What are the Techniques for Boron Removal from Commercial RO Plant?
Reverse osmosis (RO) is a popular water treatment technology used to remove dissolved salts and other contaminants from water. While effective at removing most impurities, standard RO membranes struggle to adequately remove boron. This is concerning as boron can be toxic to plants at concentrations above 0.5 mg/L. Commercially operated RO systems designed to generate high quality permeate for irrigation therefore require additional treatment to reduce boron levels. We will explores various boron removal techniques that can be implemented after RO treatment to ensure irrigation water meets stringent effluent quality guidelines.
Boron Chemistry and Toxicity
Boron is a naturally occurring element present in rocks, soil and water. It is predominantly found as undissociated boric acid B(OH)3 at the pH range of most natural waters. At alkaline pH, boron converts to borate ions B(OH)4-.
Boron is an essential micronutrient for plants, however excessive concentrations can be toxic. Sensitive crops such as citrus can exhibit damage at boron levels above 0.5 mg/L in irrigation water. WHO guidelines specify that boron should not exceed 0.5 mg/L in drinking water, and this guideline is widely adopted for agricultural irrigation.
Standard RO membranes achieve boron rejection rates of just 80-90%. Permeate boron concentrations may still exceed acceptable levels for irrigation usage without further polishing.
Boron Removal by Ion Exchange
Ion exchange using boric acid selective resins is the most established post-RO boron removal technique. Boron selective resins contain functional groups that preferentially adsorb boric acid over other aqueous species. Operating in hydrogen form, the resins exchange H+ ions for B(OH)3 molecules.
Boron selective resins can reliably reduce boron to less than 0.1 mg/L from a 1 mg/L feed concentration with appropriate design and operation. Due to preference for boric acid, systems perform better at acidic pH. Partial acidification of the RO permeate may be necessary to optimize boron uptake.
Fouling of ion exchange resins can occur due to organic matter, scaling salts, iron and microbial growth. Effective pre-treatment is essential to prevent fouling and extend resin lifetimes. Typical service runs are 1-2 years before resin change-out is required. Disposal of spent resins should consider boron loadings.
Boron Removal by Adsorption Processes
Adsorption using activated alumina and other media can also achieve effective boron reduction after RO treatment. At a pH range of 6-9, boron exists in an anionic form which can be removed by anion exchange reactions.
Activated alumina demonstrated good capacity for boron but requires pH adjustment to approximately 6 for optimal performance. Boehmite process alumina at natural feedwater pH showed poorer kinetics but good equilibrium capacity for boron removal.
Functionalized sorbents and metal oxides have also shown promise as adsorbents. Zirconium and titanium based media exhibit preferential uptake of boron over other aqueous ions. Hybrid sorbents combining metal oxides with polymers are also under development.
As with ion exchange, adequate pre-treatment is necessary to prevent surface fouling and blockage of adsorption sites. Typical sorbent lifetimes are 1-3 years depending on influent water quality and system design.
Second Pass RO for Boron Reduction
Two-stage RO with interstage boron removal has been proposed to achieve extremely low residual boron levels. The permeate from the first RO stage may contain 1-2 mg/L boron. This is fed to a column of ion exchange resin or adsorbent to reduce boron to less than 0.1 mg/L.
The polished effluent then undergoes a second RO pass to reduce boron below 0.01 mg/L. The double barrier of chemical adsorption and RO membranes results in greater overall boron rejection. The second RO stage also captures any boron that leaks from the interstage treatment column.
While effective, two pass RO is expensive due to increased membrane and chemical costs. It is generally only warranted when boron must be reduced to levels below 0.1 mg/L. Careful system design optimization is necessary to make this approach economically feasible.
Performance Monitoring and Compliance Testing
Routine testing of permeate boron levels is necessary to demonstrate adequate removal by polishing systems. Quarterly compliance sampling is typical, but more frequent monitoring provides greater operational oversight.
Online boron analyzers linked to process control systems can enable real-time process optimization. However, care must be taken to ensure accuracy and reliability of online boron measurements. Compliance sampling should utilize approved laboratory methods.
If boron levels are observed to spike, steps can be taken such as media change-out, acidification or reducing throughput. Permeate blending with low boron water can also maintain compliance. Such actions ensure irrigation water standards are consistently achieved.
Conclusion
Removing boron from commercial RO plants requires effective post-treatment such as ion exchange, adsorption or second pass RO. Each approach has advantages and disadvantages that must be weighed for specific applications. Processes can be optimized through proper system design, pH adjustment, pre-treatment and boron monitoring. With appropriate polishing technology, RO systems can generate irrigation grade water that meets stringent boron effluent limits.
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