Surface water quality deterioration is a serious problem in many rapidly urbanizing catchments in developing countries. There is currently a lack of studies that quantify water quality variation (deterioration or otherwise) due to both socio-economic and infrastructure development in a catchment. This paper investigates the causes of water quality changes over the rapid urbanization period of 1985–2009 in the Shenzhen River catchment, China and examines the changes in relation to infrastructure development and socio-economic policies. The results indicate that the water quality deteriorated rapidly during the earlier urbanization stages before gradually improving over recent years, and that rapid increases in domestic discharge were the major causes of water quality deterioration. Although construction of additional wastewater infrastructure can significantly improve water quality, it was unable to dispose all of the wastewater in the catchment. However, it was found that socio-economic measures can significantly improve water quality by decreasing pollutant load per gross regional production (GRP) or increasing labor productivity. Our findings suggest that sustainable development during urbanization is possible, provided that: (1) the wastewater infrastructure should be constructed timely and revitalized regularly in line with urbanization, and wastewater treatment facilities should be upgraded to improve their nitrogen and phosphorus removal efficiencies; (2) administrative regulation policies, economic incentives and financial policies should be implemented to encourage industries to prevent or reduce the pollution at the source; (3) the environmental awareness and education level of local population should be increased; (4) planners from various sectors should consult each other and adapt an integrated planning approach for socio-economic and wastewater infrastructure development.

Micro–nanobubbles (MNBs) can generate ·OH in situ, which provides a new idea for the safe and efficient removal of pollutants in water supply systems. However, due to the difficulty in obtaining stable MNBs, the generation efficiency of ·OH is low, and the removal efficiency of pollutants cannot be guaranteed. This paper reviews the application research of MNB technology in water security from three aspects: the generation process of MNBs in water, the generation rule of ·OH during MNB collapse, and the control mechanisms of MNBs on pollutants and biofilms. We found that MNB generation methods are divided into chemical and mechanical (about 10 kinds) categories, and the instability of the bubble size restricts the application of MNB technology. The generation of ·OH by MNBs is affected by the pH, gas source, bubble size, temperature, and external stimulation. And the pH and external stimulus have more influence on ·OH generation in situ than the other factors. Adjusting the pH to alkaline or acidic conditions and selecting ozone or oxygen as the gas source can promote ·OH generation. MNB collapse also releases a large amount of energy, during which the temperature and pressure can reach 3000 K and 5 Gpa, respectively, making it efficient to remove ≈90% of pollutants (i.e., trichloroethylene, benzene, and chlorobenzene). The biofilm can also be removed by physical, chemical, and thermal effects. MNB technology also has great application potential in drinking water, which can be applied to improve water quality, optimize household water purifiers, and enhance the taste of bottled water. Under the premise of safety, after letting people of different ages taste water samples, we found that compared with ordinary drinking water, 85.7% of people think MNB water is softer, and 73.3% of people think MNB water is sweeter. This further proves that MNB water has a great prospect in drinking water applications. This review provides innovative theoretical support for solving the problem of drinking water safety.

Wastewater collection systems (WCSs) not only play an indispensable role in urban life but also significantly contribute to greenhouse gas (GHG) emissions. Based on extensive literature research, this study (1) summarizes current research on the production mechanisms, influencing factors, control techniques, and quantitative estimates of GHGs emitted from WCSs and (2) presents initial estimates of total GHG emissions from WCSs in China. A variety of factors affect GHG production, but standard methods are still lacking to quantify GHG emissions from WCSs. China’s WCSs emit approximately 3.86–15.35 Mt of CO₂eq annually (equivalent to 5.1–20.2% of the GHG emissions from wastewater treatment). Thus, GHG emissions from WCSs are significant and deserve attention. Control of GHGs in WCSs can be achieved through the application of chemical agents, while the construction of a green stormwater infrastructure can further facilitate GHG reductions. This review provides valuable insights for policymakers to consider in future policy planning related to GHG reduction and the improved operation of WCSs. Future research should focus on quantifying the impacts of various factors and accumulating field data on GHGs in various regions to facilitate the development of standardized calculation methods.