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.
