

Determination of CT values. Calculation of CT values is a complicated

procedure depending on the conﬁguration of the whole treatment

system, type and number of point applications, and residual concen-

tration of disinfectant (Pontius, 1993).

A water utility may determine the inactivation efﬁciency based on one

point of disinfectant residual measurement prior to the ﬁrst customer,

or on a proﬁle of the residual concentration between the point of disin-

fectant application and the ﬁrst customer. The ﬁrst customer is the

point at which ﬁnished water is ﬁrst consumed. To determine compli-

ance with the inactivation (Giardia and viruses) requirements, a water

system must calculate the CT value(s) for its disinfection conditions

during peak hourly ﬂow once each day that it is delivering water to its

ﬁrst customers. To calculate CT value, T is the time (in minutes) that

the water moves, during the peak hourly ﬂow, between the point of dis-

infectant application and the point at which C, residual disinfectant con-

centration, is measured prior to the ﬁrst customer. The residual

disinfectant concentration, C, pH, and temperature should be meas-

ured each day during peak hourly ﬂow at the efﬂuents of treatment

units (each segment or section) and at the ﬁrst customer’s tap. The seg-

ments may include the inﬂow piping system, rapid mixer, ﬂocculators,

clariﬁers, ﬁlters, clearwells, and distribution systems.

The contact time, T, is between the application point and the point of

residual measured or effective detention time (correction factor from

hydraulic retention time, HRT). The contact time in pipelines can be

assumed equivalent to the hydraulic retention time and is calculated by

dividing the internal volume of the pipeline by the peak hourly ﬂow rate

through the pipeline. Due to short circuiting, the contact time in mixing

tanks, ﬂocculators, settling basins, clearwells, and other tankages should

be determined from tracer studies or an equivalent demonstration. The

time determined from tracer studies to be used for calculating CT is T

In the calculation of CT values, the contact time, designated by T

, is

the time needed for 10% of the water to pass through the basin or reser-

voir. In other words, T

is the time (in minutes) that 90% of the water

(and microorganisms in the water) will be exposed to disinfectant in the

disinfection contact chamber.

The contact time T

is used to calculate the CT value for each sec-

tion. The CT

cal

is calculated for each point of residual measurement.

Then the inactivation ratio, CT

cal

/CT

99.9

or CT

cal

/CT

should be calcu-

lated for each section. CT

99.9

and CT

are the CT values to achieve

99.9% and 99% inactivation, respectively. Total inactivation ratio is

the sum of the inactivation ratios for each section (sum CT

cal

/CT

99.9

). If

the total inactivation ratio is equal to or greater than 1.0, the system

provides more than 99.9% inactivation of Giardia cysts and meets the

disinfection performance requirement. In fact, many water plants meet

Public Water Supply 477

the CT requirement for 3-log Giardia inactivation because the plant con-

ﬁgurations allow for adequate contact time.

Calculation of CT values is a complicated procedure and depends on

the conﬁguration of the entire treatment system, type and number of

point applications, and residual concentration of disinfectant (US EPA,

1999). The water temperature, pH, type and concentration of disinfec-

tant, and effective retention time (ERT) determine the CT values. Flow

patterns and tank conﬁgurations of ﬂocculators, clariﬁers, and clearwells

(location of inﬂuent and efﬂuent pipe, and the bafﬂing condition) affect

the ERT. The ERT is different from the HRT in most instances.

The theoretical contact time, T

theo

, can be determined as (assuming

completely mixed tank):

theo

⫽ V/Q (5.184)

where T

theo

⫽ time (⫽ HRT), minutes (min)

V ⫽ volume of tank or pipe, gallons or m

Q ⫽ ﬂow rate, gallons per minute, gpm or m

The ﬂow rate can be determined by a theoretical calculation or by using

tracer studies. An empirical curve can be developed for T

against Q.

For the distribution pipelines, all water passing through the pipe is

assumed to have detention time equal to the theoretical (Eq. (5.184)) or

mean residence time at a particular ﬂow rate, i.e., the contact time is

100% of the time the water remains in the pipe.

In mixing basins, storage reservoirs, and other treatment plant

process units, the water utility is required to determine the contact

time for the calculation of the CT value through tracer studies or other

methods approved by the primacy agency, such as the state EPA.

The calculation of contact time T for treatment units is not a

straightforward process of dividing the volume of tank or basin by the

ﬂow through rate (Eq. 5.184). Only a partial credit is given to take into

account short circuiting that occurs in tanks without baffles. The

instructions on calculating the contact time (out of the scope of this

manual) are given in Appendix C of the Guidance Manual (US EPA,

1989a).

Bafﬂing is used to maximize utilization of basin volume, increase the

plug ﬂow zone in the basin, and to minimize short circuiting. Some form

of bafﬂing is installed at the inlet and outlet of the basin to evenly dis-

tribute ﬂow across the basin. Additional bafﬂing may be installed within

the interior of the basin. Bafﬂing conditions generally are classiﬁed as

poor, average, or superior and are based on results of tracer studies for

use in determining T

from the theoretical detention time of a tank. The

/T fractions associated with various degrees of bafﬂing conditions are

478 Chapter 5

shown in Table 5.7. In practice, theoretical T

/T values of 1.0 for plug

ﬂows and 0.1 for mixed ﬂow are seldom achieved due to the effect of dead

space. Superior bafﬂing conditions consist of at least a bafﬂed inlet and

outlet, and possibly some intrabasin bafﬂing to redistribute the ﬂow

throughout the basin’s cross section. Average bafﬂing conditions include

intrabasin bafﬂing and either a bafﬂed inlet or outlet. Poor bafﬂing con-

ditions refer to basins without intrabasin bafﬂing and unbafﬂed inlets

and outlets.

The procedure to determine the inactivation capability of a water plant

is summarized as follows (US EPA, 1989a, 1990, 1999; IEPA, 1992):

1. Determine the peak hourly ﬂow rate (Q in gpm) for each day from

monitoring records.

2. Determine hydraulic detention time: HRT ⫽ T ⫽ V/Q.

3. Calculate the contact time (T

) for each disinfection segment based

on bafﬂing factors or tracer studies.

4. Find correction factor, T

/T, look at the actual bafﬂing conditions

to obtain T

from Table 5.6.

5. Compute effective retention time (ERT) ⫽ HRT ⫻ (T

/T).

6. Measure the disinfectant residual: C in mg/L (with pH, and tem-

perature in °C) at any number of points within the treatment train

during peak hourly ﬂow (in gpm).

7. Calculate CT value (CT

cal

) for each point of residual measurement

(using ERT for T) based on actual system data.

8. Find CT

99.9

or CT

99.99

value from Tables 5.7 or 5.9 (from Appendix E

of the Manual, US EPA, 1989a) based on water temperature, pH,

residual disinfectant concentration, and log

removal ⫽ 3 or 4.

Public Water Supply 479

TABLE 5.7 Classiﬁcation of Bafﬂing and T

T Values

Condition of bafﬂing Description of bafﬂing T

Unbafﬂed (mixed ﬂow) No bafﬂe, agitated basin, very low length-to-width 0.1

ratio, high inlet and outlet ﬂow velocities

Poor Single or multiple unbafﬂed inlets and outlet, no 0.3

intrabasin bafﬂes

Average Only bafﬂed inlet or outlet, with some intrabasin 0.5

bafﬂes

Superior Perforated inlet bafﬂe, serpentine or perforated 0.7

intrabasin bafﬂes, outlet weir, or perforated launders

Perfect (plug ﬂow) Very high length-to-width ratio (pipeline ﬂow), 1.0

perforated inlet, outlet, and intrabasin bafﬂes

SOURCE: Table C-5 of the US EPA Guidance Manual (US EPA, 1989a)

9. Compute the inactivation ratio, CT

cal

/CT

99.9

and CT

cal

/CT

99.99

for

Giardia and viruses, respectively.

10. Calculate the estimated log inactivation by multiplying the ratio

of step 9 by 3 for Giardia and by 4 for viruses, because CT

99.9

and CT

99.99

are equivalent to 3-log and 4-log inactivations,

respectively.

11. Sum the segment log inactivation of Giardia and viruses (such as

rapid mixing tanks, ﬂocculators, clariﬁers, ﬁlters, clearwell, and

pipelines) to determine the plant total log inactivations due to

disinfection.

12. Determine whether the inactivations achieved are adequate. If the

sum of the inactivation ratios is greater than or equal to one, the

required 3-log inactivation of Giardia cysts and 4-log viruses inac-

tivation have been achieved.

13. The total percent of inactivation can be determined as:

y ⫽ 100 ⫺ 100/10

(5.185)

where y ⫽ % inactivation

x ⫽ log inactivation

Tables 5.8 and 5.9 present the CT values for achieving 99.9% and

99% inactivation of G. lamblia. Table 5.10 presents CT values for

achieving 2-, 3-, and 4-log inactivation of viruses at pH 6 through 9

(US EPA, 1989b). The SWTR Guidance Manual did not include CT

values at pH above 9 due to the limited research results available at

the time of rule promulgation. In November 1997, a new set of pro-

posed rules was developed for the higher pH values, up to pH of 11.5

(Federal Register, 1997).

Example 1: What are the percentages of inactivation for 2- and 3.4-log

removal of Giardia lamblia?

solution: Using Eq. (5.185)

y ⫽ 100 ⫺ 100/10

as x ⫽ 2

y ⫽ 100 ⫺ 100/10

⫽ 100 ⫺ 1 ⫽ 99(%)

as x ⫽ 3.4

y ⫽ 100 ⫺ 100/10

3.4

⫽ 100 ⫺ 0.04 ⫽ 99.96(%)

Example 2: A water system of 100,000 gpd (0.0044 m

/s) using a slow sand

filtration system serves a small town of 1000 persons. The ﬁlter efﬂuent turbidity

480 Chapter 5

values are 0.4 to 0.6 NTU and pH is about 7.5. Chlorine is dosed after ﬁltra-

tion and prior to the clearwell. The 4-in (10 cm) transmission pipeline to the

ﬁrst customer is 1640 ft (500 m) in distance. The residual chlorine concen-

trations in the clearwell and the distribution main are 1.6 and 1.0 mg/L,

respectively. The volume of the clearwell is 70,000 gal (265 m

). Determine

Giardia inactivation at a water temperature of 10°C at the peak hour ﬂow of

100 gpm.

solution:

Step 1.

An overall inactivation of 3 logs for Giardia and 4 logs for viruses is

required. The Primacy Agency can credit the slow sand ﬁlter process, which

produces water with turbidity ranging from 0.6 to 0.8 NTU, with a 2-log

Giardia and virus inactivation. For this example, the water system meets

the turbidity standards. Thus, disinfection must achieve an additional

Public Water Supply 481

TABLE 5.8

CT Values [(mg/l)min] for Achieving 99.9% (3 log) Inactivation

of Giardia lamblia

Disinfectant,

Temperature, °C

mg/L pH 0.5 or <1 5 10 15 20 25

Free

chlorine

≤0.4 6 137 97 73 49 36 24

7 195 139 104 70 52 35

8 277 198 149 99 74 50

9 390 279 209 140 105 70

1.0 6 148 105 79 53 39 26

7 210 149 112 75 56 37

8 306 216 162 108 81 56

9 437 312 236 156 117 78

1.6 6 157 109 83 56 42 28

7 226 155 119 79 59 40

8 321 227 170 116 87 58

9 466 329 236 169 126 82

2.0 6 165 116 87 58 44 29

7 236 165 126 83 62 41

8 346 263 182 122 91 61

9 500 353 265 177 132 88

3.0 6 181 126 95 63 47 32

7 261 182 137 91 68 46

8 382 268 201 136 101 67

9 552 389 292 195 146 97

ClO

6–9 63 26 23 19 15 11

Ozone 6–9 2.9 1.9 1.43 0.95 0.72 0.48

Chloramine 6–9 3800 2200 1850 1500 1100 750

SOURCE: Abstracted from Tables E-1 to E-6, E-8, E-10, and E-12 of the US EPA Guidance

Manual (US EPA, 1989a)

482 Chapter 5

TABLE 5.9 CT Values [(mg/L) min] for Achieving 90% (1 log) Inactivation

of Giardia lamblia

Disinfectant,

Temperature, °C

mg/L pH 0.5 or <1 5 10 15 20 25

Free chlorine

≤ 0.4 6 46 32 24 16 12 8

765 4635231712

892 6650332517

9 130 93 70 47 35 23

1.0 6 49 35 26 18 13 9

770 5037251912

8 101 72 54 36 27 18

9 146 104 78 52 39 26

1.6 6 52 37 28 19 14 9

775 5240262013

8 110 77 58 39 29 19

9 159 112 84 56 42 28

2.0 6 55 39 29 19 15 10

779 5541282114

8 115 81 61 41 30 20

9 167 118 88 59 46 29

3.0 6 60 42 32 21 16 11

787 6146302315

8 127 89 67 45 36 22

9 184 130 97 65 49 32

Chlorine 6–9 21 8.7 7.7 6.3 5.0 3.7

dioxide

Ozone 6–9 0.97 0.63 0.48 0.32 0.24 0.16

Chloramine 6–9 1270 735 615 500 370 250

SOURCE: Abstracted from Tables E-l to E-6, E-8, E-10, and E-12 of the US EPA Guidance

Manual (US EPA, 1989a)

1-log Giardia and 2-log virus removal/inactivation to meet the overall

treatment efﬁciency.

Step 2. Calculate T

at the clearwell (one-half of volume used, see Step 7(a)

of Example 5)

can be determined by the trace study at the peak hour ﬂow or by

calculation

⫽ V

/Q ⫽ 0.1 ⫻ (70,000gal/2)/(100gal/min)

⫽ 35 min

Step 3. Calculate CT

cal

in the clearwell

cal

⫽ 1.6 mg/L ⫻ 35 min ⫽ 56 (mg/L) min

Step 4. Calculate CT

cal

/CT

99.9

From Table 5.8, for 3-log removal for 1.6 mg/L chlorine residual at 10°C and

pH 7.5

99.9

⫽ 145 (mg/L) min (by linear proportion between pH 7 and 8)

then

cal

/CT

99.9

⫽ 56/145 ⫽ 0.38

Step 5. Calculate contact time at transmission main

where A is the cross-sectional area of the 4-in pipe, and v is the ﬂow velocity.

5 15.5 min

T 5 length/v 5 1640 ft/106 ft/min

5 106 ft/min

v 5

9.28 ft

/min

0.0872 ft

A 5 3.14s4 in/2/12 in/ftd

5 00872 ft

5 9.28 ft

/min

Q 5 100,000 gal/d 3 1 ft

/7.48 gal 3 1 day/440 min

Public Water Supply 483

TABLE 5.10

CT Values [(mg/L) min] for Achieving Inactivation of Viruses

at pH 6 through 9

Disinfectant Log

Temperature, °C

mg/L inactivation ≤ 1 5 10 15 20 25

Free chlorine 2 6 4 3 2 1 1

3964321

41286432

Chlorine 2 8.4 5.6 4.2 2.8 2.1 1.4

dioxide 3 25.6 17.1 12.8 8.6 6.4 4.3

4 50.1 33.4 25.1 16.7 12.5 8.4

Ozone 2 0.9 0.6 0.5 0.3 0.25 0.15

3 1.4 0.9 0.8 0.5 0.4 0.25

4 1.8 1.2 1.0 0.6 0.5 0.3

Chloramine 2 1243 857 643 428 321 214

3 2063 1423 1067 712 534 365

4 2883 1988 1491 994 746 497

SOURCE: Modiﬁed from Tables E-7, E-9, E-ll, and E-13 of the US EPA Guidance Manual

(US EPA, 1989a)

Step 6. Calculate CT

cal

and CT

cal

/CT

99.9

for the pipeline

cal

⫽ 1.0 mg/L ⫻ 15.5 min

⫽ 15.5 (mg/L) min

From Table 5.8, for 3-log removal of 1.0 mg/L chlorine residual at 10°C and

pH 7.5

99.9

⫽ 137 (mg/L) min

then CT

cal

/CT

99.9

⫽ 15.5/137 ⫽ 0.11 (log)

Step 7. Sum of CT

cal

/CT

99.9

from Steps 4 and 6

Total CT

cal

/CT

99.9

⫽ 0.38 ⫹ 0.11 ⫽ 0.49 (log)

Because this calculation is based on a 3-log removal, the ratio needs to

be multiplied by 3. The equivalent Giardia inactivation then is

3 ⫻ total CT

cal

/CT

99.9

⫽ 3 ⫻ 0.49 log ⫽ 1.47 log

The 1.47-log Giardia inactivation by chlorine disinfection in this system

exceeds the 1-log additional inactivation needed to meet the overall treatment

objectives. If a system uses chloramine and is able to achieve CT values for

99.9% inactivation of Giardia cysts, it is not always appropriate to assume

that 99.99% or greater inactivation of viruses also is achieved.

Example 3: For a 2000-ft long, 12-in transmission main with a ﬂow rate of

600 gpm, what is the credit of inactivation of Giardia and viruses using chlo-

rine dioxide (ClO

) residual ⫽ 0.6 mg/L, at 5°C and pH 8.5.

solution:

Step 1. Calculate CT

cal

/CT

99.9

for transmission pipe

T ⫽ ␲r

L/Q

⫽ 3.14 (0.5 ft)

(2000 ft) (7.48 gal/ft

)/(600 gal/min)

⫽ 19.6 min

where r is the radius of the pipe and L is the length of the pipe.

cal

⫽ 0.6mg/L ⫻ 19.6 min

⫽ 11.7 (mg/L) min

At 5°C and pH of 8.5, using ClO

, from Table 5.7

99.9

⫽ 26 (mg/L) min

cal

/CT

99.9

⫽ 11.7/26 ⫽ 0.45 (log)

484 Chapter 5