Process for forming a tank for biogas plants, comprising at least the following steps: - providing a plurality of reinforcing elements, each of them having a reticular structure, each reinforcing element comprising: • a plurality of first elements spaced from each other and having an elongated shape according to a respective main extension direction; • a plurality of second elements spaced from each other, which substantially extend along a direction transversal of said first elements, second elements having an elongated shape according to a respective main extension direction; - providing a predetermined quantity of ground; - forming a containing site comprising at least one base and at least one side wall mainly of ground, transversally emerging from base, side wall having an inner face which, together with the base, defines a containing cavity, having a closed outline, arranged to receive a predetermined quantity of organic matter configured to generate a biogas following an anaerobic bacterial fermentation, the containing site further comprising a free edge placed opposite in respect to the base and delimiting an opening of the containing cavity; characterized by the fact the step of providing the ground side wall comprises at least the following sub-steps: • forming a first ground compacted layer having a rising surface and an upper abutment surface, the upper abutment surface extending around the base to define a closed outline; • positioning on the upper abutment surface of the first ground layer, a plurality of reinforcing elements, said plurality of reinforcing elements extending around the base to define a closed outline; • connecting to each other said reinforcing elements to define a continuous layer of elements suitable for covering at least partially the upper abutment surface of the first ground layer; • after having positioned the reinforcing elements, positioning a second ground layer, so that the reinforcing elements are at least partially interposed between the first and second ground layers (25; 26), said second ground layer having a corresponding rising surface and a corresponding upper abutment surface, the upper abutment surface extending around the base to define a closed outline; each ground layer and the respective reinforcing elements associated to it, defining a reinforced layer, the process provides to repeat said sub-steps to define a plurality of reinforced layers overlapping each other so that these latter define the inner face of side wall. - providing a plurality of reinforcing elements, each of them having a reticular structure, each reinforcing element comprising: • a plurality of first elements spaced from each other and having an elongated shape according to a respective main extension direction; • a plurality of second elements spaced from each other, which substantially extend along a direction transversal of said first elements, second elements having an elongated shape according to a respective main extension direction; • a plurality of first elements spaced from each other and having an elongated shape according to a respective main extension direction; • a plurality of second elements spaced from each other, which substantially extend along a direction transversal of said first elements, second elements having an elongated shape according to a respective main extension direction; - providing a predetermined quantity of ground; - forming a containing site comprising at least one base and at least one side wall mainly of ground, transversally emerging from base, side wall having an inner face which, together with the base, defines a containing cavity, having a closed outline, arranged to receive a predetermined quantity of organic matter configured to generate a biogas following an anaerobic bacterial fermentation, the containing site further comprising a free edge placed opposite in respect to the base and delimiting an opening of the containing cavity; • forming a first ground compacted layer having a rising surface and an upper abutment surface, the upper abutment surface extending around the base to define a closed outline; • positioning on the upper abutment surface of the first ground layer, a plurality of reinforcing elements, said plurality of reinforcing elements extending around the base to define a closed outline; • connecting to each other said reinforcing elements to define a continuous layer of elements suitable for covering at least partially the upper abutment surface of the first ground layer; • after having positioned the reinforcing elements, positioning a second ground layer, so that the reinforcing elements are at least partially interposed between the first and second ground layers (25; 26), said second ground layer having a corresponding rising surface and a corresponding upper abutment surface, the upper abutment surface extending around the base to define a closed outline;