Alberta water treatment plant maintained production during UV installation
Ultraviolet disinfection system.
The City of Lethbridge, Alberta
operates a 150 million litre
per day water treatment plant,
which supplies 77,000 residents,
as well as commercial, industrial,
institutional and agricultural sectors.
To address growing concerns
regarding drinking water quality,
Alberta Environment established more
stringent standards, including 4 log
(99.99%) inactivation of Giardia, protists
common in raw water supplies.
The City was faced with significant
and expensive chemical disinfection
upgrades to meet this standard, and
sought a more economical solution.
Ultraviolet (UV) disinfection for
waterborne cysts such as Giardia and
Cryptosporidium offered a solution.
Not only would UV allow the City to
defer an intermediate 3 log inactivation
requirement, saving $500,000, but
the City would also be able to reduce
the projected capital budget for 4 log
inactivation from $20 million to $5
million.
The stringent regulatory deadline
challenged the design team of
Associated Engineering, CH2M HILL,
and the City stakeholders to meet an
aggressive implementation schedule.
To meet the deadline, the City decided
to manage construction, thus expediting
implementation. This approach
allowed the City to start construction
two months earlier than a conventional
design-bid-build project. Long delivery
items were pre-ordered, including
the UV reactors, large diameter butterfly
valves, and magnetic flow meters.
The UV disinfection facility was
designed for 150 million litres per day
to match the plant capacity. Five, 600
millimetre diameter Trojan UVSwift
reactors with a capacity of 50 million
litres per day each were installed to
meet the capacity, plus a minimum 50%
redundancy requirement. Chemical
injection points for chlorine were
installed upstream and downstream of
the reactors. Fluoride and ammonia
injection points were installed downstream
of the reactors for induction with
a hydraulic mixer. As an added benefit,
UV disinfection allowed the City to
maintain chloramination in the distribution
system, which was a consumer
preference over free chlorination.
Ammonia used for chloramination had
to be kept downstream of the UV reactors
due to potential interference with
the UV transmittance.
To maintain drinking water production,
the City needed to keep the water
treatment plant clearwell on-line during
the construction tie-in. The City
and the design team explored a number
of options for bypassing the clearwell
during construction, but none of these
was deemed feasible.
Piping layout for ultraviolet reactors.
An innovative tie-in detail was
developed that allowed the plant to
remain on-line during construction. The live tie-in involved pre-drilling 50
mm diameter pilot holes through the
clearwell walls to guide divers from
the inside. Divers then attached 900
mm diameter top hats over each pilot
hole. These top hats were raised blind
flanges, gasketted around the edge and
bolted to the inside of the clearwell
wall. Isolation valves on the pilot holes
were then opened to relieve the pressure
behind the top hats; the pressure
from the water inside the clearwell
effectively sealed the top hats to the
wall.
The top hats formed an annulus
against the wall that allowed a coring
machine to drill a 750 millimetre
diameter hole from the outside into
this annulus. Once the cores were completed,
the outlet and inlet piping headers
to the clearwell were installed,
grouted in place, and a cast-in-place
concrete thrust structure was poured to
hold the header in position. The top
hats were then removed from the
inside of the clearwell.
Construction was completed and
the plant commissioned by December
2003, meeting the regulatory deadline.
The project, originally estimated at $5
million, was completed $900,000
under budget.
"The City of Lethbridge is very
pleased with our new facility", reports
Doug Kaupp, the City's Water Utility
Manager. "It (the UV facility) has
operated flawlessly for the first year
and has served to reassure the community
that our drinking water is of the
highest quality."
Contact: Lianna Mah, P.Eng. who is
with Associated Engineering Limited
e-mail: aetoday@ae.ca
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