Purification plant to biologically purify waste water

A plant for the biological purification of waste water, said plant including in sequence a unit for decomposing organic matter, a first sludge sealing unit, a nitrification unit and a sludge settling unit, characterised in said plant includes an aerated primary settling unit prior to said units,

• wherein said plant further includes an aerated primary settling tank (1), a primary chamber (2) having an immersed bio filter (3), said bio filter being aerated by a diffuser (4) positioned beneath the filter unit, said plant further including a settling tank (5) collecting the sludge from the biological process as said sludge is fed back into the primary settling tank (1) by means of a pump system (6), and said plant further including a vertical settling tank (7), a immersed bio filter (3′) aerated by a diffuser (4′) positioned beneath the filter unit, said plant also including a secondary settling tank (9) from which purified water is discharged through an outlet,
• wherein the aerated primary settling tank is divided by a partition device not extending to the bottom, includes a diffuser being positioned at the bottom for providing an air inlet, and said tank having a volume of between 50L and 25 m3.