Alkaline hydrothermal cracking effect and substance transformation characteristics of caprolactam-containing sludge

Caprolactam is a crucial chemical intermediate, but its wastewater treatment process generates a significant amount of caprolactam-containing sludge. This study represented the first exploration of the effects of alkaline hydrothermal technology on the cracking and transformation of substances in this sludge. The cracking effect of caprolactam-containing sludge during hydrothermal treatment increased with rising reaction temperature and longer reaction time. With NaOH dosage in hydrothermal treatment increasing from 0 to 2 wt%, the volatile suspended solids (VSS) removal rate of the sludge increased from 44.5% to 74.8%, soluble chemical oxygen demand (SCOD) in the cracking liquid increased from 12772 mg/L to 22976 mg/L, and ammonia nitrogen concentration increased from 398.0 mg/L to 851.2 mg/L. However, the addition of Ca(OH)₂ did not significantly affect the changes of sludge suspended solids, VSS and SCOD concentration, but increased the leaching of ammonia nitrogen (up to 745.0 mg/L). This was due to the secondary flocculation of Ca²⁺, which rebound with dissolved non-proteinaceous organic matter. Increasing temperature, reaction time, and alkaline dosage all enhanced the fluorescence intensity of dissolved organic matter (DOM). Moreover, higher reaction temperature and alkaline dosage reduced the proportion of proteinaceous products in DOM while increasing the proportions of fulvic acids, soluble microbial metabolites, and humic acid-like substances. The study provided crucial theoretical support for engineering application of this technology.