The Significance of Chemical Phosphorus Removal Theory for Engineering Practice

Abstract

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.

Application of Membrane Bioreactor Processes for Achieving Low Effluent Nutrient Concentrations

“…These metal hydroxides will continue to remove phosphorus through surface complexation (Smith et al, 2007) in the MBR and remove phosphorus. These metal hydroxides will continue to remove phosphorus through surface complexation (Smith et al, 2007) in the MBR and remove phosphorus.…”

“…Well established design principles regarding mixing at the point of chemical injection, location of chemical injection points, and chemical dose control must be observed (Smith et al, 2007). Adequate control over chemical dosing is key to reliable performance of the system, as shown in the different case studies above.…”

Newly developed membrane contactor-based N and P recovery process: Pilot-scale field experiments and cost analysis

“…Phosphorus removal with coagulation takes place in three main possible scenarios: metal-PO 4 formation, metal-hydroxy-PO 4 complexes formation, and adsorption onto metal hydroxides (Manamperuma et al, 2016). However, the formation of metal-PO 4 is unlikely to occur in the typical wastewater treatment environment (Smith et al, 2007) because the alkalinity of wastewater neutralizes the acidity of the coagulants, producing hydrous metal oxides. Given that our reject streams are tested in such an environment, this mechanism can be ruled out.…”