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The City of Marco Island takes pride in providing excellent quality drinking water for its residents. City has continued to expand and enhance its treatment capabilities, and we feel our system provides the highest quality of water for the people of Marco Island.
Here is a summary of the drinking water treatment.
Aquifer Storage Wells for Raw Water Supply to Lime Plant and Irrigation
We have six (6) ASR wells with a total influent and storage capacity of 1.5 million gallons per day (mgd) nine (9) million gallons per day (mgd). The average zone for storing the water is from 735 to 770 feet below grade. Before raw water from Marco Lakes is pumped into the ASR wells it is filtered in one of six (6) dual media (sand and charcoal) pressure filters and treated with sodium hypochlorite and ammonia to form chloramines (disinfectant). A seventh ASR well of the same capacity has been completed and piping and additional pumping capacity will be installed to add this new well to the existing system. The time allowed for filling the ASR wells depends on the water level in Hendersen Creek. Currently we are able to fill the ASR wells for about 100 days.
Brackish Raw Water Supply for the Reverse Osmosis (RO) Plant
The brackish raw water for the RO Plant is from 18 wells with average depth of 540 feet below grade. Total capacity of withdrawl is approximately 8.2 mgd when the plant is producing its maximum production of 6.0 mgd potable 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 tot he distribution system.
Marco Island Reverse Osmosis Plant
The plant has a permitted cacpacity of 6.0 mgd of potable water. To produce 6.0 mgd the raw wtaer 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 current getting some baseline data and will be changing over within a feww 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 amonia 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 to the atmosphere.
Marco Shores Lime Water Treatment Plant
This plant is permitted for 700,000 gallons per day. It is similar to the Marco Island Lime Plant but it uses chlorine gas instead of sodium hypochorite as the source for the chlorine addition. The plant will be shut down in about six to nine months and bulk potable water will be purchased from Collier County for distribution to the customers at Marco Shores.
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