Sludges and Residues from Water Treatment

A challenge to the sustainability of water treatment processes has to do with sludges and generally semisolid residues collected or produced during water treatment. It is important to dispose of such material properly or preferably find uses for it. 

A variety of chemical sludges are produced by various water treatment and industrial processes. Among the most abundant of such sludges is alum sludge produced by the hydrolysis of Al(III) salts used in the treatment of water, which creates gelatinous aluminum hydroxide  

Alum sludges normally are 98% or more water and are very difficult to dewater. 

Both iron(II) and iron(III) compounds are used for the removal of impurities from wastewater by precipitation of Fe(OH)3. The sludge contains Fe(OH)3 in the form of soft, fluffy precipitates that are difficult to dewater beyond 10% or 12% solids. 

The addition of either lime, Ca(OH)2, or quicklime, CaO, to water is used to raise the pH to about 11.5 and cause the precipitation of CaCOj, along with metal hydroxides and phosphates. Calcium carbonate is readily recovered from lime sludges and can be recalcined to produce CaO, which can be recycled through the system. 

---