Brackish water treatment in saline intrusion areas by capacitance reduction technology

In order to contribute to solving the shortage of drinking water in the drought-salty season of Mekong Delta provinces, scientists from the Institute of Tropical Technology (Vietnam Academy of Science and Technology) and the University of Natural Sciences (Vietnam National University, Ho Chi Minh City) have successfully researched, designed and manufactured brackish water treatment equipment based on the principle of Capacitive Deionization - CDI, with many advantages: low fabrication cost and power consumption, with high desalination efficiency. This is the result of the project KC.02.24/16-20, of the Program KC 02 "Researching the application and development of new material technology".

The Mekong Delta is located in the lower Mekong River with an area of more than 40,000 km2 and is heavily affected by climate change, directly affected by high tides and saline intrusion. Over the past years, climate change has become more and more intense, and the water storage of many hydroelectric projects on the Mekong River has resulted in deep salinity intrusion and deterioration of water quality. The problem of water scarcity occurs in most provinces in the Mekong Delta.

Map of prediction of salinity distribution in the Mekong Delta (2020)

Finding technologies to convert brackish water into fresh water at an affordable cost is a sustainable solution that enables people to live with drought and saltwater intrusion. To contribute to solving this problem, scientists from Institute of Tropical Technology (Vietnam Academy of Science and Technology) and University of Natural Sciences (Vietnam National University, Ho Chi Minh City) researched and manufactured brackish water treatment equipment system using deionization capacitance - CDI technology, capacity 5-7 m3/day, contributing to overcome the shortage of domestic water due to drought and saline intrusion.

The team has been at the forefront in the technology of fabricating multi-functional materials with high salt-treatment efficiency from coconut shell charcoal and carbon nanotubes (CNTs) used as electrode materials for industrial desalination equipment according to technology CDI. This is an advanced desalination technology, with great potential to gradually replace RO membrane technology. CDI technology has many advantages such as low manufacturing and operating costs, simple equipment operation, and high desalination efficiency. The operating principle of the deionization capacitance technology includes two processes, including electro-adsorption (water filtration) and desorption (regeneration, salt type from the electrode).

Principle of operation/ electrochemical adsorption (A) and ion desorption/ electrode regeneration (B) of the CDI

Composite electrode material is the determining factor in the desalination efficiency of CDI equipment. In the study, the electrodes used for the CDI desalination equipment were optimized (composition and process) from agricultural by-products with abundant reserves, available in the Mekong Delta as coconut shell charcoal combined with multi-wall carbon nanoparticles (MWCNTs) with outstanding features of stability, porosity and salt adsorption capacity. Moreover, research also shows that this electrode has the ability to adsorb heavy metal ions and organic compounds as well as handle water hardness.

Porous composite electrode from coconut shell charcoal and CDI module

On the basis of the fabricated composite electrodes, the research team designed the desalination module with 250 electrodes connected in series and designed the fabrication scheme of the CDI equipment system. CDI water purification equipment has been successfully designed and installed with a control system that converts brackish water into fresh water up to the standards of the Ministry of Health. The equipment system consists of 45 details, including some basic details such as chassis system, water supply pump, control valve, fine filter system, DC power supply, control cabinet, PLC display screen, system desalination (CDI module) and storage tank. 

In which, the programmed control cabinet can be operated automatically or manually with simple techniques and operations. The water filtration capacity is about 5m3/ day. The maximum capacity can be up to 7m3/ day with the amount of wastewater accounting for only about 16% of the input water (this value is much smaller than> 50% when using RO technology). In particular, the equipment system has been tested with saline water samples at Cang Long, Cau Ke and Gia Rai in the provinces of Tra Vinh and Bac Lieu. Effective desalination at the 4g/ l salinity boundary opens up potential for practical application of the equipment system in the Mekong Delta.

Diagram of CDI equipment system principles

Completed CDI equipment system, practical operation with salty water inlet sample. A: Main equipment system; B: Saline water tank; C: Finished water storage tank; D: Drain valve

Although only in the initial RD studies, the successful design and manufacture of the CDI desalination system with locally available materials, reasonable cost and high desalination efficiency has important scientific and practical significance, contributing to solving the severe freshwater shortage in the provinces of the Mekong Delta, bringing direct effects to the region's socio-economic development.

Translated by Tuyet Nhung
Link to Vietnamese version



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