Challenge

Dauphin Island’s 0.49 million-gallons-per-day (mgd) (1860m2/d)-wastewater treatment facility, located at the entrance to the island, served its purpose faithfully and effectively since its upgrade in 1984.  However, after over 15 years of service, the facility found itself faced with several obstacles.

1. Corrosion and wear had taken its toll on the system’s performance.
2. Heavy rainfall often overwhelmed the near-capacity system with excessive infiltration.
3. Projected population growth would very soon demand a larger treatment system.
4. Discharge limits were soon expected to tighten, making the current treatment system sub-standard.

Solution

The Dauphin Island Water and Sewer Authority determined, after evaluation of the projected population growth for the next 10 to 20 years, that the wastewater treatment capacity needed to be doubled to approximately 1 mgd (3800 m3/d).  This team also evaluated several options for upgrading the facility, including:

Options	Determination
A. Renovating the existing 0.49-mgd plant and adding an additional new .49 mgd plant.	No. Expensive
B. Adding lagoons and aeration basins.	No. Facility property was too small (entire site was less than 2 acres
C. Replacing the entire existing plant with a new 1-mgd advanced wastewater treatment facility.	Yes.  Technically and economically the best choice.  This choice could provide effluent quality good enough for reuse.  The authority wanted to minimize discharges into the estuary where Dauphin Island is located.

After selecting the technology, the Water and Sewer Authority selected Siemens Water Technologies to manufacture and construct the new plant.  The Davco brand by Siemens was selected for several reasons.

• The Davco brand systems are corrosion resistant.
• The company offers several advanced treatment technologies.
• Siemens has incomparable capabilities and can provide a guarantee to meet the authority’s desired effluent limits.

Siemens installed a new .98 mgd (3710 m3/d) Davco steel field-erected treatment system designed to achieve a total biological oxygen demand (BOD), total suspended solids (TSS), nitrogen, and phosphorus limits to 5-, 5-, 3-, and 1-mg/L levels respectively.  This system was actually two circular mirror-imaged plants including the following in each one:

1. Digester
2. Surge tanks
3. Five oxic zones
4. Five anoxic zones
5. Integral clarifiers

Following the biological plant, two Gravisand® filtration systems were provided to provide high level water polishing prior to pumping to the old final clarifier, which was refurbished to serve as a holding tank for reuse water.

Results

The new plant started up in mid-June of 2002, just in time for the Independence Day holiday surge.  The crystal clear plant effluent is being pumped for irrigation to a nearby golf course.  Even after sewer lines were filled with an inordinate amount of salt and sand caused by storm surges from Hurricane Hanna and other tropical storms, the treatment facility did not upset.

As of November 2002, Dauphin Island had 1839 customers and is now approaching the projected customer base for 2005, according to the average historical growth rates.  The facility is working perfectly, with daily flows reaching as high as .81 mgd (3066 m3/d).  Benefits resulting from the new biological treatment plant include:

• The bay, surrounding estuaries and coastal Alabama residents are protected from potential wastewater hazards.
• The island’s treatment needs are being met economically.
• And, the Dauphin Island Golf Course now has all the water it needs for its pond and sprinkler system.

For additional information on the Dauphin Island wastewater treatment facility, see the original article “A Golden Reuse Opportunity:  Plant expansion benefits island two ways” in the January 2003 issue of Water Environment & Technology magazine.