Although the effluent met the contract specifications, fluctuations in the water temperature and the amount and intensity of daylight affected algae concentrations in the pond. Since no one thought algae could survive in the low-pH pond water, this problem was unexpected. A seepage ditch along the edge of the gypsum stack contained less algae since it was not subject to sunlight; it also had lower turbidity. Modifications were made to the feed piping and ditches so the system could take feed water from this ditch.
To reduce the rate at which submicron-sized algae fouled the RO membranes, the 5.0 micron prefilters were replaced with 1.0 micron cartridges. Another filtration system was then placed upstream to decrease the cost of changing these cartridge filters. Continuous microfiltration (CMF) technology from USFilter and ballasted flocculation clarification technology from Krüger Inc., Cary, N.C., were chosen and a three-week pilot study was conducted. The membranes, however, experienced some permanent fouling, even after a clean-in-place procedure was performed. Next, a ballasted flocculation clarifier was tested using hypochlorite at dosing rates between 2.5 mg/l and 4.0 mg/l to kill the algae, and bentonite to adsorb/destabilize the suspended particles. This process chemistry proved to be the solution for removing submicron algae.
Move to close
Steady state operations were achieved after nearly one full year of operation. During this time, two Actiflo clarifiers, also from Krüger, were installed, and the contractor patched, relined and re-bedded the media filters, and added additional RO capacity. From July 2003 to May 2004, the system produced an average of more than 490 gpm; once the improvements were made, the system was often able to process 550 gpm.
As the volume of free water in the ponds decreased during the latter months of Phase III, the RO concentrate began to impact the ponds. The pH, turbidity and algae increased, requiring daily feed water testing so adjustments could be made to the pretreatment upstream of the clarifiers.
In November 2003, the site moved from emergency-response mode to closure mode. Individual ponds or cells would be closed by lining them with a high-density polyethylene (HDPE) liner.
The RO system effluent met the contract specifications and demonstrated reliable performance, while the mobile equipment allowed flexibility for expansion. At Piney Point, RO technology to-date has treated more than 400 million gal. of process water, ultimately discharged to Tampa Bay. The ponds will be lined this year and the remaining free and interstitial water will be treated and disposed of using the RO system and the lime precipitation equipment.
As the lined ponds collect rainwater, the Piney Point site will help solve the Tampa Bay area’s water supply shortage. A site that once threatened disaster for a coastal estuary will be transformed into a reservoir that can hold 1.2 billion gal. of fresh water.
Bill Perpich, Jr., is a Tampa, Fla.-based industrial sales engineer for USFilter. He has 20 years of experience in the water treatment, groundwater remediation and solid/hazardous waste treatment industries. He has authored and presented several papers on groundwater treatment. E-mail him at PerpichB@usfilter.com.