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
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.
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).
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 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
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