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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. [ABSTRACT FROM AUTHOR]

To explore the effect of rural black-gray water treatment by a soil infiltration system on the soil environment of garden vegetable fields, a three-compartment septic tank+infiltration trench infiltration system was established in Zhanglaorenzhuang in the Ninghe District of Tianjin City on 13 July 2020. Vegetable garden with four treatment groups, namely, control（CK）, cabbage（BC）, cucumber（HG）, summer radish（SLB）, were built above the soil infiltration system simultaneously to explore the influences of infiltration trench system on the following physical and chemical indicators：the total nitrogen（TN）and total phosphorus（TP）content, a hygienic index using fecal coliforms and ascaris egg mortality, a biological index, and indicators of vegetable hygiene. Results showed that the TN content in two soil layers of the vegetable field in each treatment group showed an increasing and then decreasing trend for the physical and chemical indicators. For the BC, HG, and SLB groups, the TN content in the topsoil increased by 589.77%, 356.93%, and 70.00%, respectively, and in the deep soil by 266.46%, 718.22%, and 148.70%, respectively, from planting to harvesting. The TP content in the vegetable fields of all treatment groups showed an increasing, then decreasing, and then increasing trend. For the topsoil, the TP content of the CK group decreased by 9.66% but increased in the other three treatment groups by 0.14%, 4.53%, and 3.09%. For deep soil, the TP content in four treatment groups decreased by 16.63%, 8.65%, 18.89%, and 4.35%, respectively. In terms of biological indicators, the treatment of rural black-gray water by the soil filtration system promoted the diversity and richness of the soil bacterial community in deep vegetable fields （P<0.05）and decreased the relative abundance of pathogenic microorganisms. Moreover, the fecal coliforms and mortality of ascarid eggs of the garden vegetable soil met the Hygienic requirements for harmless disposal of night soil（GB 7959—2012）. In addition, the fecal coliform content of the three types of vegetables was less than 3 MPN·g-1, whereas the ascarid egg mortality was 100%. In conclusion, the soil infiltration system treatment of rural black-gray water can significantly increase the soil TN and TP content in vegetable fields during vegetable planting. The soil hygiene indicators of vegetable fields before and after the treatment of rural black ash water by the soil infiltration system meet the harmlessness requirement. The treatment of rural black-gray water by a soil infiltration system can improve the growth and reproduction environment of soil microorganisms in vegetable fields, reducing the risk of disease-treating microorganisms to the environment. Thus, building a garden on top of a soil infiltration system is possible without compromising vegetable safety [ABSTRACT FROM AUTHOR]

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