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The raw water used in the drinking water plant is pumped from five wells. One of them supplies approximately 3.8 × 103 m3/d groundwater with low salt concentration (Middle Potomac Well). The other four wells supply approximately 18.9 × 103 m3/d groundwater with a higher salt concentration (Lower Potomac Well). The water quality of both types of wells is presented in Table 1. Treatment with RO removes nearly all of the salts in the Lower Potomac Well water. To maintain a normal range of salt contents in the potable water, the RO effluent is blended with the groundwater pumped from the Middle Potomac Well as shown in Figure 1. The concentration of total dissolved solids (TDS) in the brackish water is approximately 2,500 mg/L, which increases to approximately 13,000 mg/L after being concentrated. To avoid the discharge of this highly concentrated brackish water, a two-stage FO is suggested for the upgrading of the RO system in James City County.

Table 1

Water quality of the well waters, concentrate, and the STW used in the experiment

Water quality parametersMiddle Potomac WellLower Potomac WellConcentrateSTW
pH 7.9–8.2 7.6–7.8 8.3 7.5 
Temperature (°C) 13 13 13 10–30 
Conductivity (μS/cm) 1,500 4,500 25,430 9.1 
TDS (mg/L) 1,000 2,300 12,500 280 
Na+ (mg/L) 417 1,100 6,127.4 – 
Ca2+ (mg/L) 6.1 20.3 105 18.0 
Mg2+ (mg/L) 1.5 4.2 25 14.5 
Iron (mg/L) 0.01 0.02 0.1 0.05 
SiO2 (mg/L) 41 29 164 – 
Cl (mg/L) 340 1,250 7,625 – 
Alkalinity (mg/L) 320 340 1,910 52.0 
Turbidity (NTU) <1.0 <1.0 <1.0 5.2 
THMs (μg/L) ND ND ND 105.0 
HAAs (μg/L) ND ND ND 15.5 
TOC (mg/L) 0.06 0.05 0.19 11.2 
Water quality parametersMiddle Potomac WellLower Potomac WellConcentrateSTW
pH 7.9–8.2 7.6–7.8 8.3 7.5 
Temperature (°C) 13 13 13 10–30 
Conductivity (μS/cm) 1,500 4,500 25,430 9.1 
TDS (mg/L) 1,000 2,300 12,500 280 
Na+ (mg/L) 417 1,100 6,127.4 – 
Ca2+ (mg/L) 6.1 20.3 105 18.0 
Mg2+ (mg/L) 1.5 4.2 25 14.5 
Iron (mg/L) 0.01 0.02 0.1 0.05 
SiO2 (mg/L) 41 29 164 – 
Cl (mg/L) 340 1,250 7,625 – 
Alkalinity (mg/L) 320 340 1,910 52.0 
Turbidity (NTU) <1.0 <1.0 <1.0 5.2 
THMs (μg/L) ND ND ND 105.0 
HAAs (μg/L) ND ND ND 15.5 
TOC (mg/L) 0.06 0.05 0.19 11.2 

–, data not available; ND, under determination limit.

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