Chemically sludge is obtained when precipitating soluble or insoluble chemicals present in the water or any kind of effluent coming out of chemical industries. Generally iron was used as an precipitate pollutants present in waste water. the final product after precipitation is called chemical sludge which is filled in a land into heap or used in road construction.
Biological sludge is formed when microorganisms are used to degrade toxic chemicals present in waste water. The excess biomass formed is termed as biological sludge which contains adsorbed toxic chemicals in their cells, toxic metabolites formed during degradation of wastes, dead biomass etc.