TABLE OF CONTENTS Interest in the removal of nutrients from wastewater effluent has increased in recent years. Part of this is due to the success of the wastewater industry in removing gross contaminants such as BOD, TSS, O&G and toxic chemicals from point sources. Now that the gross effects of pollutant loadings have largely been eliminated, we are able to examine less dramatic indicators of ecosystem stress.
Nutrient loads into the nation's waterways are now recognized as a major source of the remaining pollution that is degrading some aquatic ecosystems. New chemical and biochemical technologies are emerging for the reduction of effluent nitrogen and phosphorus concentrations.
As discussed in earlier sections of this chapter, phosphorus can be a very destructive environmental contaminant. Phosphorus is used extensively in industrial manufacturing and agriculture, and is a principal contaminant in treated sewage discharges. Both chemical and biological technologies exist for phosphorus removal.
Phosphorus can be chemically precipitated from primary or secondary treatment effluents by the addition of lime, alum, and iron salts. For biological treatment systems, the addition of alum and iron salts should be between the aeration step and the final clarifier. Ferric chloride in dosages of about 10 mg/l is the most commonly employed precipitating agent for phosphorus in treating municipal wastewater [22] achieving about 90 percent removal.
For chemical precipitation with lime, effluent phosphorus is a function of pH, as shown in Figure 9.1. Lime is less efficient than ferric chloride, used at dosages of 500 to...
