Optimizing Biofiltration for Reuse Applications

Interview with Gayathri Ram-Mohan and Kristan VandenHeuvel

Biofiltration is a well-established advanced treatment approach to produce high-quality drinking water. As more utilities consider reuse applications to supplement their water supplies, guidance is needed on reuse treatment train optimization. Through two WRF Tailored Collaboration (TC) projects, Gwinnett County (GA) and The Water Tower, a non-profit innovation hub located in Gwinnett, have investigated ways to optimize the combination of ozone with biologically active filtration (BAF). WRF sat down with Gayathri Ram Mohan, Senior Environmental Engineer at Gwinnett County; and Kristan VandenHeuvel, Director of Applied Research and Engagement at The Water Tower, to hear more about what they have learned.

Can you each tell me about yourself—how long have you worked at Gwinnett and The Water Tower and what is your role there?

Gayathri Ram-Mohan: I’ve been with Gwinnett County DWR since 2016. My background is in advanced wastewater treatment, reuse and biosolids handling. My primary focus is process optimization at all three water reclamation facilities owned and operated by Gwinnett County, including the F. Wayne Hill Water Resources Center. F. Wayne Hill is an advanced water resource recovery facility that is permitted to return up to 50 MGD of highly treated effluent to Lake Lanier.

Kristan VandenHeuvel: I have been with The Water Tower since it launched in 2019. The Water Tower, comprised of two non-profits, was born out of Gwinnett County Department of Water Resources to help expedite the implementation of innovative water and wastewater solutions. The Water Tower focuses on four key areas including applied research, technology innovation, workforce development, and community engagement. Our collaboration with Gwinnett DWR includes working together on applied research and the development of The Water Tower’s Lake Lanier Watershed 5-Year Research Plan.

Let’s talk about your completed TC project, Ozone Biofiltration Direct Potable Reuse Testing (4777). What were the drivers for your utility to pursue this project, and why did you think the TC program was the right vehicle?

Ram-Mohan: Gwinnett has never been one to wait and react—we have always been on the leading edge of applied research for water supply resilience. Gwinnett County’s primary drinking water source is Lake Lanier, which is vast, thousands of square miles, but less than 5% of that watershed is within our boundary. We are highly motivated to ensure that the lake water quality does not deteriorate.

For decades, Gwinnett has practiced indirect potable reuse (IPR) utilizing effluent from F. Wayne Hill. Many utilities who practice IPR use a full advanced treatment (FAT) train, incorporating steps like  microfiltration/ultrafiltration (MF/UF), reverse osmosis (RO), ultraviolet (UV) light disinfection, and advanced oxidation (AOP). While this model produces high-quality water, it has a high capital and energy cost. In addition, we are an inland utility, and brine disposal is expensive.

Taking all that into account, we wanted to investigate ozone biologically active filtration (BAF), which would eliminate the issues associated with RO concentrate disposal. In this project, among several other aspects of potable reuse, we also investigated what ratios of advanced treated water blended with the potable water supply met primary and secondary drinking water standards. Denise Funk (now with Brown and Caldwell) was the PI for project 4777.

This research was helpful both in terms of fine-tuning our treatment plants, be it F. Wayne Hill or our drinking water treatment plant. The research team did a detailed economic analysis of how much it would cost us to implement ozone-BAF for direct potable reuse as compared to a FAT process. We found that this approach would cost less than 50% of an MF/RO process on a dollars-per-million-gallons-treated basis.

VandenHeuvel: Gwinnett County serves close to a million people and trying to advance innovation while protecting public health can be difficult. Programs that allow you to be nimble, like the TC Program, provide an avenue for projects that might not happen otherwise.

If Gwinnett or another utility has a small budget for a project, and they want to expand it and collaborate with other utilities, the TC Program is a great option because you can take your small budget, have it matched, and draw on the expertise of other utilities. It creates a collaborative environment and makes the project even more impactful.

Gayathri, you mentioned the cost savings aspect. Since this project wrapped up in 2018, have you been able to drive these findings forward and realize these cost savings?

Ram-Mohan: As a result of project 4777, we found that the 15% blend of effluent from F. Wayne Hill with Lake Lanier water met our criteria in terms of primary and secondary maximum contaminant levels (MCLs) for drinking water treatment. As we supplemented with more F. Wayne Hill effluent, we noticed some parameters needed fine tuning. One of them was nitrates. Optimizing biological treatment at F. Wayne Hill helped us achieve the desired target concentration, while simultaneously optimizing biological phosphorus removal and reducing our metal salt reliance, all of which have contributed towards significant cost savings for the County.

Let’s switch gears to your ongoing TC project, the biofilter transformation project (5092). Could you talk about the drivers for that project?

Ram-Mohan: In 2016 we installed new GAC media in some of the empty contactors at F. Wayne Hill. Subsequently, the plant experienced water quality challenges during the transition from granular activated carbon (GAC) to biologically activated carbon (BAC). While filter performance in terms of bulk organics removal seemed to have improved immediately upon commissioning, poor treated effluent water quality was observed as the adsorptive capacity diminished (which we expected to happen over a 3–4-month period). Typically, we would have seen a smooth transition to a biofilter and continued bulk organics removal. In this case, in addition to lack of organics removal, it appeared that some level of desorption from the spent media occurred. One of the main drivers for implementing preozone-BAC in our process train is to treat recalcitrant organics and consistently meet a chemical oxygen demand (COD) NPDES limit of 18 mg/L. The filter effluent quality observed during the transition period reduced our safety margin against this COD limit. We wanted to dive deeper and understand the root cause of this issue and whether it was a short-term or long-term one.

I know you are partnering with other utilities on this project. Can you talk about how you identified partners for this project?

Ram-Mohan: We started discussing these challenges with leading professionals at other utilities, including Bob Angelotti at Upper Occoquan Service Authority (UOSA). We investigated impacts of several parameters including but not limited to temperature, empty bed contact time, media type, etc. There were aspects of the problem statement that UOSA also wanted to understand better, so we realized this was an opportunity to partner. When we investigated available literature, there was limited information related to biotransformation of media in reuse applications. That research gap needed to be filled, and we saw an opportunity to apply for a Tailored Collaboration project. UOSA’s willingness to partner with us helped strengthen our full-scale testing methodology.

With the TC Program, utilities can realize the full potential of their financial investment, because not only are we solving our regional issue, but we are also able to collaborate and harness information from other utilities facing similar challenges.   

VandenHeuvel: While working on this project, it has been great to see how willing people and utilities are to work together. If you have similar issues and problems, there’s not a whole lot of convincing when it comes to partnerships—the attitude is, how can we all work together in the best way to get the outcome that we want? That’s what WRF and the Water Tower are trying to facilitate—bringing all the right people together to find solutions that advance the industry.

Is there anything you’d share with a utility planning to apply to the TC Program for the first time?

Ram-Mohan: As a scientist in a utility setting practicing applied research, my top priority is to optimize existing processes. WRF’s TC Program helps maximize the application potential of utility-led projects while simultaneously increasing the return on subscriber dollars. In project 4777 and the ongoing project 5092, we have worked to advance the science of ozone biofiltration in reuse applications. Filling that gap using vehicles like the Tailored Collaboration Program to learn more and share guidance with fellow utilities is an important step in the right direction.

VandenHeuvel: Utilities see the everyday operation of their plant and know what issues and challenges need to be addressed. If you suspect other utilities might have similar challenges, then it could be worth looking into the TC program. Not only does the program provide resources for utilities to potentially solve their problem, but it allows them to make connections with new utilities. By working together, we can find solutions and bring helpful ideas to the table.

Whenever a TC project is published, it reaches a wide audience of subscribers that the participating utility might not be able to reach by themselves. Others can start thinking about how they might build off your work, so it makes a snowball effect of advancing science in the water industry.