HYBRID SYSTEMS

In the design of a constructed wetland system the choice of plant configuration depends on many factors: the main ones are the depurative goals, the morphology of the area and the type of wastewater.

As you can see from descriptions reported in the relative pages, each typology of constructed wetland (HF, VF, FWS systems) presents particular characteristics from the point of view of functioning and depurative efficiencies. It is obvious, therefore, as in design phase, according to the binds imposed by the particular situation (available areas, morphology, characteristics of the wastewater, requested effluent's quality objectives), the choice of combining different plant typologies comes to be often recommended (if not obliged). A hybrid system, in fact, can be able to harness the potentialities of each typologies of constructed wetland strongly reducing negative aspects of single components.

The possible solutions are numerous; in the following some of them are described.

• VF + HF : system with vertical subsurface flow disposed in two basins fed in discontinuous alternate way, followed by a horizontal subsurface flow system; the introduction of the horizontal subsurface flow system has the aim of obtaining a more efficient de-nitrification of the effluent of the vertical system.

• HF + VF : horizontal subsurface flow system, followed by a vertical subsurface flow system disposed in two basins fed in discontinuous alternate way; the stage with horizontal subsurface flow has the function of removing most of the organic load and the suspended solids remained after the primary sedimentation; the stage with vertical subsurface flow ensures a high oxidation and an efficient nitrification without having undesired problems of clogging. This scheme can be accomplished by a recycling of the effluent at the beginning of the system, so to obtain a more efficient de-nitrification.

• HF + VF + HF + FWS : system with horizontal subsurface flow, followed by a vertical subsurface flow system, by another horizontal subsurface flow system and by a free water surface system; the ulterior horizontal subsurface flow system realizes the de-nitrification of the wastewater, while the free water surface system, beside completing the removal of nitrogen components, further accomplish the removal of microbial load.