Problem: The Boca Raton (Fla.) Wastewater Treatment Plant needed to meet increasing population demands while maintaining a high level of treatment.
Solution: The plant installed a continuous-backwash sand-filtration system to increase reuse water and treat water to below discharge limits.
For 85 years, the City of Boca Raton, Fla., maintained contaminant levels well below federal wastewater treatment discharge standards, protecting aquifer recharge sites and the Florida Everglades from contamination. But with growing populations, demand has exceeded the capacity of the treatment plant and threatened its ability to maintain a high level of treatment.
To continue providing a high level of service, the city decided to expand its wastewater treatment plant. In 2009, the city began working with Eckler Engineering Inc. (Coral Springs, Fla.) on plans for the expansion.
In anticipation of future regulations and to protect the environment, the plant was designed to consistently discharge effluent with turbidity measuring less than 1 nephelometric turbidity unit (NTU) — the statewide standard required less than 2.2 NTU.
For its tertiary treatment processes, the plant had been using aeration and clarification followed by 24 continuous-backwash sand filters. Because these filters had operated easily and performed well, they became the basis of design for the plant expansion, said Jeremy Rasband, an applications engineer at WesTech (Salt Lake City).
Installing a new technology
As a parallel to the existing sand filters, the city and its engineers decided to install 32 continuous-backwash sand-filter modules provided by WesTech. In these up-flow moving-bed filters, secondary clarifier effluent enters near the bottom, and solids are filtered as the water flows up through the media bed. As the filtrate reaches the top, it passes over the effluent weir and leaves the tank. A portion of the filtrate is diverted through the sand washer and used for cleaning and conveying the waste solids.
The filters can be installed in a multimodule sand-bed configuration or as a freestanding unit. Because of the high flow at Boca Raton, the typical sand-bed configuration was the most efficient design, Rasband said.
Boca Raton received a total of eight concrete basins with four modules per basin to handle a maximum daily flow of 26,300 L/min (6950 gal/min) at a filtration rate of 179 L/m2•min (4.4 gal/ft2•min).
The filters provide uninterrupted flow due to the continuous backwash. This eliminates the need for large backwash holding tanks and backwash pumps. The system was designed to provide a minimal wastestream for disposal or further treatment, Rasband said.
System exceeds expectations
The system installed at the plant removes 96% of total suspended solids and 98% of carbonaceous biochemical oxygen demand, on average, said Eddie Catalano, an operator at the Boca Raton plant.
The filter units have proven to be effective, averaging a turbidity concentration of about 0.4 NTU without the addition of polymer. Once a month, 5 mg/L of polymer is dosed, and the plant achieves turbidity as low as 0.3 NTU, Catalano said. Throughout the past year, WesTech has made quarterly visits to optimize the filters and ensure that they are performing well.
During a full-scale performance test, the filter influent was spiked to artificially increase the influent turbidity to 10 NTU. The continuous-backwash filters consistently reduced the concentrations to less than 1 NTU, Rasband said.
With the new filters, Boca Raton has been able to increase the amount of reuse water produced and treat water to well below its discharge limits, Rasband said.
“[Boca Raton is] very happy with the performance and service,” Catalano said.
The continuous backwash filters have enabled Boca Raton to exceed its goal of protecting nearby aquifers and the Florida Everglades.
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