The variation in phosphorus (P) speciation
of sewage sludge throughout
three wastewater treatment plants (WWTPs) was obtained by combining
sequential P extraction with optical and scanning electron microscopy
(SEM), chemical analyses, powder X-ray diffraction (PXRD), and 27Al and 31P nuclear magnetic resonance (NMR) spectroscopy.
The WWTPs combine chemical P removal (CPR) and enhanced biological
P removal (EBPR) and were compared to understand the effect of iron
(Fe) dosing with and without codosing of aluminum (Al) and thermal
hydrolysis on the P speciation. 31P NMR showed comparable
inorganic orthophosphate (ortho-P, 53–60% of total P) and organophosphate
(organic-P, 37–45%) in primary sludge, whereas polyphosphate
(poly-P, 23–44%) from poly-P accumulating organisms (PAOs)
was mainly observed in the secondary sludge. Inorganic ortho-P (90–98%)
dominated after anaerobic digestion, which degraded poly-P and most
organic-P. The inorganic ortho-P was mainly Fe bound P (Fe-P), especially
after anaerobic digestion (71%). Codosing of Fe and Al led to two
comparable fractions: Fe-P (38%) and P sorbed on amorphous Al (hydr)oxides
(38%). Vivianite was identified in all samples by microscopy and chemical
extraction but was PXRD amorphous in 12 out of 17 samples. Thus, vivianite
may be more common in sewage sludge than previously known.
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