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THE SIGNIFICANCE OF CHEMICAL PHOSPHORUS REMOVAL THEORY FOR ENGINEERING PRACTICE

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Achieving phosphate removal to very low levels recently refocused the attention of the wastewater engineering profession on the design principles and various mechanisms that can produce effluent phosphate concentrations below 10 μgP/L (with TP being usually less than 100 μgP/L or 0.10 mgP/L). Current equilibrium models used for design (WEF, 1998) are based on dissociation and solubility principles and cannot account for variable precipitate stoichiometries and time dynamics. DCWASA initiated a research project to investigate the potential chemical, physico-chemical and physical mechanisms and their consequence on engineering design and operation of these systems. For iron mediated phosphate removal the iron is added to the wastewater stream in concentrated/acidic form. The alkalinity of the wastewater stream neutralizes the acidic ferric solution and results in rapid precipitation of hydrous ferric oxides (HFO). Simultaneous with HFO precipitation is soluble phosphate removal by co-precipitation and by adsorption of phosphate onto existing HFO particles. Chemically, co-precipitation and chemisorption can be viewed as the same process; phosphorus in phosphate and iron in HFO share one or more oxygen atoms. Important but frequently ignored factors for design of chemical phosphorus removal systems include sorbent to sorbate (Fe/P) ratio, G-value (mean velocity gradient) and reaction time (system SRT). The reaction time is important for both slower diffusion reactions that enhance P removal, and aging of metal flocs that reduce active surface sites and reduce the HFO sorbent capability for P removal. Experimental methods, results and the equilibrium part of the model is presented.
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Document Type: Research Article

Publication date: 2007-01-01

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