Page 679 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

634 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

filtration to give an average dose, C(O 3 ) 1.5 mg=L. Disin- Required
fection takes place in four reactors where ozone is diffused by (a) Calculate the rate of chlorine feed into the
porous plates, with u(avg) 12 min. At the end of the solution feeder. (b) Calculate the maximum concentra-
reactors, C(O 3 ) 0.4 mg=L, which is the setpoint for control; tion of HOCl in the solution feeder. (c) Assuming the
if a higher level is detected, the ozone dosage is reduced and maximum concentration of chlorine in the solution-
vice versa. After ozone, chlorine is added for both secondary feeder, determine the associated solution feeder flow,
disinfection and maintaining a residual in the distribution Q(solution).
system, C(Cl 2 ) 0.4 mg=L (Langlais, 1991, p. 229). As to 19.2 Estimating Hypochlorite Concentration of Giardia
the effectiveness of ozone disinfection, data provided by Cyst Inactivation
Langlais (1991, p. 230) showed significant reductions of Given
bacteria that were monitored, for example, Ca(OCl) 2 is used as the primary disinfectant following
filtration for drinking water. The average detention time
in the clear well basin is 20 min and is 10 min in the
Microorganism Units Post-Filtration Post-Ozonation pipeline prior to the first customer. The water tempera-
ture is as low as T ¼ 0.58C in the winter. Let the required
Heterotrophic (#=mL) 1580 3
Plate Count log R(disinfection) ¼ 2.
Total coliform (#=100 mL) 3580 0.2 Required
Fecal coliform (#=100 mL) 198 0.2 Based on the spreadsheet, Table CD19.4, calculate
Fecal streptococci (#=100 mL) 32 0 the required Ct and then the needed Ca(OCl) 2 concen-
tration.
19.3 Contact Time for Chlorine Disinfection
19.5.3 ULTRAVIOLET LAMPS Given
A drinking water treatment plant applies chlorine to the
The issue in operation of UV lamps is to ensure that the
flow into a clear well. The mean detention time in the
radiation intensity is maintained. Malley (2000, p. 8) listed
some of the UV operation and maintenance issues as: calibra- clear well, a ‘‘plug flow’’ reactor, is t(clear well) ¼
30 min. Conditions are: pH ¼ 7.8, T ¼ 58C.
tion and=or replacement of UV sensors; lamp and sleeve
Required
cleaning; lamp replacement determination; day-to-day per-
Determine feasible combinations of 8C and pH that will
formance monitoring protocol. The latter includes continuous
result in 2-log inactivation of Giardia lamblia cysts.
monitoring of equipment performance such as sensors, lamp-
19.4 UV Reactor Design
out indicators, lamp hours, ballast temperatures. The monitor-
ing data should be maintained in an easy-to-read tabular Given
3
format, using plots where feasible. Data obtained by sensors UV is to be installed in an existing WTP. Q ¼ 0.44 m =s
may be shown in real-time plotted form directly on the com- (10 mgd). The reactor is to be installed in a 1220 mm
puter screen. Other data may be plotted easily if the recording (48 ) pipeline section; a length of 4.0 m (13.1 ft) may be
00
includes date, time, and monitored values as columns within a used for the reactor, if needed. Cryptosporidium parvum
spreadsheet. The ensuing discussion is a brief overview of a is the target organism and log R(specified) ¼ 3. Lamps
few of the issues. are to be ‘‘low-pressure.’’
Fouling, for example, biofilms or chemical precipitants Required
may occur on the quartz surfaces of lamps causing a decrease (a) Determine the average UV radiation intensity, I,in
2
2
in the radiation intensity. Control may be by physical cleaning mJ=cm (i.e., mW s=cm ) that must be provided by the
or by chemical bath. Sensors that detect UV intensity, I,at UV lamps. Estimate the power input required for the
specified index locations will confirm that operation is con- lamps. (b) Set up a spreadsheet to explore the trade-off
tinuing as intended. The sensors must be cleaned also at between u and I(avg) in order to provide design that you
frequent intervals, and calibrated. may feel is more reasonable and for redundancy. All of
this may be done most easily by means of a
spreadsheet algorithm.
PROBLEMS 19.5 Mathematical Model for UV Reactor
19.1 Wallace and Tiernan’s Chlorination Apparatus Given
A UV reactor of length, Z o , and area, A, has a detention
Given
time, u, for flow Q.
A solution-feed apparatus is to be used to chlorinate a
3
1.00 m =s (22.8 mgd) water supply. The chlorine con- Required
centration is to be 3.00 mg HOCl=L, dissolved into a Set up a mathematical model for a steady-state UV
solution-feeder by a diffuser; the resulting solution is reactor for a spreadsheet solution.
then to be fed into a pipeline between the filters and the Hint: Sections 4.3.3 and 4.3.4.5, describes how this is
clear well. done. Also, Equation 15.36 is a finite difference form of