Reverse Osmosis And Granular Activated Carbon - How Does It Work?

Dec 23, 2019

Almost two and a half years ago, the public learned that the Chemours Company was discharging a variety of toxic chemicals into the Cape Fear River. Now on both sides of the river, utilities will need to spend tens of millions of dollars on water filtration systems. The Cape Fear Public Utility Authority has chosen granulated activated carbon. In Brunswick County, it’s reverse osmosis.  Both technologies come with a hefty price tag.

At the CFPUA’s Sweeney Treatment Plant off North 5th Avenue between Smith Creek and the Northeast Cape Fear River - the sound of construction.

The utility is building a granular activated carbon facility. It will measure some 18,000 square feet. Dr. Detleff Knappe of N.C. State University, whose team of researchers found the PFAS contamination a few years ago, explains how this type of filtration works.

“So with granular activated carbon, which I call G-A-C, other people call GAC, that's a process where contaminants that are in the water get absorbed. And absorption means that the chemicals that are in the water accumulate on the surface of the activated carbon, and activated carbon has some pretty amazing material in the sense that a teaspoon full of that material has a surface area that is the size of a football field. So there was a lot of internal surface area on that material that can accumulate contaminants.”

The basics. Water for the Cape Fear Public Utility Authority comes from the Cape Fear River. Most of it. Some does come from wells, which is filtered at the Richardson Plant. From the river, the CFPUA grabs it. It pours into the existing filtration system.

Before we all learned of GenX and PFAS, the CFPUA already had a filtering process for the water that gets to the spigots of tens of thousands of residents and businesses.

It starts with adding ozone gas, used to remove some of the organic material in the water. That’s followed with flocculation. Yes – flocculation.

It’s a process that causes unwanted destabilized particles to clump together and drop out. It settles into a sedimentation basin. After getting the heavy material out, the water moves on to biological filters. It ends at the UV facility, where ultraviolet light hits the water to disinfect and destroy viruses. Finally add a bit of chlorine, and the river water goes into the system as drinkable.

Carel Vandermeyden is Deputy Executive Director of CFPUA.

He says ozone, UV, and bio filtration are already considered advanced technology. 

“The unfortunate part is that none of those advanced treatment technologies, can remove PFAS. And that's why we have to add an additional treatment process to what we already have, to address the PFAS in our water.”

He says the benefit of GAC is all that carbon can be recycled. After 400 days or so, each bed of carbon gets recharged, regenerated. Vandermeyden says G-A-C will be sent offsite to undergo a process where it is subjected to extreme heat, which destroys the PFAS that were absorbed while retaining the carbon. Regenerated G-A-C can then be returned to service, with the same adsorbent capacity as if new.

The new system will cost CFPUA $46 million and then $2.9 million per year to maintain. Customers will pay for the new facility, although CFPUA is currently in litigation to force Chemours to eventually pay for all the costs.

Across the river in Brunswick County, reverse osmosis is the name of the game. It’s a lot more expensive.

“Absolutely. And that's because there's a higher capital cost, there's a higher energy usage as well. When it comes to the entire treatment process, you have a lot of wastewater that you have to deal with one way or another.”

That’s Mike McGill, President of WaterPIO. He used to work for CFPUA and also H2GO – the regional water utility that serves 11,000 customers in northern Brunswick County. WaterPIO works with water utilities across the country, which are dealing with contamination and other environmental issues.

H2GO is building a reverse osmosis facility for its customers. Besides being more costly – RO comes with something else: wastewater.

Tom Speth is associate director for the center for environmental solutions and emergency response at the U.S. Environmental Protect Agency.

“And for RO, the waste stream is, with regard to the concentrates. So about 20% of the flow into the plant, approximately, needs to be a wasted and sent somewhere and permitted to be sent somewhere. And that's not an insignificant amount of flow. And right now for PFAS, there aren't a lot of treatment technologies that can handle that more concentrated flow at those relatively high flow rates.”

Here’s how it works. RO pushes the water through membranes, made of polymers. Dense material. It does this with a lot of water pressure. These membranes are able to stop the contaminants, sort of like they hit a wall and fall away. During this process, about 20% of the water falls away. It’s very concentrated with PFAS compounds. It has to go somewhere.

However in the case of H2GO, which does not anticipate a utility increase to cover the project, there will be less wastewater. That’s because H2GO will be drawing its water from deep well, confined aquifers – not the river. Well water is cleaner than what’s coming downstream. H2GO Spokesperson Tyler Wittkofsky says any wastewater from the RO process is expected to have salinity approximately equal to seawater and is to be permitted and discharged into the Brunswick River.

Meanwhile, for Brunswick County, the full expansion project at Northwest Water Treatment Plant including the reverse osmosis system will cost an estimated at $137 million. The county has around 43,000 water customers.

Those customers will likely see a rate increase. The Board of Commissioners plans to consider an increase to cover existing costs and additional operating costs.

Brunswick County has a three-year permit from the state’s Department of Environmental Quality to discharge the RO wastewater back into the Cape Fear River. The County will monitor contaminant levels at the outflow, under DEQ guidelines.

During the RO process, all minerals are also stripped out of the water – which is not a good thing. Mike McGill.

“Correct. One of the, one of the reasons why RO – another problem with RO - is it strips out your corrosion control, which leads to the lead issue. It can cause issues with corrosion control programs because it does strip everything out.”

To fix that, utilities add minerals back to the finished water, at the end of the RO process. That’s what both Brunswick County and H2GO plan to do.

While neither process is perfect, the experts say G-A-C and RO are good enough for the job at hand. Again, Detleff Knappe.

“I think, you know, both of the treatment choices that G-A-C and the reverse osmosis treatment will improve water quality, by removing PFAS and other contaminants that are in the water. So, both the Brunswick County residents and the Wilmington residents will benefit from cleaner drinking water through the installation of those technologies.”

The CFPUA G-A-C plant is scheduled to be running in early 2022. H2GO has not yet finalized its timeline for their RO project. Brunswick County expects to be online in 2022.

Vince Winkel, WHQR News.

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