Biogeochemical Sequestration of Phosphorus in a Two-Layer Lignocellulose-Based Soil Treatment System

On-site wastewater treatment systems can contribute to the oversupply of phosphorus (P) to aquatic systems which represents a key factor for the development of eutrophic conditions and associated environmental issues, such as harmful algae blooms. This study provides novel insight into the biogeochemical processes that control P sequestration in a nitrogen-removing biofilter, a saturated two-layer lignocellulose-based soil treatment system. The concentrations of dissolved phosphorus pools at different depths within the system were investigated. Low effluent total dissolved phosphorus (TDP; 0.02-0.06 mg P L-1) and dissolved inorganic phosphorus concentrations (DIP; <0.008 mg P L-1) were observed suggesting efficient P attenuation. Analyses of different sequentially extracted solid-phase P pools revealed that P adsorption only played a minor role (<1% of total DIP removal). In the nitrification layer, P was likely sequestered into Fe and Al (hydr)oxides through deep deposition and recrystallization reactions, and by dissimilatory iron reduction followed by authigenic iron-phosphorus mineral precipitation. Organic matter P uptake also occurred in this zone. In the underlying denitrification layer, P removal by calcium phosphate precipitation was likely the dominated process.