As a reproduction of natural wetlands, the soil moisture balance equation (Rodriguez-Iturbe, 2000) can be used to describe the hydraulic behavior of these systems.INFILTRATION I(s,t)Wastewater is pumped over the filter and let percolate by gravity allowing suspended solids to be retained in the upper part of the filter. This source of water is regular and it is distributed in several batches per day in order to allow the wastewater to infiltrate. The feeding period is followed by a longer repose period which purpose is the aeration and drying of the filter. Figure 2 reproduces a typical feeding-repose period. Precipitation is an additional stochastic source of water in the system.EVAPORATION E(s,t)In an environment with abundance of water, plants are not affected by water stress and evapotranspiration reaches its maximum levels. Transpiration is the main responsible for this parameter, and its influence is more important during the repose period, helping the substrate to reach dry conditions. LEAKAGE L(s,t)The drainage of water depends on the infiltration capacity of the filter. Water moves vertically in function of soil moisture and the pressure exerted by the column of water generated in each bath. Each feeding period will force the humidity front (fig 3) downwards, so the infiltration capacity will depend on the frequency and intensity of feeding periods.Progressive accumulation of suspended solids on the surface of the filter threatens the longevity of the system. Clogging affects the infiltration capacity of the system, reducing the porosity that controls the pass of water and air through the filter. The development of biomass on the porous media may also affect the infiltration capacity. Macrophytes, on the other hand, help to keep the hydraulic conductivity of the system through root development.
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