Removal of orthophosphate and dissolved organic phosphorus from synthetic wastewater in a combined struvite precipitation-adsorption system

Abstract

In secondary and advanced tertiary phosphorus removal processes from wastewater, orthophosphate phosphorus (P-OP) is nearly completely eliminated, and dissolved organic phosphorus (DOP) becomes the dominant fraction. Therefore, DOP contributes substantially to the total phosphorus (TP) in effluents, and hence, any improvement in TP removal requires improvements in DOP elimination. Accordingly, the challenge is to remove DOP to achieve increasingly lower phosphorus concentrations below the concentrations reliably attainable with currently available technologies. Nevertheless, no specific technologies have been implemented to target the removal of this fraction. This work proposes a struvite precipitation process to remove P-OP combined with the removal of DOP by adsorption on struvite. The adsorption capability of phytic acid (PA) on struvite yielded a final DOP concentration <0.55 mg·L−1 when starting from an initial DOP concentration of 70 mg·L−1. Struvite that was previously prepared also exhibited P-OP adsorptive capability. This effect contributes to increasing removal efficiency from 59.5 % with struvite precipitation as the only removal process, to 99.2 % in a combined precipitation-adsorption system with added adsorbent for: P-OP: 70 mg·L−1, Mg/P-OP = 1.5 and 1.5 mmol·L−1 NaOH. The SEM-EDS results of the selected regions of recovered precipitates revealed that the particles were composed of C, N, Mg, P and O. Surface atomic composition did not coincide with fresh adsorbent (pure struvite) and differed from one zone to another. Moreover, the signal of C in the surface of some samples, the absence of N on some surface regions and the P-PA quantified by extraction with H2SO4, are evidences that support the hypothesis of the phytate and orthophosphate adsorption.