Iron and phosphorus speciation in Fe-conditioned membrane bioreactor activated sludge

Area of Science:

• Environmental Engineering
• Water Treatment Technologies
• Materials Science

Background:

• Membrane bioreactors (MBRs) utilize iron dosing to meet effluent phosphorus limits.
• Understanding the iron and phosphorus solids formed in MBRs is crucial for wastewater phosphorus recovery.
• MBR complexity and iron salt reactions pose challenges to characterizing these solids.

Purpose of the Study:

• To investigate the nature of iron and phosphorus solids formed in MBRs after iron salt addition.
• To determine the speciation of iron and phosphorus under different dosing conditions (ferrous vs. ferric, anoxic vs. aerobic).
• To elucidate the mechanisms of phosphorus removal and potential for phosphorus recovery.

Main Methods:

• Bench-scale membrane bioreactors (MBRs) were operated for four months with either ferrous or ferric iron dosing.
• X-ray absorption spectroscopy (XAS) at the Fe K-edge and P K-edge was used to analyze iron and phosphorus species.
• Extended X-ray absorption fine structure (EXAFS) spectroscopy was employed for detailed mineralogical analysis.

Main Results:

• Iron in conditioned sludges was consistently in the +III oxidation state, irrespective of the initial iron salt form (Fe(II) or Fe(III)) or dosing location.
• An Fe(III)-phosphate species was the dominant iron species, with lepidocrocite and ferrihydrite as secondary phases.
• Approximately 50% of phosphorus formed a distinct Fe-PO4 mineral, while the other 50% was adsorbed onto iron oxyhydroxide phases.

Conclusions:

• Both co-precipitation and adsorption mechanisms contribute to phosphorus removal by iron in MBRs.
• The findings provide insights into the mineralogical forms of phosphorus in iron-conditioned MBR sludges.
• This understanding is vital for optimizing phosphorus recovery strategies from wastewater treatment.