Analysis of phosphorus flux in reed beds at chemically dosed wastewater treatment works

Abstract

Phosphorus (P) as a macronutrient contributes to eutrophication. In the UK, iron (Fe) and aluminium (Al) salt dosing is a well-established wastewater treatment strategy for its removal. Small works with a population equivalent (p.e.) under 2,000 often employ horizontal subsurface flow constructed wetlands (HSSF CWs) as a means for subsequent tertiary (3°) treatment. Although these significantly improve the final effluent’s quality, P release has, on occasions, been observed. This study attempts to contribute to a better understanding of P flux mechanisms in reed beds and to outline a mitigation strategy countering P release. The literature review identified that, in given circumstances, the P concentration gradient, redox conditions, pH and Fe cycling are key potential factors governing P flux. The field survey revealed that secondary (2°) P effluent concentration negatively correlated with P release from the reed bed. In laboratory scale sludge reactor series, P concentration in wastewater was observed to be buffered by molecular diffusion driven by a concentration gradient in the sludge-wastewater interface. The instantaneous equilibrium point appeared to lie in 0.1 to 0.5 mg/L interval in the first 10 minutes, shifting to 1 mg/L in the next 8 hours and higher in the later stages. In biologically active systems, the shift of the equilibrium point seemed to be dominated by changes in redox potential linked to simultaneous microbial utilisation of oxygen (O2) and nitrate (NO3 - ), eventually leading to a reduction of Fe (III) and sulphate (SO4 2- ), with subsequent P release. The start of Fe (III) reduction coincided with reductive depletion of nitrate-nitrogen (NO3-N) below 1 mg/L. In systems with limited biological activity, P release was linked to disassociation from Fe-P compounds under decreasing pH. In an experiment assessing hydrodynamics, an increase was recorded in Fe and P flux fluctuation due to convection. Based on the findings, maintaining hydraulic residence time (HRT) under 24 hours and reed bed influent in concentrations above 0.5 mg/L total phosphorus (TP) and 15 mg/L NO3-N is proposed as a means to prevent or delay P release.