Aquatech’s Concentrate Recycle Reverse Osmosis (CRRO™) is a process that operates on the concept of osmotically assisted reverse osmosis. Unlike conventional RO, where the permeate concentration approaches zero, CRRO, to start with, has a permeate-side equal to or slightly lower salinity solution to reduce the osmotic pressure gradient across the membrane. This enables water transport evenly and facilitates permeate flow even when the osmotic pressure is higher.

The process produces a permeate product with 40-50% salt rejection in a single stage.  When multiple CRRO units are linked in series, this process progressively enables the recovery of lower concentrations than feed from high salinity brines and has been successfully used to further concentrate RO brines to more than 20% salt concentration.

Figure 1: Conventional RO vs. CRRO

The concept also allows reduction of osmotic pressure by generating higher permeate concentration like sea water RO feed utilizing Seawater Reverse Osmosis (SWRO) reject as the feed water. The permeate water generated in this water can be mixed with an upstream SWRO feed or treated in a separate downstream seawater desalination plant. The CRRO process may use a hollow fiber or spiral flat sheet membrane. Though each membrane uses a slightly different approach in the flow scheme, both approaches involve the progressive concentration of brine while diluting the permeate to allow for treatment through an SWRO system.

In the CRRO process, high salinity water is introduced into a CRRO module at high hydraulic pressure. A low-pressure product with a moderate total dissolved solids (TDS) concentration flows on the permeate side of the module. The high-pressure feed and low-pressure on the product side establish a hydraulic pressure gradient that drives water transport across the membrane, concentrating the feed and diluting the product side salt concentration. The concentrated feed is rejected from the CRRO process and can be used for further requirements like evaporation or disposal.

Different CRRO incorporated processes are available. The objective is to minimize the brine solution and to achieve a recovery of about 75-80%. As an integrated CRRO process, this recovery is obtained with various feed pressure and several stages. Figure 2 is based on recycling a concentrated RO stream after diluting a part of it into the CRRO process.

Figure 2: Aquatech’s CRRO process integrated with an RO process

The proposed CRRO system (as shown in Figure 2) is an integrated system consisting of an RO system followed by a multi-staged CRRO process. In this process, SWRO reject/brine is directly fed to the brine concentrate system. A fraction of the RO brine/reject is used as dilute stream (DS) carrier water and another as feed solution (FS) to CRRO. Permeate, which is now diluted brine, is recirculated back to the RO feed stream.

• Lower Applied Hydraulic Pressure – CRRO can be operated at lower applied hydraulic pressures of 70-80 bars, which permits the dewatering of more saline feed streams without exceeding the burst pressure of the membrane module.
• Increased Brine Concentration – CRRO can concentrate the brine to more than 20%, nearly closer to what a thermal brine concentrator can achieve.
• Lower Capital Cost as compared to other options.
• Improved Process Reliability – CRRO utilizes the existing, proven equipment like membranes, pressure tube, HP pumps and ERI.
• Smaller Footprint as compared to other options.
• Increased Energy Efficiency – CRRO is more energy efficient, but a secondary purification process is required to lower the salinity of the diluted water to freshwater standards. It does not require steam.