Modeling transport, fate, and removal kinetics of nitrate and orthophosphate using recycled adsorbents for high and low-flow stormwater runoff treatment.

Excessive nitrate and orthophosphate carried by the stormwater runoff potentially lead to eutrophication in surface water bodies. Various green infrastructures are used that commonly consider the biological treatment of nutrients from the runoff. Due to the leaching and clogging complexities in biological mechanisms, the selection of high-flow, eco-friendly, and recycled adsorbents has been advocated to promote the physiochemical treatment of nutrients as an alternative. In this study, column experiments were conducted to investigate the transport, fate, adsorption equilibria, and reaction kinetics of nitrate (NO3-N) and orthophosphate (PO4-P) onto three recycled adsorbents - recycled concrete aggregate (RCA), recycled crushed glass (RCG), rice husks (RH), and a layered media (LM), under high and low-flow conditions. The non-reactive solute transport in columns was investigated through the bromide tracer test. The HYDRUS-1D model was used to estimate adsorption coefficients and reaction kinetics of pollutants in unsaturated media columns. Our results indicated the maximum superficial pore velocity (v = 4.40 cm/s) and dispersion (α = 2.50 cm) in RCA at the low-flow condition. Overall, NO3-N removal at the exhaustion was low in all columns, ranging between 1 and 25%. Conversely, orthophosphate removal was significant (p < 0.05) in RCA (≤94%) under low flow conditions with increased reaction kinetics (kr,d = 3.45 min-1, kr,s = 0.55 min-1) and enhanced adsorption capacity at saturation (qmax = 1.87E+05-2.33E+05 mg/kg). In conclusion, the dissolved-phase reaction kinetics (kr,d) played a significant role apart from the physisorption for the satisfactory removal of orthophosphate in RCA.

Nutrients and solids removal in bioretention columns using recycled materials under intermittent and frequent flow operations.

This research investigated the fate and removal of nitrite (NO2-N), nitrate (NO3-N), orthophosphate (PO4-P), and total suspended solids (TSS) in two bioretention columns, which were designed with three recycled materials. The first column was packed with Recycled Concrete Aggregate (RCA). The second column was a Layered Media (LM), which has layers of RCA with crushed glass and rice husks. The columns were tested under intermittent and frequent operations of synthetic runoff with low and high feed concentrations. The effect of inflow concentration, antecedent dry days (ADD), column age, and the anticipated number of events (EN) was also statistically analyzed on the performance of columns. Depending on column types, nutrient removal was significantly (p < 0.05) increased under frequent flow operations by 26-53% over intermittent. However, TSS removal was notably (p < 0.05) increased by 23-35% under intermittent operations over frequent. Overall, LM showed an increased NO2-N (92 ± 2%) and NO3-N (88% ± 2%) removal under low feed frequent operations and TSS removal (97% ± 2%) under initial intermittent operations. On the contrary, RCA showed a maximum of 99% PO4-P removal under high feed frequent operations. Results showed that the nutrient outflow concentration was found to have a negative correlation with EN and column age and a positive correlation with ADDs throughout the experiments.