When the City of Tarpon Springs, Florida, decided to build its own water treatment plant, it selected reverse osmosis.

The city had purchased water from Pinellas County but wanted an independent supply, says Ronald Claunch, the RO plant’s lead operator. With its own plant, the city can set prices and capital budgets that allow for future development, says Earl Nash, water division manager.

Construction began in 2013, and the plant opened in 2015. CDM Smith created a 30% design, Tetra Tech handled the remainder of the design, and Wharton-Smith was the general contractor. The design-build approach enabled process adjustments based on communication among the city team, designer and contractor.

Their work earned two Outstanding Water Treatment Plant awards from the Florida Section AWWA, in 2016 and 2018. Claunch earned a 2019 Outstanding Operator Award from the Southeast Desalting Association, and Kelly Frazier, chief operator, won that award in 2017. “We’re constantly looking for a better way to get our work done,” Frazier says.

The team is proud of the city’s well management program, the caustic system installation and the plant in general.

Brackish challenge

Tarpon Springs, on the west coast of Florida, has a population of about 25,500. It’s at the north end of Pinellas County (population 975,000), which includes St. Petersburg. Because Tarpon Springs is a coastal city, the water in its aquifer is naturally brackish. RO was the best technology to make that water useable, and the regulatory environment probably would have pushed the city to RO in any case.

In the near term, the city wants to drill more wells. “We started with the wells necessary to start the plant,” Nash says. “The RO system can produce more water than we are able to feed it at the moment, and that’s for our future development.”

Because the aquifer is of poor quality and high conductivity, Claunch observes, more wells would spread raw water demand over a larger wellfield and provide the flexibility to take wells out of service for maintenance.

The operators’ work includes well rehabilitation as part of a well management program that began in 2018 when capacity in some wells began to decrease. Maintenance and instrumentation staff members handle basic rehabilitation with a 12% hydrochloric acid that is pushed into the well and allowed to sit for 24 hours.

“Florida is basically lime rock, and acid helps open some of the rock formation to get more water to the well,” Frazier says. Operators perform bleaching to reduce iron and bacteria.

Aggressive water

A sodium hydroxide system was installed in 2017 to give better alkalinity control. “One of the challenges with RO is that after water goes through, it’s aggressive; it has very little hardness in it or none at all,” Nash says. “We have to put some back in.”

When the plant opened, operators did that with calcium hydroxide and carbon dioxide. Too much calcium hydroxide creates turbidity, yet calcium hydroxide is also cheap and is now used for large adjustments. Operators use sodium hydroxide to make fine adjustments and to control turbidity.

Now in the design phase is the addition of solar panels to offset some of the plant’s power needs. In case the power grid fails, the plant has a 2.5 MW diesel generator (Caterpillar Inc., Electric Power Division) that can power the RO system and some of the wells on the plant site. Ten portable generators (Blue Star) can be moved to remote well sites to keep pumps operating, and more portables are available from the city if the need arises. “We’re vulnerable to hurricane season, and we want to be prepared,” Claunch says.

The people who make the facility work besides those already mentioned are:

Ray Page, utilities superintendent

Michele Koziol, water division coordinator

James Garner, lead operator; John Giordono, Jacqueline Douglas, Lowell Quarterman and Mark Van Der Horst, operators; and James Schaff, operator trainee

Cassandra Arter, compliance officer

Justin Economos, maintenance technician, and Jeff Geary, instrumentation technician

Designed for the future

Raw water comes into the plant through a 20-inch pipe fed by 19 wells: some on site and the most distant one about 3 miles away. Water first flows through 5-micron spiral-wound cartridge filters (Tri-Dim) in housings from Fluytec. There are four housings with 150 filters in each. This step traps large debris that would clog the membranes. At this stage, pH is adjusted with sulfuric acid and an anti-scalant (American Water Chemicals) to keep iron in solution.

Next, Flowserve pumps send water through the RO filters (Harn R/O Systems with DuPont Water Solutions membranes). Groups of filters are assembled into skids, of which there are three. Each skid has a two-stage filter arrangement. The first stage is 42 pressure vessels with seven membrane cartridges in each; the second stage is 21 pressure vessels, each also with seven membrane cartridges.

One skid is always offline for rotation, while one or two operate depending on tank levels and wellfield management. Treated water goes to a forced-air degasification tower (Jacobs Air Water Systems) to remove hydrogen sulfide, which is common in Florida groundwater.

Next in line are biological scrubbers (Jacobs Air Water Systems) that use bacteria on a growth media to break down the hydrogen sulfide. These are not in use at the moment because while there is enough hydrogen sulfide to require degasification, there isn’t so much that it must be scrubbed before release, Nash says.

Biological scrubbers are more economical to operate long-term than a chemical-based system, Claunch says. If future wells contain enough hydrogen to require scrubbing, the system would be seeded with bacteria just as a wastewater system is.

From the degassing towers, water flows to the clearwell. The first section is a contact chamber for chlorination, and the second section is for introduction of conditioning chemicals. An on-site, 5 million-gallon storage tank is designed as a tank within a tank: It has two concentric circular tanks of 2.5 million gallons each that can be operated independently if needed, so one can be taken offline for maintenance.

Brine waste (concentrate) from the RO process is sent to a 1,200-foot-deep injection well offsite. Water in that deep aquifer is of similar conductivity and is so deep that private wells in the area cannot tap it.

A better system

The Tarpons Springs plant is the first RO facility that Claunch has operated, and he prefers it to other types. It’s also more challenging; many groundwater treatment facilities don’t have to do as much to their raw water, Nash says.

Frazier observes, “We spend a lot of time on sampling events. In the beginning, one of the challenges was dosing lime appropriately.” There was also the challenge of learning a new system and the day-to-day challenge when more well pumps are switched on to fill storage tanks. Each well has water with its own characteristics, so changing the raw water feed changes the conductivity. That affects pressure in the system, which affects everything in the plant.

Nash notes, “This plant was totally new for the city, and all the team members have done a really good job from startup to where we are, with day-to-day learning and improving as we go.”