Underground PE “SEI” tanks to contain not particularly aggressive fluids. horizontal cylindrical monolithic or modular full pass electrofused welded structure, the constant thickness and the strenghtening provided by the vertical and horizontal ribbing guarantee the perfect mechanical seal. Furthermore, there are bolt holes near the support feet to anchor the tank to the ground/reinforced concrete slab. An adequit number of accesses will be positioned on the upper generatrix of the basin with bolt holes of 400/600mm in diameter and a bayonet locking cap. The tank is suited for storing drinking water and other reactants, according to the polyethylene compatibility charts.