The impact of sewage composition on the soil clogging phenomena of vertical flow constructed wetlands.

The infiltration rate and therefore the principal function of a sand based vertical flow constructed wetland (VFCW) is influenced by the content of suspended solids (SS) and chemical oxygen demand (COD) of the waste water supply. In this study there were three operating conditions defined as "No Clogging"; "Partly Clogging" and "Clogging". Investigations on 21 VFCWs approved analytical differences between these conditions. The content of SS and especially particles > 50 microm are considered to play a key role. These particles are of the same size as the pores in which seepage mainly occurs. Thus their potential for surface blocking is high. It is concluded that the construction and size of the primary settling has to ensure that the mean concentration of SS after settling does not exceed 100 mg l(-1). The results of this study indicate that the area of the VFCW should be designed for a maximum loading rate of 5 g m(-2) d(-1) and the COD load should not exceed 20 g m(-2) d(-1).