Microbial-induced calcium carbonate precipitation (MICP) is a new soil remediation technology, which can improve the physical and mechanical properties of soil by transporting bacterial solution and cementation solution to loose soil and precipitating calcium carbonate precipitation between soil particles through microbial mineralization. Based on this technique, the effects of different fine particle content and pore ratio on the physical and chemical properties of silt after reinforcement were studied. The content of calcium carbonate, the ability of silt to fixed bacteria, unconfined compressive strength (UCS), permeability coefficient and microstructure of the samples were determined. The results showed the following: In the process of calcium carbonate precipitation induced by microorganisms, more than 50% bacterial suspension remained on the surface of silt particles and their pores. The higher the bacterial fixation rate of silt, the more CaCO₃ was generated during the solidification process. The bacterial fixation rate and CaCO₃ content both decreased with the increase in the pore ratio and increased with the increase in the fine particle content. XRD and SEM images show that the calcium carbonate is mainly composed of spherical vaterite and acicular cluster aragonite. There is an obvious correlation between unconfined compressive strength and CaCO₃ content of silt. When CaCO₃ content accumulates to a certain extent, its strength will be significantly improved. The unconfined compressive strength of silt A with pore ratio of 0.75 and fine particle content of 75% is 2.22 MPa when the single injection amount of cementing fluid is 300 mL. The permeability coefficient of cured silt can be reduced by 1 to 4 orders of magnitude compared with that of untreated silt. In particular, the permeability of MICP-treated silt A is almost impermeable.

The physical and mechanical properties of rocks can be reduced significantly by an acidic environment, resulting in engineering weaknesses, such as building foundation instability, landslides, etc. In order to investigate the mechanical properties of rocks after hydrochemical erosion, a chemical damage constitutive model was established and used to analyze chemical damage variables and energy transformation. It is assumed that the strength of the rock elements obeyed Weibull distribution, considering the nonuniformity of rock. The chemical damage variable was proposed according to the load-bearing volume changes in the rock under water–rock chemical interactions. The chemical damage constitutive model was derived from coupling the mechanical damage under the external load and the chemical damage under hydrochemical erosion. In order to verify the accuracy of the model, semi-immersion experiments and uniaxial compression experiments of black sandy dolomite were carried out with different iron ion concentrations. Compared with the experimental data, the chemical damage constitutive model proposed could predict the stress–strain relationship reasonably well after water–rock interaction. The effects of water–rock interaction on the rock were a decrease in peak stress and an increase in peak strain. The peak strain increased by 4.96–29.58%, and the deterioration rate of peak strength was 0.19–4.18%. The energy transformation of the deterioration process was analyzed, and the results showed that the decrease in releasable elastic energy, U_e, is converted into dissipated energy, U_d, after hydrochemical erosion.

The utilization of livestock waste has attracted increasing attention in recent years. The presence of high levels of heavy metals is a major obstacle to the utilization of biogas as a fertilizer resource. In this study, the heavy metal contents in biogas residue, slurry, and discharged sewage from three representative farms of gooseries, henneries, and dairy farms in the Yangtze River Basin were investigated and assessed. The results demonstrated that heavy metals, including Cd, Mn, As, Cu, Pb, Cr, Zn, etc., could be detected in all biogas residues, with significantly different contents between farm types (p < 0.005). Specifically, biogas residues from the goosery and the dairy farms met “China’s Organic Fertilizer Standards” (COF Standards); however, Cd concentrations in biogas residues from hennery farms exceeded the limits by five times. The concentrations of Cd and Pb in biogas slurries from all of the farms exceeded the limits of the “China Farmland Irrigation Water Quality Standard” (CFIWQ Standard). In particular, the Pb concentrations in biogas slurry from the dairy farms exceeded the limits by 29 times, and the discharged sewage from all three farm types complied with the comprehensive sewage discharge standards in China; however, only that from the goosery farms was suitable for irrigation. Thus, it is recommended to increase the feed selection, biogas engineering, and biological-purification-supporting technology, and to carry out regular sampling inspections of the biogas residue, slurry, and discharged sewage for heavy metals, so that environmental and crop pollution risks can be reduced when they are used as sources of nutrients for eco-friendly agriculture.