This chapter focuses on chemical processes. It shows that the solubility curves observed during simultaneous precipitation of ferric or aluminum phosphates in activated sludge systems are somewhat different. The chapter aims to obtain low effluent phosphate residuals one must treat the sludge streams in such a way that the removed phosphate is not returned to the wastewater flow. Phosphate removal from wastewater involves the incorporation of phosphate into a particulate form and then the removal of the suspended solids. In the late 1960's typical raw sewage total phosphate concentrations were 10-12 mg P/L. In addition, industrial and commercial pretreatment programs removing phosphate before discharge into municipal sewers have contributed to some extent to the reduction. Calcium phosphates will only enter a digester when they are produced by lime addition to raw sewage in pre-precipitation processes. An additional sparingly soluble phosphate-containing solid can form under the conditions encountered in sludge treatment.