A reed bed is a natural filtration system that can be used to treat and improve water quality prior to discharging into the environment. It utilises the ability of a reed to transport oxygen to the soil, hence encouraging microorganisms to digest the contaminants in the effluent.
Reed beds come in various different forms and are sized based on the volume of flow per day. Two of the most popular forms of reed beds are “vertical flow” and “horizontal flow”.
A vertical flow reed bed allows wastewater to flow from one end to the other. Water flows through gravel at the base of the reed bed. The roots of the reeds reach down into the gravel to find water. Oxygen is transferred to the water via the roots, encouraging bacteria growth in this oxygen-rich environment much like in a sewage treatment plant. Water must have a long retention time (around 8 days) to enable effective cleaning.
Horizontal flow reed beds use the same biological process as vertical flow systems, however, water is dispersed evenly over the top of the gravel and allowed to percolate down through the root system collecting at the bottom where is it normally channelled out to a catch pit or inspection chamber. From this point it can be fed into another reed bed for a second (or third) phase of treatment or fed into the environment (ie a river or stream).
A pumped reed bed system is either one of the above “vertical” or “horizontal” systems but with the water fed into it using a pump. This can be advantageous, especially within vertical flow systems as water can be lifted up to then disperse back down through a second filter bed.
The maintenance of a reed bed system can be more involved than a normal sewage treatment plant. Granted, a reed bed system has no power (in non-pumped versions), however, reeds need to be cut back regularly and some systems require the water level to be adjusted to maintain optimum treatment throughout different seasons.
With this in mind, many people opt for packaged energy free systems or a sewage treatment plant that uses only the power of a 60w light bulb. These can be easier to install and maintain in the long term.
Reed beds do however work well as tertiary treatment (treatment after a sewage treatment plant) and are often used when there is a need to reduce ammonia levels prior to discharge into sensitive environments (like sites of special scientific interest).