As the global population reaches eight billion, large quantities of wastewater (domestic, industrial, livestock) need to be treated in an efficient, green, and environmentally friendly manner. Wastewater hydroponics technology (HP) can efficiently remove various pollutants (conventional and emerging pollutants, heavy metals, and microorganisms) and create economic benefits. This paper aims to systematically review the principles, applications, and limitations of wastewater hydroponics technology in the context of pollution and nutrient removal. Unlike constructed wetlands, wastewater hydroponics has been proven to be effective in removing pollutants through small-scale in situ restoration. For instance, the average removal of COD, total nitrogen (TN), total phosphorus (TP), copper (Cu), and zinc (Zn) was more than 70%, 60%, 80%, 64.2%, and 49.5%, respectively. However, HP technology still has the disadvantages of high energy consumption, complex control parameters, and low public acceptance of using wastewater for planting crops. Therefore, further research is needed to reduce system energy consumption. In addition, hybrid technologies, such as two-stage hydroponics that use aquatic plants (algae or aquatic floating weeds) to recycle pollutant-containing wastewater nutrients for hydroponics, should be further developed.

Waste glass (WG) is a solid waste with increasing reserves, and its disposal has become a global issue. The application of WG in the construction industry is one of the promising pathways for recycling WG while reducing high-cost WG landfills. This study is conducted to systematically review the potential effects of WG as both a cement and aggregate replacement on the mechanical and durability properties of cementitious composites. Different waste glass powders (WGP) can lead to various effects on both the workability and hydration of cementitious composites. 10 to 20 wt% WGP as supplementary cementitious materials (SCMs) is the optimal dosage to promote the mechanical and durability properties of the WGP–cement composites. The presence of WGP can promote the chemical resistance, freeze–thaw resistance, and elevated temperature resistance of WGP–cement composites. However, more studies and experiments are needed to provide conservative conclusions on how WGP would affect the durability properties of both normal and high-strength concrete. Sustainable concrete technology requires the use of low-carbon materials while maintaining long-term structural resilience. There is an increasing trend in recycling WGP as a cement or aggregate replacement in designing green concrete.

Urban street canyons often experience poor air quality because buildings and vegetation modify and obstruct airflow, thereby reducing the pollutant dispersion capacity within these built environments. While trees are widely recognised for their environmental (e.g., microclimate regulation and carbon sequestration) and social (e.g., aesthetics) benefits, their complex effects on pollutant dispersion remain debated. Existing numerical studies on pollutant dispersion in urban street canyons with tree canopies commonly adopted over-simplified geometries that neglected the heterogeneous sizes and spacing of individual trees, despite these features are prevalent in real-world settings. This simplification can limit the accuracy of simulated airflow and pollutant dispersion. Accordingly, this study develops a high-fidelity model that features realistic street geometry and explicit representation of unevenly distributed and irregularly shaped tree canopies to investigate their impact on roadside air quality in a real-world street canyon. The heterogeneity of real-world trees is considered, and two idealised scenarios are designed for comparison: treeless street and evenly-spaced trees. Results show that, in the real-world case with unevenly distributed trees, pollution hotspots are localised in mid-street leeward pedestrian areas (z/Z = 0.45-0.68), with peak NO₂ concentrations of ∼135 μg/m³, whereas the treeless case exhibits more uniform dispersion and lower peak concentrations (∼120 μg/m³). In contrast, the evenly distributed trees configuration produces the worst pollution, with near-ground hotspots extending over z/Z = 0.34-0.95 and peak concentrations exceeding 150 μg/m³. These findings highlight that while tree canopies in general reduce wind speed, uneven tree distribution can enhance horizontal airflows and relocate pollution hotspots in urban street canyons. These insights offer a conceptually novel strategy for roadside air quality management, shifting the focus from maximising vertical pollutant dispersion into the atmosphere or preventing pollutant transport toward the roadside to pollution hotspot relocation.