Floating islands are cleaning polluted harbours

It’s a ‘riverponic’ system, says one researcher

International Institute for Sustainable Development

The floating treatment wetland islands that are popping up in American and European harbours are showing how we can help nature heal itself. In some ways, it takes us back to a much earlier time when the Incas grew food on floating islands they built on lakes.

Floating treatment wetlands or islands are small artificial platforms that allow aquatic plants to grow in water that is typically too deep for them. Their roots spread through the islands into the water, creating dense columns of roots with lots of surface area. Not only do the plants take up nutrients and contaminants themselves, the plant roots and floating island material provide an extensive surface area for microbes to grow.

It was in 2006 that environmental toxicologist and horticulturalist Sarah White began testing the idea of whether floating mats of plants could filter pollution on retention ponds. In South Australia, Chris Walker and Terry Lucke, who had spent years working on green solutions for water treatment, patented a modular system of floating wetlands through Clarity Aquatic.

In Boston, PhD student Max Rome has found that one acre of floating wetland can absorb the nutrient pollution of from seven to 15 acres of dense urban development. He was studying wetlands that have been floating since 2020 in the historically degraded Charles River.

“The last generation did a really good job of dealing with point source pollution — and it was a huge task,” he says, referring to how the Clean Water Act reduced effluent from discharge pipes. But his generation has a new job - grappling with “ecological restoration of these degraded water bodies at the same time that we do pollution reduction,” something the wetlands could help address.

In Baltimore, a new 10,000-square-foot (900-square-meter) habitat called Harbor Wetland aims to restore aquatic life and make the harbour safe for swimming. The $14 million project is already attracting mollusks, fish and seabirds. During operating hours, it is a free floating park, with an elevated walkway, seating and shade structures.

The National Aquarium has been trying to address the problem of algae blooms in the harbour since 2010 when it installed a 200-square-foot floating wetland. It was among the first institutions to attempt such a system in a brackish tidal system.

Amelle Schultz, principal at Ayers Saint Gross, brought on to create a waterfront master plan for the National Aquarium campus in 2016, identified zones that would interpret different ecosystems of the Chesapeake Bay watershed: the coastal plain, Appalachian plateau, and the salt marsh.

In 2017, using a new design they had developed, a small prototype was installed, incorporating a pontoon-mounted planting surface that could be adjusted for buoyancy as marshy plants grew on it. Killifish and blue crabs began to show up, as well asbigger creatures that eat them, like rockfish and cormorants.

This larger marsh took several months to construct and plant, bringing in elements like elevation changes and an aeration system. The submerged part includes artificial reefs seeded with shells from nearby seafood restaurants. A current powered by 35 giant-sized bubblers runs through a shallow channel and pumps oxygen into the water.

“It’s literally like an oasis,” says Jacqueline Bershad, the National Aquarium’s vice president of planning and design. 

Other cities are also experimenting with artificial shoreline features that are more flood-resistant and friendly to wildlife than concrete or steel seawalls.

In Boston, researchers led by a Northeastern University scientist have proposed lining the waterfront in an “Emerald Tutu” of biomass modules that would be like the coastal marshes that once encircled the city. Chicago’s Wild Mile, a floating eco park launched with the Shedd Aquarium and other local partners in 2017, consists of floating mats of native plants growing in the heavily polluted Chicago River, serving as a filter while attracting marine life.

The five small islands float next to the concrete riverbank of Bubbly Creek, a stretch of the Chicago River named for gas that once rose to the surface after stockyards dumped animal waste. Bottomless baskets hanging from them will provide a place for freshwater mussels to attach. The mussels once flourished in the river.

Nick Wesley, executive director of Urban Rivers, a nonprofit working with the Shedd Aquarium believes floating systems are a natural fit for the urban environment. He calls them a “riverponic” system that requires no soil or other substrate for support.

The European project “Live Lagoons” is developing floating wetlands made of reed and willow, which are cleaning waters in the South Baltic area from substances like nitrogen or phosphorus. By doing so, they help fight an excessive growth of algae, oxygen depletion, loss of biodiversity and odour issues.

Floating wetlands “are having a bit of a moment,” says Richard Grosshans, a research scientist with the International Institute for Sustainable Development. “They function very similarly to a natural wetland: they have the same processes, plants and microorganisms, bacteria and algae, [which] naturally break down toxins. They take up nutrients and provide habitat. It’s kind of common sense to those of us who work with these types of systems.”

In Canada, 58 lakes and their watersheds in northwestern Ontario known as the  IISD Experimental Lakes Area is the world’s freshwater laboratory. It is the only place in the world where scientists can experiment on and manipulate real lakes to build a more accurate and complete picture of what human activity is doing to freshwater lakes, says the International Institute for Sustainable Development.

It does research that is aimed at keeping fresh water clean world wide for generations to come. Based on its scientific findings, it develops policy and programming solutions for governments, civil society, and industry that protect and enhance the natural environment, keep our fresh water clean, while also improving socioeconomic well-being. One example is its work with Indigenous communities on how to bring back wild rice.  

Currently it’s exploring how floating wetlands could break down oil constituents after a spill.

Sources:

How Floating Wetlands Are Helping to Clean Up Urban Waters Yale Environment 360, Nov. 22, 2022

A Floating Island in Baltimore Raises Hope for a Waterfront Revival. Bloomberg Green, Aug. 15 2024

Floating treatment wetlands: keeping our fresh water clean and healthy. International Institute for Sustainable Development website.