Project Compares Brackish Water Desalination Technologies - Part 1

July 16, 2019

About the author:

Jim Passanisi is with the City of Port Hueneme, Calif., Janet Persechino is with Ionics, Inc., Watertown, Mass., and Todd K. Reynolds, is with Kennedy/Jenks Consultants.

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In Port Hueneme, California, a state-of-the-art desalination facility uses three brackish water desalination technologies: reverse osmosis (RO), nanofiltration (NF) and electrodialysis reversal (EDR), operated side-by-side to produce over three million gallons per day (mgd) of high quality drinking water. The Brackish Water Reclamation Demonstration Facility (BWRDF) is the cornerstone of the Port Hueneme Water Agency’s (PHWA) Water Quality Improvement Program. In addition to providing desalted water for local use, the BWRDF also serves as a full-scale research and demonstration facility.

It is usually a difficult task to compare the long-term performance and operating costs of three technologies due to variables in source water quality, plant capacities, labor, power and chemical costs. Operating three full-scale desalination technologies in parallel at the same site has made a direct comparison possible. During the course of the plant’s operation, the PHWA will collect data on long-term cost and performance characteristics of the three membrane systems.

Project Purpose and Goals

Prior to the implementation of the PHWA’s Water Quality Improvement Program, PHWA’s retail customers (City of Port Hueneme [COPH], Channel Islands Beach Community Services District [CIBCSD], Naval Construction Battalion Center—Port Hueneme [NCBC], and Naval Air Weapons Station—Point Mugu [NAS]) were concerned with the long-term reliability and water quality of their existing water supplies. Each of these water purveyors utilized brackish groundwater from the Oxnard Plain Groundwater Basin, a critically over-drafted basin that is under active basin management.

The groundwater management plan called for reductions in groundwater extractions by 25 percent over 20 years. Groundwater was extracted from local deep aquifer wells along the Coast that were increasingly subject to seawater intrusion, or was delivered from inland upper aquifer wells by the United Water Conservation District. The total dissolved solids level of these water sources is normally greater than 1,000 mg/L. Although the groundwater met the primary drinking water standards of the California Department of Health Services, it was highly mineralized and was aesthetically undesirable. Furthermore, customers were burdened with added costs for water softeners and bottled water, as well as the indirect costs including shortened plumbing and appliance life and staining of glassware and laundry.

In response to increasing overdraft of the local groundwater basin, seawater intrusion and poor groundwater quality (especially during drought years), PHWA implemented its Water Quality Improvement Program. The program involves demineralization of the local groundwater that is used in conjunction with imported California State Water Project (CSWP) water.

Implementation of this innovative program has provided significant benefits to more than 55,000 people within Ventura County, Calif. Benefits include improved water quality, joint use of facilities and obtaining a long-term, safe, reliable and environmentally sustainable high quality water supply that meets current and proposed water quality drinking standards under the Safe Drinking Water Act. Shared use of facilities by PHWAs customers has eliminated the need for individual agency projects. Long-term water supply reliability for PHWA’s customers has been improved by access to both demineralized groundwater from local sources and imported CSWP water. The delivery of imported State Water Project water has allowed PHWAs customers to reduce their groundwater extractions from coastal wells threatened by seawater intrusion and minimize the capacity of the demonstration facility. Relocating groundwater extractions from the coastal area to inland recharge areas also is minimizing seawater intrusion. In addition, by demineralizing the water prior to distribution, it reduces the need for home water softening and reverse osmosis units. It also improves the potential to desalinate wastewater for future reclamation.

Timeline and Cost

The PHWA’s Water Quality Improvement Program was implemented over a six-year period starting in 1993 and culminating with the startup of the BWRDF, a joint powers wholesale water agency, in 1999. The first several years involved establishing the contractual agreements to form the PHWA and to annex the agency to the CSWP to facilitate importing surface water. Design of the BWRDF was completed in late 1996 and facility construction was completed in late 1998. During the period of the BWRDF construction, the PHWA also constructed several major pipelines to deliver raw and treated surface water to the facility and to deliver treated and blended water to the customers. The cost of the BWRDF was $5.7 million. The cost of PHWA Water Quality Improvement Program also included approximately $7 million for raw and treated water pipelines and additional legal and annexation costs. Since the BWRDP also serves as a full-scale brackish membrane research and demonstration facility, the United States Bureau of Reclamation (USBR) funded approximately 25 percent of the cost of the facility.

Desalination Demonstration

The three membrane treatment processes (RO, NF and EDR) operate side-by-side to produce a total of 3 mgd of treated and blended water as shown in Table 1.

Pretreatment Equipment

The source water for the BWRDF is chlorinated groundwater from inland, upper aquifer wells that are under the influence of source water and operated by the United Water Conservation District (UWCD). These wells are recharged with surface water from the Santa Clara River through spreading basins. Typical source water characteristics are presented in Table 2. Pretreatment of the source water is required to remove relatively large particulate matter and free chlorine ahead of the membranes and to adjust the water chemistry to preclude chemical scaling of the membranes.

The source water is drinking water quality (except for the salts) and has very low turbidity. The average Silt Density Index (SDI), a measure of the water’s likelihood to cause particulate fouling of the membrane, is very low at less than 0.5. However, because the source water comes from a well field where different wells are started and stopped in the system, the BWRDF could see periodic episodes of relatively large particles in the source water. The starting of a well pump can cause rust flakes and particles that have settled out in the pipeline to become resuspended. Therefore, pretreatment filtration of the source water is required to protect the membranes from these periodic episodes.

The membranes are protected from damage from relatively large particles by an automatic backwashing, bag filtration system with a 5- to 10-micron nominal removal. The automatic backwashing filter system was selected to permit continuous plant operation and minimize the labor and maintenance time devoted to the pretreatment filter. There was concern that a standard 5- to 10-micron cartridge type filtration system could require excessive cartridge replacement or even become blinded with particles due to the well pump operations.

While the EDR membranes can tolerate low levels of free chlorine, the source water must be dechlorinated to protect the RO and NF membranes from oxidant damage by the free chlorine. Sodium bisulfite, a reducing agent, is added to the source water after the pretreatment filter system to remove the chlorine. An oxidation-reduction potential (ORP) analyzer is used to monitor the water. Another option is to add ammonia to convert the free chlorine to chloramines. The EDR, RO and NF membranes can tolerate the chloramines and have been shown to help minimize biofouling of the membranes.

If a salt’s concentration exceeds solubility limits, precipitates can form mineral scale on the membranes. Acid and antiscalant typically are added to prevent mineral scale accumulation on the RO, NF and EDR membranes. In the RO and NF systems, the source water flows along the length of the membrane and the TDS concentration increases as product water passes through the membranes. Acid and/or antiscalant are added to the source water. In the EDR system, the TDS on the concentrate side of the membrane increases as cations and anions pass through the EDR membranes. In this case, acid and/or antiscalant are added to the concentrate recycle stream.

Hydrochloric acid is required ahead of the EDR system. Currently, the RO and NF systems do not require acid addition based on a source water pH less than 7.5 and TDS levels. Hydrochloric acid feed systems are provided for the RO and NF systems should they require acid addition in the future. Each membrane system requires a small amount of antiscalant addition. Different antiscalants have been tested and are fed to each system based on recommendations from the manufacturers and operational experience. Each membrane treatment system has a dedicated acid and antiscalant chemical metering pump to permit different chemical feed rates and to accurately monitor the chemicals used by each of the three membrane systems.

Reverse Osmosis System

The RO membranes remove total dissolved solids (TDS) from the source water. Osmosis is a natural process in which water passes through a semipermeable membrane from the side with a low TDS concentration to the side with a high TDS concentration. Reverse osmosis is a pressure driven process that raises the water pressure on the high TDS (source water) side of the membrane to well above the osmotic pressure and forces the water to flow through the membrane to the low TDS (product water) side of the membrane. The RO membrane permits the passage of water molecules but is a barrier to most of the ions in the water. As the source water flows along the membrane, the TDS is further concentrated and finally discharged as a reject stream from the process.

The BWRDF RO system is a two-stage process with 14 first stage vessels and seven second stage vessels, each with six elements per vessel. The concentrated reject stream from the first stage membranes is the feed water to the second stage membranes. The RO membrane elements are thin film composite, Filmtec BW40LE-440 elements. The product recovery for the RO system, defined as the product water out of the system divided by the source water entering the system, is approximately 75 percent. The RO pressure required to desalt source water of approximately 1,000 mg/L TDS is about 160 psi. The RO product water has a TDS of about 15 mg/L; however, this is much lower than the treated water objectives of 370 mg/L TDS and 150 mg/L of hardness. In order to produce the desired treated water quality, source water is bypassed around the RO system and blended with the low TDS RO membrane product water to produce one mgd of desalted water.

Because of project capital cost limitations, the RO and NF feed pumps are fixed speed pumps with a modulating pressure control valve, and the RO and NF systems do not have any energy recovery. The operational efficiency of the RO and NF systems would be improved with variable frequency drives (VFDs) on the feed pumps and an energy recovery system on the moderately high-pressure reject stream.

The TDS concentration in the reject water from the RO, NF and EDR systems is about three to four times the TDS concentration in the source water. The reject water for all three membrane systems at the BWRDF is discharged to the headworks of an adjacent wastewater treatment plant and discharged to the ocean through an existing outfall.

Data collection at the BWDRF is fully automated. The plant Supervisory Control and Data Acquisition (SCADA) system monitors system flows, pressures, water quality, chemical use and power consumption for each membrane system. Plant staff keeps track of operation and maintenance time and costs for each system.

Nanofiltration System

The NF membrane system operates just like the RO system to remove TDS from the source water. However, the NF system allows larger particles through the membrane than the RO system does (0.001 to 0.01 microns for the NF membranes, 0.0001 to 0.001 microns for the RO membranes). As a result, the NF system does not require as high a pressure to produce the same volume of product water as the RO. However, the larger pores also permit more monovalent salts to pass through the NF membrane.

The BWRDF NP system is a two-stage process with 15 first stage vessels and seven second stage vessels, each with six elements per vessel. The concentrated reject stream from the first stage membranes is the feed water to the second stage membranes. The NF membrane elements are thin film composite, Filmtec NF90-400 elements. The product recovery for the NF system is approximately 73 percent. The NF pressure required to desalt source water of approximately 1,000 mg/L TDS is about 140 psi. The NF product water has a TDS of about 20 mg/L; however, this is still lower than the treated water objectives. As with the RO, source water is bypassed around the NF system and blended with the low TDS NF membrane product water to produce one mgd of desalted water.

Electrodialysis Reversal System

Electrodialysis is an electrically driven process that uses a voltage potential to drive charged ions through a semipermeable membrane, reducing the TDS in the source water. The process uses alternating, semipermeable cation (positively charged ion) and anion (negatively charged ion) transfer membranes in a direct-current (DC) voltage potential field. As the source water flows between the cation and anion membranes, the DC voltage potential induces the cations to migrate toward the anode through the cation membrane, and the anions to migrate toward the cathode through the anion membrane. The cations and anions accumulate in the reject water side of the membranes and low TDS product water is produced. The electrodialysis reversal system periodically reverses the polarity of the electric field, and consequently the dilute and concentrate compartments, to help flush scale forming ions off the membrane surface and minimize membrane cleaning.

The product water from an EDR system does not pass through the desalting membrane as it does in an RO or NF system. This reduces the potential for particulate fouling of the EDR system and is not a regulatory issue for groundwater desalting. However, for desalting applications that also require treatment to meet the Surface Water Treatment Rule, the EDR system would require an additional filtration process for microbiological removal.

The BWRDF EDR membrane treatment system produces one mgd of desalted water using 15 membrane stacks. Each membrane stack contains 600 cell pairs of ion-exchange membranes and flow spacers.

The product recovery for the EDR system is approximately 85 percent (some source water is added to the reject water loop to keep the dissolved ion concentrations low enough to prevent mineral scale formation). The EDR system, unlike the RO and NF systems, uses no filtered raw water to blend with the product water. The EDR system adjusts the voltage field potential to meet the treated water quality objectives. The result is water with a TDS concentration that just meets the treated water quality criteria and optimizes the EDR treatment system’s electrical efficiency.

Post Treatment

The pH in the RO and NF systems product water typically is decreased due to the removal of some of the dissolved alkalinity in the water. The BWRDF RO and NF system product water is between 5.8 and 6.0 pH units. The EDR system does not affect pH as much and the product water pH typically is much closer to source water pH. Sodium hydroxide (caustic soda) is added to raise the pH of the desalted water to approximately 8.0 pH units for corrosion control. Sodium hypoclorite and ammonia are added to provide a chloramine disinfection residual in the PHWA’s distribution system. The BWRDF also has a 600,000-gallon treated water storage tank and a booster pump station to deliver the desalted groundwater and low TDS imported surface water to their customers. Typical treated water quality is shown in Table 3.

Part two of this article compare the treatment methods after one year of use at the site..

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