Water Supply

The City of Marco Island takes pride in providing excellent quality drinking water for its residents. The City has continued to expand and enhance its treatment capabilities, and our system provides the highest quality of water for the people of Marco Island.

Water Supply

The City of Marco Island's has four (4) water supply sources to meet the drinking water and irrigation demands of the community.  All 4 water sources are considered "alternative" water supplies .   Alternative water supplies are sustainable, nontraditional water sources, which include: stormwater, brackish ground water, reclaimed water, and Aquifer Storage & Recovery (ASR).

1. Surface/Stormwater 
   Source Water Facility (SWF) – is located just North of US-41 along Collier Blvd.  This facility is a 205-arce site that contains 2-lakes, that are connection to a stormwater collection creek (Henderson Creek).  The creek runs along the eastern part of the property.  Approximately 90% of the creek water enters the City’s lakes through sidewall infiltration between the creek and the lakes.  Additional creek water can enter the lakes through a direct connection that is controlled by a sluice gate.  Approximately, 75% of the City’s potable water comes from this water source.
    
2. Aquifer Storage and Recovery (ASR)
   During the rainy season (typically mid-June to the end of November) Henderson Creek Canal (HCC), which borders the east side of the SWF, has a high inflow of stormwater runoff. Located at the SWF is seven (7) ASR wells.  ASR wells allow the City to store fresh surface water underground, approximately 735 to 770 feet below grade.  Correspondingly, bank infiltration from the HCC into the two lakes is sufficiently high during the rainy season, which allows the withdrawal from the two lakes to simultaneously meet the consumer demand of 6-mgd and 9-mgd to fill the ASR wellfield. The maximum realistic storage rate is 13.5 mgd.

   The ASR wellfield storage zone is between two clay layers, 730 to 780 feet below ground surface and the native water has a chloride content of about 2,900 ppm.  The fresh water forms a bubble in the brackish water formation.  The stored water (with a chloride content of 70 to 150 ppm) is maintained at about 1-billion gallons in a bubble shape with a diameter of about 4,000 ft. During the dry season (December to June) the water elevation in the lakes is often low and it limits the water withdrawal rate. The additional flow (2 to 5 mgd) needed to meet consumer demand is made up by recovering water from the ASR well field.
    

3. Brackish Raw Water Supply
   The City's raw brackish water supply is treated by reverse osmonsis (RO) membranes at the City's South Water Treatment Plant (SWTP).  The facilities water is acquired by a Mid-Hawthorn aquifer brackish well field with 15 operating wells. The total capacity of the operating wells with the largest well out of service is 9.871 mgd, which is more than the 8.0 mgd needed to feed the SWTP and produce 6.0 mgd, its permitted capacity, at 75% recovery of raw water as RO permeate.  The SWTP  utilized high water pressure (350 psi) to force brackish raw water through RO membranes.  

    

4. Reclaimed Water

Marco Island Lime Treatment Plant

The plant has a permitted capacity of 6.67 mgd. All the raw water (from Marco Lakes and/or blended with raw water from the ASR wells) is treated in a vessel (called the Lime Reactor) with hydrated lime along with alum (aluminum sulfate). This forms a settled lime sludge (i.e., limestone) and clear overflow. The overflow is treated with sodium hydpochlorite and ammonia to form chloramines. Up to 5 mgd is sent to downflow sand filters for polishing and up to 1.67 mgd can go to a Zenon membrane system for polishing. The combined effluent from the sand filters and Zenon membrane system goes to the storage tanks (three 0.5 million gallon tanks). Typically about 3 to 3.5 mgd of potable water from the Lime plant is pumped from the storage tanks at the Lime Plant to the storage tanks at the RO Plant for blending with the potable water from the RO Plant and stored (in one of the three tanks at the RO Plant 5 million gallons total capacity) before being pumped to the distribution system.

Marco Island Reverse Osmosis Plant

The plant has a permitted capacity of 6.0 mgd of potable water. To produce 6.0 mgd the raw water to the plant needs between 8 to 8.2 mgd of raw water (from the 18 brackish water wells on Marco Island). Brackish water is rough filtered to remove sand. The raw water is treated with a scale inhibitor and sulfuric acid to reduce the pH. (Note, we are currently getting some baseline data and will be changing over within a few weeks to a new scale inhibitor that does not require the sulfuric acid addition,) The raw water then goes through six (6) polishing filters to remove very fine materials and is then pumped to a high pressure and sent to one (or all) of the six (6) dual stage reverse osmosis membrane trains. The reject water is pumped to the Marco Island Wastewater Plant for injection into the deep (3200 ft) well. The permeate from the membranes is treated with sodium hypochloride and ammonia to form chloramines and then contacted with air in one of two (2) degasifiers to remove hydrogen sulfide. The water is then sent to the storage tanks and the offgas from the degasifiers goes to a scrubber to remove the hydrogren sulfide from the air before it is vented into the atmosphere.