Water and Wastewater Engineering
Подождите немного. Документ загружается.
12-20 WATER AND WASTEWATER ENGINEERING
12-3. Estimate the area of membrane required for Bar Nunn’s new membrane filtration
plant. The manufacturer of one of the membranes they have selected for testing has
provided the following data:
M a ximum TMP 103 kPa
R
m
3.1 10
10
cm
1
Based on their pilot tests, Bar Nunn has selected the following operating parameters:
Operating TMP 75% of maximum TMP
T e mperature 20 C
Q
max
181.7 m
3
/ d
12-4. Bar Nunn has investigated an alternate manufacturer to the one identified in
Problem 12-3. The alternate manufacturer offers a vacuum operated system that
uses a maximum TMP of 55 kPa. With the exception of the TMP, the operating
conditions are the s ame as those given in Problem 12-3. The c apital cost of mem-
brane is $1,100/m
2
for both membranes, but the operating costs are less for the
vacuum sy stem because of the lower TMP. If the vacuum system operating costs
are $16,500 per year and those for the pressure system are $33,000 per year and
Bar Nunn uses 6.00% interest and a 10-year membrane life to evaluate alternatives,
which sys tem
should be chosen based on these costs alone?
12-5. Yeehaw’s winter water demand is 10,400 m
3
/ d, and its summer demand is 15,600 m
3
/ d.
If the membrane filtration system is designed for the winter demand, can the system
meet the summer demand with the same area of membrane? The proposed winter and
summer design conditions are:
M a ximum TMP 152 kPa
Operating TMP 75% of maximum TMP
R
m
2.94 10
12
m
1
Winter water temperature 4 C
S ummer water temperature 20 C
12-6. Snowshoe’s winter water demand is 7,600 m
3
/ d, and its summer demand is
10,200 m
3
/ d. If the membrane filtration system is designed for the winter demand,
can the system meet the summer demand with the same area of membrane? The pro-
posed winter and summer design conditions are:
M a ximum TMP 200 kPa
Operating TMP 75% of maximum TMP
R
m
3.2 10
12
m
1
Winter water temperature 1 C
S ummer water temperature 15 C
12-7. A membrane manufacturer’s data sheet reports the following:
Initial specific flux 1.24 L/h · m
2
· kPa at 20 C
Flux 33 L/h · m
2
R
m
2.9 10
12
m
1
What is the TMP for these conditions?
MEMBRANE FILTRATION 12-21
12-8. Determine the number of racks for the Spartanburg membrane filtration system for
the “worst case scenario” of one rack off-line for CIP plus one rack off-line for back-
wash given the following design requirements:
Q
max
30,300 m
3
/ d
J 0.127 m
3
/h · m
2
J
max allowable
0.170 m
3
/h · m
2
Cleaning cycle 30 min
T i me off-line to clean 150 s
M e mbrane area per module 15 m
2
M o dules per rack 90
12-9. Determine the number of racks for the town of Agate’s membrane filtration system
for the worst case scenario of one rack off-line for CIP plus one rack off-line for
backwash given the following design requirements:
Q
winter
7,600 m
3
/ d
Q
summer
10,200 m
3
/ d
Winter water temperature 1 C
S ummer water temperature 15 C
J 0.159 m
3
/h · m
2
for winter temperature
J
max allowable
0.267 m
3
/h · m
2
for winter temperature
Cleaning cycle 30 min
T i me off-line to clean 150 s
M e mbrane area per module 15 m
2
M o dules per rack 4 0
12-7 DISCUSSION QUESTIONS
12-1. Because of the change in viscosity of water, a membrane design for cold water will
have a higher flux and be able to treat a greater flow rate at a higher temperature.
True or false?
12-2. What is the difference between irreversible and reversible transmembrane membrane
pressure?
12-3. Describe the parameter values you would need to compare membranes from
different
manufacturers.
12-8 REFERENCES
AWWA (2005) Microfiltration and Ultrafiltration Membranes for Drinking Water, AWWA Manual
M53, American Water Works Association, Denver, Colorado.
Bergman, R. A. (2005) “Membrane Processes,” in E. E Baruth (ed.), Water Treatment Plant Design,
McGraw-Hill, New York, pp. 13.1–13.49.
Carmen, P. C. (1937) “Fluid Flow through Granular Beds,” Transactions of Institute of Chemical
Engineers, London, vol. 15, p. 150.
Chella
m, S. and J. G. Jacangelo (1998) “Existence of Critical Recovery and Impacts of Operational Mode
on Portable Water Microfiltration,” Journal of Environmental Engineering, vol. 124, no. 12,
pp. 1211–1219.
12-22 WATER AND WASTEWATER ENGINEERING
Ferry, J. D. (1936) “Statistical Evaluation of Sieve Constants in Ultrafiltration,” Journal of General
Physiology, vol. 20, pp. 95–104.
Howe, K. J. and M. M. Clark (2006) “Effect of Coagulation Pretreatment on Membrane Filtration
Performance,” Journal of American Water Works Association, vol. 98, no. 4, pp. 133–146.
Jacangelo, J. and C. A. Buckley (1996) “Microfiltration,” in J. Mallevialle, P. E. Odendaal, and M. R.
Wiesner (eds.), Water Treatment Membrane Processes, American Water Works Association and
McGraw-Hill, Denver, Colorado, p. 11.25.
Jacangelo, J., N. Patania, J. M.Laîné, W. Booe, and
J. Mallevialle (1992) Low Pressure Membrane
Filtration for Particle Removal, American Water Works Association Research Foundation and
American Water Works Association, Denver, Colorado.
Jacangelo, J. G., S. Adham, and J. Laine (1997) Membrane Filtration for Microbial Removal, AWWA
Research Foundation and American Water Works Association.
J ucker, C. and M. M. Clark (1994) “Adsorption of Aquatic Humic Substances on Hydrophobic
Ultrafiltration Membranes,” Journal Membrane Science, vol. 97, pp. 37–52.
MWH (2005) Water Treatment: Principles and Design, John Wiley & Sons, Hoboken, New Jersy,
pp. 882, 892–894.
Pressdee, J. R., S. Veerapaneni, H. L. Shorney-Darby, J. A. Clement, and J. P. Van der Hoek (2006)
Integration of Membrane Filtration into Water Treatment Systems, American Water Works
Association Research Foundation Report No. 91103.
Rose, H. E. (1945) “On the Re
sistance Coefficient–Reynolds Number Relationship of Fluid Flow through
a Bed of Granular Material,” Proceedings of the Institute of Mechanical Engineers, vol. 153, p. 493.
U. S. EPA (2005) Membrane Guidance Manual, U.S Environmental Protection Agency, Washington,
D.C., pp. 2–15.
Yao, K. M., M. T. Habibian, and C. R. O’Melia (1971) “Water and Waste Water Filtration: Concepts and
Applications,” Environmental Science and Technology, vol. 5, no. 11, pp. 1105–1112.
13-1
13-6 CHAPTER REVIEW
13-7 PROBLEMS
13-8 DISCUSSION QUESTIONS
13-9 REFERENCES
DISINFECTION AND FLUORIDATION
13-1 INTRODUCTION
13-2 DISINFECTION
13-3 EMERGENCY DISINFECTION
13-4 FLUORIDATION
13-5 OPERATION AND MAINTENANCE
13
CHAPTER
13-2 WATER AND WASTEWATER ENGINEERING
13-1 INTRODUCTION
D i sinfection is used in water treatment to reduce pathogens to an acceptable level. Disinfection is
not the same as sterilization. Sterilization implies the destruction of all living organisms. Drinking
water does not need to be sterile to be safe to drink.
Three categories of human enteric pathogens are of concern in drinking water: bacteria,
viruses, and amebic cysts. Disinfec
tion must be capable of destroying all three.
In the United States, five agents have found common use in disinfecting drinking water:
(1) free chlorine, (2) combined chlorine, (3 ) ozone, (4) chlorine dioxide, and (5) ultraviolet
irradiation. These are the subject of this chapter.
Fluoridation, which is also discussed in thi
s chapter, refers to the addition or removal of fluoride
from drinking water to maintain an optimum concentration to reduce tooth decay.
13-2 DISINFECTION
Disinfection Chemistry
Free Chlorine. Chlorine is the most common disinfecting chemical used. The term chlorination i s
often used synonymously with disinfection. Chlorine may be used as an element (Cl
2
), as sodium
hypochlorite (NaOCl), also known as bleach, as calcium hypochlorite [Ca(OCl)
2
], also known as
HTH
®
, or as chlorinated lime (CaOCl
2
).
When chlorine is added to water, a mixture of hypochlorous acid (HOCl) and hydrochloric
acid (HCl) is formed:
Cl g H O HOCl H Cl
22
()
+
(13-1)
This reaction is pH dependent and essentially complete within a very few milliseconds. The pH
dependence may be summarized as follows:
• In dilute solution and at pH levels above 1.0, the equilibrium is displaced to the right and
very little Cl
2
exists in solution.
• Hypochlorous acid is a weak acid and dissociates poorly at levels of pH below about 6.
Between pH 6.0 and 8.5 there occurs a very sharp change from undissociated HOCl to
almost complete dissociation:
HOCl H OCl
(13-2)
• Chlorine exists predominantly as HOCl at pH levels between 4.0 and 6.0.
• Below pH 1.0, depending on the chloride concentration, the HOCl reverts back to Cl
2
a s
shown in Equation 13-1 .
• At 20 C, above about pH 7.5, and at 0 C, above abou t pH 7.8, hypochlorite ions (OCl
)
predominate.
• Hypochlorite ions exist almost exclusively at levels of pH around 9 and above.
DISINFECTION AND FLUORIDATION 13-3
Chlorine existing in the form of HOCl and/or OCl
i s defined as free available chlorine or free
chlorine.
Example 13-1. If 15 mg/L of HOCl is added to a potable water for disinfection and the final
measured pH is 7.0, what percent of the HOCl is not dissociated? Assume the temperature
is 25 C.
Solution:
a . The reaction is shown in Equation 13-2 . From Appendix A, find that the p K
a
i s 7.54 and
K
a
10 2 88 10
7 548.
.
b. Writing the equilibrium constant expression
K
a
[][ ]
[]
HOCl
HOCl
and substituting the values for K
a
and [H
]
288 10
10
8
7
.
[][ ]
[]
OCl
HOCl
Solving for the HOCl concentration
[] []HOCl OCl
3 47.
Because the frac tion of HOCl that has not dissociated plus the OCl
that was formed
by the dissociation must, by the law of conservation of mass, equal 100% of the original
HOCl added:
[][] (HOCl OCl of the total HOCl added
100% to the solution)
then
3 47 100.[ ][ ]OCl OCl
%
4 47 100.[ ]OCl
%
[]OCl
100
447
22 37
%
%
.
.
13-4 WATER AND WASTEWATER ENGINEERING
and
[] ( )HOCl 3 47 22 37776.. .%%
Comments:
1 . Note that the concentration of HOCl was not used in the problem solution!
2. For different temperatures, the percentages will be different because K
a
i s a function of
temperature.
H ypochlorite salts dissociate in water to yield hypochlorite ions:
NaOCl Na
OCl
(13-3)
Ca OCl Ca OCl()
2
2
2
(13-4)
The hypochlorite ions establish equilibrium with hydrogen ions (in accord with Equation 13-2 ).
Thus, the same active chlorine species (HOCl and OCl
) and equilibrium are established in water
regardless of whether elemental chlorine or hypochlorites are used. The significant difference
is in the resultant pH and its influence on the relative amounts of HOCl and OCl
e xisting at
equilibrium. Elemental chlorine tends to decrease pH; each mg/L of chlorine added reduces the
alkalinity by up to 1.4 mg/L as CaCO
3
. Hypochlorites, on the other hand, always contain excess
alkali to enhance their stability and tend to raise the pH somewhat. To optimize disinfecting action,
the design pH is in the a range 6.5 to 7.5.
Free chlorine is relatively stable in pure water. It reacts slowly with naturally occurring organic
matter (NOM) and rapidly with sunlight. The photolytic reac
tion is with hypochlorite. The reac-
tion products are oxygen, chlorite ion, and chloride ion (Buxton and Subhani, 1971).
Chlorine/Ammonia Reactions. The reactions of chlorine with ammonia are of great signifi-
cance in water chlorination processes. When c hlorine is added to water that contains natural or
added ammonia (ammonium ion exists in equilibrium with ammonia and hydrogen ions
), the
ammonia reacts with HOCl to form various chloramines. The reactions between chlorine and
ammonia may be represented as follows (AWWA, 2006):
NH HOCl NH Cl H O
Monochloramine
3 22
(13-5)
NH Cl HOCl NHCl H O
Dichloramine
222
(13-6)
NH Cl HOCl NCl H O
Trichloramine
22 3 2
(13-7)
DISINFECTION AND FLUORIDATION 13-5
The distribution of the reaction products is governed by the rates of formation of monochlora-
mine and dichloramine, which are dependent upon pH, temperature, time, and initial Cl
2
:NH
3
ratio.
In general, high Cl
2
:NH
3
ratios, low temperatures, and low pH levels favor dichloramine formation.
Chlorine also reacts with organic nitrogenous materials, such as proteins and amino acids, to
form organic chloramine complexes. Chlorine that exists in water in chemical combination with
ammonia, or organic nitrogen compounds, is d
efined as combi ned available chlorine or com-
bined chlorine. The sum of the c oncentrations of free chlorine and combined chlorine is called
total chlorine.
The oxidizing capacity of free chlorine solutions varies with pH because of variations in
the resultant HOCl:OCl
ratios. This is also true for chloramine solutions as a result of varying
NHCl
2
:NH
2
Cl ratios. Monochloramine predominates at high pH levels.
Chlorine Dioxide. Chlorine dioxide is a stable free radical that, at high concentrations, reacts
violently with reducing agents. It is explosive with a lower explosive limit * (LEL) reported vari-
ously between 10 and 39 percent. Thus, virtually all applications require synthesis on-site. Chlorine
dioxide (ClO
2
) is formed on-site by combining chlorine and sodium chlorite. One of three alter-
native reactions may be employed (MWH, 2005):
222
22 2
NaClO Cl g ClO g NaCl() ()
(13-8)
22
22
NaClO HOCl ClO g NaCl NaOH ()
(13-9)
5 44 5 2
222
NaClO HCl ClO g NaCl H O ()
(13-10)
Under alkaline conditions chlorine dioxide forms chlorite
()ClO
2
and chlorate ()ClO
3
ions
(Gordon et al., 1972):
22
223 2
ClO OH H O ClO ClO
(13-11)
The typical reaction of chlorine dioxide in water is a one-electron reduction (Haas, 1999):
ClO e ClO
22
(13-12)
Ozone. Ozone is a pungent, unstable gas. It is a form of oxygen in which three atoms of oxygen
are combined to form the molecule O
3
. Because of its instability, it is generated at the point of
use. Ozone may be generated by photochemical, electrolytic, and radiochemical methods, but it
is most commonly generated by a discharge electrode. Either pure oxygen, purchas ed as liquid
oxygen (LOX), or the oxygen in the air, is dissociated by the impact of electrons from the dis -
charge electrode. The atomic oxygen then combines with atmospheric oxygen to form ozone in
the following reaction:
OO O
2 3
(13-13)
*Mixtures of gases with air that have concentrations above the LEL can be ignited.
13-6 WATER AND WASTEWATER ENGINEERING
When LOX is used, 5 to 8 percent by volume of the air exiting from the apparatus will be ozone.
The resulting ozone-air mixture is then diffused into the water that is to be disinfected.
If ambient air is used as a source of oxygen (as opposed to liquid oxygen), trace levels of
water can react with the nitrogen in the air to form nitric acid
:
O N O H O HNO
3 222 3
2
hv
(13-14)
The nitric acid then corrodes the ozone generator. The implication for design of ozone systems is
that the process must include a method for drying ambient air to a very low moisture content.
Redox Reactions. Chemical disinfectants are oxidants. Because they undergo oxi
dation-reduction
reactions, this allows comparison of the disinfectants based on their oxidizing power. This is of
particular interest in comparing chlorine compounds. The relative amount of chlorine present in
these compounds may be expressed as percent available chlorine. The percent available chlorine
of a
compound is the electrochemical equivalent amount of Cl
2
. It is a measure of the oxidizing
power of the compound in comparison to Cl
2
. It is calculated as
% available chlorine
Equivalent weight of C
llincompound
Equivalent weight of compoun
2
dd
(13-15)
The equivalent weight of a compound in an oxidation-red uction reaction is calculated using
its ox idation-reduction half reaction. A selected list of half-reactions is given in Table 13-1 .
E xample 13-2 demonstrates the calculation of percent available chlorine.
TABLE 13-1
Selected half reactions
Reaction
Ca(OCl)
2
2H
4e
2Cl
CaO H
2
O
CaOCl
2
2e
2Cl
CaO
Cl
2
(g) 2e
2Cl
ClO
2
2H
2
O 5e
Cl
4OH
H
2
O
2
2H
2e
2H
2
O
HOCl H
2e
Cl
H
2
O
MnO H 5eMn4HO
24
2
8
NaOCl H
2e
Cl
NaOH
NH
2
Cl H
2
O 2e
Cl
NH
3
OH
NHCl
2
2H
2
O 4e
2Cl
NH
3
2OH
O
3
2H
2e
O
2
H
2
O
SO H e H S g H O
4
2
10 8 4
22
()
Sources: Weast, 1983; LaGrega et al., 2001; Snoeyink and
Jenkins, 1980.
DISINFECTION AND FLUORIDATION 13-7
Example 13-2. Estimate the percent available chlorine in
Ca (OCl)
2
.
Solution:
a . The appropriate half reactions from Table 13-1 are
Cl g e Cl
Ca OCl H e CaO H O Cl
2
22
22
24 2
()
()
b. From the half reactions calculate the equivalent weights of Cl
2
and
Ca (OCl)
2
Equiv Wt of Cl
GMW of Cl
No of electron
..
2
2
.
ssconsumed
2 35 45
2
35 45
().
.
Equiv Wt of Ca OCl
GMW of Ca OCl
No o
..
.
()
()
2
2
ffelectrons c onsumed
142 98
4
35 45
.
.7
c. The percent available chlorine is
%%available chlorine
35 45
35 745
100 99 1
.
.
.() 77%
Comments:
1 . Although the weight percent of chlorine in Ca(OCl)
2
is barely half of the weight of the
compound, the oxidizing power is virtually equivalent to chlorine gas.
2. It is possible for a compound to have a % available chlorine greater than 100%. For these
compounds the implication is that they have greater oxidizing power than chlorine gas.
Ultraviolet (UV) Radiation. Table 13-2 outlines the s pectral ranges of interest in photo-
chemistr
y.
The energy associated with electromagnetic radiation may conceptually be thought of as
photons. The energy is related to the wavelength of the radiation (Einstein, 1905):
E
hc
(13-16)