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Islamorada Waste Water Treatment Plant in Plantation Key, Fla. was historically serviced by a traditional septic tank system. However, the facility is on a coral island that does not have the capacity to handle a high amount of sewage due to little soil and very porous underlying rock. The septic system in place was contaminating surrounding waters, which led to algal blooms and fecal coliform contamination in some areas, restricting recreational use of the water.

The island began the process of determining a better treatment technology that would meet the needs of the island and also comply with the strict design guidelines. The island proceeded to obtain funding to build a communal system to eliminate the septic tanks over time.

The plant was constructed as a design-build project, with various teams vying for the contract. The proposal selection was largely based on the merits of the Enviroquip MBR technology .
The MBR system is capable of meeting the strict discharge limits within a small footprint, reducing the visibility of the treatment plant. Ease of operation was an important deciding factor for the island, as the plant would be maintained on an intermittent basis by a contract operator.

The treated effluent is delivered to either a reuse storage tank or a local aquifer. When the effluent is for reuse, it passes through ultraviolet (UV) disinfection and then irrigates baseball fields and other green spaces in the community. This additional step is not necessary when the effluent is injected into the well.

The Process

Raw wastewater from a vacuum collection system is pumped to the plant headworks, passes through a coarse screen and flows into the equalization (EQ) tanks. Wastewater is pumped from the EQ tanks through fine screens into the MBR process at the pre-anoxic (PRAX) tanks, where it is mixed with recycled activated sludge.

In the absence of dissolved oxygen, microorganisms convert nitrate to nitrogen gas in a process known as denitrification. Meanwhile, BOD is consumed to synthesize cell mass. The denitrified wastewater then flows to the SymBio basins by gravity.

In the SymBio basin, fine-bubble aeration is used to provide oxygen for nitrification to converting ammonium to nitrate. A portion of the activated sludge then flows by gravity into the post-anoxic (POAX) basins for denitrification. The remainder of the flow is pumped back into the PRAX basins for denitrification, completing one internal nitrate recycle stream.

From the POAX tanks, mixed liquor then flows by gravity to the MBRs, where biological treatment is completed, and the solids are separated from the clean water. In each MBR, integral diffusers furnish air for cleaning and mixing requirements. The added air also supplements the oxygen supplied in the SymBio basins for biological treatment. Thickened mixed liquor from the MBRs is pumped back to the PRAX basins to complete a second internal recycle loop. This serves to stabilize mixed-liquor concentrations and enhance denitrification.

The plant has been in operation since July 2006. It has proven to be a viable solution to Islamorada’s needs.

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