Modeling phosphate transport and removal in a compact bed filled with a mineral-based sorbent for domestic wastewater treatment

Highlights

• Removal of PO₄ in filters for on-site wastewater treatment was modeled.

• The hydro-geochemical transport code PHREEQC was used.

• The models could successfully simulate PO₄ removal measured in laboratory.

• CaO, wollastonite and calcite were the phases dissolved from the material.

• The only precipitated Ca–P compound suggested by the models was ATCP (Ca₃(PO₄)₂).

Abstract

Phosphorus filter units containing mineral-based sorbents with a high phosphate (PO₄) binding capacity have been shown to be appropriate for removing PO₄ in the treatment of domestic wastewater in on-site facilities. However, a better understanding of their PO₄ removal mechanisms, and reactions that could lead to the formation of PO₄ compounds, is required to evaluate the potential utility of candidate sorbents. Models based on data obtained from laboratory-scale experiments with columns of selected materials can be valuable for acquiring such understanding. Thus, in this study the transport and removal of PO₄ in experiments with a laboratory-scale column filled with a commercial silicate-based sorbent were modeled, using the hydro-geochemical transport code PHREEQC. The resulting models, that incorporated the dissolution of calcite, kinetic constrains for the dissolution of calcium oxide (CaO) and wollastonite (CaSiO₃), and the precipitation of amorphous tricalcium phosphate, Ca₃(PO₄)₂, successfully simulated the removal of PO₄ observed in the experiments.

Introduction

Filters containing mineral-based alkaline materials have been shown to be viable options for removing dissolved phosphorus (PO₄) from pre-treated domestic wastewater in on-site facilities. Numerous materials could be used in such filters (Johansson Westholm, 2006, Vohla et al., 2011), but full-scale field-based investigations are costly and time-consuming. Thus, the potential utility, performance and longevity of various materials for this purpose have been mostly investigated in laboratory-scale batch and column experiments (Johansson Westholm, 2006, Vohla et al., 2011). A problem associated with this approach is that conditions in full-scale applications may differ considerably from laboratory conditions. Thus, robust models of the transport of solutes and removal of PO₄ in laboratory-scale filter columns are required for screening candidate materials, investigating probable scale-up effects and optimizing operational parameters. However, in order to model the retention of PO₄ in filters successfully, a better understanding of PO₄ removal mechanisms and reactions that could lead to the formation of P-compounds in them is required. In previous research, some reactive phases (Gustafsson et al., 2008) and some P compounds generated in selected filter materials have been identified (Eveborn et al., 2009, Gustafsson et al., 2008). Generally, for instance, calcium-based filter materials release calcium ions to the solution phase through kinetically constrained dissolution processes and calcium-P compounds are subsequently precipitated at high pH (Gustafsson et al., 2008).

Despite such recent advances, the PO₄ retention and P product formation mechanisms are still not fully understood. Thus, the overall aim of the study presented here was to develop a model describing the transport and removal of PO₄ in a reactive filter using the hydro-geochemical transport code PHREEQC (Parkhurst and Appelo, 2011). The modeling focused on the removal of the PO₄ in a compact filter filled with a calcium-silicate sorbent. However, the developed model also provides more general indications of possible reactions involved in the removal of PO₄ from filtered solutions.