With China’s ongoing economic development and increasing emphasis on environmental protection, the number and treatment capacity of sewage plants is increasing annually. Simultaneously, sludge production is increasing. In recent years, researchers have investigated various approaches to the environmental and economic analysis of sludge treatment and recycling systems (STRS). These investigations did not take the universal law of different capacities for environmental impact and STRS economics into account. The aim of this study was to analyze the scale effect of STRS with different technologies (i.e., incineration, aerobic composting, used in material (brick), anaerobic digestion) on the environment and economy. Moreover, the cost–benefit impact of introducing a carbon- trading mechanism into the STRS to achieve carbon neutrality was analyzed. After reducing carbon emissions through by-products of STRS, the carbon emission quota can be sold, which will generate income. The results show that the break-even scales for incineration, anaerobic composting, used in building material (brick), and anaerobic digestion are 54,899, 6707, 48,775, and 4425 t/y, respectively. The break-even scale of each system decreased after the introduction of the carbon trading system into the STRS. These findings could provide critical technical information for superior decision-making in sewage sludge recycling systems.

Spatiotemporal analysis of heavy metal pollution and risks in soils from a shut-down electroplating plant was carried out. Two batch samples were tested in December 2020 and August 2022, obtained from different sampling positions and depths. The results show that Cu, Pb, As, and Ni were the main pollutants, and their spatial distribution characteristics in December 2020 and August 2022 were similar. The pollution depth of Cu, Pb, As, and Ni was mainly concentrated within 1.0 m from the ground. In the horizontal direction, the four kinds of pollutants had some relatively concentrated pollution areas and many dispersed pollution points. In December 2020, Cu contamination was heavy, and the comprehensive ecological risk was slight. In August 2022, Cu contamination was heavy, Pb contamination was moderate, and the comprehensive ecological risk was moderate as well. Both As and Ni posed carcinogenic risks in both samples. Cu, Pb, As, and Ni migrated very slowly under the action of rainfall infiltration. Soil pollution was primarily attributed to wastewater discharge, while dispersed pollution sites were mainly caused by the landfilling of production waste. The research findings hold significant implications for heavy metal pollution control and risk prevention in the soil of electroplating enterprises.

With Chinas ongoing economic development and increasing emphasis on environmental protection, the number of sewage plants is increasing annually and sludge production is increasing. This study analyzed the scale effect of sludge treatment and recycling systems (STRS) with different technologies (i.e., incineration, aerobic composting, used in material, anaerobic digestion) on the performance of environment an socio-economy by life cycle assessment (LCA) and life cycle cost (LCC). The LCA results showed that aerobic composting had a better impact on climate change (1.53E03kg CO₂ eq/p/yr) than other methods, while the whole life cost (WLC) results showed the scenario of using in material had the less cost in four scenarios. Moreover, the socio-economic impact of introducing a carbon ecological compensation mechanism into the STRS to achieve carbon neutrality was analyzed taking Tianjin as an example. In the future, it is recommended to consider and incorporate the environmentally friendly impacts of STRS with various scales into the carbon ecological compensation mechanism.