The benefits of bioreactors

Researchers from the University of Illinois, led by Assistant Professor Laura Christianson, are encouraging wider adoption of bioreactors as a nitrate management strategy. Christianson has put together a special issue of the Journal of Environmental Quality focusing on bioreactors, with 15 articles in the issue summarising bioreactor technology and its benefits.

Bioreactors are passive filtration systems that capitalise on a bacterial process known as denitrification to remove 25–45% of the nitrate in water draining from farm fields. Research on and installation of bioreactors has accelerated in the past decade, but Christianson and her colleagues are urging a move past proof of concept towards large-scale deployment.

“Bioreactors are one of the most effective edge-of-field practices, but until now, they haven’t been rolled out on a large scale,” said Christianson.

The typical arrangement for a 16–32 ha field is a large (30 x 6 m) pit situated just ahead of where drainage pipes flow into ditches or streams. The pit is filled with carbon-rich organic material — usually wood chips, but sometimes corn cobs, biochar or other matter. Denitrifying bacteria make their homes in the organic material and utilise its carbon as an energy source to convert nitrate in the water to nitrogen gas. 

According to Christianson and other contributors to the issue, flow rates can significantly affect the efficiency of bioreactors. During low-flow periods, water can be held in bioreactors for too long, setting up conditions for different bacteria that create noxious hydrogen sulfide gas. In high-flow periods, water may move through too quickly for efficient nitrogen removal.

“Tile drainage systems never flow at a consistent rate,” Christianson explained. “Bioreactors have to be designed strategically to optimise retention time and maximise nitrate removal without undesirable by-products.”   

Temperature and seasonal changes also affect how well bioreactors work, with Christianson noting that the critical period for nitrate loss is early spring, before plants are growing and taking up nitrogen.

“Snowmelt puts a significant amount of water through a bioreactor, depending on where you are,” she said. “And because snowmelt and early spring drainage water is cooler, the bacteria aren’t as efficient.”  

Christianson and her colleagues are now calling for more field-scale research to optimise design for the set of conditions unique to each field.