The performance of vertical flow constructed wetland systems with special reference to the significance of oxygen transfer and hydraulic loading rates

The paper reviews the development of the vertical flow (VF) reed beds/constructed wetlands over the past 20 years. The performance of VF systems (and their use within hybrid systems) is analysed by reference to a number of brief case studies. The oxygen transfer rate (OTR) achieved is absolutely critical to the sizing of the systems. The author reviews the reported OTRs and comments on the existing design equations proposed for calculation of the area of beds. The 1st generation of VF systems used a set of parallel beds that were dosed one at a time in rotation and then rested for a period of days because there was considerable concern (based on early experience) that they would become clogged. In the past 10 years a number of new designs of 2nd generation VF beds have been built which make use of a single bed and hence operate without any resting periods. The hydraulic loading rate and the selection of the bed media, which are critical to the design and hence successful operation of these 2nd generation compact VF beds, are described. It is now possible to produce a very high quality of effluent from VF beds alone sized at 2 m2/pe when treating domestic sewage.