How can membranes help solve water scarcity and improve resiliency?

Christian Sanders is an envi­ron­men­tal engineer who has worked on water cleaning projects all over the world and is one of the leading experts pushing membrane filtration and separation tech­nolo­gies to new heights. 

In this article, he answers the following 3 questions: 

• What are the major challenges associated with membrane separation processes? 
• How do we produce the maximum amount of clean water from a given water source? 
• How has public perception of water recycling changed as the technology has advanced? 

Energy costs and brine disposal are really the two biggest hurdles. When it comes to costs, over the last 50 to 100 years, we've been relying on conven­tional sources of water, and generally we can use gravity for a lot of those treatment processes. As we pivot to alternative sources of water for meeting potable demands, additional treatment is often required.  This additional treatment has increas­ingly included the use of reverse osmosis (RO) membranes which are a highly effective barrier against dissolved salts and a wide variety of water quality cont­a­m­i­nants.  Compared with conven­tional treatment processes, RO treatment requires a significant amount of pressure to drive water across the semi-permeable membranes, and the cost of providing that hydraulic pressure comes at a cost. What we have seen, then, is a significant increase in the cost of bringing on new supplies of water. To recoup these additional energy costs, municipal agencies are enacting rate rises to pay for them. 

Another factor to consider with RO treatment is that it generates a concentrated waste stream in the form of brine. If you’re near the ocean, ocean discharge is an option. If you’re at an inland location, however, brine disposal options are more constrained.  They include evaporation, conveyance to a river, or pumping it into the ground.  In most cases, the disposal of brine adds additional complexity to the project and increases the overall cost of the water further.

The quality of the source water, as well as the cost of treatment, ultimately determines what recovery is practical for a project.  As we drive towards higher recovery targets to minimize our waste streams, while at the same time utilizing more saline source waters, we have to deal with the impli­ca­tions of concen­trat­ing up sparingly soluble salts to the point where they precipitate out of solution and foul the membranes.  Scaling results from dissolved salts concen­trat­ing up on one side of the semi-permeable membrane facil­i­tat­ing the separation process.

When we look at recovery, we are trying to find a sweet spot in which as much water can be recovered without putting operations at the limits of what is possible. The goal of a well-designed treatment process to achieve a buffer between what recovery is theo­ret­i­cally achievable and where a system is designed to operate. This ultimately depends on the source water quality.

In general, when wastewater or groundwater is the source water, recoveries between 85% and 90% are typical, though slightly higher values have been achieved success­fully.   With seawater, which is much higher in dissolved salts, the targeted recovery is much lower, 40%-45% being typical.

Public perception of using membranes for water reuse has completely transformed as the technology advances in this rapidly evolving and dynamic field. Public outreach is still crucial for completing these projects, but large orga­ni­za­tions and agencies are 100% on board with potable reuse now, especially in my state of California. Further, states are now generating regulations to guide these processes which is providing certainty to agencies and the public alike. As it stands now in the US, concerns of public perception regarding water reuse appear to be in the rearview mirror as the industry and the communities it serves embrace the move towards treating water in new ways to improve water resiliency.