Rapid cultivation of sulfur-based autotrophic denitrification granular sludge and enhanced nitrogen removal from ionic rare earth mining wastewater

This study addressed the issue of elevated ammonia nitrogen pollution in wastewater from ionic rare earth mining operations. Given the high costs and limited efficiency of conventional biological denitrification methods, the sulphur-based autotrophic denitrification (SAD) process was explored as a cost-effective alternative to improve nitrogen removal performance. A sequencing batch reactor was operated over 70 days to cultivate and maintain granular sludge, successfully enriching sulphur-oxidising bacteria (SOB). During this cultivation phase, the average sludge particle size increased from 228.69 μm to 709.95 μm, the granulation rate reached 84.56%, and the relative abundance of the SOB genus Thiobacillus rose to 13.57%. An orthogonal experimental design was employed to optimise the operational parameters of the SAD granular sludge. Single-factor experiments were first conducted to assess the effects of sodium bicarbonate concentration (0-2000 mg/L), sludge concentration (3500-6000 mg/L), reaction duration (2-12 h), and sodium sulfide concentration (0-300 mg/L) on the removal efficiencies of total inorganic nitrogen (TIN). The results indicated that sodium bicarbonate concentration was the most influential factor. Subsequently, an L₉(3⁴) orthogonal experiment was designed to determine the optimal operational conditions: 1600 mg/L sodium bicarbonate, 5000 mg/L sludge concentration, 8 h of reaction time, and 37.5 mg/L sodium sulphide. Under these optimised conditions, the TIN removal efficiency reached 67.64%. Economic analysis demonstrated that the unit denitrification cost of the SAD process was 31.83% lower than that of the heterotrophic denitrification process, highlighting its potential as a low-carbon and efficient solution for treating rare earth mining wastewater.