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Some Guidelines for pH Control of a P-recovery Pellet Reactor

Emphasis in recent years has been focused on improving processes which lead to enhanced phosphate recovery. This paper studies the precipitation features of calcium phosphate in a fluidized bed reactor in a concentration range between 4 and 50mg/L, and establishes the conditions for optimum phosphate removal efficiency. A two level modeling methodology is proposed and used for the determination of a control strategy of the unit pilot. INTRODUCTION Phosphorus can be found under various chemical forms in urban wastewater, which represents about 30 to 50% of the total refusal of P: insoluble or dissolved organic phosphorus, orthophosphates (until 70% sometimes) and condensed inorganic phosphates. In France, the average concentration of phosphorus in domestic wastewater is within the range 15-25 mg/l, which may strongly vary from day to day, even during day. The discharge of phosphorus in the aqueous natural environment leads to an excessive development of algae and, generally to a pH increase, thus corresponding to eutrophication. Consequently, the phosphorus reduction in the rivers is considered as a key factor of the fight against pollution. The principal legislative tool in Europe for fighting against eutrophication is the EC Urban Waste Water Treatment Directive (271/91/EEC). This action came into force in 1991 and enabled waterbodies to be classified as Sensitive Areas if they display symptoms of eutrophication. In this context, the precipitation of phosphate salts is a solution used as a P-recovery process for effluents with a low concentration in inorganic phosphorus. In the last years, several works have been devoted to calcium phosphate precipitation in the so-called pellet reactor (Hirasawa and Toya, 1990, Morse et al., 1998, Seckler et al., 1996). In this paper, calcium phosphate precipitation is investigated in a fluidized bed reactor in a concentration range between 4 and 5 mg.L-1. More precisely, the objective of this paper is to propose some guidelines for pH control of the reactor. First, the pilot unit used in this study is presented and a discussion is proposed from the observed experimental results to propose a feasible process control variable. The principles of a two modeling approach stage are presented and a possible regulation loop is finally proposed. PROCESS DESCRIPTION The process is based on calcium phosphate precipitation obtained by mixing a phosphate solution (orthophosphoric acid and demineralized water) with calcium ions and a base. More precisely, it involves a fluidized bed of sand continuously fed with two aqueous solutions, 26 Proceedings of IMPHOS-IWPPC Jena 2002 Phosphorus Research Bulletin Vol. 15 (2004) CaCl2 and KOH (Figure 1). Calcium phosphate precipitates upon the surface of sand grains. Simultaneously, small particles, i.e., "fines", leave the bed with the remaining phosphate not recovered in the reactor. A layer of fines which has agglomerated is observed at the upper zone of the fluidized bed. The main experimental observations are the following ones: at high velocity, fines leave the fluidized bed while at lower values, a layer of fines is systematically observed at the upper level of the bed. In both cases, two regimes are identified. First, a steady state regime is observed for the liquid phase leaving the bed. Measurements of the P concentration at the outlet zone show that constant values are rapidly obtained (20 min for all the runs performed). Second, a steady state regime (as measured by the fluidized bed height) is also observed in the bed after a maximum of 3 hours for all the experimental runs. More precisely, this regime is assumed to be reached when the P concentration of both the recovered sand grains and of the fine layer exhibits a constant value. These experimental observations lead to define two performance indicators of the reactor for the process. The phosphate removal efficiency (ƒÅ) of the reactor and the conversion of phosphate from the liquid to the solid phase (X) are defined respectively as: