Behavior of inorganic elements during sludge ozonation and their effects on sludge solubilization

The behavior of inorganic elements (including phosphorus, nitrogen, and metals) during sludge ozonation was investigated using batch tests and the effects of metals on sludge solubilization were elucidated. A decrease of ∼50% in the ratio of sludge solubilization was found to relate to a high iron content 80–120 mgFe/gSS than that of 4.7–7.4 mgFe/gSS. During sludge ozonation, the pH decreased from 7 to 5, which resulted in the dissolution of chemically precipitated metals and phosphorus. Based on experimental results and thermodynamic calculation, phosphate precipitated by iron and aluminum was more difficult to release while that by calcium released with decrease in pH. The release of barium, manganese, and chrome did not exceed 10% and was much lower than COD solubilization; however, that of nickel, copper, and zinc was similar to COD solubilization. The ratio of nitrogen solubilization was 1.2 times higher than that of COD solubilization (R2 = 0.85). Of the total nitrogen solubilized, 80% was organic nitrogen. Because of their high accumulation potential and negative effect on sludge solubilization, high levels of iron and aluminum in both sewage and sludge should be considered carefully for the application of the advanced sewage treatment process with sludge ozonation and phosphorus crystallization.

Research highlights
► A decrease in the ratio of sludge solubilization was related to the high iron content.
► P precipitated by Fe and Al was more difficult to release with pH decrease.
► The release of Ba, Mn, and Cr did not exceed 10% and lower than COD solubilization.
► The release of Ni, Cu, and Zn was similar to COD solubilization.
► Of the total nitrogen solubilized, 80% was organic nitrogen.