There is some truth to the old environmental engineering adage, “the solution to your pollution is dilution.” But as water and energy become more valuable, old ideas are being challenged. For industrial clients, this means a greater emphasis on recovering water. At the same time, energy generated from non-fossil fuel sources is becoming more desirable. This has lowered the threshold at which wastewater shifts from a headache to a resource. This shift will incentivize developing technologies that focus on harnessing this resource rather than diluting it. One such technology is Forward Osmosis (FO).
Forward Osmosis vs. Reverse Osmosis
Reverse Osmosis (RO) is a well-known method of removing salt from water. Pressure is applied to the side of the membrane with a high salt concentration. This creates the osmotic potential to push water through the membrane, leaving the salt behind. FO works in the opposite direction, pulling water through the membrane by placing an even saltier (100,000-250,000 ppm) draw solution on the other side. This is the natural, low-energy osmotic process with flow across the membrane driven by salt concentration differential, not pressure. The incoming draw solution is more concentrated, resulting in a diluted draw solution on the other side. The diluted draw solution must be regenerated by removing the water or solute, which is where modern, forward osmosis systems shine.
One method of regenerating the draw solution is using a high-pressure RO to remove the salt from the draw solution. This may seem counter-intuitive because the purpose of an FO system is an alternative to RO for removing salt from water. However, the draw solution can be made using only divalent salts such as MgCl, which RO membranes easily remove. Also, the diluted draw solution is a mixture of pure water and the selected salt, without other impurities to clog the membrane. Other methods include unique materials that come out of the solution under specific heat and pressure conditions. Depending on the client’s goals and facility characteristics, these proprietary processes can offer benefits over RO regeneration.
What are the benefits of FO?
Regardless of how the diluted draw solution regenerates, FO systems have some significant advantages over conventional RO when managing wastewater. Most importantly, FO systems routinely recover over 90% of wastewater, compared with 75-80% for an RO system. This goes a long toward helping clients with sustainability mandates or minimum liquid discharge (MLD) regulations. It also means the concentrated reject is twice or greater in concentration than an RO system, making it more valuable for other uses. For instance, twice as much biological oxygen demand (BOD) means twice as much energy per volume for an anaerobic digester.
FO systems can also handle stronger and more complex wastewater. The effectiveness of RO systems decreases drastically as the concentration of contaminants in the wastewater increases. For this reason, most RO systems have several pre-treatment steps upstream, such as micro-, ultra-, or even nanofiltration, to remove larger particles. But FO systems are designed to remove water from wastewater, allowing particles to continue toward the reject stream instead of blocking membrane pores. This also results in less frequent backwashing cleanings. When an FO membrane does need to be cleaned, it is a simpler process due to the lower operating pressure. Particles are not embedded into the membrane under pressure, making them easier to remove with the cleaning solution. This further increases the longevity of the FO membrane because those embedded particles in an RO membrane lead to irreversible performance loss (or permeability).
FO is not for everyone
For all of their advantages, FO systems do have some drawbacks. They generally use more energy per unit of wastewater treated and can have a higher capital cost, depending on the type of draw regeneration. The best situation for an FO system is typically managing a wastewater stream that is strong and complex. An FO system can recover water for reuse and generate a stronger wastewater that changes waste into a valuable product for another user. No single technology is appropriate for every situation. A skilled water or wastewater engineer can help you determine which solution is right for you, and FO is a welcome addition to the toolkit.
