Usually the primary and secondary treatments are sufficient to meet wastewater effluent standards. However, if water produced is required to be of higher water quality standards (in case the water to be put to some direct reuse) then advanced wastewater treatment is carried out.
A wide variety of methods are used in advanced waste treatment, which include the removal of (a) Suspended solids, (b) BOD, (c) Plant nutrients, (d) Dissolved solids and (e) toxic substances. These methods may be introduced at any stage of the total treatment process as in the case of industrial wastewaters or may be used for complete removal of pollutants after the secondary treatment.
The wastewater treatment processes are basically concentrating or thickening processes on which the suspended solids are removed as sludge’s. The impurities in the wastewater are concentrated into solid form and are then separated from the bulk liquid. This concentrated form is referred to as sludge. Whereas the dissolved solids are first converted into suspended solids which are subsequently removed as sludge’s.