Nanofiltration Membrane Technology Series – Use Of Nanofiltration As Pretreatment For Desalination Plants

ABSTRACT/SYNOPSIS

Desalination of seawater provides a climate-independent source of water for critical human needs and economic development (industry and agriculture in particular). It is an effective way to secure water supplies against the effects of climate change, a growing population and drought. Desalination is a process of removing dissolved salts from water in order to produce processed, potable water for domestic use. The limited availability of fresh water sources coupled with the increasing population and associated high water demands has led the water industry professionals to evaluate alternate sustainable options of drinking water such as desalination of seawater. The growing need for a safe, reliable and consistent supply of water has made many manufacturing industries and water treatment plants to look for efficient desalination plants. Although thermal desalination has remained the primary technology of choice in the Middle East, membrane processes, such as reverse osmosis (RO), have rapidly developed since the 1960\’s and currently surpass thermal processes in new plant installations. However, one of the inherent drawbacks of RO is its prone to membrane fouling because of the high total dissolved solids concentration in seawater. As such, pre-treatment units have become more vital since they can effectively filter the seawater before the feed reaches the main reverse osmosis (RO) unit. This episode discusses the application of Nanofiltration as a pretreatment measure in desalination water plants. It is the second part of a two-part series titled “Nanofiltration Membrane Technology” by Sani Water. Sani Water has been at the forefront of providing their customers with the latest products and advanced water treatment technologies available in the drinking water industry. With “Nanofiltration Membrane Technology Series”, Sani Water aims to educate and inform their customers about the fundamentals and applications of nanofiltration membrane technology and use of nanofiltration technology as pretreatment for desalination plants. 

DESALINATION PROCESS

One of the main applications of nanofiltration (NF) is in the pretreatment stage of seawater desalination. NF has high rejection rates for divalent ions and could eliminate the scaling species that pose serious fouling problems in seawater desalination. One major impediment to the widespread application of seawater desalination technology (thermal-based or membrane-based) is its relatively high production costs. However, many recent advancements in seawater desalination technology have led to appreciable declines in water production costs. One such advancement is the application of nanofiltration (NF) as a pretreatment technique for both thermal and membrane-based processes. NF targets the removal of scale-forming agents (mainly sulfates) that adversely affect desalting processes.

REFLECTION POINTS

APPLICATION AND BENEFITS OF NANOFILTRATION AS PRETREATMENT FOR DESALINATION

Feed pretreatment is one of the major factors determining the success or failure of a desalination process. Different methods of pretreatment for desalination process were suggested by many researchers. Traditional pretreatment is based on mechanical treatment (media filters, cartridge filters) supported by extensive chemical treatment. In the past, conventional pretreatment (i.e., coagulation, flocculation, acid treatment, pH adjustment, addition of anti-scalant and media filtration) was usually used. The main problem in using conventional pretreatment is corrosion and corrosion products. For example, in the acid dosing system, corrosion of metallic surfaces and corrosion products will roughen the surface of the equipment, which provides active sites for precipitation of more scale deposits. In addition, this pretreatment is known to be complex, labor intensive and space consuming. 

Pressure-driven membrane processes [microfiltration (MF), ultrafiltration (UF), and nanofiltration NF] are the new trends in designing a pretreatment system. The introduction of NF as a pretreatment will lead to a breakthrough in the application of RO and MSF because it has implications for the desalination process itself, and not only on quantity of the feed water. MF can remove suspended solids and lower the silt density index (SDI) while in UF, not only suspended solids and large bacteria are retained, but also (dissolved) macromolecules, colloids and small bacteria. 

NF membranes can be used to treat water by removing contaminants and organics such as natural organic matter, disinfection by-product, pesticides, dissolved uranium, arsenite, chromate, and other metals. In addition, NF was used as a pretreatment in the desalination process to prevent RO membrane fouling, prevent scaling (both in RO and MSF) and lower the required pressure to operate RO plants. Fouling problems will lead to higher operation costs: higher energy demand, increase of cleaning and reduced lifetime of the membrane elements. The separation mechanism of NF has been identified mostly due to charge and steric effects. A large majority of modeling work on NF has been by models based on the extended Nernst-Planck equation. A good model will lead towards better prediction and optimization of NF membrane processes.

REFERENCES

1. Subramani, A., & Jacangelo, J. G. (2015). Emerging desalination technologies for water treatment: a critical review. Water research, 75, 164-187.
2. https://www.keiken-engineering.com/en/desalination-plants-for-water-treatment/
3. http://www.awa.asn.au/AWA_MBRR/Publications/Fact_Sheets/Desalination_Fact_Sheet.aspx#:~:text=Benefits%20of%20desalination,a%20growing%20population%20and%20drought.
4. Abdelkader, B. A., Antar, M. A., & Khan, Z. (2018). Nanofiltration as a pretreatment step in seawater desalination: A review. Arabian Journal for Science and Engineering, 43(9), 4413-4432.
5. Hilal, N., Al-Zoubi, H., Darwish, N. A., Mohamma, A. W., & Arabi, M. A. (2004). A comprehensive review of nanofiltration membranes: Treatment, pretreatment, modelling, and atomic force microscopy. Desalination, 170(3), 281-308.
6. https://wcponline.com/2010/08/18/pretreatment-technologies-for-reverse-osmosis-and-nanofiltration/