Operational Investigation of the Role of Precipitated Solids in Full-Scale Tertiary Chemical P Removal from Wastewater Effluents

Chemical P removal is commonly implemented in wastewater treatment to remove P to very low levels (<0.1 mg P/L). The dose of precipitant (Al or Fe) needed is several times the stoichiometric dose in order to achieve a filtered effluent level of < 30 μg TP/L. The mechanistic basis for P removal using chemical precipitant addition is generally considered to be more than simple precipitation. The role of adsorption and/or complexation in removal of reactive or unreactive phosphorus to the already formed chemical precipitates or complexes has been investigated. Potential operational efficiency gains resulting from age of chemically precipitated solids and the recycle of these solids to the process stream was undertaken at the Iowa Hill Water Reclamation Facility which employs the DensaDeg® process (IDI, Richmond, VA) for tertiary chemical P removal. The effect of solids age was found to be insignificant over the solids retention time of 2 to 8 days, indicating that the solids were unaffected by the aging effects of decreasing porosity and surface acidity, and that the bulk of solids were retained in the clarifier blanket, hence providing no advantage in the removal mechanisms from increased concentration of solids. When solids recycle was redirected from the traditional location of the flocculation reactor to a point just prior to chemical addition in the chemical mixing reactor, lower effluent soluble P concentrations at lower molar doses of aluminum were achieved. At laboratory scale, the chemical sludge showed both high capacity and rapid kinetics for P sorption. Saturation concentrations were approximately 29 mg sRP/g solids and removal reactions were modeled well by nth order equations with n ≥ 3.