UF membranes retrofit vintage pumping station in Sudbury
By Paul Graham,Toby Brodkorb, and Scott Lenhardt
The second stage of membrane treatment incorporates reinforced,
hollow fibre membranes that are ideal to treat the reject water from
the first stage. This two-stage design enables the plant to operate at
99 per cent efficiency.
The City of Greater Sudbury
has become a thriving community
centrally located in
Northeastern Ontario. Well
known for its nickel mining industry it
is also a centre for governance, medical
referral, education, retail and
many service industries - a community
that is committed to diversification
and sustainability.
The former Regional Municipality
of Sudbury has won several international,
national, and provincial awards
for its successful environmental initiatives.
In 1992, the City was honored by
the United Nations for the Sudbury
Regional Land Reclamation Program,
which led to the planting of over eight
million new trees in the local area.
Greater Sudbury is also a leader in
local action planning to promote the
reduction of greenhouse gases.
Black water blues
Though Greater Sudbury’s water
had always met provincial water quality
standards, the City’s long history of
providing an unfiltered water supply
had resulted in an accumulation of
manganese and other substances, such
as iron, in parts of the City’s water distribution
system. This led to seasonal
problems with water discolouration
and, on some days, residents complained
about a black tinge in the water
caused by the manganese.
New provincial drinking water standards
also made filtration of all City
surface water supplies a necessity. The
Ontario Drinking Water Standards,
introduced in 2000, created a legally
enforceable standard, to guarantee
water that is safe, palatable, and protective
of public health. Not only
would potable water need to be free of
disease-causing organisms and dangerous
concentrations of toxic chemicals,
but it would also have to be aesthetically
acceptable. Aesthetic parameters
established by the legislation control
the taste, smell, and colour of the
water.
Working with consultants, CH2M
HILL, the City studied several options
for improving water quality. One
option was the possibility of upgrading
its David Street Water Treatment Plant.
Located minutes from the City’s downtown
core, the plant has long provided
the community with high quality water
from Ramsey Lake. The site was ideal
due to its proximity to both the water
source and the customers, but posed
some significant challenges because of
its limited size. It would be difficult to
convert the plant to a full water treatment
plant using conventional technology.
As a result, water managers quickly
began to consider retrofitting the
existing plant with ultrafiltration (UF)
membrane technology because of its flexibility of design, compact footprint,
and high quality water production
capabilities.
Working within a small footprint
After prudent evaluation and pilot
testing, the City of Greater Sudbury
selected ZENON ZeeWeed® immersed
hollow fibre UF technology for the
David Street site. It was ideal for this
plant for three main reasons:
The location near housing and
parkland made it impossible to significantly
expand the footprint. UF membrane
technology allowed the City to
construct a full water treatment plant
on the existing site.
By retrofitting the David Street
Plant, the City was able to save significant
capital costs. Distribution of
treated water is also more cost-effective
since the plant is located in the
heart of the City. This minimizes
pumping costs since customers are in
such close proximity.
The innovative siphon design of
the plant further minimized construction
and ongoing operational costs.
Since gravity is used to draw water
through the hollow fibre membranes,
the plant does not require permeate
pumps, which saves the cost of the
pumps and associated equipment such
as valves and variable frequency
drives. Moreover, the ongoing
expense of maintaining and operating
the pumps is also eliminated. The
City has concentrated on energy efficiency
for years and is very proud of
the efficient design.
Two-stage gravity assisted process delivers 99% efficiency
The City of Greater Sudbury is constrained
by limitations on the amount
of water it can draw from Ramsey
Lake. The water taking permits allow
27,300 m3/day (7.2 MGD) on a monthly
average basis and 40,000 m3/day
(10.5 MGD) in any 24-hour period.
This limit is set by the Ontario
Ministry of the Environment to protect
the shoreline and aquatic life of the
lake. Recreational activity and aesthetic
concerns also play a part in the
plant’s water taking restrictions and it
was desirable to minimize the water
lost in the process operations.
Therefore, the 40,000 m3/day (10.5
MGD) David Street WTP incorporates
a two-stage UF membrane design that
enables the plant to produce final treated
water equal to approximately 99 per
cent of the pumped raw water. This
makes the plant one of North
America’s most sophisticated in terms
of water utilization.
At the start of the process, raw
water is sampled and then treated with
sodium hypochlorite. Four vertical turbine
pumps lift water from the lake
into inline strainers where it passes
though a 0.5 mm screen, on its way to
the first filtration stage. If the raw
water manganese levels exceed 0.2
mg/L, sodium permanganate may be
added instead of sodium hypochlorite.
In the first filtration stage, the water
passes through four process trains,
each with four cassettes, of ZeeWeed
1000 membranes, ZENON’s unsupported
hollow fibre membranes that
are specifically designed to treat water
with low concentrations of suspended
solids. With a nominal pore size of
0.02 microns, the membranes create a
physical barrier to suspended particles and pathogens in the
raw water, and produce
permeate that
consistently meets the
Ontario Drinking Water
Standards.
Filtered water, or
permeate, flows by
gravity siphon into
the surge/backpulse
tank that is located
below the primary
membrane tank. The
difference in height
between the tanks creates
the hydraulic gradient
needed to
siphon the water
through the membrane
fibres and into
the surge/backpulse
tank, which also acts
as a chlorine contact
tank. Gravity flow is also used to
transport backwash water, used during
periodic membrane cleaning, into a
reject tank.
Transfer pumps direct water from
the reject tank up to the secondary
membranes for further treatment. Two
process trains, each with four cassettes
of reinforced, vertically-oriented
ZeeWeed 500 membranes, are used at
this stage. With a nominal pore size of
0.04 microns, the reinforced structure is
ideal for the higher solids concentrations
of the reject water. Permeate from
the second stage flows to the
surge/backpulse tank by gravity siphon,
while reject water is discharged by
gravity into the City’s sanitation system.
The final plant water is treated by
chlorination and fluoridation. A corrosion
inhibitor is also added. The water
is then pumped into the distribution
system after passing through UV disinfection
units.
Automation offers efficiencies
Virtually every operational and
maintenance process in the new David
Street WTP is automatically controlled.
Only one operator is on site
during the day shift, while overnight
the plant is remotely monitored from
the water works control room at the
Wanapitei Water Treatment Plant.
Membranes are regularly cleaned
by forcing permeate water back
through them to dislodge any adhering
particles. Aeration is used to scour
debris from the fibres and provides
mixing within the process tank to
maintain solids in suspension. If membrane
fouling has reduced permeability
below specified performance levels,
chemical cleaning can be performed
in situ to restore the membranes back
to optimum permeability levels.
During this process, one train can be
taken off line for cleaning, while the
operating flux is increased in the others
to compensate.
Membrane integrity is directly
monitored by automatic pressure hold
tests. It is also indirectly monitored by
equipment that tracks turbidity and
particle count levels. In the unlikely
case that levels of either rise, an alarm
notifies the operator who then takes
appropriate action.
All three levels of government -
municipal, provincial, and federal -
came together to provide funding for
the retrofit of the David Street WTP.
Each tier contributed one third of the
cost, with Greater Sudbury covering
its share out of the capital budget
and a special reserve fund for large
projects.
Today, the David Street WTP provides
about 40 per cent of Greater
Sudbury’s daily water needs, typically
producing about 18 MGD (68,100
m3/day) of potable water. As the City
continues to grow, residents and businesses
can be assured that the investments
in clean, safe drinking water
will support the needs of this growing
city for years to come.
Paul Graham, P.Eng., is Manager of
Environmental Innovation and
Energy Initiatives, City of Greater
Sudbury;
Toby Brodkorb, P.Eng., is
Project Manager, CH2M HILL; and
Scott Lenhardt, P.Eng., is Product
Manager, ZENON Environmental
Inc.
Contact e-mail: paul.graham@greatersudbury.ca
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