Changes of phosphorus species during (hydro) thermal treatments of iron-rich sludge and their solubilization mediated by a phosphate solubilizing microorganism.

This study systematically evaluated phosphorus (P) solubilization from pyrochar and hydrochar derived from both raw sludge and iron-rich sludge. The data indicated, that an increase in thermal treatment temperature and the presence of iron promoted the accumulation of P in both pyrochar (derived at 300, 500, and 800 °C) and hydrochar (derived at 100, 200, and 280 °C). After incubating pyrochar and hydrochar with a phosphate solubilizing microorganism (PSM) (Pseudomonas aeruginosa) for 30 days, PSM significantly promoted the solubilization of P in pyrochar and hydrochar synthesized at low temperatures rather than those at high temperatures, with a 59 % increase for the pyrolysis of raw sludge at 300 °C than that pyrolyzed at 800 °C and a 62 % increase for the hydrothermal treatment of raw sludge at 100 °C than that treated at 280 °C. And the phenomena were more obvious on the char samples derived from iron-rich sludge. The mass balance of different P species in the solid and liquid phases indicated that after incubating with PSM for 30 days, NaOH-P was the main P solubilized from the solid phase of pyrochar and HCl-P was the main P solubilized from the solid phase of hydrochar. Considering P availability to plants, the preliminary economic analysis indicated that the hydrothermal treatment of iron-rich sludge at 100 °C showed the highest economic benefits for P recovery, with the net cost of 28.79 USD/ton wet sludge. This study was useful in giving novel insights into the reuse of char samples as P fertilizer, and also suggested the importance of Pseudomonas aeruginosa and other bacteria in sludge application, particularly in terms of P solubilization.