The invention concerns a modular system comprising a plurality of identical modules (1) which are produced as injected low-density plastics monopart bodies and can therefore float in water, the modules (1) have coupling means (3, 4) for forming by interconnection of a plurality of modules (1) a floating surface or sheet for macrophyte plants. To that end, provided in each module (1) are vessels (5) which are kept rigid by means of arms (6), in a reticular structure of triangular templates, the inter-coupling of a plurality of modules (1) forming a surface floating on water such that the growth on the vessels (5) of the plants which will become macrophytes brings about the purification of the water in which the surface of the floating structure is located.
