For fans of reverse osmosis (RO), there is good news, and there is better news.

The good news: RO technology — which involves pushing water under pressure through a semipermeable membrane —does a great job of treating tough contaminants in our drinking water. In addition to treating typical contaminants like chlorides and nitrates, it has the potential to treat emerging contaminants like perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA).  Depending on the contaminant, concentration and the type of RO system selected, RO can remove up to 99% of contaminants.   

The better news: Methods for disposing of these concentrated contaminants are growing in number and becoming more effective.

To dispose of high concentrations of harmful contaminants, many landlocked water suppliers are injecting RO contaminants in deep wells drilled hundreds of feet below the earth’s surface. Storage capacity in these formations, however, is limited, and other industries are using the wells for similar purposes. These factors, along with the rising fluid storage volumes in disposal formations, are leading cities like Hutchinson, Kansas, to take a proactive approach to reducing or eliminating its RO concentrate disposal by deep well injection.

Finding Alternatives for Hutchinson

Having injected RO concentrate in deep wells for nine years, Hutchinson asked us to look for alternative, longer-term methods of RO concentrate disposal.

We considered seven. For example, we investigated current water quality and considered ways to improve source water management so that the city could increase recovery through the RO units and reduce the volume of concentrate injected into the deep wells.

We also considered disposal methods that eliminated deep well injection altogether, discharging the RO concentrate to the sanitary sewer system for treatment at the wastewater treatment plant (WWTP) or implementing technologies like electrodialysis metathesis or crystallizers.

One approach, for example, called for the RO concentrate to have a dedicated pipeline that allowed for the concentrate to blend with effluent from the WWTP. Because the sulfates and chlorides in the concentrate can kill the microorganisms needed for treatment of municipal wastewater, it would need to be blended with the effluent leaving the plant, rather than the influent entering it. The blended effluent could be discharged into local streams at permitted concentrations. This strategy, coupled with the potential for beneficial reuse, transforms the concentrate — currently an expensive waste product — into a value-added solution. 

The “Total Water Solution”

For Hutchinson, the exploration of concentrate disposal options led to a “total water solution” that could keep concentrate water in the water cycle, maintain the integrity of local streams and reduce or eliminate the use of the deep wells for concentrate disposal with a relatively inexpensive capital and O&M cost.

This solution will take time to implement. In the meantime, the city will pilot zero-cost operational changes identified in this study that will reduce the volume of RO concentrate injected into the injection wells.

The lesson for current or future RO operators is simple. Waste streams from water treatment are very important parts of your project, and it’s worth your time to explore and consider your disposal alternatives. There are likely many opportunities to optimize your disposal method that may help reduce your environmental footprint.

 

Water treatment processes are complex and must be customized to each community’s environment. A thorough process evaluation can help determine an effective path to safe, reliable water supplies.

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by
Michaela Rempkowski, PE, is a chemical engineer for Burns & McDonnell who specializes in water, wastewater and reuse treatment process design. Having worked on water, wastewater and reuse treatment facilities in Georgia, Kansas, Missouri, Montana, Oklahoma, New Mexico, Tennessee and Texas, she brings a broad and unique perspective to treatment process selection.