The removal of chemical oxygen demand from primary-treated domestic wastewater in subsurface-flow reed beds using different substrates

Subsurface-flow experimental reed beds were designed and built based on a combination of two design methodologies. Four different growing media were used with a combination of topsoil, gravel, river sand, and mature wastewater biosolids compost to determine the best substrate for chemical oxygen demand removal. Eight units were constructed, two for each material. One bed for each pair was planted with Typha latifolia plants commonly known as cattails. Primary-treated domestic wastewater was continuously fed to the beds for more than 6 months. The best performance was achieved by the gravel reed beds, with an average removal rate higher than 50%. Soil-based beds containing topsoil and sand only managed to attain removals of approximately 10%. The reed beds containing compost in their substrate produced the worst treatment, mainly because of leaching of organic substances from the compost. Primarily as a result of channel flow, all beds showed significant deviation from the designed retention time. There was no significant difference in the performance of planted and unplanted reed beds.