Characterisation of runoff pollutant removal in biological retention systems by orthogonal experiment

Abstract

Biological retention system is an outstanding stormwater management feature. Multiple factors affect its runoff pollutant removal performance. In this study, a multi-factor orthogonal experiment was designed to identify critical factors and elucidate their mechanisms. The factors included influent flow (A), influent concentration (B), ponding zone height (C), filter material height (D), saturated zone height (E) and soil mass fraction (F). The sequence and degree of the factors as follows: B(**) > A(**) > E(**) > D(**) > F(**) > C(*) for ammonia nitrogen (NH4+ − N), B(**) > E > F > D > A > C for nitrate nitrogen (NO3− − N), B(**) > E(**) > A(**) > D(**) > F(**) > C(**) for total phosphorus (TP) and B(**) > E(*) > D(*) > C(*) > A(*) > F for chemical oxygen demand (COD). Among all factors, the influent concentration has the greatest influence on the effluent concentration of the system. For design parameters of a biological retention system, height of filter material and saturated zone had the greatest impact on effluent concentration. The trend of effluent concentration with influent concentration indicates that biological retention systems are not suitable for severely polluted areas, and their pollutant removal performance attenuate with influent increase. For excellent pollutant removal, height of filter and saturated zone should be 1000 and 400 mm respectively. 1:9 or 3:7 is considerable choice for the ratio of soil to sand in the filter material. The findings provide a comprehensive understanding on pollutant removal in biological retention systems and support for their design and promotion.