Abstract
Phosphate recovery from sewage sludge is essential in a circular economy. Currently, the main focus in centralized municipal wastewater treatment plants (MWTPs) lies on struvite recovery routes, land application of sludge or on technologies that rely on sludge incineration. These routes have several disadvantages. Our study shows that the mineral vivianite, Fe2(PO4)3 × 8H2O, is present in digested sludge and can be the major form of phosphate in the sludge. Thus, we suggest vivianite can be the nucleus for alternative phosphate recovery options. Excess and digested sewage sludge was sampled from full-scale MWTPs and analysed using x-ray diffraction (XRD), conventional scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), environmental SEM-EDX (eSEM-EDX) and Mössbauer spectroscopy. Vivianite was observed in all plants where iron was used for phosphate removal. In excess sludge before the anaerobic digestion, ferrous iron dominated the iron pool (≥50%) as shown by Mössbauer spectroscopy. XRD and Mössbauer spectroscopy showed no clear correlation between vivianite bound phosphate versus the iron content in excess sludge. In digested sludge, ferrous iron was the dominant iron form (>85%). Phosphate bound in vivianite increased with the iron content of the digested sludge but levelled off at high iron levels. 70–90% of all phosphate was bound in vivianite in the sludge with the highest iron content (molar Fe:P = 2.5). The quantification of vivianite was difficult and bears some uncertainty probably because of the presence of impure vivianite as indicated by SEM-EDX. eSEM-EDX indicates that the vivianite occurs as relatively small (20–100 μm) but free particles. We envisage very efficient phosphate recovery technologies that separate these particles based on their magnetic properties from the complex sludge matrix.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 312-321 |
| Number of pages | 10 |
| Journal | Water Research |
| Volume | 144 |
| DOIs | |
| State | Published - Jul 20 2018 |
Bibliographical note
KAUST Repository Item: Exported on 2020-10-01Acknowledgements: This work was performed in the TTIW-cooperation framework of Wetsus, European Centre Of Excellence For Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Frysla^n, the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. We thank the participants of the research theme “Phosphate Recovery” for their financial support and helpful discussions. Furthermore, we acknowledge the valuable support, discussions, friendly welcome and warm coffee from all operators and staff of the municipal wastewater treatment plants we visited. Many thanks for providing various resources to Dr. Bernd Heinzmann from the Berliner Wasserbetriebe, Dr. Christian Kabbe from the KompetenzZentrum Wasser Berlin and Thomas Kurz from the Stadtentwässerungsbetriebe Köln. Additionally, we thank the Kemira employees Outi Grönfors, Wout Barendregt and Jens Weyermanns for valuable support during sludge sampling in Cologne and Espoo and for providing valuable information about the treatment parameters of these plants. We thank the two anonymous reviewers whose comments/suggestions helped to improve and clarify this manuscript.
