Variation in Phosphorus Speciation of Sewage Sludge throughout Three Wastewater Treatment Plants: Determined by Sequential Extraction Combined with Microscopy, NMR Spectroscopy, and Powder X-ray Diffraction

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 27 Al and 31 P 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. 31 P 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.