Page 678 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Disinfection 633

19.4.5.4.2 Ballasts impetus came about in 1998 after the discovery that UV
The lamp ballasts are transformers that control the lamp inactivated Cryptosporidium parvum at moderate dosage, for
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power. Two types are electromagnetic and electronic; the example, dose(UV) 6mJ=cm . This discovery further post-
latter generate less heat and have a longer life than the former. poned the implementation of filtration for many cities that had
avoided filtration through waiver provision in regulations
19.4.5.4.3 UV Lamp Controls (Surface Water Treatment Rule, Federal Register, Vol. 54
Control systems should include UV intensity sensors, cali- (124):27486, June 29, 1989). The foregoing has led to the
brated to UV intensity; flow meters for each reactor; pressure planning for one of the largest UV installations, that is, the
gages for pressurized systems; hour meters to record lamp New York City water supply. The Consultants Corner,
operation time. The control system should be able to shut JAWWA, Vol. 95(12):47 provided details. The contract was
down the UV lamps and flows in the event of high reactor $35.7 million to Hazen and Sawyer, and CDM for permitting,
temperature, low UV dose, or a higher flow rate, that causes design, and design services during construction for an
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the dose to fall below that specified. 8 million m =day (2 bgd) facility for 9 million customers,
with the completion scheduled for 2009 and the construction
19.4.5.5 UV Design Guidelines cost estimated to be about $500 million. The source was the
UV design criteria for wastewater reclamation in California Catskill and Delaware reservoir system, which provided about
specified that under the worst operating conditions, the system 90% of the water to the New York City system. The project
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should deliver a minimum UV flux of 140 mW s=cm for the allowed New York City to maintain its non-filtration status
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maximum week flow and 100 mW s=cm at the peak hour for that precludes the need to a massive filtration facility, which
the maximum day flow. The minimum dose must be based on could cost about $2 billion.
the following conditions (White, 1999, p. 1388):
. UV lamp output 0.70 new lamp output 19.4.8 SUMMARY
. Transmittance through quartz sleeves 0.70
UV has been favored over chlorine for primary disinfection
. UV flux density calculation method ¼ point source
since about the late 1990s for several reasons: (1) the tech-
summation
nology is effective in inactivating most organisms, including
. UV dose to be achieved with a minimum of three UV
Cryptosporidium parvum at costs that are competitive with
banks in series
other technologies; (2) disinfection by-products are not
formed; (3) transport and storage of hazardous chemicals is
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For 100 dose 120 mW s=cm , pilot testing with a hori-
avoided (except for a much smaller rate of use of chlorine in
zontal lamp configuration showed that 4-log inactivation of
order to provide a residual for the distribution system.)
poliovirus was achieved, that is, log(N=N 0 ) ¼ 4. Backup
power should be provided for continuous and reliable oper-
ation and the UV reactors should be designed for plug flow 19.5 OPERATION
(White, 1999, p. 1390).
Operation of the several kinds of disinfection systems involves
monitoring, inspection, maintenance, and reporting. The topic
19.4.6 COSTS
is extensive and only some key issues are reviewed here.
Malley (2000, p. 8) gave capital and operating=maintenance
costs:
19.5.1 CHLORINE OPERATION
Aspects of chlorine operation include pacing chlorine feed
Cost per Unit of Installed Capacity Cost per Unit of Water Treated
rate to water flow with fine adjustment based on a ‘‘setpoint’’
$=m 3 $=gal $=m 3 $=1000 gal for chlorine residual, maintaining a negative pressure in the
Low High Low High Low High Low High chlorine storage room, providing a response procedure for
chlorine leaks, maintaining a sodium hydroxide purge vat for
12.2 21.1 0.05 0.08 0.0013 0.008 0.005 0.03
chlorine gas, etc. In wastewater chlorination, de-chlorination
has been common since the 1980s, using sulfur dioxide gas
(Bernhardt, 1991). Other reducing agents such as sodium
These costs, he stated, were as low as one-fifth the cost of
thiosulfate, sodium bisulfate, or sodium sulfite are possible
ozone and one-tenth the cost of membrane filtration (MF) and
but not common.
would require only a small fraction of the space required for
the other processes.
19.5.2 OZONE OPERATION
19.4.7 CASE
At the Neuilly-sur-Marne WTP, 1 of 12 plants that provides
While UV has been gaining acceptance in both drinking water drinking water to Paris, ozone is generated by 4 ozonators,
and wastewater disinfection since the early 1990s, a major each with a capacity of 30 kg=h. Ozone is applied after rapid