Q&A: Treating Reverse Osmosis Reject with the Atlantis RDI™ Capacitive Deionization System

Patrick Curran, CEO, Atlantis Technologies

What is reverse osmosis used for?

Reverse osmosis (RO) is a water desalination technology that uses a semipermeable membrane to separate ions, molecules and larger particles from water. Reverse osmosis is widely used and recognized as a leading technology in the desalination process for low, medium, and high salinity applications. Improvements in RO technology have reduced costs and made the process even more viable for commercial applications in water treatment and reclamation.

Are there limitations or challenges to using RO?

The concentrated waste stream after separation – “RO reject” - is high in salinity and RO anti-scalant chemicals. Second stage RO systems can be installed to process the RO reject, but it is a more expensive and difficult application and many customers are satisfied with the recovery of the first stage.

Are there options to treat the RO Reject?

There are 4 practical methods for treating RO reject.

  1. Second stage RO system: Typically expensive because the salinity is higher and usually requires seawater membranes and high energy.
  2. Thermal evaporation: Equipment is expensive due to exotic corrosion-resistant materials of construction and operating cost is high due to maintenance of this type of equipment and energy required to boil water.
  3. Evaporation ponds: Low capital and operating costs, but not practical in most locations due to geography constraints and weather.
  4. Capacitive deionization: The latest generation of capacitive deionization from Atlantis Technologies can economically desalinate water with lower capex and opex than other options and a higher recovery and process water up to 100,000 ppm.

How does capacitive deionization work?

Radial deionization is an improved and patented form of capacitive deionization developed over the last 10 years by Atlantis Technologies. With the Atlantis radial deionization – RDI™ - system, water is passed between two oppositely charged supercapacitors which remove the salt. Once full, the capacitor polarity is switched and a low-volume, high-concentration brine is produced.

This desalination process is done at lower cost of ownership and higher clean water recovery than other capacitive deionization and reverse osmosis systems. RDI cylinders and system can be placed in parallel to increase volume or in series to process high salinity water. The system can also partially desalinate water (TDS shaving), further improving economics for applications such as cooling towers and waste discharge.

Where in the process does Atlantis RDI system fit?

Depending on the circumstances, the Atlantis RDI system can pretreat water for RO or treat the RO reject. For example, if the water is greater than 45,000 ppm, which is too high for RO, the RDI can perform a partial reduction and hand off to RO for final desalination. Or, the RDI can treat the RO brine as described above, or both.

How is RDI different from other capacitive deionization systems?

The RDI supercapacitor design allows for water to flow across up to 1 meter of continuous supercapacitor material. Other capacitive deionization systems only allow for flow across 10 cm of material, or 1/100th of the distance. The long distance enables greater total dissolved solids (TDS) reduction and allows processing of very high TDS streams and flow rates.

The RDI desalination system also incorporates proprietary operating techniques that enable high flux rates, improved fouling resistance, improved energy conservation, higher recovery/brine concentration, and lower capital costs. The RDI supercapacitors also incorporate the latest generation materials for extremely high salt capacity.

Is RDI used in other applications?

RDI can be used in a wide variety of applications where only partial desalination is needed (cooling tower feed, RO reject, discharge compliance), high hardness (FGD, oil, gas, mining, brackish groundwater), high recovery applications (cooling tower, groundwater), and high contaminants. The RDI has no inlet limits for alkalinity, hardness, or silica and generous specs for organics and TSS.

How much does it cost?

While specific capital and operating costs depend on inlet flowrate, inlet salinity, and outlet target, the capital cost of an RDI system is about equal to an RO system and about half of a thermal system. Average operating costs (energy, chemicals, maintenance) are about half of RO and one quarter of thermal.

How can using RDI actually reduce water treatment costs?

Hauling brine for disposal can be a significant expense. A personal care products manufacturer in Corona, CA, recently installed an RDI system to process RO reject to reduce the brine required to be hauled away for disposal. The customer reduced their brine disposal by 80%, saving $100,000 in disposal costs with a 6-month payback on the equipment.