Processes of nitrogen removal from rainwater runoff in bioretention filters modified with ceramsite and activated carbon.

Conventional bioretention filters lack satisfactory performance in nitrogen removal. In this study, we used a mixture of cultivated soil and river sand as the bioretention filter to remove nitrogen pollutants from simulated rainwater runoff. To improve its permeability and nitrogen removal performance, both activated carbon and ceramsite were used as additives. The nitrogen removal processes and its mass accumulation in the modified bioretention filters were studied. The contribution of adsorption and biotransformation processes, together with the effects of percolate rate on nitrogen removal performance was explored. The results showed that an activated carbon layer in the bioretention filters could obviously improve nitrogen removal efficiencies, but its location made no significant difference in nitrogen removal performance. Bioretention filters modified with 20% of ceramsite could achieve the optimal percolate rate and nitrogen removal efficiencies. At given conditions, the average removal efficiencies of ammonium nitrogen (NH3-N), nitrate-nitrogen (NO3-N), and total nitrogen (TN) by the modified bioretention filter reached 80.27%, 41.48%, and 59.45%, respectively. During the leaching processes, organic nitrogen originated in the filter materials can be mineralised into NH3-N, then be denitrified and completely removed in the anaerobic environment under flooding conditions. Biotransformation in the modified bioretention filters caused a reduction of NH3-N removal efficiency by 15.41% and an increase of NO3-N removal efficiency by 31.03%. The modified bioretention filter can withstand a long-term operation. Compared with NO3-N and TN, the pollutant of NH3-N in rainwater runoff is not easy to form a mass accumulation in the modified bioretention filter.Highlights The modified bioretention filter showed high percolation rate and nitrogen removal.Hydraulic residence time is a critical design parameter to achieve nitrogen removal.NH3-N is not easy to form a mass accumulation in the filler media as NO3-N.Biodegradation increased NO3-N removal efficiency by 31.03% at given conditions.

Modeling the sources and retention of phosphorus nutrient in a coastal river system in China using SWAT.

The Soil Water Assessment Tool (SWAT) was used for exploring the sources and retention dynamics of phosphorus nutrient in the river system of the Yong River Basin, China. The performance of the SWAT model was assessed. The retention dynamics of phosphorus nutrient in the river continuum and the factors contributing to those patterns were studied. The results showed that an average of 1828 tons of TP entered the river network of the Yong River Basin annually and in-stream processes trapped 1161 tons yr-1 of TP in the watercourse, which accounted for 63.5% of the annual TP inputs. The TP retention rates in the river network ranged from 3.08 to 63.43 mg m-2 day-1. An average of 666.9 tons of TP was delivered from the estuary to the East China Sea annually. The unit area riverine exports of TP ranged from 102.21 to 244.00 kg km-2 yr-1. The river network is a net sink for TP and is going through a phosphorus accumulation phase. The results confirm that the river system has a considerable phosphorus retention capacity that is highly variable on a spatiotemporal scale. Because of the cumulative effect of continued phosphorus removal along the entire flow path, the retention fractions of phosphorus removed from all streams at the basin scale is considerably higher than that of an individual river portion. The variations of hydrological regimes, water surface area, unit area inputs of phosphorus, and the concentrations of suspended sediments have a great influence on phosphorus retention.

[Analysis of Rainwater Runoff Pollution Characteristics of Various Typical Underlying Surfaces in Ningbo].

The pollution of rainwater runoff in urban areas can cause nutrient enrichment and eutrophication in receiving waters. To explore the pollution characteristics of rainwater runoff in Ningbo, eight sampling campaigns were carried out during rainfall events from 2009 to 2019. Samples of rainwater runoff were collected from underlying surfaces of roofs, squares, grassland, main roads, and the roads in commercial streets and residential districts. The concentrations of runoff pollutants, their sources and correlations, and first flush effects were studied using frequency statistical analysis and correlation analysis. The average event mean concentrations of the chemical oxygen demand (COD), ammonia nitrogen, total nitrogen, total phosphorus, and total suspended solids (TSS) in the rainwater runoff in Ningbo were in the ranges of 23.88-102.31, 0.40-1.69, 3.41-8.71, 0.09-0.50, and 37.6-323.4 mg·L-1, respectively. Apart from the square surfaces, the COD and total nitrogen pollution of the underlying surfaces was severe. The ammonia nitrogen concentrations from the roof, commercial street, and residential district surfaces were significantly higher (P<0.05) than those from the square, grassland, and main road surfaces. The concentrations of total phosphorus from the commercial street, main road, and grassland surfaces were significantly higher (P<0.05) than those of the other underlying surfaces. The correlations of TSS with COD, ammonia nitrogen, and total phosphorus showed that the pollutants and TSS have the same original sources in roof, square, main road, and commercial street runoff, while ammonia nitrogen and total nitrogen have the same original sources in runoff from grassland and residential areas. Under the meteorological conditions of light and moderate rain, the first flush effects of ammonia nitrogen in the runoff of roofs and grassland were observed clearly, whereas this was not the same for the pollutants of total nitrogen and total phosphorus in the residential area, main road, and square runoff.