The Importance of Slow Kinetic Reactions in Simultaneous Chemical P Removal

Laboratory and full-scale experiments were performed in order to investigate the effect of mixing, as represented by the mean velocity gradient, G-value (Grady et al, 1999), and time dynamics in chemical phosphorus removal. The results prove that the chemical reactions occurring during phosphorus removal upon addition of trivalent metal salts (Fe, Al) cannot be described with the currently applied equilibrium theory (WEF, 1998). Two reactions are postulated: a) a significant part of the phosphate, depending on the applied G, is removed instantaneously, or within the first few minutes. This fast reaction however is not a simple equilibrium precipitation, since it involves varying stoichiometry and varying residual concentrations; b) a smaller part of the total initial phosphate is removed by a slow kinetic reaction (termed adsorption). The time constant of this reaction is in the order of hours to days. Pure metal hydroxide flocs, formed without the presence of phosphate, also remove phosphorus kinetically. The role of this reaction is more expressed in plants where mixing at dosage point is inadequate, particularly during simultaneous phosphorus removal. The slow reaction has increased relevance when residual phosphate concentrations are in a very low range, below 0.1 – 0.2 mgP/L, a range that is an increasingly common permitted requirement for wastewater treatment facilities. Thus, the slow reaction can be used with advantage to achieve very low phosphorus residuals via simultaneous precipitation. Additionally, this reaction may be important for tertiary blanket clarifiers and for tertiary clarifiers with internal sludge recirculation.