Optimization of Ventilation Pipe Design Parameters for Aerobic Composting Based on CFD Simulation and Compost Maturity Evaluation

Composting is a primary method for organic waste treatment, and ventilation is crucial for composting performance. The pipe design parameters influence ventilation efficiency, while the interactions between these parameters remain unclear. Conventional response surface methodology (RSM) is limited by discrete experimental points, potentially affecting optimization accuracy, whereas computational fluid dynamics (CFD) provides a spatially resolved analysis of airflow fields within the entire compost pile. Herein, this study combines RSM and CFD to exploit their complementary strengths for more reliable optimization. A 17-run Box–Behnken design (BBD) was conducted to evaluate compost maturity under different pipe design parameters, including pipe diameter (11, 12, 13 mm), adjacent orifice spacing increment (0.5, 0.75 and 1 cm) and orifice diameter (2, 2.5 and 3 mm). Meanwhile, CFD simulations were performed to quantitatively evaluate the effects of different design parameters on airflow distribution characteristics. An experimental validation was additionally conducted under the optimal configuration. Results indicated that the optimized design parameters reduced the coefficient of air speed variation (CASV) by 55.37%. Pipe diameter, adjacent orifice spacing and orifice diameter had significant effects on germination index and humus (p < 0.01), whereas the significance of the interaction terms differed among response variables. The influence ranking on germination index and humus was as follows: pipe diameter > adjacent orifice spacing > orifice diameter. To achieve maximum composting effectiveness, the optimal pipe design parameters were determined as an adjacent orifice spacing increment of 0.78 cm, an orifice diameter of 2.83 mm, and a pipe diameter of 12.71 mm. Relative to the 17 BBD experimental runs, these optimal parameters resulted in maximum observed improvements of 18.66 percentage points in final GI and 63.36 g/kg in final HS, while the humic acid-to-fulvic acid ratio increased by 1.27. This study provides a theoretical basis for optimizing compost ventilation pipe design and improving composting performance in aerobic composting systems.