Treatability of a Highly-Impaired, Saline Surface Water for Potential Urban Water Use

: As freshwater sources of drinking water become limited, cities and urban areas must consider higher-salinity waters as potential sources of drinking water. The Salton Sea in the Imperial Valley of California has a very high salinity (43 ppt), total dissolved solids (70,000 mg/L), and color (1440 CU). Future wetlands and habitat restoration will have signiﬁcant ecological beneﬁts, but salinity levels will remain elevated. High salinity eutrophic waters, such as the Salton Sea, are difﬁcult to treat, yet more desirable sources of drinking water are limited. The treatability of Salton Sea water for potential urban water use was evaluated here. Coagulation-sedimentation using aluminum chlorohydrate, ferric chloride, and alum proved to be relatively ineffective for lowering turbidity, with no clear optimum dose for any of the coagulants tested. Alum was most effective for color removal (28 percent) at a dose of 40 mg/L. Turbidity was removed effectively with 0.45 µ m and 0.1 µ m microﬁltration. Bench tests of Salton Sea water using sea water reverse osmosis (SWRO) achieved initial contaminant rejections of 99 percent salinity, 97.7 percent conductivity, 98.6 percent total dissolved solids, 98.7 percent chloride, 65 percent sulfate, and 99.3 percent turbidity.


Introduction
Many inland bodies of water suffer from rising salinity which can harm biota and impair or prevent beneficial water use [1]. Salinization occurs when salts and minerals in soil are mobilized from clearing natural vegetation [2], when fresh water is diverted for irrigation [3], there are ongoing or reoccurring drought conditions [4], or as a result of municipal wastewater discharges [5]. As freshwater sources of drinking water become limited, cities and urban areas must consider higher-salinity waters as potential sources of drinking water.
The Salton Sea is a large, shallow saline lake in an arid desert area of Southern California. It was formed by an accidental diversion of the Colorado River into the Salton Sink between 1905 and 1907. The lake is the largest and lowest inland water body in California with a total surface area of 980 km 2 , a maximum depth of approximately 15 m, and approximately 70 m below mean sea level. It is a closed-basin lake with no outlet, sustained by irrigation return flows and municipal wastewater discharges. Initially a freshwater lake, a high nutrient loading from agricultural runoff, continuous municipal wastewater treatment effluent discharges, and no natural outflow has resulted in a steady decline in water quality over many decades [6]. Diversion of agricultural water to municipal use beginning 1 January 2018 further threatens water quality, but is also expected to result in further shrinking the size of the Salton Sea [7].
The Salton Sea has been the subject of significant research, and its water quality deterioration is well characterized [8][9][10][11]. Despite being a hypereutrophic, hypersaline water body, the Salton Sea provides a significant ecological function, and is a vital habitat for migrating birds. Development and implementation of plans to remediate the Salton Sea ecosystem has been an ongoing challenge X X X -X X

Jar Testing
A series of jar tests were conducted following ASTM D 2035-08, Standard Practice for Coagulation-Flocculation Jar Test of Water [23]. Stock solutions were prepared for each coagulant at a concentration of 10,000 mg/L. A jar test was conducted for each coagulant at doses of 0, 10, 20, 30, 40, and 50 mg/L. Coagulants were added with rapid mixing for 2 min, slow mixing for 30 min (tapered at Water 2018, 10, 324 4 of 14 10 min intervals), and settling for 45 min. Aliquots were taken from each jar for analysis of turbidity, pH, alkalinity, and color.
The effectiveness of filtration pretreatment was assessed by filtering settled jar test samples through filters with consecutively smaller nominal pore sizes. Sand filtration was simulated by passing settled water through Whatman 40 (8 µm) paper filters (Fisher Scientific, Waltham, MA, USA). Microfiltration was simulated by passing settled water through a 0.45 µm membrane filter followed by a 0.1 µm membrane filter using vacuum filtration.

Bench RO Treatment
The effectiveness of RO treatment was assessed at the bench scale by passing approximately 75 L of filtered Salton Sea water through a sea water reverse osmosis (SWRO) unit described in Table 2. Samples of feed water, permeate and concentrate were collected and analyzed according to the sampling plan presented in Table 1. For comparison, an identical SWRO treatment test was performed on Pacific Ocean water. Feed water was pumped through the SWRO system at 20 L/h for 60 min at 58 to 66 bar transmembrane pressure (TMP). Samples were taken for analysis of the SWRO feed water, permeate, and reject stream. Initial contaminant rejection was assessed by taking composite samples over the 60 min operating period. TMP, permeate flow, and reject water flow were monitored during the test. The volume of water treated (20 L) and duration of testing (60 min) was limited, due to having to transport raw water to the laboratory for testing. Given the short-term of testing, normalization of test results was not necessary.

Analytical Methods
All analyses were performed at the CBU environmental engineering laboratory. The analytes and analytical methods used are presented in Table 3. Standard Methods [25], U.S. Environmental Protection Agency (EPA) methods [26], or their equivalent as developed by Hach [27] and Micrology Laboratories [28], were used. Quality assurance (AQ) and quality control (QC) measures were followed along standard laboratory practices for instrument calibration according to the manufacturer's instructions. Filtration through a 0.45 µm membrane filter was performed prior to ultraviolet absorbance (UVA) (Hach DR5000 UV VIS, Loveland, Colorado, CO, USA) and 254 nm (UV254) (Hach DR5000 UV VIS, Loveland, Colorado, CO, USA). All experiments and analyses were performed at laboratory temperature (22 • C).

Results and Discussion
Results of water quality testing, cartridge filtration, jar testing, and SWRO bench testing are presented below. Analytical results for all water quality tests are presented in Tables 4 and 5 for SSW and POW, respectively.

Cartridge Filtration
Salton Sea water and Pacific Ocean water were filtered through a 30 µm cartridge filter prior to performing SWRO bench tests. Turbidity removal of 54 percent was achieved for SSW. No significant removal of turbidity was achieved using a cartridge filter for POW because of the low raw water turbidity.

Water Quality Test Results
Consistent with prior studies, the Salton Sea water quality was found to be highly saline (43 ppt). The SSW chloride and total dissolved solids (= total solids − suspended solids) concentrations were 38,000 mg Cl − /L and 70,000 mg/L, respectively. In contrast, POW chloride and total dissolved solids concentrations were 18,800 mg Cl − /L and 39,434 mg/L, respectively.

Jar Test Results
The Salton Sea is highly colored. Results of jar testing are presented in Figures 2-7. Coagulation-sedimentation proved to be relatively ineffective for lowering turbidity with no clear optimum dose for any of the coagulants tested (Figures 2-4).
ACH generally increased pH ( Figure 4) and alkalinity ( Figure 5), whereas ferric and alum lowered pH and alkalinity. Alum was most effective for color removal (28 percent) at a dose of 40 mg/L.    ACH generally increased pH ( Figure 4) and alkalinity ( Figure 5), whereas ferric and alum lowered pH and alkalinity. Alum was most effective for color removal (28 percent) at a dose of 40 mg/L.    ACH generally increased pH ( Figure 4) and alkalinity ( Figure 5), whereas ferric and alum lowered pH and alkalinity. Alum was most effective for color removal (28 percent) at a dose of 40 mg/L.

SWRO Treatability Results
The feed flow rate during SWRO testing was 75 L/h. The permeate flow rate was 7.95 L/h and 9.2 L/h during treatment of SSW and POW, respectively. An average recovery of 10.6 percent and 12.2 percent was achieved for SSW and POW, respectively.

SWRO Treatability Results
The feed flow rate during SWRO testing was 75 L/h. The permeate flow rate was 7.95 L/h and 9.2 L/h during treatment of SSW and POW, respectively. An average recovery of 10.6 percent and 12.2 percent was achieved for SSW and POW, respectively.

SWRO Treatability Results
The feed flow rate during SWRO testing was 75 L/h. The permeate flow rate was 7.95 L/h and 9.2 L/h during treatment of SSW and POW, respectively. An average recovery of 10.6 percent and 12.2 percent was achieved for SSW and POW, respectively.

SWRO Treatability Results
The feed flow rate during SWRO testing was 75 L/h. The permeate flow rate was 7.95 L/h and 9.2 L/h during treatment of SSW and POW, respectively. An average recovery of 10.6 percent and 12.2 percent was achieved for SSW and POW, respectively. SWRO water quality test results for SSW and POW are presented in Tables 4 and 5, respectively. Salinity (Figure 8), conductivity (Figure 9), total dissolved solids (TDS) (Figure 10), chloride (Figure 11), sulfate (Figure 12), and turbidity ( Figure 13) were all removed. SWRO contaminant rejection is summarized in Figure 14.

Discussion
High salinity, TDS, sulfate, chloride, and color make treatment of Salton Sea water challenging. High sulfate concentrations coupled with warm temperatures and low redox potentials are present much of the year. These conditions result in sulfate reduction, producing hydrogen sulfide, which effects the iron geochemistry of the lake [9]. Lake mixing events during the summer have adverse effects on the fish and invertebrates in the Sea, as well as migrating birds feeding on them [11].
Coagulation of SSW with alum was found to be most effective for color removal at dosages characteristic of drinking water treatment, although residual color was still very high. Primary production in the Salton Sea is limited by phosphorus. Treating Salton Sea inflow water with alum to remove soluble phosphorus has been considered [29], but requires higher chemical dosages than considered here. The Salton Sea is supersaturated with respect to calcite and gypsum [30,31], potentially limiting the long-term feasibility of SWRO treatment to meet drinking water quality standards.
Drinking water utilities using RO can consistently meet treated drinking water regulatory limits [32,33]. To ensure regulatory compliance, water utilities establish site-specific treated water quality goals that are more strict than regulatory requirements. A comparison of the RO permeate quality achieved here to typical drinking water quality goals is provided in Table 6. Bench tests of SWRO effectively removed contaminants examined here from SSW, except for chloride and TDS. POW permeate only exceeded the chloride water quality goal. Chloride and TDS are regulated in the United States as National Secondary Drinking Water Regulations, which are recommended limits to ensure that drinking water is palatable. Permeate exceeding the recommended limit for chloride and TDS does not pose an adverse health risk, but may affect the taste of the water for some people.
The U.S. Environmental Protection Agency (USEPA) requires filtration and disinfection treatment for surface water sources of drinking water [33]. Jar tests indicate 0.1 μm pore microfiltration pretreatment will consistently achieve a treated water turbidity limit of 0.3 NTU. The membrane fouling potential is very high for SSW, which must be further assessed. To lessen chemical

Discussion
High salinity, TDS, sulfate, chloride, and color make treatment of Salton Sea water challenging. High sulfate concentrations coupled with warm temperatures and low redox potentials are present much of the year. These conditions result in sulfate reduction, producing hydrogen sulfide, which effects the iron geochemistry of the lake [9]. Lake mixing events during the summer have adverse effects on the fish and invertebrates in the Sea, as well as migrating birds feeding on them [11].
Coagulation of SSW with alum was found to be most effective for color removal at dosages characteristic of drinking water treatment, although residual color was still very high. Primary production in the Salton Sea is limited by phosphorus. Treating Salton Sea inflow water with alum to remove soluble phosphorus has been considered [29], but requires higher chemical dosages than considered here. The Salton Sea is supersaturated with respect to calcite and gypsum [30,31], potentially limiting the long-term feasibility of SWRO treatment to meet drinking water quality standards.
Drinking water utilities using RO can consistently meet treated drinking water regulatory limits [32,33]. To ensure regulatory compliance, water utilities establish site-specific treated water quality goals that are more strict than regulatory requirements. A comparison of the RO permeate quality achieved here to typical drinking water quality goals is provided in Table 6. Bench tests of SWRO effectively removed contaminants examined here from SSW, except for chloride and TDS. POW permeate only exceeded the chloride water quality goal. Chloride and TDS are regulated in the United States as National Secondary Drinking Water Regulations, which are recommended limits to ensure that drinking water is palatable. Permeate exceeding the recommended limit for chloride and TDS does not pose an adverse health risk, but may affect the taste of the water for some people. The U.S. Environmental Protection Agency (USEPA) requires filtration and disinfection treatment for surface water sources of drinking water [33]. Jar tests indicate 0.1 µm pore microfiltration pretreatment will consistently achieve a treated water turbidity limit of 0.3 NTU. The membrane fouling potential is very high for SSW, which must be further assessed. To lessen chemical fouling of RO, softening pretreatment should be considered for lowering calcium and total hardness feed concentrations. The recovery achieved (10 percent) is typical for a single membrane element under these test conditions. The recovery that could achieved by a membrane array treating Salton Sea water must be further evaluated.
In general, high-salinity eutrophic waters are difficult to treat, and are typically avoided as water supply sources. Lower-salinity surface waters, ground waters, and even desalinated sea water are generally preferred when available. The SWRO bench tests conducted here examined only initial contaminant rejection from SSW and POW in the laboratory. Results suggest that treatment of Salton Sea water to meet potable water quality goals is possible, at least at the bench scale. Based on these results, an integrated membrane system consisting of microfiltration, membrane softening, and SWRO is the most promising for treating Salton Sea water. Additional pilot testing at the Salton Sea will be necessary to assess the long-term feasibility of SWRO treatment of Salton Sea water for potential urban water use.
Coastal area desalination plants typically dispose of concentrate by ocean discharge. Concentrate disposal options are more limited for desalination of inland waters such as the Salton Sea, where ocean discharge is not feasible. If large-scale desalination of Salton Sea water is pursued, concentrate disposal must be addressed, regardless of the specific desalination technology selected [34,35]. Concentrate disposal options include well injection, evaporation ponds, or further concentration using vapor compression evaporation. Importation of water from Mexico into the Salton Sea, suggested as a long-term remediation solution [15], could also impact the feasibility of RO desalination and brine disposal for potable water production.
Countries in the Arabian Gulf rely on desalination using distillation technologies (multi-effect and multi-stage) [36]. Should geothermal energy development at the Salton Sea progress to the point of providing an economical supply of energy, distillation technology may provide a viable alternative to RO. Currently, RO is the most economical of desalination technologies for potable water treatment. In December 2015, the largest desalination plant in the United States began operation at Carlsbad, California. The plant provides 50 million gallons per day of RO-treated Pacific Ocean water to the customers of the San Diego County Water Authority [37], further demonstrating the feasibility of SWRO for producing potable water.