Swansea Desalination Facility
The Swansea Water District (SWD) owns and operates 11 source wells and its production capacity is inadequate to meet growing demands. The district decided to develop a new brackish water source and build a new innovative $17.6 million membrane treatment facility. The 2.2 mgd facility includes a 1.4 mgd brackish seawater desalination plant and a 0.8 mgd membrane filtration groundwater treatment plant.
HDR's project role included treatment piloting, design and construction administration services to Hoyle, Tanner and Associates, Inc., the lead local firm. The facility's purpose is two-fold: to improve finished water quality from the Vinnicum wellfield and to treat water from a new, alternative water source, the tidally-influenced Palmer River. Because of water quality and salinity differences, two treatment trains were proposed. The Vinnicum wellfield raw water requires microfiltration/ultrafiltration to remove organics, iron and manganese, while the Palmer River raw water requires microfiltration/ultrafiltration to remove organics and pathogens, followed by reverse osmosis to remove dissolved salts. An analysis of the raw water composition and required flow rates was performed to confirm the feasibility of membrane filtration to produce finished water of acceptable quality and quantity.
Following this analysis, HDR led a comprehensive piloting study to compare membrane systems. The pilot study included two different seasons of piloting at the Palmer River, a single season at the Vinnicum wellfield, and compared membranes from two different manufacturers. Bids were taken and the Pall Corporation was selected to provide the membrane equipment. To meet stringent intake requirements, the plant employs a constructed infiltration system intake on the Palmer River, upstream of Narragansett Bay. Approximately 4 mgd of water will be withdrawn during daily low tide events up to two six-hour periods per day to obtain the lowest possible salinity.
In addition to wide TDS swings, the highly variable raw water can have TOC levels of 15 to 20 mg/L and color in excess of 100 CU. The pretreatment system consists of a 500 m strainer followed by the addition of sodium hydroxide to reach a 6.8 pH, coagulation with 15 to 20 mg/L of an aluminum-ferric coagulant blend, and 10 minutes of flocculation. Two Pall Microza MF skids operating at 97 percent recovery provide feedwater to three RO trains equipped with Toray membranes. Each RO train is designed to run at a constant feed rate and will operate between 50 to 55 percent recovery, depending on feed water salinity. RO concentrate will be collected in a storage tank prior to its diffused discharge to the river. The discharge will coincide with flood/high tide events and will be limited to a 32,000 mg/L TDS. The tidal cycles will be programmed into the plants SCADA system and the control valve will open to maximize the flow at peak high tide. Under the maximum raw water withdrawal, a discharge of 3,700 gpm (233 L/s) over each of two tidal events is anticipated. The discharge rate will be held constant under all operating conditions to achieve the required diffuser discharge velocity to meet mixing zone requirements.
The MF membranes require a periodic back pulse cycle using sodium hypochlorite solution to prevent biofouling, with a more intensive cleaning cycle once per month, and the RO membranes will undergo CIP leaning with citric acid three or four times a year. The cleaning solutions will be neutralized and dechlorinated before being blended with the RO concentrate prior to discharge. A separate Pall MF system will be used to treat groundwater from the existing Vinnicum well field, and the filtrate can be blended with the RO permeate based on system demands.
Post treatment includes the addition of soda ash and sodium hydroxide for pH/alkalinity adjustment before and after the clearwell. Sodium hypochlorite is also added for disinfection, to meet CT requirements and maintain a chlorine residual in the distribution system. Sodium fluoride and the option to feed orthophosphate for corrosion control will also be provided.
Finished water is stored in the 0.170 MG (567 m2) clearwell constructed below the plant process floor. The clearwell detention time will ensure a minimum of 0.5-log Giardia inactivation.