Numerical model as a simulation tool was used to describe the pollutants transformation and degradation process in constructed wetlands (CWs). It can help provide insight into the “black box” and increase the understanding of the complex processes in CWs. In the last few decades, several process-based numerical models were developed to depict the pollutants removal processes in CWs, which include biochemical model, hydraulic model, reactive-transport model, plants model, clogging model, and coupling model combining two or more sub-models. However, there was a long way to go before fully understanding the decontamination mechanisms of CWs. On the one hand, single or a composite model coupling a small number of sub-models cannot fully reveal the decontamination processes. On the other hand, a comprehensive model including all sub-models of current cognition involves numerous parameters, most of which are interaction and cannot quantitatively determined, thus making the model complex and leading to diffuse interaction. Therefore, in order to describe the reaction processes in CWs more accurately, it is expected that all parameters should be quantified as far as possible in the future model. This study aims to provide a review of the numerical models of CWs and to reveal mechanism of decontamination. Based on the advantages and disadvantages of existing models, the study presented the improvement method and future research direction: (1) new detection/monitoring technique or computing method to quantitatively assess the parameters in CWs models, (2) correcting the simulation errors caused by the assumption of Activated Sludge Models (ASMs) and developing a complete biofilm reaction sub-model, (3) simplification of the comprehensive model, and (4) need of emerging pollutants modeling.

Novel alum sludge-based floating treatment wetland (FTW) was developed to enhance the purification performances of natural water bodies, i.e., rivers, lakes, and ponds. Polyurethane was applied to foam the lightweight alum sludge based-substrate (PU-AL) of FTW through the response surface method. Three FTWs configurations were created for a half-year lab-scale operation, and the PU-AL FTW presents the greatest purification performance in the removal rate of chemical oxygen demand (COD) of 62.58 ± 6.65%, total nitrogen (TN) of 53.31 ± 4.65%, and total phosphorus (TP) of 45.39 ± 4.69%. PU-AL substrate could enhance the nutrient removal performance of existing FTW by providing a proper media for microbial and plants’ growth. This study provides a good solution and showcase not only from a natural water restoration point of view but also from the waterworks sludge management view for a better understanding of FTWs and good applications in engineering practice.

Alum sludge is an inevitable by-product from the water purification process, which had been applied as substrates in some constructed wetlands with good performance, especially for phosphorus (P) adsorption. The raw alum sludge is similar to a clay lump with an irregular shape, and there is a concern of it leaching into water. For better reuse, herein, some sludge was fired to produce alum sludge ceramsite (ASC) with a uniform spherical shape via a four-step process of kneading the sludge ball, air drying, preheating at 400 °C for 10 min, and firing at 600 °C for 5 min. Meanwhile, an air-dried alum sludge ball (adASB) was manufactured for comparison. The physicochemical properties and P adsorption ability of ceramsite were investigated subsequently. Through XRD and FT-IR tests, there was no obvious difference between ASC and adASB on the phase structure, but there was a certain amount of Al-OH group loss on the surface of ASC. The structure of ASC was still amorphous, similar to adASB, while ASC possessed more micropore structure and a bigger specific surface area than adASB. Adsorption experiments showed the P adsorption behaviors of ASC and adASB were much similar, and their adsorption kinetics were in accordance with the two-step adsorption kinetics rate equation and pseudo-second-order kinetics equation. The maximum adsorption capacities of ASC and adASB fitted by the Langmuir model were 1.66 mg/g and 1.89 mg/g, respectively. It should be pointed out that, compared with other adsorbents, the ASC produced in this study still had a greater ability to adsorb P. Therefore, ASC should have a great application potential for P removal in wastewater treatment in China.

Microbial communities regulate water quality and biogeochemical cycling in rivers, but their responses to geomorphological factors remain unclear. Water samples were collected in August 2024 (summer wet season) from the Wuding River, and metagenomic sequencing was used to investigate microbial community changes and the influences of geomorphological factors. Environment (nutrients, etc.,) exhibited significant spatial heterogeneity with temperature ( p < 0.01), total organic carbon (TOC, p < 0.001), dissolved organic carbon (DOC, p < 0.001), chemical oxygen demand (COD, p < 0.05), total phosphorus (TP, p < 0.001) and suspended solids (SS, p < 0.001), which were significantly higher downstream than upstream. Pseudomonadota , Cyanobacteriota , and Actinomycetota were the most important microbial phyla, and Cyanobacteriota ( p = 0.016) was significantly more abundant upstream than downstream. The linear discriminant analysis effect size (LEfSe) revealed 8 and 10 biomarkers upstream and downstream, respectively. Upstream microbial communities were adapted to oligotrophic and high-light environments, whereas heterotrophic, carbon-metabolizing communities occurred downstream. Significantly higher ACE ( p < 0.05), Chao1 ( p < 0.05), Shannon ( p < 0.001), and Pielou’s evenness ( p < 0.001) indices were observed downstream than upstream. The relative abundance of genes associated with carbon cycling (the methane metabolism pathway, TCA cycle, and rTCA cycle) was greater downstream than upstream, as was the relative abundance of nitrogen functional genes. Elevation affected the upstream microbial communities, whereas temperature, TP, TOC, and nitrate nitrogen (NO ₃ -N) affected the downstream communities. The results improve our understanding of how geomorphology drives the environmental factors and then governs the microbial community and their carbon and nitrogen cycling pathways.

River sediments are important carbon reservoirs in terrestrial–aquatic systems, and a thorough understanding of the factors that influence the sources and distribution of organic carbon in river sediments can help us understand the carbon cycling process in river ecosystems. In this study, 21 river sediment samples were collected along the upstream–downstream gradient of the Wuding River Basin. The study revealed that (1) the sources of DOC in the Wuding River Basin were spatially heterogeneous, with plankton being the largest source of DOC upstream (39%), whereas terrestrial sources were the largest source of DOC downstream (54%). (2) Influenced by geomorphic conditions and land use, the gradually increasing hydrodynamic conditions from upstream to downstream prompted the migration of surface soil organic matter and nutrients such as effective nitrogen (AN), total phosphorus (TP), and total nitrogen (TN) to the river channel, resulting in an increase in the concentration of suspended sediment, which led to the accumulation of organic carbon in the downstream section of the river. (3) Using macrogenomic techniques, microbial - driven carbon cycling processes were identified and predicted. Downstream has greater carbon - cycling potential than upstream. Upstream DOC is positively correlated with anaerobic carbon fixation and methanogenesis (p < 0.001). Downstream DOC is negatively correlated with the rTCA cycle (p < 0.001), inhibiting autotrophic fixation as microbes use available carbon. The results of this study provide data to support ecological restoration, carbon sink enhancement, and water quality assurance in high-sediment rivers.