Stormwater biofilter response to high nitrogen loading under transient flow conditions: Ammonium and nitrate fates, and nitrous oxide emissions

Effect of tire wear particle accumulation on nitrogen removal and greenhouse gases abatement in bioretention systems: Soil characteristics, microbial community, and functional genes

Tire wear particles (TWPs), as predominant microplastics (MPs) in road runoff, can be captured and retained by bioretention systems (BRS). This study aimed to investigate the effect of TWPs accumulation on nitrogen processes, focusing on soil characteristics, microbial community, and functional genes. Two groups of lab-scale bioretention columns containing TWPs (0 and 100 mg g-1) were established. The removal efficiencies of NH4+-N and TN in BRS significantly decreased by 7.60%-24.79% and 1.98%-11.09%, respectively, during the 101 days of TWPs exposure. Interestingly, the emission fluxes of N2O and CO2 were significantly decreased, while the emission flux of CH4 was substantially increased. Furthermore, prolonged TWPs exposure significantly influenced the contents of soil organic matter (increased by 27.07%) and NH4+-N (decreased by 42.15%) in the planting layer. TWPs exposure also significantly increased dehydrogenase activity and substrate-induced respiration rate, thereby promoting microbial metabolism. Microbial sequencing results revealed that TWPs decreased the relative abundance of nitrifying bacteria (Nitrospira and Nitrosomonas) and denitrifying bacteria (Dechloromonas and Thauera), reducing the nitrification rate by 42.24%. PICRUSt2 analysis further indicated that TWPs changed the relative abundance of functional genes related to nitrogen and enzyme-coding genes.

Simultaneous nitrate and phosphorus removal in novel steel slag biofilters: Optimization and mechanism study

The simultaneous nitrate (NO3--N) and phosphorus (P) removal systems are considered to be an effective wastewater treatment technology. However, so far, there are few studies on system optimization to improve NO3--N and P removal. In this study, nine simultaneous NO3--N and P removal biofilters (SNPBs) were constructed to treat simulated wastewater. In order to optimize the NO3--N and P removal, different material loading positions were set: (1) red soil, steel slag, and rice straw (RSR), (2) steel slag, red soil, and rice straw (SRR), and (3) red soil, rice straw, and steel slag (RRS). Results showed that the above three treatments had mean removal efficiencies of 58%-91% for NO3--N and 55%-81% for TP, with the best N and P removal occurring in the SRR. The TN mass balance indicated that microbial removal was responsible for 78.2% of the influent TN in the SRR biofilter. The key microorganisms were Enterobacter, Klebsiella, Pseudomonas, Diaphorobacter, and unclassified_f_Enterobacteriaceae, which accounted for 61.9% of the total microorganisms. The main P-removal mechanism was the formation of Al-P, Fe-P, and Ca-P in red soil or steel slag layer. In addition, the decrease of SRR effluent pH from 11.86 in 1-7 days to 7.75 in 8-50 days indicated that red soil and rice straw had a synergistic effect on water pH reduction. These results suggest that a reasonable combination of steel slag with red soil and rice straw not only simultaneously removes NO3--N and P but also additionally solves the problem of high pH caused by steel slag.

In situ monitoring of internal water storage reveals nitrogen first flush phenomena, intermittent denitrification, and seasonal ammonium flushing

Internal water storage (IWS) can be included in bioretention practices to increase storage capacity or promote denitrification-the microbial reduction of nitrate to nitrogen gas. IWS and nitrate dynamics are well studied in laboratory systems. However, the investigation of field environments, consideration of multiple nitrogen species, and determination between mixing versus denitrification is lacking. This study employs in situ monitoring (∼24 h duration) of water level, dissolved oxygen (DO), conductivity, nitrogen species, and dual isotopes of a field bioretention IWS system for nine storms events over a year period. Rapid peaks in IWS conductivity, DO, and total nitrogen (TN) concentrations occurred along the rising limb of the IWS water level and indicated a first flush effect. TN concentrations generally peaked during the first ∼0.33 h of sampling and the average peak IWS TN concentration (C_max = 4.82 ± 2.46 mg-N/L) was 38% and 64% greater than the average TN along the IWS rising and falling limb, respectively. Dissolved organic nitrogen (DON) and nitrate plus nitrite (NO_x) were the dominant nitrogen species of IWS samples. However, average IWS peak ammonium (NH₄⁺) concentrations August through November (0.28 ± 0.47 mg-N/L) demonstrated statistically significant shifts compared to February through May (2.72 ± 0.95 mg-N/L). Average lysimeter conductivity measurements were more than ten times higher February through May. The sustained presence of sodium observed in lysimeters, from road salt application, contributed to NH₄⁺ flushing from the unsaturated media layer. Dual isotope analysis showed denitrification occurred for discrete time intervals along the tail of the NO_x concentration profile and the hydrologic falling limb. Longer antecedent dry periods (17 days) did not correlate to enhanced denitrification but did correspond to more leaching of soil organic nitrogen. Results from field monitoring highlight the complexities of nitrogen management in bioretention systems. First flush behavior into the IWS suggests management to prevent TN export is most critical during the onset of a storm.