An underwater device, able to favour the sea auto-cleaning capacities, is herein described. This system, called MUDS (marine underwater depuration system), consists of a percolating filter and is placed at sea over an urban sewage outflow of a submarine pipeline. Due to the density difference, the water effluent flows through the percolating filter: this favours the mixing and a prompt recycling of organic matter, activating a marine trophic web. Rich microbenthic communities develop on the MUDS, both interstitially, inside the filter, and on the structure. The community mainly consists of ciliates, nematodes, harpacticoid copepods and polychaetes, all of which being organisms that increase the depuration efficiency by consumption of organic matter. This structure acts also as a deterrent for the illegal trawling activity in the area, and attracts large numbers of several finfish species, thus working as a fish aggregating device (FAD). It is possible to utilise this underwater device for medium littoral towns with strong differences in effluent discharges during the year, where the use of land-built effluent treatment plants is too expensive.