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How to Build a Floating Trash Island

Artificial floating islands can be used to clean urban bodies of water. Any polluted canal, river, estuary, lake in a city park, or storm water retention pond would benefit from a floating island.

Image: Illustration by Juan Martinez.
Image: Illustration by Juan Martinez.
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TEST Floating islands occur in nature when part of a lake’s bank breaks away from the shore and floats around, sometimes for years. The islands act as the lake’s liver, purifying its waters. The roots of plants hold the island together and dangle down into the lake’s waters, creating a habitat for bacteria, algae, zooplankton, and other critters. These organisms, as well as the plants themselves, play a role in the uptake of nutrients and degradation of toxins in the water.

Storm water runoff has a large negative impact on urban water quality. Before rushing into storm drains, the rain washes across parking lots, roadways, and people’s backyards, picking up a load of pollutants and debris. This toxic mixture contains motor oil, gasoline, plastic bottles, floating trash, dog poop, lawn fertilizer, and pesticides.

Storm water runoff is difficult to control. Unlike a single heavy polluter, such as a factory, known as a “point source polluter” because its pollution comes from a single “point” which can be treated and regulated, the toxins and debris picked up by storm water runoff originate from multiple sources which are harder to track down.

Buoyant debris

Storm water is referred to as “non-point source pollution”. During heavy rains, storm sewers can be overwhelmed and overflow into water bodies. The combined effects of pollution and choking debris threaten the aquatic life in these ecosystems.

Floating trash islands are one strategy to remediate pollution in any body of water affected by storm water runoff. Building one involves binding together buoyant debris often found along the banks of polluted bodies of water, chiefly capped plastic bottles and Styrofoam chunks. (Which also serves to reuse this trash).

Water plants are then strapped onto the floating media and allowed to develop their root systems. Most water plants do not need to be rooted in soil in order to thrive, and some even do better with their roots freely suspended in water.

Image: A “young” floating island. As it matures, the plants will grow throughout the plastic fencing
Image: A “young” floating island. As it matures, the plants will grow throughout the plastic fencing
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Building a floating island

Supplies needed :

  • Lots of floating plastic bottles or Styrofoam
  • A roll of plastic construction fencing
  • Zip ties
  • Water plants. Irises, bulrush, pickerel rush, arrowhead, duckweed, and watercress are good choices, but just about any water plant will work.
  • Anchor and rope; or mooring

How to :

  • Roll the plastic fencing into a tube with a diameter of at least a foot. Zip tie it closed along the side at one end, leaving the other end open. The length of the tube will be equal to the circumference of the island. Islands with a diameter of 5 to 10 feet and circumference of roughly 16 to 31 feet work well. (Circumference = diameter x 3.14)

  • Stuff plastic bottles or Styrofoam into the open end of the tube, filling it. Pack them in snugly, while still allowing the tube to be bent. Make sure the bottles are tightly capped, otherwise they could fill with water and cause the island to sink.

  • Bend the tube into a ring and zip tie the ends together, sealing the tube and completing the circle.

  • Stretch plastic fencing across the center of the ring and zip tie it in place.

  • If water plants are in containers, remove them and wash off any gravel. Gently place the plants upright on the fencing in the island’s center, touching the inside edge of the bottle-stuffed ring. Work the roots through the holes in the fencing so they will dangle into the water. Zip tie larger roots to the plastic fencing. If possible, try to work the leaves or stems of the plants through the sides of the ring to help to keep them upright. The plants should cover the surface of the island without choking each other out. A combination of plants can be used to encourage diversity. Take care not to introduce an invasive species in the area.

  • Choose a location. Ideally, the island should be put in full sun.

  • Choose an anchoring system. The island must be secured so that it doesn’t float away or drift into a shady area. It can either be anchored to the bottom or placed on a mooring that keeps it in the same location. An anchor can be as simple as a rope tied to a gallon jug filled with concrete. A mooring can be constructed from a metal poke stuck through the floating island and mounted on a concrete base sitting at the water’s bottom. A kiddie pool or similar shaped container can be used as a mold to make the base. Stand the pole upright in the center of the mold and fill the mold with several inches of concrete. Once the concrete has set, the pole should be locked into the base and the mold can be popped off. The wide bottom of the base will keep the pole from falling over.

  • Once a location and an anchoring system have been chosen, position the island by boat or by foot, wearing waders.

Water birds

As the island matures, the plants will grow throughout the plastic fencing. Their dead leaves and stalks will add to the island’s mass. Eventually, the shade produced by the plants will protect the bottles from degrading in the sun. Soon after it has been launched, giant sheets of algae will begin to form off the bottom of the island, aiding in water purification. If the body of the water is healthy enough to support fish, minnows will take shelter in the roots of the island. Water birds may even nest in it.

Floating islands can still be used in ponds that are dry for part of the year. To do so, install a mooring system in a plastic stock tank greater in diameter than the island. When the water level drops, the island slides down the mooring and comes to rest inside the water-filled stock tank. The tank may need to be manually filled depending on the length and intensity of the dry season. As the water in the pond returns, the island rises with the water level. This method provides a reservoir for many types of life in the pond. Fish, frogs, and birds would leave or perish without it.

Air pumps

Solar-powered air pumps can be attached to islands to remediate eutrophied bodies of water. When a body of water has an excess of nutrients, typically from fertilizer run-off or sewage, algae feed off these nutrients and explode in population. After the nutrients are used up, the algae die and are eaten by bacteria. The growing bacteria populations deplete the oxygen in the water, frequently resulting in fish die-offs. This process is known as eutrophication.

It can be mitigated or reversed by increasing the amount of oxygen in the water. This can be done by mechanically aerating the water with an air pump. The pump is placed on the island. Airstone diffusers are connected to the pump with rubber hosing and submerged as deep as possible directly beneath the island.

A floating island’s impact on overall water quality depends on the maturity of the island and its size relative to the water. With enough floating islands, it could be possible to restore vitality to an urban aquatic ecosystem.

Excerpted from “Toolbox for sustainable City Living: A Do-It-Ourselves Guide” by Scott Kellogg and Stacy Pettigrew (South End Press, 2008). Reprinted with permission from the authors and the publisher. Buy the book // About the authors.


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Matthew Murphy

DIY, floating trash island, low-tech, no-tech, survivalism, scott kellogg, stacy pettigrew, juan martinez

Howdy Just thought that I would put my 2 cents in. The design for the low-tech floating island has major drawbacks. The use of plastic bottles, which photodegrade, is not even remotely environmentally friendly. Add to this the problem of the island eventually falling apart because of lack of structural integrity and we arrive at a problem, not a solution. The bottles could be housed in a cement ’ donut ‘, thus sealing them from interaction with the water, air and sunlight, thus delaying the problem of eventual degradation but, inevitably, there will still be an environmental negative in the use of such material. I would suggest a rethink on promoting this as a project.

Kris De Decker

Matthew, you are right that plastic bottles eventually photodegrade, but as far as I understand, this process takes at least 100 years.

You don’t put these islands in some far away lake and forget about them. This is a method to clean up urban bodies of water in your own surroundings, which means that you can monitor them.

It does not take 100 years to do the cleaning job, so after some years you take them out of the water again. If they get in a bad shape before the job is done, you take them out and install new ones.

I. M. Moderate

Since the cans and bottles seldom come with caps, especially tight caps, I can see you are already so labor intensive with melting and testing seals that this must be intended more of a case of “practical art” commentary than truly practical solution.

People are always ready to put far more labor into art and political commentary projects than into a project that is far more efficient but purely mundane. I prefer the rare case where these folk help engineers make mundane stuff blend into the environment or appear beautiful.

And I have to agree with the first guy that the first incarnation of this sort of project probably sank or broken apart within few weeks. However, each incarnation would probably last longer as the leadership relearned practical engineering lessons the hard way.

One of those engineering lessons would be that humans in boats are going to mess with this floating island. The most innocent probably being fisherman after the fish gather underneath the float.

Martin Ellis

Could I suggest that a raft be made from some type of bundled reed? similar to those used in Iraq. Although perhaps some further boyancy cuold be afforded with coating the reeds with a ersin from pine trees or the like?

We see reed mats and rolls used to filter silt from stormwater or surface water runs… so assume that works.

But I would rather see one big polyetheleen drum used as a primary float than a bunch of plastic bottles. Only becasue it is only one piece of litter that can be removed later.. and tethered.

Although what you use for your teathers and hold lines is probably just as risky?

I have seen a system used at Auckland Sewer treatmetn ponds.. they use a geogrid product.


I was just thinking…if the dirt was thick enough, with plants with well well-established root systems, could the “island” become strong enough to held itself together? It would have to be plenty big, and wouldn’t float very much, but then you could take the bottles off and leave only the nets and itself to hold it together. You might need some wood planks on the bottom to give it a little flotation and stability, but it might be effective.

Chris Berens

Love this low tech clean up (plastic bottles and water) tip. I think we’ll try it here in Cape Town :-)

Low cost too, just the plastic fencing and cable ties to buy. When the island becomes overgrown you could wrap a larger ring around the perimeter or joist hoist it out with a crane and build a new one.

Perhaps the solar pump could bring water to each island and let it fall through the roots

Chris Bosch

Use plastic containers and fill w/ expanding foam insulation to make flotation units… virtually unsinkable.

Rusty Fox

I love the concept! No comments other than that, but a question:

Would the use of water plants which naturally float be able to support the entire structure at some time in the future, enabling it to continue floating even after bottles/styrofoam have broken down?

The first plant I can think of immediately to fit that bill is the water hyacinth (Eichornia crassipes) - and in Australia it is considered a noxious pest.

Tamar Shaddeau

Birds get caught in that plastic fence stuff…going to cover it with something else or pull the island out…