Modeling and optimization of siphon downflow hanging sponge reactor using ASM1 for enhanced nutrient removal in municipal wastewater treatment.

Siphon downflow hanging sponge (siphon-DHS) reactors offer an energy-efficient alternative for biological nutrient removal. However, their performance is highly sensitive to internal configuration. Here, siphon-DHS was modeled using Activated Sludge Model No. 1 (ASM1) to systematically optimize zone volume ratios of the reactor for enhanced municipal wastewater treatment. Four reactor configurations (A, B, C, and D) with varying aerobic, changing, and anaerobic zone volume ratios were operated and evaluated through water quality and microbial community analysis. Reactor D, which had the highest performance index (59%), was selected for model calibration. Zone-specific first-order rate constants derived from experimental data revealed the highest rates for chemical oxygen demand (COD) removal (k1 = 0.624 h-1) and nitrification (k1 = 0.571 h-1) in aerobic zones and exceptional denitrification (k3 = 1.288 h-1) in anaerobic zones. Systematic optimization using equal-weighted and denitrification-prioritized approaches identified an optimal configuration of 0.55:0.10:0.35 (aerobic:changing:anaerobic) for denitrification prioritization, achieving 84% COD removal, 81% nitrification, and 91% denitrification-representing a 68% percentage-point improvement in denitrification over the baseline. Dynamic Simulink simulations elucidated the process distribution and substrate utilization across zones. Meanwhile, sensitivity analysis revealed that minimizing the changing zone to 10% while maximizing sharp aerobic-anaerobic interfaces enhanced overall performance. The calibrated model provides a scalable, energy-efficient framework for siphon-DHS reactor design, offering significant potential for sustainable municipal wastewater treatment.