Sludges typically generated by a municipal sewage treatment system are turned into compost with the reed bed system.
Reeds act in two ways to alter the character of metals present in the sludge.
Firstly, their root system provides oxygen which boosts the population and activity of naturally occurring micro-organisms which, in turn, mineralize the sludge. Secondly, the plants grow rapidly in this nutrient-rich medium and absorb some of the minerals, as well as drawing water from the sludge. Meanwhile, the processes of evaporation, drainage and plant uptake combine to transform the sludge into a stable humus-like fertilizer material, which can be used to seal sanitary landfill cells or as a soil amendment. In conventional sand drying beds, dried sludge forms a hard, impermeable crust that needs to be removed before more sludge can be added. In a reed bed, the dense mat of rhizomes provides channels through which water can percolate to the underdrains.