Full-liquid pig manure is characterized by the high total solids (TS) content and complex non-Newtonian flow behavior, leading to variation in mechanical performance, transport energy demand, and land application. The rheological constraints can often hinder the efficiency of manure handling, particularly in cold regions where low temperatures exacerbate flow resistance. This study aims to quantitatively investigate rheological properties and model construction of full liquid pig manure. A stepwise experiment was also adopted to combine the single-factor simulations with the response surface method (RSM). A systematic optimization of parameters was finally implemented after simulation. Among them, 1) single-factor experiments were conducted using TS levels ranging from 0.24% to 15.29%, temperatures from 5 to 35℃, and shear rates from 2 to 600 s^-1. Steady-state rheological tests confirmed that pig manure was a typical pseudoplastic fluid. TS was identified as the dominant factor on the apparent viscosity, with a critical threshold at 4.28% TS, leading to the transition from weakly non-Newtonian to strongly shear-thinning behavior. 2) A critical shear rate of approximately 158 s^-1 was determined using the rheological curves. The flocculent structure of the fluid was fully disrupted beyond this threshold, and the viscosity approached a stable limit (<10 mPa·s). 3) A three-factor, three-level Box-Behnken Design (BBD) was employed to optimize the process specifically for engineering conditions (High TS >10%, and Low Temperature 5-15 ℃). A log-transformed quadratic regression was constructed to accurately predict apparent viscosity. 4) RSM analysis revealed that there were significant main effects of temperature (P<0.01) and a significant interaction between TS and temperature (P<0.05), contrary to previous single-factor observations with negligible temperature effects. There was a strong concentration-dependent thermal sensitivity: While temperature had the minimal effect at low TS, the coupling of low temperature (5 ℃) and high TS (>10%) also triggered a "low-temperature locking effect." There was an exponential increase in viscosity due to reduced molecular thermal motion and enhanced hydrogen bonding networks among solid particles. Numerical optimization was performed to minimize transport energy consumption for the highly efficient nutrient loading. The optimal parameters were identified as: TS of 10.0%, temperature of 15.0 ℃, and shear rate of 136.1 s^-1 under winter operating constraints. Validation experiments showed that there was a relative error of less than 5% between predicted and measured values, indicating the high reliability of the model. Notably, the optimal shear rate (136.1 s^-1) was lower than the critical value (158 s^-1), indicating the ideal viscosity reduction at an energy-efficiency inflection point without pursuing excessive rotational speeds. In conclusion, there were synergistic effects of TS, temperature, and shear rate on manure rheology. The improved model can provide a theoretical basis for the precise matching of pump power and mixer design to prevent dead zones. Crucially, heat tracing (maintaining ≥15°C) was a more energy-efficient strategy than only increasing pumping power to overcome the "low-temperature, high-solid" viscosity barrier in the winter transport of high-solids manure.

Livestock and poultry manure is often used as organic fertilizer in the farmland surrounding large-scale farms, which is an effective method for recycling manure in agricultural settings. However, the salt content of livestock and poultry manure has not received sufficient attention in terms of the risk of salinization of agricultural land. Before applying livestock and poultry manure to fields, it is crucial to determine its total salt content or water-soluble salt content for salt analysis and management of saline-alkaline land. Unfortunately, there is a lack of standards and methods for determining the total salt content of livestock manure both domestically and internationally. Given the absence of standardized detection methods, the non-uniformity of determination methods, and the loss of potential salt residue, we conducted experimental research by gravimetric method. This study aimed to determine the total salt content in livestock and poultry manure and validate the feasibility, with the goal of providing a more accurate and reliable means of assessing the quality of livestock and poultry manure for agricultural production. Firstly, we conducted a completely randomized trial to optimize the sample-to-water ratio, oscillation parameters, leaching method, and leachate uptake. Subsequently, an orthogonal experimental design was used to evaluate three key parameters, including oscillation frequency, centrifugation time, and leachate uptake. Nine treatment combinations were formed to optimize the conditions for manure leachate extraction. Finally, a spiked recovery test was performed to assess the precision and accuracy of the method. The results indicated that the optimal conditions for determining the total salt or water-soluble salt content in livestock manure, with solid manure as the primary source, were as follows: a ratio of 1:5 for sample-to-water, oscillation at 150 r/min for 3 minutes, centrifugation at 4 000 r/min for 8 minutes, and leachate uptake of 20-50 mL. Moreover, it has been demonstrated that centrifugation can effectively replace the existing air suction filtration method for obtaining leachate. There was a significant correlation observed between the turbidity of the leachate from livestock and poultry manure and the centrifugation parameters, as well as the results of total salt determination. Leachate uptake volume had the greatest influence on the total salt content in the leachate from livestock and poultry manure, oscillation frequency took the second place, and centrifugation time had the weakest effect. According to the gravimetric method, we determined the total salt content of six different livestock manure samples under these optimized parameter conditions. The measured results exhibited relative standard deviations (RSDs) ranging from 3.7% to 13.8% and recovery rates ranging from 89.0% to 106.1%, meeting the requirements of analytical quality control in the laboratory. The established gravimetric method for determining the total salt content in livestock manure offers advantages such as low reagent usage, cost savings, high accuracy and precision, and practicality. It can be applied on a large scale to test the total salt content in livestock manure, thereby improving efficiency. This study provides a scientific foundation for the research and development a technology of salt reduction, and contributes to the sustainable development of integrated crop-livestock agriculture.