Hydrochar is considered to be a good adsorbent for the separation of metal ions from aqueous solutions. However, the yield of hydrochar from raw straw is generally low, because the hydrothermal carbonization occurs via dehydration, polymerization, and carbonization. In this work, various hydrochar samples were prepared from rice straw with nitrogen and phosphorus salt; moreover, toilet sewage was used instead of nitrogen, and phosphorus salt and water were used to promote the polymerization and carbonization process. The modified carbon was characterized using XRD, XPS, SEM, and FTIR, and the adsorption capacity was investigated. A significant increase in hydrochar yield was observed when toilet sewage was used as the solvent in the hydrothermal carbonization process. The adsorption capacity of N/P-doped rice straw hydrochar for Cd²⁺ and Zn²⁺ metal ions was 1.1–1.4 times higher than that those using the rice straw hydrochar. The Langmuir models and pseudo-second-order models described the metal adsorption processes in both the single and binary-metal systems well. The characterization results showed the contribution of the surface complexation, the electrostatic interaction, the hydrogen bond, and the ion exchange to the extraction of Cd²⁺ and Zn²⁺ using N/P-doped rice straw hydrochar.

Rural toilet reform in cold areas is the focus and difficulty of China’s toilet revolution. In this paper, by carrying out the septic tank gradient burial depth test and regular seasonal monitoring of the operation effect of rural septic tanks in cold areas, the frost protection effect and treatment effect of rural toilets in cold areas are clarified, which provide important references for the rural toilet revolution and its practical significance in cold areas of China. Based on controlled tests and correlation analysis methods, different gradient burial depths of septic tanks (40, 60, and 80 cm) and different seasonal operation effects (spring, summer, autumn, and winter) were studied to analyze the influence of factors such as burial depths and temperatures on the effectiveness of anti-freezing and treatment of septic tanks. The results found that the septic tank effluent indexes of the six cold-zone test households could not meet requirements of Class Ⅰ-B in the discharge standard of pollutants for municipal wastewater treatment plants (GB 18918-2002). (The criteria for Class I-B are as follows: COD emission limit of 60 mg/L; SS emission limit of 20 mg/L; TN emission limit of 20 mg/L; TP emission limit of 1 mg/L; NH₃-N emission limit of 8 mg/L); The average removal of suspended solids (SSs), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), NH₃-N, and NO₃-N by septic tanks was 79.7%, 21.8%, 10.5%, 13.5%, 9.76%, and 2.38%, respectively; the water temperature was significantly positively correlated with SS, TN, and TP; the double-layer insulated septic tank with medium burial depth (60 cm) could operate normally in cold areas in winter. The treatment effect of septic tanks on various indicators basically shows that summer > autumn > spring > winter. Temperature is the main limiting factor affecting the treatment effect of septic tanks.

A field trial was conducted in Tianjin to assess the impact of “three-compartment septic tank (SPT) + soil wastewater infiltration system (SWIS)” on vegetable crop soil, determine the SPT effluent quality, and establish the consumer safety of vegetables grown above the SWIS. The effluent total nitrogen (TN), total phosphorus (TP), ammonium-N (NH₄⁺-N), chemical oxygen demand (COD), and 5-d biochemical oxygen demand (BOD₅) levels all varied largely every month. The average COD failed to meet the criteria of the Standard for Irrigation Water Quality (No. GB5084-2021) but significantly influenced bacterial community distribution. Hierarchical clustering disclosed seasonal variation in SPT effluent. SWIS treatment of rural black-grey water significantly affected both the vegetable soil TN and TP content, and it promoted microbial community diversity and richness in deep soil. The treatment also increased the relative abundances of the beneficial bacterial genera Thiobacillus and Arthrobacter by more than 320% and decreased the relative abundance of the pathogenic bacterial genus Streptomyces in vegetable soil by more than 20.33%. The faecal coliform levels and ascaris egg mortality rates in the vegetable crop soils lay within published human health and safety thresholds both before and after SWIS treatment. All vegetable crops grown above the SWIS were fit for human consumption. The VC level in the vegetables planted in experimental households were higher than those for the vegetables planted in ordinary households. The present work provides reasonable theoretical and empirical bases for optimising the “SPT + SWIS” process and SPT discharge standards in rural areas.