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.
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.
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.
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
A "young" floating island. As it matures, the plants will grow throughout the plastic fencing
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.
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.
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.
© Scott Kellogg and Stacy Pettigrew / Illustration by Juan Martinez
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.
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