Sewage sludge hydrothermal treatment by microwave irradiation combined with alkali addition

This article focuses on the effects of microwave treatment on sewage sludge with alkali addition. Both the solubilization of organic matters and the settleability of sludge were investigated. It was found that the combined treatment provided a rapid and efficient process to release organics from sludge flocs. Anaerobic digested sludge with low organic content (37%) obtained better results as compared to fresh sludge with high organic content (56%). For digested sludge, 50–70% of VSS was dissolved into a solution at 120–170 °C with a dose 0.2 g NaOH/g-DS within 5 min, and 80% of the total COD was released to supernatants. The liquor soluble COD concentration increased up to 20 g/L. Nearly 60% of nitrogen dissolved at 170 °C. For fresh sludge, solo microwave heating reduced 40% of VSS at 170 °C within 1 min. Adding 0.05 g NaOH/g-DS increased the VSS dissolution ratio to 50% and got 35% of SS reduction. After 1 min microwave treatment with alkali, the treated digested sludge provided a significant settleability improvement. Nevertheless, the high organic content of fresh sludge lessened the improvement of settleability behavior.