Reed beds were mainly used and seen as a process of reducing the level of biochemical oxygen demand (BOD) and suspended solids in sewage effluent.
Over the years, they have been increasingly used for other forms of treatment, and are currently being considered as an alternative solution for phosphorous removal in small sewage treatment plants.
Phosphorous is a limiting nutrient for plant growth in order to avoid eutrophication in lakes and rivers.
The process is viewed as a sustainable solution, due to its capacity to operate on low to almost no energy input, using only the hydraulics associated with gravity.
However, to achieve the levels of phosphorous being considered by the Environment Agency in certain areas, the extent of the reed bed required is proving significant, requiring a substantial area and involving in some cases the purchase of additional land outside the existing sewage treatment works, many times greater than that occupied by the works itself.
The removal efficiency of phosphorous depends upon the type of media used, which can vary between gravel, steel slag, etc.
Recent research suggests that the phosphorous is removed by chemical action as the principal process. However, this is limited to the composition of the media.
There are still unanswered questions that may require further research. Such questions are: what to do with the media after it has reached saturation, including safe disposal, also the durability of the bed and media replacement costs, all of which need to be taken on board and included in any cost comparison.
Reed beds have and always will provide a simple, effective and ‘natural’ way of providing basic treatment. However, it must be appreciated that they are not a panacea, and based upon the research carried out to date with respect to phosporous removal, to extend their treatment capability as an alternative solution in place of the more established systems, needs to be approached with some caution.