The second largest depuration plant in Prato is Calice which was started in 1985 and is located in the western part of the municipality, on the border with the Municipality of Agliana. The plant was structurally adapted in 2000 and almost doubled in 2002.
The sewage drained in the west of the city and those coming from the nearby Municipality of Montemurlo converge treatment plant of Prato-Calice (190,000 P.E.).

The Calice depuration plant, on weekdays can treat up to 39,000 cubic meters/day and can break down up to 20,000 kg/day of COD and 1100 kg/day of surfactants. Basically it consists of equalization, primary sedimentation, denitrification, biological oxidation, secondary sedimentation, clear final flocculation, sand filters and ozonation. The sludge line consists of gravity thickening, mechanical dewatering with centrifuge and filter press. The resulting sludge from the purification process can amount up to 10,000 t/year with a variable humidity between 64 and 74%.