Page 680 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Disinfection 635

Equation 19.40, which should be modified to the steady- ACKNOWLEDGMENTS
state form, that is, DC=Dt ¼ 0 for left side. Also, the

kinetic term, qX=qt, is replaced by the UV kinetic term, Kevin Gertig, water resources and treatment operations
manager, City of Fort Collins, Colorado, has helped exten-
Equation 19.34 in which I is constant along the length of
sively with this chapter, as with others, for example, in pro-
the reactor and k is as determined in Section 19.3.7.4.3.
viding photographs and reference items from the FCWTP
Table 15.10 illustrates the pattern of spreadsheet set-up.
library, and in various discussions and site visits related to
19.6 Application of a Mathematical Model for UV
the functioning of facilities.
Reactor
Dr. James Malley, University of New Hampshire, gra-
Given
ciously provided his powerpoint presentation for a 2002
Let Cryptosporidium parvum oocysts be the target
AWWA (Rocky Mountain Section)-sponsored UV Workshop
microorganism for a UV reactor for a municipal was-
in Denver, Colorado, which was consulted and cited to clarify
tewater. Assume that 4-log inactivation is required.
and amplify various issues on UV disinfection.
For calculation purposes, assume N o ¼ 10,000
Dr. Dean Gregory, director of environmental technologies,
oocysts=L (a typical concentrations of Cryptosporid-
CDG Environmental, LLC, Denver office, reviewed Section
ium oocysts is N o 200 oocysts=L). Let Q ¼ 0.88 19.3.8 on chlorine dioxide and added considerable knowledge
3
m =s(10 mgd), v ¼ 0.5 m=s, and assume the reactor
on the topic pertinent to practice.
length, Z o ¼ 3.0 m, and assume D= v ¼ 1.4 cm (or
0.014m). AweiroraParshall flume is used to
maintain a minimum depth (so that the UV lamps GLOSSARY
are always covered with water).
Absorbance: Defined as, A ¼ ln(I o =I), where I o is the incident
Required radiation, and I is the radiation at any given distance
Determine the concentration profile based on Equation from the occurrence of the incident radiation. See
19.56 in finite difference form, using a spreadsheet for also Transmittance.
the profile calculations. Determine the required average Actinometry: A photochemical reaction, for which the quan-
in UV intensity in the reactor such that the 4-log inacti- tum yield, is known; thus, the measurement of the
vation occurs by the end of the reactor. chemical yield after exposure to light allows the
19.7 Estimating Effect of Dispersion determination of the photon flow (Bolton, 2001,
Given p. 19).
Let Cryptosporidium parvum oocysts be the target Available chlorine: White (1999, pp. 221–223), considers
microorganism for a UV reactor for a municipal waste- this term a misnomer and that it ‘‘has no place in
water. For calculation purposes, assume N o ¼ 10,000 the field of water and waste treatment.’’ Its origin
oocysts=L (a typical concentrations of Cryptosporidium according to White had to do with the idea of
3
oocysts is N o 200 oocysts=L). Let Q ¼ 0.88 m =s comparing the bleaching or disinfecting power
(10 mgd), v ¼ 0.5 m=s, and assume the reactor length, of different chlorine compounds as measured by
Z o ¼ 3.0 m, and assume D= v ¼ 1.4 cm (or 0.014 m). the starch-iodide test (the iodometric method) in
A weir or a Parshall flume is used to maintain a min- which iodine is liberated.
imum depth (so that the UV lamps are always covered Breakpoint chlorination: Satisfaction of the reaction
with water). demand for HOCl as an oxidant by reducing sub-
stances, for example, HS ,SO 3 ,NO ,Fe ,
2
2
2þ
Required
ammonia, and organic compounds. Upon satisfac-
(a) Calculate the dispersion profile for a frontal wave of
tion of this demand, the ‘‘free chlorine’’ is available
salt moving through the reactor (b) Calculate the disper-
for disinfection.
sion profile for a pulse wave of salt moving through the
Bromine: (1) A halogen element that reacts with water to
reactor. (c) Estimate the Ct for the 10% of flow, u(10)
form HOBr and OBr . Discovered in 1827 and

that has moved through the reactor first. Estimate the
named after the French word brome. (2) Nonmetallic
associated concentration of oocysts, N that remain
halogen element that is isolated as a deep red
viable as they leave the reactor in the first 10% of
corrosive toxic volatile liquid of disagreeable odor
flow. Compare the finite difference solution with the
(http:==www.merriam-webster.com=). MW ¼ 79.904
mathematical solution.
g=mol. Disinfection properties are similar to
Hint: Let [qN=qt] r ¼ 0, such that Equation 19.56
chlorine except that bromine is effective as HOBr
becomes the same as Equation 4.5, that is, at pH < 8.5 (as compared with pH < 7.5 for chlor-
2
qN qN q N ine); bromine also reacts with ammonia in a
¼ v þ D . This expression can be set
qt qZ qZ 2 sequence of reactions similar to chlorine, that is,
o
up in a finite difference form and solved by a forming bromamines.
spreadsheet algorithm. The mathematical solution is Calcium hypochlorite: Ca(OCl) 2 is a white granular
seen in Equation 4.6. powder with 70% ‘‘available chlorine.’’ ‘‘Available