How duckweed might be a solution to pollution in Lake Winnipeg

A group of Manitoba scientists is looking into whether a tiny native water plant could help reduce toxic algae blooms on Lake Winnipeg.

As the City of Winnipeg grapples with the massive task of cutting off the flow of phosphorus from the North End Sewage Treatment Plant, the scientists hope to show cost-effective green technology can help treat the many smaller sources of the algae-feeding nutrients.

Earlier this week, research scientist Richard Grosshans and his team from the International Institute for Sustainable Development harvested 33 kilograms of duckweed they had grown in the sewage lagoon at Dunnottar, Man., a village of 760 people, 65 kilometres north of Winnipeg on the west shore of Lake Winnipeg. 

The tiny leafy plant is thirsty for phosphorus and nitrogen, and Grosshans wants to see how well duckweed and floating islands of local marsh plants work to clean the water.

The early results look promising, he said.

"So far, the growth on the islands and the growth of the duckweed is just mind-blowing," he said. "Within three weeks it completely filled our containment area."

The duckweed will be dried and sent for analysis at the University of Manitoba to find out how much phosphorus it absorbed. 

If Grosshans can show that the duckweed and floating islands of cattails, marsh grasses and sedges effectively reduce the nutrients in the water, the potential impact could be huge, he said. 

"There are hundreds of these wastewater lagoons out there across the province, and many of them were never designed, never monitored for phosphorus removal," he said.

The raw sewage is dumped into a primary lagoon, where the solid matter settles to the bottom before flowing into a secondary lagoon, where the duckweed and floating islands do their work.

While other municipalities invest in expensive chemical treatment systems that can cost millions of dollars to reduce their phosphorus outputs, systems that use duckweed, also known as lemna, could potentially do the job in a way that won't break precariously balanced budgets. 

"The upfront capital investment is 100 times cheaper," said Dunnottar Mayor Rick Gamble, who is working with Grosshans and his partner Chris Penner of the design firm Scatliff Miller Murray.

Penner convinced Gamble to go ahead with the project, said the Dunnottar mayor, who initially was worried the duckweed would spread and become a nuisance. The system uses containment structures to keep the duckweed from growing wild, and the harvested plant can be used as animal feed.

North Dakota example

Although Grosshans isn't sure whether a lemna system could be adapted to treat wastewater from a municipality with the population of Winnipeg, there have been some larger-scale applications of the technology.

"There's actually a wastewater treatment facility in North Dakota which uses duckweed, which we are sort of modelling our idea off of," he said.

The lemna system in Devils Lake, N.D., treats wastewater from roughly 10,000 people, using a "serpentine-shaped canal system" at a site covering about 60 acres, says a report on the city's website. It removes up to 91 per cent of the phosphorus from the wastewater, the report says.

The total cost of the entire system was about $4.5 million when it was set up in the late 1980s. 

The lemna system is the final step of the water treatment process, and "polishes" the wastewater to the point where it can be discharged into Devils Lake, said Mike Grafsgaard, city engineer for Devils Lake.

Later in the summer, once the duckweed has covered the canal system, floating mechanical harvesters scoop it up.

Since it is a biological system, it requires minimal upkeep, Grafsgaard said.

"Realistically, the plant does most of the work for us."

One advantage of duckweed is that it grows quickly, doubling its mass every three to five days.

The Devils Lake system can treat 13.25 million litres of wastewater per day. Winnipeg's North End Sewage Treatment Plant treats about 195 million litres of wastewater a day.

Grosshans expects to have the results from his first duckweed harvest back within the next few weeks. He plans to have another harvest after that, which will give him a clearer idea of how well it's working. 

Next year, he plans to create a system that will allow him to test exactly how much phosphorus is in the water released from the lagoon. 

Gamble envisions potential applications of the technology beyond municipalities.

Farmers could use water retention ponds to catch runoff from their fields and use lemna systems to filter it before its released into waterways, he said.

It's a relatively simple potential solution to a complicated problem, one that could appeal to cash-strapped municipalities.

"In the long term, it would be the cheapest way to go," Gamble said.