Desalination (Brackish Water TDS Reduction)
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.
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 antiscalant 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.
The Benefits Of Electro-dialysis For Brackish Desalination
Today’s generation of state-of-the-art 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 easily stacked on an ED skid with a very small footprint. In addition, ED ‘tunability’ offers operators (designers) 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 according client preference.