The influence of phosphorus fractions in bottom sediments on phosphate removal in semi-natural systems as the 3rd stage of biological wastewater treatment

The research was carried out in two semi-natural systems (the polishing ponds in Swarzewo and the free water surface constructed wetland in Zarnowiec) in Poland. They were built as the 3rd stage of a conventional mechanical–biological wastewater treatment plant. These systems were built to improve the quality of the effluent of treated wastewater.In the polishing ponds and FWS wetland system, suspended solids, organic matter as well as nitrogen are removed, but there is problem with phosphorus removal. The aim of this paper was to determine phosphorus removal from treated wastewater as well as the characteristics of the bottom sediments with respect to phosphorus fractions and their transformation due to seasonal changes.The total efficiency of the phosphorus removal from the wastewater in Swarzewo was only 23.7%, while in Zarnowiec the total concentration of phosphorus in the effluent increased.It was found that in Swarzewo phosphorus bound with organic matter (NaOH–P fraction) had the largest impact on the deposition process and the release of phosphorus from the sediments to the wastewater, while in Zarnowiec–phosphorus loosely absorbed on the particle surface (NH4Cl–P fraction) and bound with iron (Fe–P fraction) was the most significant.The sediments from the semi-natural systems are a rich source of phosphorus, which is directly assimilable by plants. In the sediments from Swarzewo, the mobile forms (the sum of NH4Cl–P, Fe–P, NaOH–P) constituted 79% of the total phosphorus. The sediments from Zarnowiec are characterized by higher concentrations at these forms (about 96.0%). The sediments are capable of releasing phosphorus, so they should be scooped. The relatively low concentration of heavy metals and high concentrations of nitrogen and mobile fractions of phosphorus indicate, however, that after hygienization, the analyzed sediments can be potentially used as an initial product for the production of fertilizer.