Between 1973 and 1997, the Plant City, Fla., Water Reclamation Facility underwent five upgrades. However, there had not yet been any significant improvements to the biological treatment section of its 8-million-gal-per-day (mgd) wastewater treatment system. The system was not designed to meet some of the current regulatory requirements 100% of the time. In 2004, Plant City began design for an expansion of the facility. After a 2005 pilot study for a fine bubble diffuser, Environmental Dynamics Intl. (EDI) was selected to provide an aeration system set up in three oxidation ditches at the plant.
The upgraded facility is designed to treat 10 mgd with a peak flow of 27 mgd, according to Patrick Murphy, chief plant operator for the city of Plant City. Wastewater enters the facility at the headworks, where it undergoes preliminary treatment. The water then is pumped to new 130,000-gal anoxic tanks that excite nutrient-removing bacteria and trap heavier grit.
From the anoxic tanks, the water flows into three aeration basins, where four 400-hp single-stage blowers introduce dissolved oxygen to remove waste nutrients, and six mixers in each basin keep the flow pattern moving around the tank. Here, 496 fine bubble diffusers from EDI are mounted to the floor of each aeration basin, for a total of 1,488 diffusers installed. These diffusers provide vertical mixing and oxygen transfer to break down raw sewage constituents and destroy pathogenic organisms. EDI’s system was selected after the study found its diffuser tubes showed ”excellent” air distributions and profiles.
“The measurement of diffuser physical properties provides an indication of the type and magnitude of changes in the diffuser material itself that may occur during service,” Murphy said. “EDI provided three types of diffuser tubes for pilot testing: polyurethane and two EPDM formulations, EPDM 1 and EPDM 2. Both the EPDM 1 and EPDM 2 diffusers showed measurable but small changes in [their] physical properties. The polyurethane diffusers had ‘good to excellent’ air distributions, and, except for changes in permanent set, had negligible changes in [their] physical properties.”
These favorable air release profiles and air distributions mean uniformity of the diffuser’s air release, according to Murphy, as well as a more effective oxygen transfer to support the microbiology, and less likelihood of diffuser fouling.
Since the fine bubble diffuser system was installed, there have been minimal issues.
In the Aeration Basin
Mixed liquor suspended solids overflow the aeration basin through a splitter box, and ferric sulfate is introduced to induce phosphorous removal. The flow then is diverted to selected secondary clarifiers. Solids are pumped from the bottom of the clarifiers back to the aeration basins, and scum is skimmed off the top and sent back for retreatment. From the clarifiers, effluent flows to the continuous upflow filter, and immediately is measured for turbidity, followed by chlorine injection for disinfection. The effluent then passes into chlorine contact chambers.
Effluent meeting state reuse standards is pumped from the chambers into ground storage tanks for use by the facility’s reclaimed water customers. Any excess water is diverted to the East Canal after dechlorination and re-aeration. Water that does not meet reuse standards automatically is rejected into reject storage basins and re-entered into the treatment process.
The aeration system started up in 2007, and the upgraded Plant City WRF was placed into service in February 2008. The plant has been able to operate with only two of the three aeration basins running at a time. Despite the plant being designed for a flow of 10 mgd, it currently records an annual average flow of 4.7 mgd.
“Plant City’s utilities director had the foresight to have a pilot study performed to make sure the city ended up with diffusers that would work satisfactorily in our waste contributions,” Murphy said. “The training and operating manual documentation were done well and provided assurance of operating and maintenance procedures for sustainability of the equipment. The support provided from EDI and the sales representative (EnviroSales of Florida) has been superb.”