Struvite Precipitation modelling using Extended Quadrature Method of Moments and Drift-Flux Model

Abstract

The primary fertilisers used in the agricultural industry are phosphorus based. The production of phosphorus is mainly based on the mining operations, however it is a current consensus that in 50 − 100 years time the quality and the availability of phosphorus ores will decrease substantially and the deposits will be either completely depleted or the quality of the obtainable phosphorus will be too toxic to be used in the agriculture. Taking into account the current population growth trends the food production will have to be increased even more and therefore the global food shortage is an event of high potential. To address that alternative sources of phosphorus based fertilisers must be found and numerous technological research have been taking place to limit the impact that phosphorus shortage might cause in the future. The aim of this study is to present a potential technology that could be used for phosphorus recovery. It is known that there is a significant amount of phosphorus in the wastewater therefore most of the technological developments have been focussed towards the recovery of phosphorus from the wastewater streams. The Hydrodynamic Vortex Separator (HDVS) is a product developed by Hydro International® that is primarily used for solid-liquid separation. However this study proposes the potential use of the HDVS as a reaction vessel for phosphorus recovery through magnesium-ammonium-phosphate precipitation. The study presents a development and application of a mathematical model of the two-phase system that is coupled with the formation of a dispersed phase through a precipitation process. Hydrodynamics of the system is modelled using an Eulerian approach, specifically the drift flux mixture model (DFM)[81]. The precipitation process and the change of the dispersed phase properties, namely the mean diameter of the particulates, is modelled using the Population Balance Equation (PBE), which is solved using the Extended Quadrature Method of Moments (EQMOM) method. The affects of the changes observed in the dispersed phase properties were coupled to the hydrodynamics using two new settling models that depend on the size of the particulate phase. The model has been applied to a horizontal settling tank and the HDVS geometries in order to understand the potential of the precipitation process in these type of wastewater treatment tools. In both of the cases the model predicted reasonable values of the dispersed phase mean diameter.