Spent pickle liquor, as a source of ferric ion, was added to primary treatment facilities and found to be effective in removing more than 60% of the phosphorus contained in the sewage. When advanced means for more complete removal of the insolubilized phosphates are available, removals in excess of 90% are indicated. In addition, overall suspended solids removals averaged 61.5% and BOD and COD removals averaged 61.6 and 55.3 percent during a 23 month test period. Required iron dosage ranged from the theoretical minimum of 2.7 to a maximum of approximately 3.1 weights of ferrous iron per weight of phosphorus. The weight increase in sludge solids due to chemical precipitation of phosphorus was 100 lbs/million gallons/mg/liter of phosphorus in the influent. Total costs for treating a raw sewage containing 13 mg/l phosphorus are given.

Of a number of methods available for phosphate removal from waste water, chemical precipitation (often called coagulation) using aluminum, ferric iron, ferrous iron and lime is considered to be the most economical. The objective of the present study has been two-fold: To evaluate the effects of pH and reactant concentration on the rate and efficiency of phosphate removal from synthetic and secondary effluent wastewaters and on the nature of the precipitates formed; and to compare the data on the Fe(II)-phosphate precipitation system with the available data for Fe(III)- and Al(III)-phosphate systems. Most of the experiments in the study were conducted on pure orthophosphate solutions and in the absence of dissolved oxygen. Only a limited number of tests were made of the use of ferrous iron for orthophosphate removal from wastewaters containing dissolved oxygen.