Cationic polyacrylamide (CPAM) is a commonly used flocculant for water treatment. Factors that affect the flocculation effect and can be controlled manually include the type and dosage of CPAM, wastewater pH, stirring time and settling time, and their reasonable setting is critical to the flocculation effect of CPAM. In this paper, the optimal flocculation conditions of a novel CPAM were studied. First, single-factor tests were conducted to preliminarily explore the optimal range of factors that influence CPAM flocculation, and then response surface methodology (RSM) tests were performed to accurately determine the optimums of the influencing factors. The results showed that the flocculation effect was better when the intrinsic viscosity was larger or the cationic degree of CPAM was higher. The CPAM dosage, wastewater pH and stirring time significantly impacted the flocculation effect, and inflection points were observed. A model that could guide CPAM-8.14-40.2 flocculation was obtained by RSM tests. The model optimization showed that the optimal flocculation conditions of CPAM-8.14-40.2 for treating wastewater prepared with kaolin were as follows: the CPAM dosage, wastewater pH and stirring time were 5.83 mg·L⁻¹, 7.28, and 5.95 min, respectively, and the turbidity of the treated wastewater was reduced to 6.24 NTU.

A combined process that involves adjusting pH value, adding calcium chloride and flocculation was used for the removals of zinc (Zn), total phosphorus (TP), and chemical oxygen demand (COD) from phosphatizing wastewater. The result of single-factor experiment showed that the optimum process conditions used for precipitation were phosphating wastewater pH (pHI) of 10.0 and CaCl₂ dosage of 25.0 g L⁻¹, and for flocculation were cationic polyacrylamide (CPAM) dosage of 20.0 mg L⁻¹ and pH (pHII) of 8.0. Using the optimum process conditions to treat phosphating wastewater, Zn, TP, and COD were reduced to 0.44, 0.33, 38.0 mg L⁻¹, respectively. Response surface methodology (RSM) was employed to optimize the factors of pHI, CaCl₂ dosage, CPAM dosage, and pHII to maximize COD removal efficiency. The significant order of factors that affect COD removal efficiency was as CPAM dosage > pHI > CaCl₂ dosage > pHII. The interactions between CPAM dosage and pHII, pHI and pHII are relatively significant. The optimization results of RSM experiment showed that the COD removal efficiency of 82.1% could be achieved with the optimal conditions: pHI of 9.5, CaCl₂ dosage of 29.0 g L⁻¹, CPAM dosage of 25.2 mg L⁻¹, and pHII of 7.5.

The construction of water-saving renovation projects can bring substantial benefits to the development of agriculture, but it may also be accompanied by negative impacts, especially in a large-scale irrigation district. Hence, there is always controversy, and it is vital and necessary to investigate the effectiveness of water-saving renovation. In this study, the Yahekou irrigation district, one of the largest districts in China, is selected as the case to explore the beneficial influence and adverse impact. Kriging interpolation, Pearson correlation analysis, and linear regression analysis are employed to study the temporal evolution, spatial distribution, and domain influencing factors. The results show that the water-saving renovation project in the Yahekou irrigation district had significant water-saving benefits during the period of 1998–2021, and the average annual water use of the irrigation district decreased by 61%. Canal lining is an important influencing factor for the decrease in irrigation water use, with a correlation coefficient of −0.538, B of −18.669, and R² of 0.290. Furthermore, the water-saving renovation of irrigation districts is not the dominant reason for changes in groundwater depth. Meanwhile, the groundwater depth below ground level (the level DBGL) in the irrigation district increased by 82%. The combined effects of atmospheric precipitation, canal lining, river sand mining, and groundwater exploitation have led to a sustained increase in groundwater depth in the study area. The results obtained from this study can provide valuable and scientific reference for agricultural water resource management over the large-scale irrigation district. This article focuses on the impact of water-saving renovation on ecological and technical indicators such as water use and the groundwater DBGL. However, the impact of water-saving renovation in irrigation districts is multifaceted; subsequent research can explore the impact of water-saving renovation projects on society and the economy.

Rural domestic sewage, originating from human activities that involve the extraction and utilization of natural resources, is an inherent component of the ecological cycle in nature. Therefore, its disposal methods should align and harmonize with the laws governing nature’s evolutionary processes. This study conducted a comprehensive investigation on the domestic sewage facilities in representative villages located within the water source protection area of the middle route of the South-to-North Water Diversion Project. Taking Village A’s domestic sewage treatment station as a case study, an analysis was performed to assess its operational status and identify existing issues. The consideration of rural domestic sewage treatment should encompass factors such as the generation and discharge of household wastewater, the characteristics of water quality, discharge regulations, the natural and social environment, as well as post-completion operations and maintenance modes. We also proposed source reduction measures for the reuse of gray water in domestic sewage treatment in Village A, along with integrated treatment approaches involving biochemical treatment, landscape integration, and farmland irrigation for black water. These measures not only achieve effective treatment outcomes but also foster harmonious coexistence between humans and nature. Moreover, they align with the principles of ecological civilization while considering rural revitalization and promoting green agricultural development.

The quartz sand-enhanced coagulation (QSEC) is an improved coagulation method for treating water, which uses quartz sand as a heavy medium to accelerate the sedimentation rate of flocs and reduce the sedimentation time. The factors that influence the QSEC effect and can be controlled manually include the quartz sand dosage, coagulant dosage, sewage pH, stirring time, settling time, etc., and their reasonable setting is critical to the result of water treatment. This paper aimed to study the optimal conditions of QSEC; first, single-factor tests were conducted to explore the optimal range of influencing factors, followed by response surface methodology (RSM) tests to accurately determine the optimum values of significant factors. The results show that the addition of quartz sand did not improve the water quality of the coagulation treatment, it took only 140 s for the floc to sink to the bottom, and the sediment volume only accounted for 12.2% of the total sewage. The quartz sand dosage, the coagulant dosage, and sewage pH all had a significant impact on the coagulation effect, and resulted in inflection points. A QSEC-guiding model was derived through RSM tests, and subsequent model optimization and experimental validation revealed the optimal conditions for treating domestic sewage as follows: the polyaluminum chloride (PAC) dosage, cationic polyacrylamide (CPAM) dosage, the sewage pH, quartz sand dosage, stirring time, and settling time were 0.97 g/L, 2.25 mg/L, 7.22, 2 g/L, 5 min, and 30 min, respectively, and the turbidity of the treated sewage was reduced to 1.15 NTU.

The high performance sedimentation tank (HPST) is an efficient water treatment technology, which accelerates the settling rate of flocculates by adding sludge, so as to increase the water treatment load of coagulation sedimentation tank. Its sewage treatment effect is affected by many factors such as sludge dosage, wastewater pH, flocculant dosage, stirring time, settling time, etc. This paper was to study the optimal conditions of HPST, first, some single-factor tests were conducted to preliminarily explore the optimal range of influencing factors, and then response surface methodology (RSM) tests were performed to accurately determine the optimums of significant factors.The results showed that adding sludge can not improve the water quality of coagulation treatment, but it can significantly accelerate the coagulation settlement process, the sludge dosage, the coagulant dosage and sewage pH all impacted significantly on its coagulation effect, and existed inflection points. A model that could guide HPST was obtained by RSM tests. The model optimization and experimental validation showed that the optimal HPST conditions for treating domestic sewage were as follows: the dosage of polyaluminum chloride (PAC) was 1.70 g/L, cationic polyacrylamide (CPAM) dosage was 2.35 mg/L, sewage pH was maintained at 8.0, sludge dosage was 10 mL/L, stirring time lasted for 5 minutes, and settling time lasted for 30 minutes. As a result of these optimized conditions, the turbidity of treated sewage decreased to 1.19 NTU.