The hydrology of a wetland is one of the most important characteristics for the efficiency of the system. It is very important that the water entering the wetland has time enough to go through the different cleaning processes and interact with the aquatic biota. The time most looked at here is the retention time which tells how long the water stays in the system, only the wetland or the whole system. This variable can be controlled by regulating the inflow and outflow together with the depth and storage. The retention time is generally calculated by dividing the volume of water in the system with the flow rate. In sub surface-flow wetlands the volume of water depends on the porosity and the absence of vegetation in the pore space. To make this calculation easier it is usual to use the clean pore space in the calculations. Therefore it has been showed that the real retention time is normally 40 to 80 per cent less than the theoretical, mainly due to preferential flow and the difference in the calculated and the real pore space (EPA, 2000). The maximal flow is normally used as it gives the lowest retention time.
It is considered important that the water level does not reach the surface in sub surface-flow wetlands (EPA, 2000). This is mainly because it leads to increased evapotranspiration which will concentrate the pollutants, and the water passing the wetland above the filter will not undergo any treatment.
Another important aspect is the velocity with which the water passes the system. If the velocity of the water is too high there will be less chance for the particles to sediment, to make complexes with other particles or to attach to the biofilm of the plants. Controlling the water depth and the flow through the system is of great importance also for the establishing of the vegetation in the wetland. One has to take rainfall and evapotranspiration in consideration to calculate the true hydraulic loading. The way the water takes through the wetland also has a great impact on the cleaning efficiency. If the water is not spread out well enough, the retention time is lowered and also it can lead to saturated parts where no more processes can occur and the water basically goes through untreated while other parts of the system are not used at all in the cleaning process.
Sedang diterjemahkan, harap tunggu..