Process Page: The Water Independence

Matt Tebow

The Water Independence for Cape Coral program was started in the late 1980s and was designed to reduce the impact that irrigation has on the Mid Hawthorne Aquifer, one of the City of Cape Coral’s main sources for drinking water supply. The city has been recognized as having one of the largest municipal residential irrigation demands in the United States, with a daily average of 31.42 million gallons per day (mgd), according to the annual reuse report for Fiscal Year 2019.

The city’s irrigation is supplied by treated wastewater from its two wastewater facilities, Southwest Water Reclamation Facility (WRF) and Everest WRF, and supplemented by freshwater canal water pumped from the city’s five freshwater canal pumping stations.

The Southwest WRF was originally constructed in 1992 using a carrousel-style oxidation ditch to treat an annual average daily flow (AADF) of 6.6 mgd. The Southwest WRF was expanded in 2008 to 15 mgd AADF and currently treats approximately 7 to 8.5 mgd seasonally.

The headworks consists of two mechanical step screens and one manual bar screen, grit removal provided by four stacked trays, vortextype grit removal units, odor control, and two cyclone/classifier units, followed by a common influent mixing channel. The secondary treatment (activated sludge) process utilizes the anaerobic-anoxic-oxic (A2O) process, including anaerobic, anoxic, and aerobic zones.

Although not a regulatory requirement, the A2O process is capable of biological removal of both nitrogen and phosphorus. Three A2O process basins receive flow from the common influent mixing channel, with each basin having an anaerobic zone followed by an anoxic zone, and then followed by an aerobic zone. Each basin has four floating mixers, with two mixed liquor internal recycle pumps, which pump from the end of the aerobic zone to the beginning of the anoxic zone. Mixed liquor from the three basins flows to a common reaeration channel; then, the combined flow is split and gravity-fed to five secondary clarifiers (two at a 100-feet diameter and three at a 120feet diameter) for settling.

The sludge pump station, with multiple return activated sludge (RAS) and waste activated sludge (WAS) pumps, discharges to the common influent mixing channel or sludge holding tanks, respectively. Effluent from the secondary clarifiers is gravity-fed to two sets of effluent automatic backwash (traveling bridge) filters. The effluent from the backwash filters then flows by gravity and is split between two chlorine contact chambers (CCC). High-level disinfection is provided by liquid sodium hypochlorite.

The reclaimed water is then stored in three 5-million-gallon (MG) tanks or, due to wet weather or low demand, pumped into the deep injection well. The WRF provides reclaimed water to the city’s service area system, also known as the Water Independence for Cape Coral system, to supplement with potable water irrigation. The WRF includes a 6.8-MG reject storage tank, where substandard reclaimed water may be directed. Biosolids processing includes the carrousel-style oxidation ditches repurposed into four aerated sludge holding tanks and three centrifuges.

The WRF is unique since it receives WAS from the city’s Everest WRF (13.4 mgd). This creates operational challenges due to the higher solids loading and increased nutrient loading from the centrifuge centrate (Everest and Southwest WAS).

The original WRF design philosophy considered total nitrogen (TN) reduction to less than 10 mg/L as N. Although not required for discharge to the reclaimed water system, the Southwest WRF historically struggled to consistently meet the original design intent of effluent TN less than 10 mg/L, while maintaining a stable process with the increased solid and nutrient loadings from the Everest WAS.

Beginning in 2018, Matthew Astorino, the