CONSTRUCTED WETLANDS WITH HORIZONTAL SUBSURFACE FLOW (HF)

SFS-h or HF systems (submerged horizontal flow) consist in basins containing inert material with selected granulometry with the aim to assure an adequate hydraulic conductivity (filling media mostly used are sand and gravel); these inert materials represent the support for the growth of the roots of emerging plants (generally plants are Phragmites australis); the bottom of the basins has to be correctly waterproofed using a layer of clay, possibly available on site and under adequate hydrogeological conditions, or, as more often happens, using synthetic membranes (HDPE or LDPE 2 mm thick); the water flow remains always under the surface of the absorbing basin and it flows horizontally thanks to a low bottom slope (about 1%) obtained with a sand layer under the waterproof layer.

During the passage of wastewater between the ryzosphere of the macrophytes, organic matter is decomposed by microbial activity, nitrogen is denitrified, if in presence of a sufficient organic content, phosphorus and heavy metals are fixed by adsorption on the filling medium; vegetation's contributes to the depurative process can be represented both by the development of an efficient microbial aerobic population in the ryzosphere and by the action of pumping atmospheric oxygen from the emerged part to the roots and so to the underlaying soil portion, with a consequent better oxidation of the wastewater and creation of an alternation of aerobic, anoxic and anaerobic zones, with a consequent development of different specialized families of microorganisms and an almost total disappearance of pathogens, highly sensitive to rapid changes in dissolved oxygen content. Submerged flow systems assure a good thermal protection of the wastewater during winter, specially when frequent periods of snow are prevented.