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 resulting form the oxidising treatment with calcium oxide in powder form, resulting in an alkaline mixture comprising hydrolysed organic material and calcium sulphate; - feeding of the mixture 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 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 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 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 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 resulting form the oxidising treatment with calcium oxide in powder form, resulting in an alkaline mixture comprising hydrolysed organic material and calcium sulphate; - feeding of the mixture 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 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 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 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 from the downstream outlet of the rotary reactor.