Sludges are produced during wastewater treatment.
Wastes first receive primary treatment where solids are removed that settle out easily.
Trickling filters, activated sludge processes and attached growth systems are used in secondary treatment. In the future, many treatment plants will be producing sludges using tertiary treatment such as chemical precipitation and filtration, in which lime and polymers are used to thicken and dewater sludges. Lime stabilization kills pathogens in two hours and immobilizes heavy metals when high pH levels are maintained. Oxyozonation is a chemical stabilization process that uses pressurized oxygen and ozone under acidic conditions. Treatment methods and chemical characteristics influence how sludges can be used.