Nitrogen and phosphorus adsorbents are widely used to mitigate agricultural non-point source pollution. However, research on adsorbents mainly involves studying chemical adsorption properties, and analyzes of the effects of adsorbent on pollutant removal has not considered the surface morphology of the adsorbent or the surface distribution of pollutants. In this study, we focus on the surface morphology of the adsorbent and the surface distribution of contaminants while examining chemical adsorption properties. The crystal composition of the adsorbent was evaluated by x-ray diffraction (XRD) characterization. Kinetic adsorption data and adsorption isotherms demonstrated that thermally modified zeolite exhibits better nitrogen adsorption. The optimal removal of nitrogen and phosphorus by thermally modified zeolite and diatomite occurred at a 3:2 ratio, reaching a removal rate of 92.07% and 84.61%, respectively. The potential adsorption mechanism of a composite adsorbent for nitrogen and phosphorus capture was investigated by Fourier transform infrared spectroscopy. Scanning electron microscopy mapping, grey image recognition, and gradient recognition confirmed a relationship between the surface morphology of the adsorbent and the distribution of surface pollutants. The larger the surface of the gradient, the more uneven it is, the more nitrogen and phosphorus sites are adsorbed on the surface, and the more nitrogen and phosphorus are adsorbed. These results suggest that thermally modified zeolite/diatomite can serve as a promising adsorbent for nitrogen and phosphorus removal in practical applications.

The “ecological ditch” (eco-ditch) is an effective measure used to alleviate agricultural non-point-source pollution. However, information is lacking about the continuous transport characteristics of internal and external nitrogen and phosphorus in the interstitial water of the bottom mud of these ditches and overlying water under dynamic continuous inflow conditions. Understanding of the effect of matrix dams and microbial communities inside eco-ditches on the continuous transport characteristics of the N and P therein needs to be improved. To determine the interception effects of eco-ditches on the transfer of endogenous and exogenous N and P, an eco-ditch combining plants and a matrix dam was built to explore the transport distribution characteristics of N and P in the intermittent water and overlying water in the bottom of the eco-ditch and in the bottom of the soil ditch. We compared and analyzed the composition characteristics of the microbiological communities along the ecological and soil ditches. The research results showed that: (1) The concentration gradient between the interstitial water and the overlying water in the soil ditch is the main reason for the transport and diffusion of pollutants. However, in eco-ditches, the absorption function of plant roots and the differences between the structures of the microbial communities destroy the correlation of this concentration gradient diffusion, especially the effect on ammonium N; (2) a large number of mycelia adhere to the surface of the matrix dam in an eco-ditch, and are conducive to the adsorption and purification of pollutants in the water; (3) Proteobacteria, Chloroflexi, Actinomycetes, and Acidobacteria were the main bacterial groups in the ditches. The aquatic plants in the eco-ditch changed the microenvironment of the sediment, and both the microbial diversity and abundance along the eco-ditch were higher than in the soil ditch.

The purification effect of a biological aerated filter (BAF) mainly comes from the microorganisms in the reactor. Understanding the correlation between microbial community characteristics and environmental factors along the filter has great significance for maintaining good operation and improving the removal efficiency of the filter. A two-stage BAF was employed to treat domestic sewage under organic loads of 1.02 and 1.55 kg/m³·d for 15 days each. 16S rDNA high-throughput sequencing technology and redundancy analysis were applied to explore the correlation between microbial community characteristics and environmental variables. The results showed that: (1) the crucial organic-degrading bacteria in the A-stage filter were of the genus Novosphingobium, which had a significant increase in terms of relative abundance at sampling outlet A3 (135 cm of the filling height) after the increase of organic load; (2) the microbial communities at different positions in the B-stage filter were similarly affected by environmental factors, and the main bacteria associated with nitrogen removal in the B-stage filter were Zoogloea and Rhodocyclus; and (3) to improve the pollutant removal performance of this two-stage biological aerated filter, a strategy of adding an internal circulation in the B-stage filter can be adopted.