Research at the wastewater treatment plant in Lawrence, Kan., is demonstrating that algae can remove nitrogen and phosphorus from wastewater. Now in its fifth year, the pilot project is helping researchers learn more about how algae removes nutrients and how it can be used to create biofuel and other beneficial products.

“I think the verdict is still out on the best way to remove nitrogen and phosphorous that’s going to use the least energy,” says researcher Belinda Sturm, assistant professor of environmental engineering at the University of Kansas. Her work in Lawrence earned an Excellence in Environmental Engineering Honor Award in 2012 from the American Academy of Environmental Engineers.

“This is adding another option to the suite of tools we have,” Sturm adds. Her work at turning wastewater into a resource is part of a larger “Feedstock to Tailpipe” program at the university, which is researching alternative and renewable fuels and technologies.

Testing the hypothesis

After positive results in preliminary laboratory studies on algae for phosphorus removal, the university approached Lawrence in 2009 to see about testing the idea. “Looking at the chemistry in which algae grow, the wastewater effluent matches up pretty well,” Sturm says. Because the Lawrence plant performs nitrification but not denitrification, the effluent has traces of nitrate and phosphate. “Our hypothesis was that we could grow algae in the effluent and remove nutrients,” says Sturm.

While bacteria are preferred for removing BOD, treatment plants are under increasing pressure to remove nitrogen and phosphorous. “That’s what is unique about this project,” says Sturm. “We’re saying we can grow algae in our effluent as a polishing step so it doesn’t interrupt BOD removal, then separate the algae so we’re not discharging it.”

The 12 mgd (average) Lawrence plant serves 90,000 people. Sturm’s project is testing a concept, so it does not use commercial-grade equipment. The four algae tanks are 2,600-gallon fiberglass cattle tanks that take effluent from the secondary clarifier and operate from April to October. “We’re not trying to get the very best productivity with reactor design,” Sturm stresses. “We’re testing some of the scientific principles and nutrient removal across the system by playing with some variables.”

Hitting the targets

One variable tested was adding carbon dioxide to enhance algae growth. “It did not, which was surprising to us, because in the laboratory it definitely helps. There may be enough total inorganic carbon in effluent that the additional carbon dioxide doesn’t help.”

The testing has shown that algae significantly reduced both nitrogen (by 61 percent) and phosphorus (by 91 percent). Over the study period, the algae tanks receive effluent with 20 mg/L nitrogen on average (ranging from 12 to 28 mg/L) and 3 mg/L phosphorus (ranging from 1.5 to 5 mg/L).

The targeted effluent levels from the state Department of Health and Environment are 8 mg/L nitrogen and 1.5 mg/L phosphorus. “We achieve the phosphorous goal,” says Sturm. “The nitrogen removal is pretty close, even in this system that is not optimized. I was pleasantly surprised at the efficiency of phosphorous removal across the system.”

A bioreactor would be expected to perform even better than the cattle tanks.

Algae is harvested from the effluent using gravity sedimentation. It is dewatered with a centrifuge and turned over to a team of chemical engineers for conversion into bio-crude oil. “That would be the ultimate end-use of the biomass,” says Sturm. “The bio-crude has very similar properties to crude oil that comes out of the ground.”

Versatile product

Algae create much more bio-crude than other possible sources, which are normally food crops. Sturm points out that bio-crude can be produced from any wastewater biosolids. “One thing we might try is taking the biosolids from the plant’s anaerobic digestion and see what type of crude oil it produces,” she says.

Another byproduct of the algae process is biochar, which is comparable to wastewater biosolids. Its main components are carbon, silica, calcium and phosphorus. It can be used as fertilizer or even as a supplement to coal as an energy source.

The final liquid waste material is similar to centrate from anaerobic digesters. “It’s a high-nitrogen waste stream, so we’re testing how to recycle that back to algae growth,” says Sturm. “There may also be other uses for it. We’re trying to close the loop with all the products we get.”

As for marketability, she says algae is a good candidate for lagoon systems and smaller wastewater treatment plants. “They might not have operators to handle anammox or enhanced biological phosphorous removal systems. Lagoon systems often have an algae composition to them anyway.”

The pilot has funding through 2014 and Sturm is seeking more research funding. She has also issued design proposals for a more sophisticated bioreactor design for the Lawrence plant. She adds, “We also have a few sites in the area where we have nitrates in groundwater, so I’d like to see if we can clean up groundwater with algae.”