Boost your Forward Osmosis solution with CounterFlow Reverse Osmosis
When using forward osmosis for brine concentration, you need a system to regenerate the draw solution to create a high osmotic pressure and at the same time extract clean water for reuse. The most effective way to do this is by using CounterFlow Reverse Osmosis, which can achieve brine concentrations of over 260,000 ppm TDS at significantly lower costs than thermal evaporators.
As contaminated high TDS effluent flows through a forward osmosis (FO) process, the highly saline draw solution is responsible for extracting clean water through the FO membrane. If clean water is not continually removed, the draw solution gradually becomes diluted and less effective as it loses osmotic pressure. This is why an effective draw recovery system is a key component of a FO process. It both extracts purified water for reuse and restores the total dissolved solids (TDS) concentration of the draw solution.
The most common way to recover water from the draw solution is reverse osmosis which typically operates up to 1000 psi/70 bar. High pressure RO systems can push this limit further to 1740 psi/120 bar but already have challenges with membrane and process stability arise not to mention higher energy cost. The salinity needed for a potent and effective draw solution in many applications easily exceeds 100,000 ppm TDS and can reach up to 200,000 ppm which makes it impossible to use traditional RO as a recovery method.
Minimize the need for costly evaporators
To reach concentrations beyond that TDS level, draw recovery processes are limited in number or still in a development stage and thus not reliable. Another option would be to use thermal evaporators either for direct treatment or additional draw concentration. However, this is an energy-intensive process with high OPEX and CAPEX cost. In addition, evaporator-based treatment systems are prone to scaling and fouling issues, which increases the risk of downtime and the need for maintenance.
A better solution for reaching higher TDS levels is the method known as osmotically assisted counter flow reverse osmosis, which uses a dilute saline solution to reduce the osmotic pressure barrier and thereby reduce the required feed pressure.
At Aquaporin, we have partnered with the water tech company Gradiant who have developed this type of solution under the name CounterFlow Reverse Osmosis (CFRO). In Gradiant’s CFRO process, the draw solution is cascaded through multiple CFRO stages, concentrating it while producing a clean water permeate for reuse.
With CFRO, it is possible to achieve draw solution concentrations up to 260,000 ppm TDS using less than 1,000 psi feed pressure. Since this eliminates the need for thermal evaporators for draw solution regeneration, the OPEX costs of a CFRO solution are also significantly lower compared to conventional thermal evaporators.
FO and CFRO – the perfect partnership
The combination of FO and CFRO technology forms the perfect partnership for industrial wastewater management and achieving zero liquid discharge reliably and economically. They can be compared to a rugged truck and a fine-tuned racecar, each with their unique strengths and limitations. By sandwiching the two technologies, they enable you to treat wastewater with extreme levels of both TDS and COD.
On one side, you have the FO doing the hard and dirty work of handling the contaminated effluent. Because the FO functions at no or low inlet pressures, it has a lower risk of fouling and the high rejection of TDS. Aquaporin Inside® FO membrane makes it possible to handle high levels of contaminants like COD.
On the other side, you have the CFRO, which like a racecar performs excellently in the cleaner atmosphere of pure saline water. Like all RO membranes, the membranes in a CFRO solution are sensitive to the quality of the water and should ideally only handle pure salt. If the feed water is contaminated, you get fouling and scaling.
By integrating FO with CFRO, you clear away the impurities before the water reaches the CFRO, which in turn delivers a top-quality draw solution that lets the FO keep function at peak efficiency.
Want to test the efficiency of FO + CFRO for yourself?
The combination of FO and CFRO has significant potential for multiple industrial and municipal applications, including textile, power, semiconductor, oil & gas, mining and food & beverage.
When using FO+CFRO for industrial waste water management, it is possible to achieve a highly effective concentration of the effluent, which both allows you to reduce the load on your thermal evaporators in zero liquid discharge systems to save energy and recover more clean water for reuse.
Each process is unique, so the best way to evaluate the full potential for your facility is by running a pilot trial before implementing a full-scale solution.