Hydraulic Flow Patterns in an On-Site Wastewater Treatment Unit Under Various Operating Conditions

The role of on-site wastewater treatment (OSWT) is increasingly important for water reuse and local sustainability, but treatment efficiency is highly dependent on hydraulic behavior and mixing. This study used validated CFD simulations and tracer experiments to analyze flow patterns and mixing performance in a six-zone OSWT unit under different operational scenarios, including inflow, aeration, recirculation, combined mechanisms, and closed-loop operation without inflow. The results show that influent flow is essential for maintaining convective transport and system-wide momentum, while aeration and recirculation enhance local mixing, but cannot fully overcome geometric dead zones. The combined use of inflow, aeration, and recirculation achieved the highest mixing efficiency and minimized the dead volume, whereas scenarios lacking inflow exhibited severe stagnation and expanded dead zones. These findings highlight the need to integrate hydraulic interventions with thoughtful reactor design to ensure effective and resilient small-scale wastewater treatment systems.