Study of the characteristics of pollutants in rural domestic sewage and the optimal sewage treatment process: a Chengdu Plain case study.

Domestic sewage is an important source of surface water pollution in the rural areas of developing countries, especially in the rural areas of China. In recent years, with the strategy of rural revitalization, China has paid increasing attention to the treatment of rural domestic sewage. Therefore, 16 villages in the Chengdu Plain were selected for the study, and seven indicators were analyzed and evaluated, including pH, five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonia nitrogen (NH3-N), total phosphorus (TP), suspended solids (SS) and total nitrogen (TN), in the water samples at the inlet and outlet of the wastewater treatment plant. The concentration of each pollutant in the rural scattered domestic sewage of the Chengdu Plain in Southwest China was obtained, and the concentration of each pollutant in domestic sewage was higher than that in summer. In addition, the preferred process for removing each pollutant was obtained by studying the effects of the treatment process, season and hydraulic retention time on the removal efficiency of each pollutant. The research results provide valuable references for the planning and process selection of rural domestic sewage treatment.

Enhancing waste activated sludge dewaterability by reducing interaction energy of sludge flocs.

How to efficiently improve waste activated sludge (WAS) dewaterability is a common challenge in WAS treatment and management throughout world. The interaction energy of sludge flocs is of great importance for sludge dewaterability. In this study, the relationship among the repulsive force of sludge flocs, hydrophilic/hydrophobic characteristics of sludge flocs, and sludge dewaterability have been quantitatively and qualitatively investigated based on extended Derjaguin-Landau-Verwey-Overbeek theory for the first time. The energy barrier of sludge flocs has good correlations with sludge dewaterability (p < 0.05). Trivalent cations (Al3+ and Fe3+) and Fenton's reagent reduced the interfacial free energy (ΔG) from 9.4 mJ/m2 of raw sludge to -34.2 (Al3+), -60.5 (Fe3+), and -63.2 (Fenton) mJ/m2, respectively, indicating that the hydrophilic surfaces of the sludge flocs converted to hydrophobic (△G < 0), and decreasing Lewis acid-base interaction energy (WAB) of sludge flocs. In addition, most of the trivalent cations (Al3+ and Fe3+) were attached to sludge flocs, leading to neutralize negative charges and mitigate electrostatic interaction energy (WR) of sludge flocs. The reduction of WAB and WR eliminated energy barrier of sludge flocs and repulsive force between sludge flocs. In comparison, monovalent (Na+ and K+) and bivalent (Ca2+ and Mn2+) cations cannot completely change the hydrophilic surface characteristic and negative charge of sludge flocs. The existed energy barrier prevented sludge flocs to agglomerate with each other, thus resulting in a worse dewaterability. This study illustrated that reducing interaction energy of sludge flocs played a critical role to improve sludge dewaterability.

Integrating sulfur, iron(II), and fixed organic carbon for mixotrophic denitrification in a composite filter bed reactor for decentralized wastewater treatment: Performance and microbial community.

Decentralized wastewater treatment in rural areas is an imperative challenge around the world, particularly in developing countries. The composite filter bed reactor is viable for decentralized wastewater treatment, but its performance on nitrogen removal often fluctuates with the unstable influent characteristics and loadings. Here, a composite filter bed reactor integrating sulfur, iron(II), and fixed organic carbon (shaddock peel) was developed and continuously operated under different conditions. The fixed organic carbon source promoted nitrogen removal with an efficiency higher than 90% and reduced effluent sulfate level by 40%, indicating that the integrated electron donors could improve the resistance and stability of the reactor. Moreover, sulfur-oxidizing bacteria (Thiomonas, Sulfuriferula, and Acidithiobacillus), iron-oxidizing bacteria (Ferritrophicum), and denitrifiers (Simplicispira and Hydrogenophaga) were identified in the anoxic/anaerobic layer of the reactor, suggesting that mixotrophic denitrification was stimulated by sulfur, iron(II), and fixed organic carbon. The findings of this study indicate that the developed reactor with the integrated electron donors could be reliable for carbon, nitrogen, and phosphorus removal and promising for the application of decentralized wastewater treatment.

An efficient hydrodynamic-biokinetic model for the optimization of operational strategy applied in a full-scale oxidation ditch by CFD integrated with ASM2.

Oxidation ditches (ODs) are often used for wastewater treatment. However, limitations of ODs like high energy expenditure and increased sludge sedimentation hinder its wide application. In this study, the computational fluid dynamics (CFD) model integrated with Activated Sludge Model No. 2 (ASM2) was proposed and applied in a full-scale OD. The integrated model provided heterogeneous information on the characteristics of hydrodynamics and biokinetics of OD, especially with respect to the simulation of total phosphorus removal by CFD-ASM2 integration model for the first time. The simulated values of flow velocities, suspended solids (SS), dissolved oxygen, chemical oxygen demand, total nitrogen, ammonium nitrogen, and total phosphorus concentrations were well validated with the measured results, with the standard deviation errors of less than 5.56%, 0.28%, 0.74%, 7.39%, 3.17%, 5.27%, and 7.40%, respectively. Based on the integrated model, four different operational strategies were simulated. The proposed operational strategy of operating 7 surface aerators and 10 submerged impellers not only met the standard discharge requirements (GB 18918-2002) but also consumed less energy by 22.3%, compared with the original strategy of operating 9 surface aerators and 13 submerged impellers. Meanwhile, this proposed operational strategy also reduced the SS concentrations in the second and fourth channels, which was beneficial to elimination of sludge sedimentation. Moreover, the proposed operational strategy was successfully applied and validated in full-scale OD. The foregoing results collectively suggest that the CFD-AMS2 integration model is numerically capable to optimize the operational strategy of ODs.