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The amount of excess sludge produced in municipal wastewater treatment plants in Japan is increasing every year as the urban population increases. Phosphorus in excess sludge could be a potential phosphorus resource since at present, phosphate rock is being exhausted all over the world. Every year, Japan imports large quantities of phosphorus from abroad but much is discharged as excess sludge. Therefore, the solubilization process, one method of recovering phosphorus from sludge, could be a promising solution. In this study, a subcritical water process, a new technology that solubilizes sludge under subcritical conditions, was applied before the phosphorus in sludge was recovered with the magnesium ammonium phosphate (MAP) process. As a result, the solubilization rate of excess sludge achieved approximately 80% and about 94-97% of the phosphorus could be recovered.

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The complex formation equilibria between several bivalent metal ions and 3,3′-bis[N,N′-bis(carboxymethyl)aminomethyl]-o-cresolsulfonphthalein (XO) have been investigated potentiometrically and spectrophotometrically. The metal ions studied here were Co(II), Ni(II), Cu(II), Zn(II), Pb(II), and Cd(II). The formation constants and visible-absorption-spectrum parameters of these metal complexes have been determined. It has been found that all these metal ions form 1:1 and 2:1 (metal: XO) complexes with XO. Moreover, the complex of each composition changes into one of several protonated ones depending on the pH of the solution. The probale structures of the complexes in solution are discussed.