Addendum to Regional Water Supply Facilities Plan Final Report (April 2011) Page: 6-19
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APRIL 201:1
installations across the United States. and
abroad for almost 100 years. However,
the cost advantage of chlorine gas
systems over other forms of chlorine for
disinfection has decreased substantially
in recent years due primarily 'to
increased costs resulting from 'the
adoption of new regulations i.e.,
Uniform Fire Code and' Risk
Management Program (RMP) and
increased material:costs.
Because of safety concerns related 'to
potential accidental releases of chlorine
gas during transport and storage, new
and stricter federal regulations have been
adopted. These regulations have
resulted in a substantial increase in the
cost of chlorine gas systems. The same
quality that makes chlorine gas a good
disinfectant also makes it extremely
toxic to humans. Although: new safety
measures are currently in effect, there.
are still risks associated with the use and
transportation of chlorine gas. It is also
important to note that the transportation
of chlorine gas is highly regulated, and
requires special transportation permits
and licensing. The trend toward more
regulations regarding the transportation
and 'storage of chlorine gas may
continue, resulting in increased cost and
difficulties associated 'with its use.
Based on previous similar project
experience, for .comparison purpose,
probable cost estimates were developed
and compared for the three chlorine
disinfection systems as shown in Table
6-3. An on-site hypochlorite generator
has the lowest cost for a chlorine
disinfection feed system. The on-site
hypochlorite generation system was
recommended for this project.Figure 6-5 shows a typical chlorine
system plan layout for a 10 MGD water
treatment plant.
UV Light
Ultraviolet (UV) disinfection 'is a
physical disinfection process, as
opposed. to a chemical disinfection
process. It uses electromagnetic
energy in the 200 to 300 manometers
(nm) wavelength range to inactivate
microorganisms. The inactivation of
microorganisms is based on the UV
dose (mWs/cm2), which is a product
of the light intensity (mWs/cm2), and
the exposure time (seconds). The UV
dose is analogous to the CT term used
for inactivation credit for chemical
oxidants. Since the UV dose is
primarily based on the light intensity,
water quality parameters that have the
most effect on UV dose are turbidity
and suspended solids that can shield
microorganisms from the UV light,
and some organic and inorganic
compounds that can absorb UV light.
Figure 6-6 illustrates a typical UV
disinfection unit.
UV disinfection has a major
advantage of little of no production of
DBPs. Studies have shown that there
is no appreciable:increase in TTHM or
HAA concentrations as a result of UV
disinfection at doses that would be
applicable in water :treatment. UV
does not depend upon typical water
quality parameters (pH, temperature)
as chemical disinfectants. The
disadvantages of UV include: '(1) little
full-scale experience in surface water
treatment, (2) does not hold a residual
and . must be followed by a residual
disinfectant for the distributionREGIONAL WATER SUPPLY FACILITIES.PLAN
6-19
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Civil Systems Engineering, Inc. Addendum to Regional Water Supply Facilities Plan Final Report (April 2011), report, May 2011; Houston, Texas. (https://texashistory.unt.edu/ark:/67531/metapth839161/m1/171/?q=%22~1%22~1: accessed August 15, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.