Advanced Surface Water Treatment Plant's Enhanced Coagulation Treatment Process Requires Tough Protective Coatings

Coatings

By the late 1990s, Florida's largest wholesale water
supplier, Tampa Bay Water (the Utility), was facing a quandary all too familiar
to managers of the nation's drinking water infrastructure. The contract
creating Tampa Bay Water required that the utility replace 68 million gallons
per day (mgd) of groundwater with new,
alternative sources. Water usage in the tri-county region (Hillsborough,
Pasco and Pinellas counties and the cities of Tampa, St. Petersburg and New
Port Richey) had soared from 177.7 mgd in 1980 to 218.5 mgd in 1995. By 1998
that number had jumped to 232.36 mgd, and was projected to continue rising as
the region grew. The Utility had to develop new sources to replace the
groundwater lost to permit reductions and to meet the region's continuing
growth.

For years, the Tampa Bay region relied on groundwater to
meet its drinking water needs. However, in the early 1990s, the region was
required by contractual mandate to reduce groundwater pumping at 11
long-producing wellfields in order to help the surrounding environment rest and
recover. New non-groundwater sources were needed.

After years of studying water supply options, the Utility
developed a $610 million Master Water Plan blueprint to meet the long-term
drinking water needs for more than two million customers in the Tampa Bay
region. In 1998, the first configuration of projects was approved. A key
component of the Master Water Plan's first phase was a strategy to deliver
surface water. Therefore, a surface water treatment plant (SWTP) was proposed.

In March 2000, the Utility entered into a $144 million,
15-year design/build/operate (DBO) agreement with a team led by USFilter
Operating Services, Houston, to carry out the design and construction of the
new state-of-the-art surface water treatment plant and to provide for its
day-to-day operation. Owned by regional utility Tampa Bay Water, the plant is
among the most technologically sophisticated in the world.

The Utility's first surface water plant also is the nation's
largest DBO water project. The new 66-mgd regional treatment plant began
operation in September 2002. Located on 435 acres in Hillsborough County, the
SWTP operates 24 hours a day, seven days a week. Tampa Bay Water officials
estimate that this public/private partnership over the 15-year term of the
contract will save the region an estimated $80 million.

Role of High-Performance Coatings in Plant Design

USFilter turned to the global, full-service consulting,
engineering and construction firm of CDM, Tampa, Fla., as its design partner.
Because of the accelerated schedule for the project, the CDM design team was
required to produce complete water treatment plant design drawings for
submittal to permitting agencies in less than six months.

From the notice to proceed, the SWTP was up and running in
about 28 months, including design, permitting and construction, according to
Project Manager and CDM Vice President Richard D. Moore, P.E.

"That's about half of the time to implement a
conventional design/build/operate project of this magnitude," Moore said.

CDM design engineers faced the formidable challenge of
protecting diverse plant operation surfaces and substrates spread over 20
acres, including carbon steel, concrete (both pre-cast and poured-in-place),
concrete masonry units (CMU), ductile iron and alloy steel.

During construction, CDM design engineers turned to Tnemec Company,
Inc., Kansas City, Mo., for answers to this challenge. Because of the
sophisticated water treatment technologies designed for the plant, Moore noted
that the design team looked for the most reliable high-performance coatings
systems for the 50,000 square feet of the plant's multiple surfaces.

Technological Sophistication

Plans called for the use of the high rate ballasted Actiflo
flocculation process. This process is particularly advantageous when treating
large flow rates with variable raw water quality--the conditions anticipated
for the regional water treatment plant. The facility treats water from the
Hillsborough and Alafia rivers as well as the Tampa Bypass Canal to standards
that exceed the current EPA Safe Drinking Water Act requirements for potable
water.

It is the largest Actiflo plant for municipal water in the
United States. The plant also utilizes ozone as the primary disinfectant and
granular activated carbon as a medium for the biological filtration, providing
advanced treatment.

The process uses sulfuric acid to lower the pH of the raw
water and ferric sulfate to coagulate the water. Fine grain sand and polymer
are added to the water in the injection tank (See diagram at left).

The water then moves into the maturation tank for further
mixing that allows the sand and flocculant particles (floc) to join together
before passing into the settling tank. In the settling tank, the floc and raw
water are completely separated. The sand's weight forces the floc to fall to
the bottom, allowing the treated water to rise to the top for further
treatment. Lamella tube settlers are used to improve the separation. At this
point, the pH level of the water is lower than normal. Lime is added to help
adjust the water's pH level back to normal and prepare the water for
disinfection.

Ozone gas then is added as the primary disinfectant. It
destroys the microorganisms including bacteria, viruses and protozoa that may
be left in the water. Ozone also oxidizes any remaining organic particles that
may be left in the water.

After disinfecting the water, the pH is raised again with
lime to prepare it for the filtration process. The water moves from the ozone
tank to the biologically active filtration area. During the biologically active
filtration process, "good bacteria" oxidize the remaining organic
molecules. Layers of sand and granular activated carbon then filter out any
remaining particles.

After the biologically active filtration process, the water
moves to a storage area (or clearwell) for a second disinfection process.
Chloramine is added to maintain the residual chlorine in the distribution
process and to ensure that the water supply remains free of bacteria.

After leaving the surface water plant, the water moves to a
blending facility where it is merged with treated water from the nearby
groundwater plant. Then, the combined water is piped to Tampa Bay Water's
member governments.

The plant is fully automated. At the control station,
changes in water quality and any changes in the process are monitored continuously.
If a problem occurs, an alarm will sound, identifying the problem area. The
automated controls allow the operators to react quickly and make any necessary
changes.

Coating Solutions

Tnemec consultants specified four categories of coatings
systems needed to protect the wide variety of substrates throughout the
treatment plant, including protective linings, exterior masonry and steel
coatings, corrosion resistant coatings and floor coatings and overlayment
systems.

Protective linings included novolac epoxies, vinyl esters,
amine epoxies, elastomeric aromatic urethanes and modified aliphatic amine
epoxy mortars. These coatings were specified for the interior of the settling,
maturation, coagulation and injection tanks, and on submerged concrete in the
gravity thickener tank, recycle surge basin and influent box. They were
selected to resist corrosive industrial and municipal wastewater and hydrogen
sulfide gas permeation as well as abrasion, cavitation and chemical exposure.

Exterior masonry and steel coatings included zinc-rich
primers, epoxies, aliphatic urethanes, acrylics, water-based acrylics, saline
water repellants, sealers and stains. These were specified for all piping and
pumps, exterior concrete, CMU, containment areas and in the maintenance and
storage buildings. These coatings and water repellants were chosen because of
their ability to resist atmospheric corrosion in a coastal environment and to
improve the long-term aesthetics of the facility.

Corrosion resistant coatings included polyamines,
amido-amines and cyclo-aliphatic epoxies as well as fiber-reinforced polymers.
These were specified for all interior and exterior pipe galleries, filters,
submerged steel, walls and ceilings of the operations building. These
corrosion-resistant coatings were required to protect against the numerous
processing chemicals used in this facility.

Floor coatings and overlayments included resinous floor
epoxies applied to the maintenance area, chemical storage area, gravity
thickener room, control rooms and bathrooms. These coatings and overlayments
were selected to decrease maintenance costs, provide slip-resistant surfaces,
increase cleanability and provide a pleasing work environment.

Throughout the plant, wherever there were piping and valves,
a tough polyurethane overcoat for UV protection was specified. Because of the
many colors available, Tampa Bay Water was able to coat the piping
infrastructure with seven colors, one for each process in the treatment cycle.

"Because of the plant's enhanced coagulation treatment
process, it was extremely important to have the right coatings to protect the
plant's extensive concrete and steel substrates against this low pH water
environment, which is aggressive and borderline corrosive," USFilter Plant
Manager David Hackworth said.

Therefore, a great diversity of coatings was needed. Besides
chemical and corrosion resistance, the coatings needed to adhere to both the
steel and concrete substrates immersed in potable water.

"The integrity and excellence of coating systems is
critical to the efficient operation of this state-of-the-art, sophisticated
facility," Hackworth said. "It's one more thing we don't have to
worry about."

Lake H. Barrett, Jr., is the director of water and wastewater operations at Tnemec in Kansas City, Mo. He holds a Bachelor's degree from Penn State University and is pursuing a Master's degree in management. He is an active member of SSAC, NACE, ASME, AWWA and WEF.

Leave A Comment

  • Web page addresses and e-mail addresses turn into links automatically.
  • Allowed HTML tags: <a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>
  • Lines and paragraphs break automatically.

More information about formatting options

By submitting this form, you accept the Mollom privacy policy.