Brackish water TDS reduction
Treating Brackish Water and high Total Dissolved Solids
The term ‘Brackish’ is a general description for water with salinity between that of fresh water and sea water. While the widely accepted range for brackish water is from 500 to 30,000 parts per million of sodium chloride in solution, most brackish water taken for treatment and purification contains less than 3,000 ppm NaCl.
Brackish water is very abundant in estuaries, where river water and sea water mix but it is also found in many lakes and aquifers. It can be a very effective resource, particularly in areas of limited fresh water availability, with many uses ranging from potable water supplies to ultra pure, industrial process water.
Typically, brackish water sources can vary significantly in salinity over time and their chemical composition can be very different from location to location. This variability in the raw water supply presents a special challenge for pretreatment and purification systems.
Brackish Water purification:
Distillation was the common water recovery process for sea water and brackish water until the 1970s, when Reverse Osmosis found a new application in large-scale desalination plants. While sites such as power stations and refineries can provide the heat required for thermal distillation techniques as a by-product, others lack a ‘low cost’ energy source and desalinate using RO systems.
Today, with expertise in multiple desalination methods, Pure Water Group can design and supply systems that are optimized according to the characteristics of the source water and the product water quality required. The preferred technologies are Reverse Osmosis (RO) or Electrodialysis Reversal (EDR).
By far the most common method of the two is RO. However, if the brackish water is in the 2,000 to 15,000 ppm range of total dissolved solids (TDS), the latest EDR technology could offer a viable alternative to RO and potentially be a better solution.
In the RO desalination process, the brackish feed is pressurized, at 250 to 400 psi, to induce water to pass through a water-permeable membrane. Dissolved salts are therefore concentrated on the pressurized side of the membrane and a proportion of the brine concentrate is discharged from the RO vessel. This maintains the osmotic pressure and prevents the RO pressure having to be increased as the concentration builds. The permeate (RO product stream) flows from the RO vessel and may undergo post-treatment, particularly if it is required for potable water use.
The main stages of a reverse osmosis system consists are (1) pretreatment, (2) pressurization, (3) membrane separation, and (4) post-treatment stabilization:
Pretreatment: The brackish feed water is pretreated to prevent fouling of the RO membranes by filtering suspended solids. The pH may be adjusted and adding a chemical softener added.
Pressurization: Electric pumps create the pressure needed to process the pretreated water effectively, according to the salinity of the feed water.
Separation: The RO membranes separate the fresh product water and the brine reject streams.
Stabilization: If post-treatment is required the permeate may be subject to pH adjustment and degasification.
Electrodialysis has been a very capable technology for the desalination of brackish water for many years but the newest systems have reduced the cost significantly.
quality, including waste water with high turbidity. Like ED, EDR equipment can regulate the amount of salt removal by adjusting the electrical potential applied to the electrodes. Automated DC power control can therefore maintain stable product water quality even under conditions of significant feed water variation.
In addition, EDR periodically reverses the electrode polarity to drive ionic flow in the opposite direction. This is analogous to ‘backwashing’ a filtration membrane to remove deposits. This self-cleaning capacity reduces scaling and extends the service life of the membrane stack. It can also reduce pre-treatment requirements and widen the area off application to include boiler blowdown and challenging process water waste recovery.
Our reference projects
For more detailed information please have a look at our reference projects.
The benefits Of Electrodialysis for brackish desalination
Today’s advanced ED (and EDR) units are often described as ‘tunable’. In other words, the electrical energy applied can be changed, according to the amount of salt that needs to be driven out of the feed water. When the feed quality improves, the energy requirement reduces and that energy is saved. For example, if water is abstracted from a river estuary, the salt content might vary by a factor of 10 or more but the output quality remains consistent with Electrodialysis.
This is not the case for RO, which requires a steady, high pressure (70 bar) – and therefore energy input – to deliver consistent quality. Operating at less than 7 bar, Electrodialyisis a quiet-running and less demanding technology, in terms of the high pressure pumps, valves and piping that are not required.
There are also benefits when it comes to cleaning and maintenance. When an ED module needs to be cleaned, the clean-in-place (CIP) procedure involves no drop in flow rate, or water quality. The electrical power to the other modules is increased to compensate, maintaining the overall system treatment capacity.
Electrodialysis modules are relatively compact and the best examples can be stacked on an ED skid with a very small footprint. In addition, ED ‘tunability’ offers operators a degree of flexibility in this respect. For a given flow rate and output quality, they can either opt to use more modules and restrict the energy consumption, or reduce the number of modules and increase the energy input.
Experience, expertise and attention
We believe there are only a few companies in the world to rival what we do.
Thanks to our extensive experience in EDI solutions for power, pharmaceuticals, hemodialysis, petrochemicals and microelectronics applications, we developed essential insights into the technology and how it meets specific requirements ‘in the field’. This is reflected in the quality of training, service and validation we provide. So why not contact us? Our advice is free!