reed_beds - The Chartered Institution of Water and Environmental Management

View Results

Reed Bed Wastewater Treatment

Reed bed technology is based upon the cleansing power of three main elements: soil dwelling microbes, the physical and chemical properties of the soil, sand or gravel, and finally the plants themselves. Of these, the microbial flora and fauna is the most important constituent.

The civil engineering surrounding a reed bed treatment zone is generally very simple, firstly an excavated void is lined to prevent seepage of wastewater to the environment, then this void is filled with appropriate gravel, soil or sand and the reeds are planted into this media. The water is delivered either over the surface of the system (vertical flow), or via a feeder trench at the front end of the system (horizontal flow).

Most reed beds in the UK are sub-surface flow systems where the water flows below the bed surface. Some systems, and commonly those in the USA, are designed for the water to flow above the surface, and treatment occurs much as it would do in a pond system, supplemented by some plant activity.

Microbial activity

Micro-organisms attach themselves to the outside of the gravel or soil particles and to the plants and plant roots. These organisms metabolise polluting chemicals, degrading and mineralising them. Commonly reed beds are used to degrade sewage, but with higher retention times even intractable compounds such as PAH, PCB, dyestuffs, amines and glycols can be treated. After the Gulf War for example, in the oil-polluted sands of the Kuwait desert, the first cleaning processes took place around the roots of annual plants, where oil-eating micro-organisms cleaned both the roots and the sand particles sticking to them (Nature, 1995).

Choosing Media

The choice of bed media, gravel, soil or sand, is dependent upon the particular application requirements. Gravel is less active microbiologically, but allows a faster throughput of water. For this reason, gravel has commonly been used in secondary and tertiary sewage treatment applications and in mine waste water treatment where the plants help to keep the water oxygenated thus encouraging the deposition of insoluble metal ions precipitates. Soil has commonly been used for primary and secondary treatment of industrial effluents. Certain soil minerals actually encourage the deposition of metal ions, phosphate and sulphate. Soils can therefore be custom-engineered to treat particular effluent streams. In addition, the ability of clay particles and humic materials to entrap polluting chemicals means that the soil system can cope with shock loads.

The Plants

The plants have three functions. Firstly, the very extensive root system creates channels for the water to pass through. Secondly, the roots introduce oxygen down into the body of soil and provide an environment where aerobic bacteria can thrive. These organisms are necessary for the breakdown of many types of compound, in particular in the oxidation of ammonia to nitrate, the first step in the biological breakdown of this compound. Thirdly, the plants themselves take up a certain amount of nutrient from the waste water. In the spring and summer, about 15% of the treatment capacity for sewage effluent occurs through this route. Most degradation of nutrients is, however, undertaken by the microbes. The plants are also capable of accumulating certain heavy metals, an area where there is currently a great deal of research.

Many of the reed bed systems in the UK have used the common reed, Phragmites australis, as the main plant in the treatment bed. It is a vigorous and geographically well spread plant, and has a large number of varieties that vary in their growing habits and tolerance to particular chemicals. Other plant species that are used include Juncus effusus, Scheonoplectus sp and Typha sp. Phragmites australis has the most extensive root system, but other plants may be suitable for specialist applications or for landscaping.