Granulated apatite filters for phosphorous retention in treatment wetlands: Experience from full-scale applications

Abstract

Since natural apatite has shown a great capacity for phosphorus retention from wastewater, a granulated apatite product was developed to control the particle size distribution of the filter material and avoid premature clogging. Since 2012, twenty full-scale granulated apatite filters have been installed in France, with the primary aim to remove phosphorus from domestic wastewater. Measurement campaigns concerning four of these full-scale filters were carried out over two years (2017-2018) to assess their performance, the maximum retention capacity and phosphorus removal kinetics after several years of operation. The filters were selected according to their age and their hydraulic characteristics (horizontal flow, vertical down flow, vertical up flow). The maximum phosphorus retention capacity was found to be approximately 7.4 g PO4-P/kg of granulated apatite. The study revealed low kinetic removal rate coefficients. It is believed that phosphorus retention occurred by adsorption and precipitation within the porosity of granules since precipitation on the granules surface was not observed by the Scanning Electron Microscopy images. Therefore, under the specific environmental conditions tested, the substrate behaves exclusively as an adsorptive-like media. The increase in pH due to alkalinity release requires the implementation of pH correction for a significant period after the commissioning of the filter (0.5–1.2 years).