Controlled low-strength material (CLSM), a widely used replacement for conventional compacted filling material, is self-leveling and self-compacting, currently used in many structural applications. The properties of a CLSM mixture made from cement, fly ash, and waste water treatment sludge (WTS) were examined in this study. The CLSM mixtures were prepared using several water-to-binder (w/b) ratios (0.5, 0.65, and 0.8) and WTS weight ratios (0, 20, 40, and 80%). A solution of 5M NaOH was employed as a mixture accelerator. The influences of WTS addition on the characteristics of both fresh and hardened CLSM were examined. Toxicity characteristic leaching procedure (TCLP) and the change in minerals and microstructures of the CLSM samples were also investigated. The experimental results demonstrated that incorporating WTS into CLSM results in significant changes in fresh unit weight and mixture workability. The CLSM properties were found to be strongly affected by both the w/b ratio and accelerator dosage. Moreover, the TCLP test confirmed that WTS can be used as a safe byproduct because the leached levels of heavy metals from raw WTS and resulting CLSM were significantly less than the permitted thresholds. Finally, the CLSM designed using a w/b ratio of 0.65, accelerator of 5%, and 40% WTS achieved the highest performance that meets current relevant Taipei City design regulations for excavatable backfilling.