Reverse Osmosis (RO) is a membrane filtration process that uses special membranes to produce fresh water from salty waters or industrial wastewaters, but at the same time a so-called RO reject or brine is produced.
In this article, we will explain how to concentrate or crystallize the RO reject and increase the recovery ratio of every RO systems by presenting the best Zero Liquid Discharge solution for any water and wastewater treatment plants.
What is RO Reject or Brine?
Reverse Osmosis (RO) Reject (or Brine) is the by-product of the membrane filtration of salty waters.
Reverse Osmosis (RO) water treatment equipment, such as REWATER, has been used for years in various industries to separate dissolved solids from water by forcing the water through a semi-permeable membrane.
The water and other molecules with lower molecular weight (specific weight of molecules allowed to pass through is dependent on the selected membrane) pass through the micropores in the membrane, yielding a purified water stream called the permeate. Larger molecules are retained by the membrane as well as a portion of the water that does not pass through the membrane. This concentrated stream is called concentrate, RO reject, or brine in the case of salty water desalination.
The RO reject can be further concentrated to enhance the recovery rate of such systems thanks to evaporators and crystallizers.
TDS Concentration of RO Reject and Concentrate (or Brine)
The typical RO reject or concentrate TDS levels are often in the range of 30,000~50,000 mg/L.
The salts in solution can range from highly soluble sodium and potassium salts to less soluble or reverse soluble calcium and barium salts. In this application a correct process design and material selection needs to be planned carefully to increase the system efficiency and life.
Reverse Osmosis Concentrate and Reject Treatment
One of the major drawbacks with RO is the volume of concentrate (reject) produced during the filtration process. Several technologies and process configurations, which are available for further treatment of RO concentrate, reduce the reject volume.
Often, concentration of RO brine is limited by scaling ions, such as CaSO4 or SiO2 and other low solubility compounds. If these scaling ions are removed, or separated, it can increase the recovery of membrane systems and reduce the volume of brine.
Treating concentrated RO brine to reduce the disposal volume further or produce zero liquid discharge solids can substantially increase capital and operating costs and requires equipment with a larger physical footprint, such as evaporators and crystallizers.
To reach the highest concentration levels with RO brine, a pre-treatment system is needed to remove scaling ions from the reject water.
Using Evaporators and Crystallizers to Dewater Reverse Osmosis Reject Streams
In industrial settings, RO is often used to remove total dissolved salts (TDS) from industrial wastewater or treated industrial wastewater and yield permeate with relatively low TDS concentrations and the reject streams can be further concentrated by evaporation and crystallization.
Specifically, the high salinity in RO reject and concentrate can make it prohibitive for discharge to a local sewer facility. Hauling the wastewater to a treatment facility can also be prohibitively expensive.
What is the best solution for RO reject?
The best solution for treating RO reject should be approached from two directions.
Firstly, maximize the recovery of lower cost membrane systems to minimize the volume for later steps. Secondly, weigh brine disposal options against thermal treatment technology to determine whether you need to concentrate your brine or treat it down to solids.
Reverse osmosis (RO) is the most widely-used saline water and wastewater treatment technology for a good reason; it is a cost-effective option for removing contaminants from water, but the concentrate or reject can be a real issue in the long run.
Treating RO Brine: to Concentrate or to Crystallize?
The options for treating RO brine depend on whether your company needs zero liquid discharge or a reduction in brine volume.
If you do not have a disposal outlet available, concentrating brine with an industrial evaporator can offer advantages. Further treatment by crystallization is usually the most expensive step and should only be considered if you need to produce solids, or face high disposal costs.
Zero Liquid Discharge (ZLD) Solution for RO Concentrate and Reject
When there is an interest or requirement to achieve zero liquid discharge (ZLD), evaporation can be used to concentrate the RO reject and concentrate, which can then by further dewatered with supplemental equipment.
In the first stage of handling RO reject and concentrate, our EVADEST evaporator is fed the reject and it yields distilled water and concentrate.
The distilled water can reach high purity levels and can be reused in the production line or simply discharged. Instead, the concentrated reject has very high salinity levels with TDS well above 260,000 mg/L and can be further treated by crystallization processes.
For increasing the energy efficiency of this system an MVR evaporator can be used instead of the classical heat pump technology.
Crystallizer for RO Reject or Brine Crystallization
YASA ET SOLIDEST crystallizer for RO reject and brine crystallization are used to meet stringent zero liquid discharge (ZLD) requirements.
SOLIDEST Crystallizers are used to concentrate feed into solid crystals and clean water. Thanks to this solid-liquid separation technology solid crystals are formed from brine or RO reject. For more information you can click here.
YASA ET evaporators and crystallizers have proven to be effective technologies for dewatering RO reject and concentrate waste streams. In brief, evaporation is a time-tested methodology for reducing the water portion of water-based waste. The evaporator converts the water portion of water-based waste to water vapor, while leaving the higher boiling contaminants behind. This greatly minimizes the amount of waste that needs to be hauled off-site.
Instead, the crystallization process further reduces the waste stream by crystallizing the solids in any RO reject and brine. The solids obtained from this process can be either reused or brought to a solid waste disposal facility.
In the case of high brine and RO reject volumes a multiple effect evaporator with integrated crystallizer can be the best solution to save energy and increase the total system efficiency.
Electrodialysis and EDR For RO Concentrate Treatment
Another option to maximize brine concentration is through selective ion removal with electrodialysis (ED) or electrodialysis reversal (EDR) systems, which can concentrate brines up to 200,000 mg/L TDS.
In the ED process, a stack consists of alternating cation and anion selective membranes between a cathode and anode. Cations are attracted to the negative cathode and pass through the cation transfer membranes. Meanwhile, anions are attracted towards the positive anode and pass-through anion transfer membranes. Hundreds of cell pairs are included in an ED stack.
The membranes are periodically cleaned by either clean-in-place or disassembling the stack. ED has been shown to be feasible and a cost-effective alternative for treating RO concentrate with low to moderate salinity.
One of the major drawbacks of utilizing ED for RO concentrate treatment is the scaling potential of sparingly soluble salts, restricting ED performance. To restrict the precipitation of inorganic salts.
A particular advantage of using ED/EDR for RO concentrate treatment is when the concentration of silica exceeds saturation limits. Since silica is uncharged under neutral pH conditions, the ion exchange membranes utilized in ED/EDR systems does not reject silica and it is passes along with the final reject stream.
However, fouling of the ion exchange membrane stack with organic matter must be prevented using appropriate pre-treatment. The use of ED/EDR for RO concentrate treatment is cost-effective only when the TDS is <3000 mg/L. Higher TDS values will result in substantial energy costs associated with these systems.
Click here for more information about YASA ET Electrodialysis and EDR systems.
What Is the Best Treatment for RO Reject and Concentrate?
The best treatment system for RO reject and concentrate often involves various stages and technologies. The correct equipment needs to be selected and carefully manufactured to reach optimal results.
The first step in designing a RO reject treatment system is to analyze the chemical composition of the fluids. Secondly, extensive testing needs to be undertaken to find the correct parameters for treating the specific brine. Finally, the right equipment for the treatment system can be manufactured and the system can be installed at the client site.
In the case of RO reject, the concentration can be performed by vacuum evaporators, such as EVADEST; it can be crystallized by SOLIDEST crystallizers; or it can be furtherly concentrated by Electrodialysis or Electrodialysis Reversal Systems.
For the right treatment system, you need the right expertise.
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