treatment, also called advanced
waste treatment, provides contaminant removal beyond that
achieved in primary (physical settling) or secondary (biological)
treatment. It may include additional removal of organic matter or
solids, reductions in the concentration of nutrients such as nitrogen
and phosphorus or treatment of toxic substances.
Nuisance growth of the
attached alga Cladophora in Lake Erie
removal is of special interest in the Great Lakes region where additions
of this plant nutrient may stimulate nuisance algal growth. Treatment
plants discharging to the Great Lakes or their tributary
waters have limits placed on the amount of phosphorus in their effluent.
Phosphorus is removed by adding aluminum (alum) or iron (ferric
chloride) salts at some point in the primary or secondary treatment
process. The salts form a precipitate with the phosphorus which
settles out in the clarifiers and is removed with the sludge.