Reed-beds

The purification of waste water and effluent using reed-beds has been successfully achieved for hundreds of years. By allowing dirty water to pass through wetlands planted with reeds and rushes, the roots of certain plants release oxygen, which helps micro organisms break down and filter out impurities. The method can ultimately produce high quality water which may be suitable for drinking. The plant biomass that grows in this system can also be harvested as a source of mulch, or perhaps as a crop in its own right.

Reed-beds may be naturally formed wetlands or artificially constructed and planted channels and beds. Given the current degree of environmental pressure on the few natural wetlands remaining, it would appear that further pressure on or usage of such wetlands is unwise. However, the deliberate building of new, well-designed wetlands/reed-beds could be a very useful enterprise, especially for treatment of effluent from dairy farms.

When micro organisms break down water pollutants, they use up oxygen. This oxygen consumption varies with different materials, and is known as the biological oxygen demand (BOD). For example, nutrient-rich wastes such as farm manures or silage effluent have a high BOD. When these pollutants find their way into waterways, the oxygen level in the water becomes seriously depleted as a result of breakdown processes, causing parts of the natural flora and fauna of the waterway to die. When the water body is small and the flow rate is slow (eg in conditions of low rainfall), this problem can be quite severe. The blue-green species of algae are then able to flourish, poisoning and fouling the water even further.

The problem of limited oxygen supply may be overcome by the use of structures such as pebble streams, rock-lined channels or waterfalls. In this environment of plentiful oxygen, micro-organisms such as bacteria, yeasts and fungi become established and thrive on the surfaces of the pebbles or rocks and consume the soluble polluting matter.

Alternatively, plants may be used to supply the oxygen necessary for micro organisms to break down pollutants. Some plants, mainly reeds and rushes, absorb atmospheric oxygen through their leaves and transfer it down hollow stems to their extensive root systems. The oxygen is then released through fine root hairs into the soil where it helps build up micro organism populations and facilitates the breakdown of organic matter. Reed-beds work most effectively when a dense layer of rhizomes and root hairs is formed. This may take about three years to fully develop.

(From Sustainable Agriculture by John Mason)