Desalination of high salinity brackish water by an NF-RO hybrid system

Highlights

• Global concentration distribution of brackish water

• Desalination properties comparison of five commercial 4040 membrane modules

• Desalination of high salinity brackish water by NF-RO hybrid system under 1.6 MPa

• Provide technical reference for freshwater production from high salinity brackish waters

Abstract

Desalination of brackish water is an important approach to overcoming freshwater shortage. 85% of brackish water contains 1–10 g/L of salt; however, most work on brackish water desalination targets at low salinity (1–5 g/L) brackish water. This work looks into membrane desalination processes suitable for high salinity brackish water. Feed waters with a high-concentration (≥10 g/L) of NaCl or mixed NaCl/MgSO₄ salts were considered, and the desalination performance of representative commercial nanofiltration membranes (VNF2 and ESNA1) and low-pressure reverse osmosis membranes (ULP100, LP100, and ESPA1) were evaluated. It was revealed that the desalination process can be operated at a pressure below 1.6 MPa, and the combination of ULP100-VNF2 arranged in series was a good option for production of freshwater from the high-salinity brackish water.

Introduction

With economic development and population growth, the demand for freshwater resources is gradually increasing. The shortage of freshwater and the safety of drinking water have become one of the biggest constraints to sustainable socio-economic development in the regions with scarce freshwater resources. One has to look inevitably for alternative water resources to alleviate the freshwater crisis. Brackish water is surface water or groundwater with a salt content generally in the range of 1–25 g/L, and its salinity is higher than freshwater (salt content <1 g/L) and lower than seawater (salt content ~35 g/L) [1,2]. The high salt content, high hardness and bitter taste make brackish water unsafe to drink by humans and livestock. Long-term consumption of high salinity brackish water can cause digestive diseases, skin infections, hypertension, kidney stones and even various types of cancers. Prolonged use of brackish water for agricultural irrigation will deteriorate the structure of the soil, which can affect the permeability and water retention performance of the soil and cause crops to grow abnormally. In addition, industrial use of brackish water is also problematic to the equipment because of the dissolved salts. Brackish water represents 1% of the overall water on the earth, and the available freshwater accounts only for 0.8% of the total water, with 96.5% of the water on the earth being seawater [3]. In spite of the large reserves of seawater, the desalination of brackish water is of significant interest because it consumes lower energy than seawater desalination [4]. Therefore, the desalination of brackish water has become an important approach to providing safe freshwater.

The locations of some reported brackish water points are listed in Table 1, and the global distribution of brackish water is sketched in Fig. 1. Brackish water is mainly found in Northwestern China, Saudi Arabia, Egypt, Turkey and the western part of the United States. The salinity of brackish water varies with regions. In China, the maximum salt content of surface water is 13.7 g/L, with an average salt content of 5.8 g/L; The maximum salt content of ground water is 12.9 g/L, with an average salt content of 4.3 g/L. Around 75% of the brackish surface water in China has a salt content below 10 g/L, and about 90% of the brackish groundwater contains <10 g/L of salts. About 80% of total brackish water in China has a salt content below 10 g/L (Fig. 2a). Globally, the surface water has a maximum salt content of 28.8 g/L (with an average salinity of 6.8 g/L), while the groundwater has a maximum salt content of 21.3 g/L (with an average salinity of 3.3 g/L). The surface water with a salt content of <10 g/L accounts for 76% of the total brackish surface water worldwide, whereas groundwater with a salinity below 10 g/L accounts for approximately 94% of total brackish groundwater (Fig. 2b). Overall, over 85% of brackish water on the earth has salt content <10 g/L. This means that if the brackish water with a salt content of 10 g/L or less can be desalinated, then >85% of the brackish water can be reclaimed, which represents an enormous water resource that can be used.

The major desalination technologies can be divided into two categories: the thermal processes (e.g., multi-stage flash, multi-effect distillation) [49], and the membrane processes (e.g., reverse osmosis (RO), electrodialysis (ED)) (Table 2). The membrane-based processes for water desalination are most widely used due to such advantageous characteristics as flexibility in water supply and processing capacity, adaptability of source water quality, small equipment footprint, simple operation, and low capital and operating costs [31,[49], [50], [51], [52]]. Desalination of brackish water by membranes has proven to be a viable process to produce clean freshwater [27,53,54], and it has been widely used in industrial manufacturing, agricultural irrigation and ecological protection in water-deficient areas [36,55]. However, as shown in Fig. 3, most of the brackish water desalination studies reported in the literature are designed for low salinity brackish water with a salt content of 1–5 g/L, and research work on the desalination of brackish water at higher salt contents is lacking.

The primary objective of this study was to design and evaluate a membrane-based desalination process for processing high salinity brackish water with a salt content of 10 g/L, in the hope of realizing desalination of 85% of brackish water resources. The desalination properties of five commercial low-pressure NF and RO membranes were compared. The desalination performance of the membranes under different operating conditions (including operating pressure, feed flow rate, and salt concentration in feed water) was evaluated in terms of water flux and salt rejection. A series experiments were also conducted to evaluate the desalination performance of various integrations of the membrane processes. This work is expected to fill in the knowledge gap of membrane-based desalination of high salinity brackish water and to provide a technical reference to the process.