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

Chemical P removal reactions consist of fast and slow processes. The long contact time of metal hydroxide flocs in simultaneous precipitation (as opposed to the relatively short contact in primary and post-precipitation) makes an efficient use of the chemical dosed, and can counteract the effect of sub-optimal mixing conditions to provide a low residual soluble P concentration in the effluent. Furthermore, the slow reaction is efficient in sequestering residual phosphates to produce a low phosphorus effluent if necessary.

There are three potential dosing points in chemical phosphorus removal in a wastewater treatment plant: 1) before the primary settler (primary precipitation or pre-precipitation) 2) into the aeration tank (simultaneous precipitation) or 3) in a tertiary step in the treated effluent, called post-precipitation. The contact time between the chemical flocs and the phosphate ions of the wastewater is significantly longer in simultaneous precipitation than in the other two methods. In primary and post-precipitation the chemical sludge forming after the chemical dose is separated and removed from the system within one HRT. In simultaneous precipitation the chemical sludge is captured and kept in the aeration tank for the SRT of the system. It is believed that this longer SRT is beneficial to chemical phosphorus removal. There have been recent attempts to increase the residence time in tertiary clarification by using blanket clarifiers or through internal sludge recirculation with resulting decrease in chemical dose or phosphorus residuals. This study attempts to explain these phenomena.

Many plants that practice simultaneous precipitation have an observation that while the process is resilient to perturbation, but once perturbed, it is difficult to make rapid improvements. This points to the importance of the slow kinetic rate that dominates in plants using simultaneous precipitation. Consequently, a series of laboratory experiments comprising over 1500 phosphate determinations were performed to identify the most important factors affecting instantaneous (“equilibrium”) and slow (kinetic) chemical P removal. The same phenomenon was further investigated at the Kecskemét WWTP (Hungary). At this site, the role of different P removal processes in pre-precipitation was investigated.

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