By Jay Landers

In a bid to revamp the digestion process at its huge Southerly Wastewater Treatment Plant, the city of Columbus, Ohio, is working with the environmental engineering firm Brown and Caldwell to assess opportunities for improving digester operations and recovering more resources from wastewater, including phosphorus and biogas.

As part of an ongoing evaluation, Brown and Caldwell and the city are researching a new approach that would utilize the WWTP’s existing acid-phase digesters in a novel manner, potentially resulting in lower operations and maintenance costs, cost-effective nutrient recovery, and higher-quality biosolids and biogas products.

In April, the city awarded Brown and Caldwell a $2.5 million contract for preliminary design services associated with the planned digester improvements. In October, the company received the notice to proceed on the preliminary design, says Dante Fiorino, P.E., the project manager for Brown and Caldwell. Scheduled to last 18 months, the wide-ranging effort will include “several pilot studies” examining a range of potential improvements, Fiorino says.

In renovating the acid-phase digesters at the Southerly facility, the city is interested in implementing “some innovative processes,” including side stream nutrient removal, says Darin Wise, the manager of the Southerly WWTP. Ultimately, Columbus wants to improve the digestion process at the facility in order to do more with it. “If possible, we want to intensify our process to prevent having to build more digesters in the future,” Wise says.

Assessing the acid-phase digesters

Serving approximately 1.3 million residents, the Southerly WWTP treats an average daily flow of 109 mgd and can accommodate a peak design flow of 440 mgd. The biological phosphorus removal facility features a dual-phase anaerobic digestion system as part of its solids handling process, which produces Class B biosolids.

The facility has six methane-phase digesters that were constructed in the late 1960s. After fulfilling their original purpose for about a decade, the digesters were used as storage tanks for nearly 25 years. In 2009, the city returned the six digesters to their original purpose and added three acid-phase digesters. (As part of a separate project, Columbus, with Brown and Caldwell as its design engineer, is currently preparing to renovate the six methane-phase digesters and add a seventh digester.)

Located immediately upstream of the methane-phase digesters, the acid-phase digesters are designed to help optimize the digestion process. They do this by separating the acid-forming stage of digestion from the methane-forming stage, which occurs in the methane-phase digesters.

In theory, separating the stages in this manner improves overall digestion performance. However, the Southerly facility’s methane-phase digesters have experienced foaming when all three of the acid-phase digesters are in operation, says John Willis, Ph.D., P.E., the vice president of wastewater solutions for Brown and Caldwell. Foaming decreases available capacity within the digesters, interferes with their operation, and requires increased operator attention. Presumably, the foaming occurs because acid-phase retention times become too long, Willis says.

Wise and his team of operators at the Southerly WWTP have learned to mitigate the downstream foaming by maintaining a lower retention time in the acid-phase digesters. However, they do not know the root biological or chemical cause of the foaming, so Brown and Caldwell is working with the University of Massachusetts Amherst to determine why the foaming occurs only in certain circumstances.

“We’re doing research to figure out what happens at a controlled-lab scale,” Willis says. Funded by the Water Research Foundation and other utilities, the research will seek to determine “which parameters are driving the failed performance,” he says.

Potential plus

This research effort also will examine the efficacy of so-called Acid+ digestion, a proprietary process for which Brown and Caldwell has filed a provisional patent.

Essentially, the Acid+ process entails recirculating sludge from methane-phase digesters to one or more acid-phase digesters that otherwise would not be in service, Willis says. In this way, stable methanogens from the methane-phase digesters can consume volatile solids in the acid-phase digester effluent, potentially resulting in earlier and intensified digestion and increased biogas generation with much smaller acid-phase digesters. Meanwhile, acid-phase biogas can be wasted directly, eliminating much hydrogen sulfide. This means the biogas generated during the methane phase is cleaner and requires less treatment to remove hydrogen sulfide.

“It's totally novel and has never been done before,” Fiorino explains.

If the Acid+ process could be deployed at full scale, it would offer multiple potential benefits, including up to a 50% increase in digester capacity, increased biogas production, a reduction of phosphorus in biosolids, and decreased expenses associated with transporting and applying biosolids to agricultural lands. “I'm viewing this as a potential home run, but we need to put in the work at the pilot scale to confirm that what the biological models are telling us is possible,” Fiorino says.

Acid+ digestion also has the potential to facilitate easier removal of struvite from the solids stream, Willis notes. Struvite, or ammonium magnesium phosphate, forms when magnesium encounters nitrogen and phosphorus under conditions of increasing pH. Unfortunately, struvite frequently collects inside pipes at WWTPs, decreasing their capacity and necessitating removal of the compound. “It ends up being a big maintenance issue,” Willis says.

Removing struvite from the acid-phase digesters would protect Southerly’s methane-phase digesters, heat exchangers, centrifuges, and other downstream processes against struvite formation.

At the same time, harvesting struvite during the acid phase would enable Columbus to capture the phosphorus-rich material for potential sale as a high-value, slow-release fertilizer product. “You’re potentially taking an operations and maintenance nuisance and turning it into a new stream of revenue for the city,” Fiorino says.

Benefits for biosolids

Of course, removing struvite during the acid-phase digestion process also means less phosphorus in the biosolids, Wise says. Such an outcome would benefit the city’s already successful biosolids program.

As of the end of 2021, Columbus was expected to “have reached five years of 100% beneficial reuse of all our biosolids,” Wise says. “For a city this big, that’s unheard of.”

“By removing the phosphorus from your biosolids, you can get a much drier cake — sometimes 3 to 5% drier,” Wise says. Such a reduction in the water content of biosolids that are being prepared for other uses would save a “tremendous amount of hauling costs,” he notes.

Reducing the phosphorus concentration in biosolids would also enable Columbus to land apply more biosolids than is currently possible, Wise says. On some of its land application sites, the city is “limited by the amount of phosphorus we can apply,” he says. “If there’s less phosphorus in the sludge, you can increase the amount of biosolids that is land applied at a given location. You stretch your land.”

Although it holds great promise, the Acid+ process still needs to be proved, Fiorino says. “There’s potential there,” he says. “But first, we have to do it in a lab. Then if the results are favorable, we’re going to try to do it at scale at Columbus.” 

The research at the University of Massachusetts Amherst is to be completed in a year, Fiorino says. “Ideally, we get this research done before this preliminary design is even complete, and we can start to incorporate findings and potentially include it as an alternative in our preliminary design,” he says.

Other possibilities

During the preliminary design phase, Brown and Caldwell and its subconsultant, Black & Veatch, also will evaluate other more conventional methods for phosphorus recovery. “We’re going to be doing an alternatives evaluation and cost-benefit analysis on those” and then work with the city to determine how best to proceed regarding phosphorus recovery, Fiorino says.

Another task to be conducted during the preliminary design entails investigating the feasibility of creating a receiving station to accept food waste and fats, oils, and grease —known as FOG — for codigestion at the Southerly WWTP.

In addition to providing Columbus with a source of revenue from tipping fees, such a program could have other benefits as well, Wise says. “FOG is like rocket fuel in a digester,” he says. “It’s pretty much 100% volatile. It breaks down really well. It produces a lot of gas.” The increased gas production potentially could enable Columbus to pursue such beneficial reuse options as cogeneration to produce energy or the treatment and sale of its biogas.

Methods for controlling and limiting the presence of emerging contaminants in biosolids also will be addressed. The family of compounds known as perfluoroalkyl and polyfluoroalkyl substances is of particular concern to Columbus, Wise says. If the biosolids were found to contain PFAS at high enough concentrations, the city could be forced to discontinue beneficially reusing its biosolids and incinerate the material instead because of concerns that PFAS pose a risk to human health and the environment.