Correlation between phosphorus removal technologies and phosphorus speciation in sewage sludge: focus on iron-based P removal technologies.

Phosphorus recovery from sewage sludge as secondary raw materials or as a direct P-rich fertiliser is one of the top frontrunner solutions to tackle Phosphorus (P) scarcity and depletion. However, the efficiency of this P recovery process greatly depends on its phosphorus dissolution potential, which in return relies on the phosphorus speciation in the sewage sludge. This article investigates the potential correlation between P speciation in sewage sludge and the iron-based P removal technologies used in sewage treatment plants (STP) through an innovative sequential extraction method based on the SEDEX method that distinguishes quantitatively between ferrous bound phosphate and ferric bound phosphate. XRD and SEM-EDX were also used to characterise P and Fe species in the studied sludge qualitatively. Principal component analysis showed that the sludge characterised by P bound to ferric iron (as the dominant P fraction) are mostly correlated with sludge produced from the CPR process (chemical phosphorus removal) and primary sludge. Moreover, sludge with a non-negligible amount of P bound to ferrous iron were correlated with sludge from the mixed EBPR-CPR process (Enhanced Biological P Removal assisted with CPR). However, Vivianite was only found in CPR sludge with Fe/P molar ratio higher than 0.6.

Mechanisms governing the dissolution of phosphorus and iron in sewage sludge by the bioacidification process and its correlation with iron phosphate speciation.

In the last two decades, phosphorus (P) recovery from sewage sludge liquors gained much interest for its high-quality product potential. However, the consistently reported constraints are the low phosphorus availability and the technical-economical difficulties to increase it through chemical acidification. This article discusses the mechanisms of phosphorus dissolution by the biological acidification process (Biological acidification or acidic fermentation) as an alternative to chemical acidification. In addition, we investigate the potential correlation between the phosphorus dissolution and iron phosphate speciation of several types of sludge from different sewage treatment plants and P removal technologies. The results show that the percentage of P dissolution by bioacidification is always higher than the P dissolution by chemical acidification at equal pH for all types of sludge except for the settled primary sludge. The highest P dissolution was recorded for the sludge from the Enhanced Biological P Removal process assisted with Chemical P Removal process (EBPR-CPR) with around 65% of P dissolution. Three mechanisms were identified as contributing to the increased P dissolution by bioacidification: P release by the Polyphosphate Accumulating Organisms (PAO), P dissolution by pH decrease, and P dissolution by a biological activity at acidic pH (3.7-4) that includes iron reduction and aluminum dissolution. The principal component analysis and Pearson's correlation indicate that P dissolution by bioacidification is negatively correlated with the P-bound to ferric iron, hence positively correlated with the P-bound to ferrous iron, which characterizes the sludge from the EBPR-CPR process. This study suggests that the choice of the P removal technology significantly influences the P recovery from sewage sludge liquors.