Denitrification System Helps Florida Plant Win Award

The Fiesta Village Advanced Wastewater Treatment (AWT) Plant in Lee County, FL, has received the 2006 Earle B.

The Fiesta Village Advanced Wastewater Treatment (AWT) Plant in Lee County, FL, has received the 2006 Earle B. Phelps Award for Best AWT facility in the state. Advance wastewater treatment plants with Tetra filters for denitrification have won the Phelps Award in the last nine of 13 years.

The Earle B. Phelps Award, established in 1964, honors outstanding treatment plants in Florida that have maintained the highest removal of pollutants. Awards are given in three categories: advanced treatment; advanced secondary facilities; and secondary treatment facilities. Plant size also is considered, with nominees being evaluated against facilities of the same relative size and treatment type.

Established in 1985, Fiesta Village is a 5 mgd advanced activated sludge wastewater treatment plant. The facility processes wastewater for the South Fort Myers and Cypress Lakes areas, producing quality effluent for reuse. The reuse system consists of approximately three miles of distribution mains that provide irrigation for golf courses and other entities. If reuse water is not needed, the plant is permitted to discharge to the Caloosahatchee River.

The Fiesta Village plant uses an oxidation ditch activated sludge process that incorporates nitrification/denitrification zones and phosphorus removal. Biological denitrification of wastewater is accomplished either by a fixed-film or a suspended growth process. The Tetra Denite® system used at Fiesta Village is a fixed-film biological denitrification process and a deep bed filtration system capable of removing suspended solids (SS) and nitrate-nitrogen (NO3-N). It is used as the final treatment step in the total nitrogen removal process to help the facility meet stringent TN discharge limits of 3 mg/liter.

The biological denitrification process, available from Severn Trent, requires one-tenth of the space used with suspended growth systems, greatly facilitating expansion or retrofitting requirements. With Denite, the denitrification process and the filtration process are combined in a single system, offering excellent process synergy. NO3-N is converted to nitrogen gas and captured within the media bed along with suspended solids and biomass formed from the denitrification reaction. The gravity filter system operates in a downflow mode to maintain excellent suspended solids removal without additional effluent polishing filters.

The specially sized and shaped granular media used in fixed-film biological reactors is an excellent support medium for denitrifying bacteria and the deep bed environment is conducive to efficient NO3-N and solids removal. The surface area of the 2-3 mm sand is high, 300 square feet per cubic foot. A six-foot media depth prevents short-circuiting and premature solids breakthrough. The contact between wastewater and biomass is excellent and hydraulic short-circuiting is negligible even during plant upsets.

The media allows for heavy capture of solids, at least 1.0 pound of solids per square foot of filter surface area before backwashing is required. The high solids capture permits extended operating periods and easily handles peak flow or plant upsets.

As solids are captured, increasing the head loss in the filters, a backwash is required to remove the solids. Because of the heavy loading capacity and media depth, deep bed vessels require heavy-duty backwashing. Concurrent backwash air and water are used during the backwash cycle. The solids slurry removed from the filter media is typically returned to the upstream biological process. Due to the filters’ high solids loading capacity, the percent of return is less than 4% (<2% typical) of the plant’s forward flow.

During the denitrification reaction, nitrogen gas accumulates in the media bed and wastewater is forced to flow around the gas bubbles in the media voids. This reduces the filter area while the size of the media improves the biomass contact and filtration efficiency. The effect of the gas bubbles increases head loss and requires periodic removal in between backwashes. Removing a reactor from service and applying backwash water for a short period of time accomplish this. This nitrogen release cycle, or bump, releases the entrapped nitrogen gas into the atmosphere, reducing the head loss. The Tetra SpeedBump® technology is used to conduct a complete system bump cycle without stopping flow to the reactors.

Conclusion

As more facilities look toward using the effluent being produced from their wastewater treatment plants as a source of reclaimed water or a supply to put back into the natural water cycle, it becomes important that their tertiary treatment is capable of meeting the strict discharge limits required in either situation. Currently, 35 percent of all Florida reuse water goes through a Tetra filter.

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