Analysis on rheological properties and model construction of full liquid pig manure

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.