Role of humic substances and alkaline in phosphorus release from sludge pre-treated by (alkali-) hydrothermal

The scarcity of phosphorus (P) resources makes the recovery of P urgent. Sludge is a secondary resource rich in P, and the release of P from it is a key step for recovery. Hydrothermal (HT) is currently a popular method for sludge pretreatment, and its combination with alkaline (alkali-hydrothermal, AHT) could reduce the energy consumption in treatment. This study tried to compare their P release profiles in treating activated sludge in which organic P (OP) and non-apatite inorganic P (NAIP) were co-existence. Apart from the OP release in cell lysis, P release from NAIP brought by the joint effect of OH^- and humic substances (HS) formed in treatment was focused. The results showed that, compared to HT treatment, more P was released when OH^- participated (AHT), and the peak P release was observed at 160 °C. Variation of P distribution in the treated sludge revealed that more P was released from NAIP in AHT than in HT. HS formed in treatments was extracted and characterized. The amount and the structure of the HS varied significantly with the treatment conditions, and there was a linear correlation ship between PO₄^3--P release and the humic acid (HA) amount in HS. Mechanism study indicated there was a synergism between HS and OH^- in promoting PO₄^3--P release from NAIP. This study linked HS produced by sludge with P release, which provided a new perspective for subsequent P recovery from sludge.