Process for treating a starting biological sludge (MBRO) with a dry fraction and a liquid fraction, wherein the dry fraction is greater than or equal to 15% by weight of the starting biological sludge, comprising: - acid conditioning of the starting biological sludge (MBRO) by means of mixing with sulphuric acid, resulting in a fluid acid sludge with a higher temperature and a lower viscosity than the starting biological sludge; - oxidising treatment of the fluid acid sludge by means of mixing with a hydrogen peroxide solution; - mixing of the fluid acid sludge (MBR1) resulting form the oxidising treatment with calcium oxide in powder form, resulting in an alkaline mixture (MBR2) comprising hydrolysed organic material and calcium sulphate; - feeding of the mixture (MBR2) comprising hydrolysed organic material and calcium sulphate inside a rotary reactor with a reaction chamber extending between an upstream inlet for feeding the mixture and a downstream outlet, further comprising an upstream opening for outputting gaseous substances; - rotating the rotary reactor so that the mixture (MBR2) is mixed and turned around and moves from the upstream inlet to the downstream outlet in the reaction chamber ; wherein during rotation of the rotary reactor and transit inside the reactor of the alkaline mixture (MBR2) from the upstream inlet to the downstream outlet, a temperature of the mixture inside the reaction chamber exceeds 90°C and the mixture is dehydrated with production of steam and an increase in the percentage of dry substance of the mixture to more than 55%, forming a dried resulting material which is a granular fertilizer (MBR3) in the form of granules with a residual moisture less than or equal to 40%, preferably of between 10% and 30%, suitable for use as a corrective for alkaline and/or saline agricultural soils; - extraction of the granular fertilizer (MBR3) from the downstream outlet of the rotary reactor. - acid conditioning of the starting biological sludge (MBRO) by means of mixing with sulphuric acid, resulting in a fluid acid sludge with a higher temperature and a lower viscosity than the starting biological sludge; - oxidising treatment of the fluid acid sludge by means of mixing with a hydrogen peroxide solution; - mixing of the fluid acid sludge (MBR1) resulting form the oxidising treatment with calcium oxide in powder form, resulting in an alkaline mixture (MBR2) comprising hydrolysed organic material and calcium sulphate; - feeding of the mixture (MBR2) comprising hydrolysed organic material and calcium sulphate inside a rotary reactor with a reaction chamber extending between an upstream inlet for feeding the mixture and a downstream outlet, further comprising an upstream opening for outputting gaseous substances; - rotating the rotary reactor so that the mixture (MBR2) is mixed and turned around and moves from the upstream inlet to the downstream outlet in the reaction chamber ; wherein during rotation of the rotary reactor and transit inside the reactor of the alkaline mixture (MBR2) from the upstream inlet to the downstream outlet, a temperature of the mixture inside the reaction chamber exceeds 90°C and the mixture is dehydrated with production of steam and an increase in the percentage of dry substance of the mixture to more than 55%, forming a dried resulting material which is a granular fertilizer (MBR3) in the form of granules with a residual moisture less than or equal to 40%, preferably of between 10% and 30%, suitable for use as a corrective for alkaline and/or saline agricultural soils; - extraction of the granular fertilizer (MBR3) from the downstream outlet of the rotary reactor. <EOS>