RO reject recovery
recovery includes the potential to reuse
Reverse Osmosis reject water
The percentage of permeate separated from the feed water (RO recovery rate) normally ranges from 50-75%, so the remaining 25-50% of the RO feed water is discharged as wastewater.
So how is the overall recovery rate improved?
One option is to reuse the water rejected by a first stage RO system and introduce it as feed to a second RO stage. However the high salinity and Total Dissolved Solids (TDS) of the first stage RO reject / RO concentrate means that a further treatment is required to prevent excessive scaling in the secondary RO membrane.
With the right membrane filtration (MF) system between the RO-RO sequence it is possible to extend the recovery rate beyond 95%, even though the TDS concentration in the initial RO reject may be in excess of 45,000 mgl-1. Rather than moving to an entirely new desalination system, it can therefore be advantageous to simply add a second RO stage, with prefiltration, as a retrofit.
The high TDS RO reject typically contains very high levels of dissolved ions, silica, organic compounds and hardness-causing calcium and magnesium salts. It is important to take the bulk of these out of solution before the reject stream is membrane filtered.
In a 2-stage RO system the membrane filtration step normally adds softener and flocculant, inducing solids to settle out of solution in a concentration vessel. In the case of a Zero Liquid Discharge (ZLD) design, the excess solids are sent to a filter press for de-watering and disposal. Further solids removed by the membrane filters are also returned to the concentration tank, while the MF filtrate goes to the second stage RO.
Drawbacks – Recovering water from RO reject / RO concentrate using this approach is very feasible but it has a high maintenance component. The system is continuously combating silt fouling, biofouling and scaling of membrane surfaces and necessitates adding chemicals as part of the process. A far simpler alternative involves Electrodialysis:
Pure Water Group’s Electrodialysis (ED) and Electrodialysis Reversal (EDR) modules can produce a high recovery rate of reverse osmosis (RO) reject water, without membrane fouling and scaling problems. It was once cost-prohibitive to use electrodialysis in this role but the new technology follows a decade of research and development that has removed the problem. Today’s new generation Electrodialysis modules have a cross-flow fluid path, high performance membranes and lower cost control technology. Modulating the applied DC power also allows tuning of the process to select a desired quantity of salt removal.
The advantage of EDR over ED is that a periodic reversal of electrode polarity produces the equivalent of backwashing of the Electrodialysis membrane, which also reduces anti-scaling pre-treatment requirements. Our EDR modules have already been used to increase the overall RO recovery to 96%.
How Does Electrodialysis Work?
The function of Electrodialysis is to divide the input water into concentrated (reject) and dilute (product) streams, transporting dissolved ions by an electrical force. Under a DC potential, negative ions (like Cl-) are attracted towards an anode and positive ions (like Na+) are attracted towards a cathode. Between the two electrodes, a series of compartments are formed by ion exchange membranes. The membranes are stacked alternately, according to their ionic charge, so that the majority of dissolved ions permeate through to the reject streams.
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. Consequently, the process is self-adjusting to fluctuations in the RO reject quality.
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 water treatment solutions for a wide range of applications, we developed essential insights into the EDI, MD and EDR technologies and how they meet specific requirements ‘in the field’. This is reflected in the quality of technical/commercial support, training, service and documentation we provide.
So why not contact us? Our advice is free!