ERDC researchers combat cyanobacterial harmful algal blooms

U.S. Army Engineer Research and Development Center
Published Dec. 18, 2023
Image of ERDC CyanoSTUNTM vessel during operation

Image of ERDC CyanoSTUNTM vessel during operation

Image of bow-mounted screen and trolling motor to protect the UV reactor from debris and maximize mixing within the UVC reaction zone

Image of bow-mounted screen and trolling motor to protect the UV reactor from debris and maximize mixing within the UVC reaction zone

The UV reactor beneath the CyanoSTUN platform (stern side depicted with weir and effluent sampling device).

The UV reactor beneath the CyanoSTUN platform (stern side depicted with weir and effluent sampling device).

The UV reactor illuminated beneath the CyanoSTUN

The UV reactor illuminated beneath the CyanoSTUN

VICKSBURG, Miss. — The U.S. Army Engineer Research and Development Center’s (ERDC) Environmental Laboratory (EL) recently completed field trials in Massachusetts, using germicidal ultraviolet light (UV-C) to sustainably suppress and potentially eradicate cyanobacterial harmful algal blooms (cyanoHABs) without using chemicals.

Many of the conventional techniques for managing cyanoHABs are challenging to deploy in a safe and timely manner, as well as difficult to scale to size. To address these limitations, Taylor Rycroft, a research environmental engineer in ERDC-EL and principal investigator of the study, is working with a team consisting of representatives from EL, the Cold Regions Research and Engineering Laboratory and the Oak Ridge Institute for Science and Education. Together, they engineered the CyanoSTUNTM (Cyanobacterial Suppression Through Ultraviolet-Light-C Neutralization) vessel.

“The CyanoSTUNTM is a small pontoon boat fitted with a wastewater-grade UV-C reactor and other modifications to optimize UV treatment capability. As the vessel traverses the cyanoHAB-contaminated water, the cyanobacteria and cyanotoxins in the top five to eight inches of the water column are irradiated with UV light at a wavelength capable of disrupting the cellular DNA of the cyanobacteria and degrading certain toxins,” said Rycroft.

The goal of the CyanoSTUNTM vessel is to give water managers the ability to rapidly and safely suspend an algal bloom for an extended period of time, or destroy it entirely, without using any chemicals. Laboratory results show UV-C can disrupt intracellular processes within cyanobacteria, leading to both reversible and irreversible damage depending on the amount of time the cells are exposed to radiation.

“UV-C has been shown to disproportionately harm toxin-producing cyanobacteria compared to other microalgae that are beneficial to the ecosystem,” Rycroft said. “In that sense, at our targeted UV-C doses, the CyanoSTUNTM is expected to knock out the ‘bad guys’ and have little effect on the ‘good guys’.”

A UV channel reactor is mounted to the bottom of the pontoon boat and braced with stainless steel crossbars.  The reactor uses ten 87.5W UV-C lamps, which are accessible by lifting a hatch cut in the deck of the vessel. A UV sensor attached to one of the lamps provides a real-time measurement of UV intensity within the reactor, and that information is then used to inform the appropriate velocity of the boat for adequate treatment.

Rycroft and his team also customized the reactor by lining the interior stainless-steel walls with high-UV254-reflectance polytetrafluorethylene diffuse reflector sheeting.

“This design was implemented to improve wall reflectance from approximately 25% to approximately 95%. The modification helps ensure the water flow paths along the reactor wall receive the highest possible UV intensity despite their distance from the lamps,” Rycroft said.

Additionally, the CyanoSTUNTM has a custom bow-mounted screen made from aluminum perforated sheeting, which prevents debris from entering the reactor and damaging the lamps. The vessel is powered by a bow-mounted electric trolling motor.

“In addition to providing thrust and directional control, the trolling motor creates turbulent flow as the water enters the reactor, thereby improving mixing and reducing the potential for dead space or laminar flow lanes near the reactor walls and corners,” Rycroft said.

Finally, electrical power is supplied to the reactor lamps and trolling motor batteries using a portable generator.

The CyanoSTUNTM uses UV-C for a targeted amount of time to inactivate cyanobacteria and reduce the concentration of cyanotoxins in the contaminated water.

“The process takes place within the channel reactor which prevents UV-C from emitting beyond the designed control volume,” said Rycroft.

The dose of germicidal UV-C administered by the CyanoSTUNTM is dependent on the UV intensity within the reactor and the residence time of the cyanobacteria as they pass through the reactor. The UV intensity is entirely dependent on the UV transmittance (UVT) of the surface water which will fluctuate depending on the type and concentration of suspended and dissolved constituents in the water. At a high UVT (or low absorbance), more UV254 radiation will reach the target cyanobacteria in the reactor than at a low UVT, when more of the emitted UV254 radiation will be absorbed and less will be available to act upon the cyanobacteria and cyanotoxins. The residence time of the cyanobacteria is controlled by adjusting the speed of the boat. The faster the boat travels, the less time the cyanobacteria spend in the reactor, and vice versa.

The vessel is currently designed for calm freshwater lakes and ponds with low wave action, but Rycroft and his team have ambitions to enhance the CyanoSTUNTM to tackle larger blooms in larger waterbodies with more challenging wave and current conditions.

“The CyanoSTUNTM is designed to combat small-scale blooms that are less than 3 acres in size. Beyond this size, exposure time of cyanobacteria – and therefore UV dose – will be compromised because the boat will need to travel at the maximum speed to cover the larger surface area,” Rycroft stated.

Early results of the field trials have indicated that that the CyanoSTUNTM can reduce the concentration of microcystin, a potent cyanotoxin, by 30-50% depending on the administered dose of UV-C. Rycroft’s team is still awaiting analytical results showing the effect of the CyanoSTUNTM on cell density and community composition across various cyanobacteria and algal species. ERDC filed a patent application for the CyanoSTUNTM in April 2023.