Quick-Change Artist

The team at a 120 mgd plant in Tampa relies on a rapid settling process, experience, and automation to produce consistent results from variable raw water sources.
Quick-Change Artist
These ozone generators (Fuji Electric) are an integral part of the plant’s primary disinfection process.

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As big as it is, the 120 mgd Regional Surface Water Treatment Plant is just part of the supply network for Tampa Bay Water, a wholesale utility serving three counties and three cities on the Gulf side of Florida.

The plant’s operating team has to cope with changeable raw-water quality, and with changes in volume dictated by Tampa Bay Water’s comprehensive strategy for supplying 2.4 million residents in its member jurisdictions.

The plant draws raw water from four sources: The 15.5-billion-gallon C.W. Bill Young Regional Reservoir, the Alafia River, and two pools in the Tampa Bypass Canal. Helping to keep the finished water supply consistent in quality is a rapid sedimentation process aided by the addition of microsand in the settling basins.

The team also relies on extensive automation to meet the En-hanced Treated Water standards in the plant’s operating contract. Veolia Water built the plant in 2002, expanded it in 2010, and operates it under a 20-year agreement, according to Mike Kuhn, P.E., Veolia area manager.

Critical need

Tampa Bay Water was formed in 1998 to provide single-source water-supply responsibility for its member governments. At the time, groundwater was the only source of drinking water, and heavy withdrawals were affecting lakes and wetlands. As part of an alternate supply plan, the agency built the reservoir, a seawater desalination plant and the surface water plant to supplement its wellfields.

Water from all these sources is now wheeled around the service territory, the mix of sources depending on cost, seasonal availability and other factors. The surface water plant, designed by the CDM Smith engineering firm, was built with a 66 mgd capacity and expanded in 2010 — with new trains of the same treatment processes and additional solids handling capacity — to its current 120 mgd capacity. The upgrade also added two 1,250 kW emergency generators (Caterpillar) able to power the plant at the 66 mgd rate.

At the heart of the facility is the Actiflo high-rate clarifier process from Veolia Water Solutions & Technologies. In this process, water is flocculated with ferric sulfate, microsand and polymer. The microsand enhances formation of robust flocs and acts as ballast, significantly reducing settling time. “We achieved significant capital savings because the Actiflo process generally has a footprint about one-fifteenth the size of a conventional settling process with similar capacity,” says Kuhn.

Water enters the plant from a combination of its four sources as dictated by Tampa Bay Water and fills two raw-water tanks with 12.5 million gallons total capacity. “In the pipe on the way to Actiflo, we inject sulfuric acid for pH adjustment to 4.0 to 4.5, which is optimum for the enhanced coagulation process. In the Actiflo basin, we can trim the pH with acid if we need to by way of a lower-capacity injection point.”

In the basin, ferric sulfate coagulant is added rather than alum. “Our source water tends to be higher in color and TOC more so than suspended materials,” says Kuhn. “We need to remove precursors for disinfection byproducts (DBPs), which means we need to knock down the TOC in the Actiflo. Our jar testing showed that ferric was much more effective at TOC removal than alum.

“We also have stipulation in our contract to produce a sludge that can be beneficially used. All the sludge we produce goes to Vigiron, our vendor, who uses it to make proprietary iron micronutrients, and potting soil materials. Their products are used by golf courses, sod producers, citrus orchards, tree farms and other farming applications. They are also incorporated in fertilizer mixes sold by fertilizer manufacturers at consumer outlets.”

The sludge is dewatered to about 18 percent solids on six belt presses, four J-Belt units (Siemens) and two Alfa Laval / Ashbrook Simon-Hartley units.

On downstream

After the Actiflo process, the water enters a basin where the pH is adjusted to about 5.5 by addition of lime (with caustic soda as a backup). The water is then fed into ozone contactors equipped with Fuji ozone generators for primary disinfection. “We need tight pH control in the ozone contactors to avoid formation of bromate,” Kuhn reports.

Another pH adjustment to 7.3 follows. The flow then passes to biological downflow filters with a top 4-foot layer of granular activated carbon (top-off material supplied by PICA, a Jacobi Carbons Company), a 6-inch layer of sand, and Leopold underdrains.

“The filters have two purposes,” says Kuhn. “The first is traditional straining to remove turbidity caused by lime addition. The second is chemical and biological action. TOCs remaining after the Actiflo process are fractured by the ozone treatment. At that point you create the opportunity for organics that can be combined with chlorine downstream to form DBPs.

“Instead, we allow the bugs that live on the activated carbon to eat those organics. The bugs are very happy in the warm climate of Florida. They’ve stayed healthy since we began operating the filters back in 2002. We have not seen any decline in the microbe populations or in the effectiveness of the treatment.”

Filtered water passes through a set of clearwells and then into a 2-million-gallon covered chlorine contact chamber, where pH is adjusted with caustic soda and sodium hypochlorite is added for chlorine residual. From there, Tampa Bay Water adds ammonia to form chloramines, and the water then enters a set of three 7.5-million-gallon finished water storage tanks for delivery to the water company’s network.

Dealing with variation

The plant’s 29-member operation, maintenance and lab team deals often with changes in source water quality and quantity. Depending on Tampa Bay Water’s needs, the plant may produce 50 million to 80 million gallons on a given day, and from a different mix of sources. Seasonal events affect the water composition.

“In winter we see water that looks much like groundwater,” says Kuhn. “The TOC is lower, but it has higher hardness. In summer, we see more color and more organics in the water, especially after a rain. Suppose we’re treating mostly Alafia River water and we just had a big rain and a whole lot of organics wash into our basins. That’s a tremendous difference in a short time in what the water looks like.

“That’s another argument for Actiflo. We need to react quickly to chemistry changes and quantity changes. Actiflo gets us there. It has a very short detention time, we can make chemical adjustments very quickly, and we can bring more basins online in a short time — generally a couple of hours. It’s a very resilient process.”

The primary process adjustment is in the ferric sulfate dosing, which relates mainly to the raw water TOC. Operators stay abreast by analyzing process control samples 24 hours a day. In addition, Tampa Bay Water keeps the plant apprised of changes in volume and the mix of sources, generally a week or so in advance.

“We have enough history to know in general what will happen if they switch to a certain balance of sources at certain times of year, or if there has been a rain recently,” says Kuhn. “We have what is likely the most automated surface water plant in the U.S., but operators still have to use the art of operation for any given water that’s coming in to achieve maximum efficiency in the chemical use.

“For example, right at the operating console we have a chart that tells us the general target ferric dose for a given TOC in the raw water. Our operators, using the process control lab, then can really nail down what the best dose is for that particular water.” Operators enter the correct dose into the plant SCADA system (Intouch/Wonderware), and the chemical feed system (designed by CDM Smith) feeds that dose.

The plant is monitored around the clock by a lead operator at one of three control panels in the operations room. A second operator makes rounds and does process w sampling. Maintenance and administrative staff generally work the day shift.

Life cycle savings

Kuhn notes that the facility was created under a design-build-operate (DBO) model that is projected to save Tampa Bay Water some $85 million over a 20-year life cycle. The savings result partly from the Actiflo process and its small footprint, saving significantly on land and construction costs, but also from multiple decisions made with an eye toward the long term.

“We looked at this not just as a construction project but as an operating entity,” says Kuhn. “It doesn’t make sense to save $1 today on the capital cost of a project if instead you could spend $2 and as a result save $5 on operations over the project life. A DBO team looks at the full picture because they have skin in the game all the way along.

“We’ve got guarantees on this facility. We guarantee power use, chemical use, and the quantities and quality of the water. So we had to be competitive to win the project, and we have to be smart about how we operate it over the long term.”


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