<p>Due to the problems of high difficulty and cost of sludge treatment and disposal, the residual sludge with high water content was treated by lysis to realize the reduction. The sludge lysis was conducted by ultravio-sonication (UVS). The effects of wavelength and power of ultraviolet (UV), and sludge concentration and alkali treatment were investigated. The results found that the power of the UV impacted the sludge lysis degree (DD_COD) more strongly than the wavelength, which could increase the amount of TP, PO₄³⁻-P, TN, NH₄⁺-N, protein and polysaccharides in the supernatant but reduce the percentage of carbon, nitrogen and phosphorus. During the lysis by UV-ultrasound, DD_COD increased slightly as the sludge concentration increased, and alkali treatment was more conducive to the dissolution of substances. Under the conditions of ultrasonic power 400 W and frequency 40 kHz, UV power 16 W and wavelength 185 nm, sludge concentration 12,000 mg·L⁻¹, pH = 11 (alkali treatment), the contents of TP, PO₄³⁻-P, TN, NH₄⁺-N were 297.1 mg·L⁻¹, 183.9 mg·L⁻¹, 522.3 mg·L⁻¹, and 58.9 mg·L⁻¹, respectively, with DD_COD reaching up to 63.02%. The improvement of sludge lysis degree was conducive to the release of substances and the reduction the moisture content, which facilitated the subsequent sludge disposal and resource utilization.</p> Graphical Abstract <p></p>