MODELING CHEMICAL PHOSPHORUS PRECIPITATION USING EQUILIBRIUM CHEMISTRY

Many wastewater treatment plants in the world are required to achieve extremely low total phosphorus effluent concentrations. Both effluent solids (containing phosphorus) and soluble phosphorus (mostly orthophosphate, OP) must be maintained at very low levels. Chemical precipitation is a widely used technology for controlling effluent OP discharge, either on its own or supplementing biological methods. The various chemical and physico-chemical mechanisms that result in extremely low residual OP levels are complex and pH dependent. Iron phosphate solubility, the dissociation state of many other chemical species, as well as adsorption of phosphate ions on hydroxy complexes and flocculation also play a role (D.W. de Haas et al., 2000, E. Pierri et al., 2000). In practice, engineering calculations frequently use an empirical precipitation model (WEF 1998). This model requires pH as input and predicts a lowest achievable OP residual of 35 μgP/L at a narrow optimum pH of 6.9 – 7.0, when an excess of ferric is added.Analysis of effluent data from the DCWASA plant (Washington, DC.), the largest chemical P removal plant in the world, showed that residuals as low as 10 μgP/L OP were regularly achieved, over a wider pH range. The WEF model was recalibrated and combined with a biological process model that incorporated weak acid/base chemistry and pH. This model was able to accurately predict pH, OP residuals and chemical sludge composition.The practical benefits associated with a mechanistic, equilibrium chemistry based model include: a) improved control of ferric dosage depending on pH conditions (chemical savings), b) better estimates of chemical sludge produced, and c) by modeling the competition between iron phosphate and iron in other forms, better understanding of the iron overdose required.