Nutrient Removal, Recovery to be Showcased at WEFTEC

Across the U.S. utilities are faced with meeting low numeric levels set by the EPA for nitrogen and phosphorous. Can these challenges be turned into an opportunity through nutrient recovery? James Laughlin looks at pilot studies in the region ahead of a conference session dedicated to the topic at WEFTEC.

Nutrient removal and recovery will be among the important topics discussed at this year's WEFTEC conference, set for Sept 30 to Oct 3 in New Orleans. Across the country, and especially in key watersheds, utilities are being challenged to meet increasingly stringent nutrient limits for wastewater discharges. At the same time, there is a growing interest in recovering nutrients from wastewater, especially phosphorus.

Ana Pena-Tijerian from the City of Fort Worth, TX, and Rajendra Bhattarai from the city of Austin, will discuss the challenges they've faced in trying to help develop nutrient criteria for the state of Texas.

Their paper, "The Texas Two-Step: Developing the Numeric Nutrient Criteria for the Second Largest State in the U.S," will discuss the challenges, and frustration, they have experienced as they've worked through this process.

Bhattarai says "a dance" has been going on between USEPA and the Texas Commission on Environmental Quality (TCEQ), with EPA insisting on strict numeric criteria for total nitrogen and total phosphorus, while Texas has put forward surrogate criteria as an alternative. In 2010, TECQ submitted criteria for 75 major reservoirs in the state but they have yet to be approved by EPA.

"We realize that what we are seeing here in Texas is not really any different from what other states are seeing or facing. The challenges are the same," says Pena-Tijerian. "The reason for this paper was to make the point that we have a common problem here. The challenges are basically the same, and maybe we can work together to solve these issues."

One major challenge has been the low numeric levels sought by EPA, which would be difficult and expensive to achieve with current technologies, Pena-Tijerian adds.

"Most of the wastewater treatment plants here in Texas are smaller than 10 mgd, so having to install the technologies needed to achieve those very low numeric criteria is just not possible," she says.

Stephanie Sansom, a supervising engineer at MWH, will present a paper that highlights the challenges faced by utilities trying to meet low numeric criteria. Her paper, "TN and TP - How Low is Low and How Do We Get There?" highlights a series of treatment plant projects designed to tackle low nutrient levels.

In one example pilot project in Florida, the treatment facility faced two different phosphorus limits -- 0.27 mg/l for reuse in wetlands applications and a very tough 0.005 mg/l for discharge to Class 3 Outstanding Florida Waters. The pilot project used MBR, chemical addition for additional phosphorus precipitation, followed by reverse osmosis and then advanced oxidation processes, Sansom says.

"The point of that plant wasn't to look at the various technologies, it was really seeing if we could actually get to that (0.005 mg/l) level by throwing everything at it," Sansom adds. "We could achieve the .27 level, but we could not achieve the .005. We came close. We got down to point .007 on one or two samples."

The facilities in her study were all discharging to sensitive water bodies and had limits that went beyond those imposed by US EPA.

"We found that MBR systems can remove dissolved phosphorus after chemical addition. Ultrafiltration has been successful at rejecting particulate sections of phosphorus and that can be enhanced by chemical addition, and then reverse osmosis has various rejections," Sansom adds.

Still, few treatment facilities would be willing or necessarily able to install an RO system to achieve extremely low nutrient levels, she says. "For every additional point or additional .5 of phosphorus or nitrogen, you have to consider how much additional energy does it cost, how much additional chemicals," Sansom says. "Obviously, there is a limit not only on technology but what treatment plants are willing to spend on capital and operational expenditures. And you have to ask, at what point does the technology or operational requirements outweigh the sustainability of the plant?"

Nutrient Recovery

While utilities are struggling to meet – or prepare for – tighter nutrient limits, there is a growing interest in resource recovery in the wastewater treatment industry. Recovering nutrients for reuse can help preserve resources and also potentially help offset the cost of treatment, says Dr. James Barnard of Black & Veatch.

Barnard will present a paper entitled "State-of-the-art Recovery Of Phosphorus From Wastewater." The paper will examine both biological and chemical recovery system and look at work underway in the US and abroad.

Barnard is recognized for his development of biological nutrient removal processes. He says phosphorus is a limited resource and will grow in value in the years ahead. While chemical precipitation can produce a viable fertilizer product, he prefers biological processes for recovering phosphorus. He says the two processes can be used in combination to help increase recovery as utilities strive to meet lower discharge limits.

As more facilities turn to biological phosphorus removal, a large fraction of the nutrient becomes concentrated in biomass. Anaerobic digestion and dewatering produce a sidestream with a high nutrient content. Recovering nutrients from that concentrated stream can be more economic than recovery from main-stream processes.

Wendell Khunjar of Hazen and Sawyer will present a paper entitled "Economic Evaluation of Alternatives for Sidestream Nutrient Removal and Recovery." He will discusses different strategies for treating and recovering nitrogen and phosphorus from that sidestream.

"We will discuss four case studies representative of treatment facilities that are experiencing really stringent nitrogen limits and have to deal with a large fraction of their nutrient load coming back in their sidestream," Khunjar says. From a nitrogen perspective, he will discuss removal of nitrogen using conventional nitrification/denitrification or using novel strategies including nitritation/de-nitritation as well as deammonification. He will also discuss recovery of both nitrogen and phosphorus in struvite.

"Struvite recovery allows you to recover up to 90% of the phosphorus but also allows you to take out about 25% of the nitrogen," Khunjar says. "In some facilities, that 25% reduction might be sufficient to allow that nutrient stream to come directly around to the head of the plant and not have an impact on mainstream treatment."

As plants begin to embrace biological phosphorus removal, they often also experience an increase in problems caused by struvite. Dealing with that nuisance and also recovering a marketable product can make economic sense, Khunjar says.

"Right now there are 20-25 plants who have done piloting or some type of evaluation and that number is growing," he says. "Everyone is very interested in understanding if it (nutrient recovery) makes sense from an economic perspective. Sometimes it does and sometimes it does not."

Conclusion

Sansom says the facilities she examined in her study were all working toward a future of tight nutrient limits.

"They are looking more long-term and how to phase in these requirements," she says. "They are not ready to stick an RO plant at the back of their facility to achieve these limits, but they are looking for ways to move forward and how can they incorporate even upstream reductions of nutrient loading in the source water."

The process of setting nutrient criteria is slow and frustrating at times, but it is an important process that requires careful deliberation, Bhattarai says. "The nutrient problem was not created overnight and we cannot solve it overnight," he says. "It's probably better to take some initial steps and, gradually, using an adaptive management strategy, work toward achieving compliance. The title of our paper is the Texas Two Step. In the end, we say the Texas Two Step still goes on," he says. "We're still dancing."

Author note: James Laughlin is the editor of WaterWorld, the sister publication of WWi covering the municipal water and wastewater markets in the US and Canada. For more information, visit: www.waterworld.com