Reed beds are a tertiary treatment with the process aim of removal of suspended and dissolved matter beyond that which the conventional secondary sewage treatment process provide. They remove 60 – 80% solids from the secondary effluent. In the operation of horizontal flow reed beds secondary treated effluent is passed uniformly over vegetation so that suspended solid matter is retained in the vegetation. Reed beds have are very good at removing BOD, ammonia and nutrients, therefore this is one of the few forms of tertiary treatment that can be used to improve poor quality secondary effluent. Very simply, it consists of an area of reeds planted in a soil or gravel medium. It traps the solids from the effluent during its travel across the bed. Reed beds planted in a soil medium are capable of removing BOD and suspended solids up to 95% with potential removal of ammonia, nitrate and phosphate. Gravel based reed beds achieve BODs of 1-4 mg/l and suspended solids of 2-8 mg/l regardless of the quality of the influent. Ammonia and nitrate reduction is also possible.
The following things need to be considered by the designer of a reed bed:-Â¨ land cost and availability;
- gravel size 3-10 mm;
- typical depth of 0.6 m;
- need a uniform slope of 1:100;
- gravel medium needs liner;
- need constructed inlet and outlet to contain the bed;
- appropriate distribution system to prevent channelling
- appropriate effluent collection system.
Typical sizes of reed beds are 1m2/pe, so they are more suitable for small, rural works. It is advised to plant 4 reeds/m2 . The most popular reed used is the common reed ( Phragmites australis) although other suitable varieties exist. Typical loadings are 0.2 m3/m2/day.