The aim of this study was to improve the removal of nitrogen pollutants from artificial sewage by a modeled two-stage constructed rapid infiltration (CRI) system. The C/N ratio of the second stage influent was elevated by addition of glucose. When the C/N ratio was increased to 5, the mean removal efficiency of total nitrogen (TN) reached up to 75.4%. Under this condition, the number of denitrifying bacteria in the permanently submerged denitrifying section (the second stage) was 22 times higher than that in the control experiment without added glucose. Elevation of the C/N ratio resulted in lower concentrations of nitrate and TN in the second stage effluent, without impairment of chemical oxygen demand removal. The concentration of nitrate and TN in effluent decreased as the abundance of denitrifying bacteria increased. Moreover, the bacterial biofilms that had formed in the sand of the second stage container were analyzed. The secretion of extracellular polymeric substances, a major constituent of biofilms, was enhanced as a result of the elevated C/N ratio, which lead to the improved protection of the bacteria and enhanced the removal of pollutants.

In this work, the influence of graphene on nitrogen and phosphorus in a batch Chlorella reactor was studied. The impact of graphene on the removal performance of Chlorella was investigated in a home-built sewage treatment system with seven identical sequencing batch Chlorella reactors with graphene contents of 0 mg/L (T1), 0.05 mg/L (T2), 0.1 mg/L (T3), 0.2 mg/L (T4), 0.4 mg/L (T5), 0.8 mg/L (T6) and 10 mg/L (T7). The influence of graphene concentration and reaction time on the pollutant removal performance was studied. The malondialdehyde (MDA) and total superoxide dismutase (SOD) concentrations in each reactor were measured, and optical microscopy and scanning electron microscopy (SEM) characterizations were performed to determine the related mechanism. The results show that after 168 h, the total nitrogen (TN), ammonia nitrogen (AN) and total phosphorus (TP) removal rates of reactors T1–T7 become stable, and the TN, AN and TP removal rates were gradually reduced with increasing graphene concentration. At 96 h, the concentrations of both MDA and SOD in T1–T7 gradually increased as the graphene concentration increased. In optical microscopy and SEM measurements, it was found that graphene was adsorbed on the surface of Chlorella, and entered Chlorella cells, deforming and reducing Chlorella. Through the blood plate count method, we estimated an average Chlorella reduction of 16%. According to the water quality and microscopic experiments, it can be concluded that the addition of graphene causes oxidative damage to microalgae and destruction of the Chlorella cell wall and cell membrane, inhibiting the nitrogen and phosphorus removal in Chlorella reactors. This study provides theoretical and practical support for the safe use of graphene.

To study the removal performance of a constructed rapid infiltration (CRI) system and its microbial community characteristics, we took the demonstration project of a CRI system that has successfully operated for 15 years as an example, aiming to analyze the CRI system's removal performances for COD, NH 4 +-N, TN and TP. Meanwhile, high-throughput sequencing technology was used for the first time to study the microbial community diversity and structure in the CRI system. The results showed that the average removal efficiencies for COD and NH 4 +-N were 75.52% and 92.94%, and the average removal efficiencies for TN and TP were respectively 39.74% and 42.78%. High-throughput sequencing technology indicated that a variety of bacterial phyla were found in CRI's bacterial communities, including Bacteroidetes, Actinobacteria and Acidobacteria, among which Proteobacteria dominated. At the genus level, a spatial variation was illustrated for the diversity and structure of bacterial communities. The dominant genera on the surface layer (0 cm) of CRI were mainly Nocardioides, Sphingomonas, Bryobacter and other microorganisms that can degrade organic matter, and the dominant genera in the inside (30-120 cm) were mainly microorganisms that play an important role in removing nitrogen. This study provided a theoretical basis for the long-term operation of a CRI system.