Effect of engineering treatment on the physical and chemical properties of livestock slurry

The combination of livestock breeding and crops planting is an important strategy for manure utilization and management. Under intensive production, the combination of livestock breeding and crops planting mainly relies on efficient modern engineering measures. However, limited studies have been carried out on the effects of engineering measures on the physic-chemical properties of slurry, which restricts the utilization of slurry as a resource. This paper reviewed recent literatures and analyzed the effects of three engineering measures (manure solid-liquid separation, anaerobic fermentation and storage) on the physic-chemical properties of slurry. The results showed that the number and size of particles, the concentration and morphology of nutrients and heavy metals in slurry changed significantly under intervention of engineering measures. In solid-liquid separation process, almost all large particles in slurry were removed and dry matter (DM), total N (TN) and total P (TP) also significantly reduced, with respective removal efficiencies of 5.7%-65% for DM, 2.7%-49% TN and 2.3%-82% for TP. Removal efficiency of DM, TN and TP as well as the rate N:P:K in slurry significantly varied with solid-liquid separation method. However, few heavy metals such as Cu and Zn were removed in solid-liquid separation process. Small particles decomposition mainly occurred in anaerobic fermentation process, which greatly reduced DM concentration in slurry. After anaerobic fermentation, the total amounts of TN and K changed little, while NH₄⁺-N concentration increased significantly with proportion of TN in the range of 46%-93%. Patches of P, Cu and Zn crystals were attached on the inside walls of the tank, while the remains transferred from liquid to solid in the anaerobic fermentation process. In the storage process, the concentrations of DM, TN, NH₄⁺-N, TP and K significantly decreased in the liquid part of the slurry, due to the decomposition and deposition. After 90 days of storage at 20-25℃, TN, TP and K concentrations in biogas slurry decreased respectively by 39%-77%, 61%-78% and 23%-54%. The study provided relevant guide on designing slurry treatment engineering and formulating comprehensive nutrient management plans in a given region. Since the area of farmland under slurry application is limited in China, livestock manure treatment usually follows multi-stage treatment technology. However, most of the current studies have focused on single engineering measure for the physic-chemical properties of slurry. Thus, it is difficult to put forward the best manure treatment scheme and treatment combination through such data. It was recommended that future works of slurry utilization in farmlands focused on:(1) the monitoring and evaluation of the response of physic-chemical properties of slurry to the entire processes of engineering treatments; (2) the development of directional regulation technology of slurry nutrient between solid and liquid phases for better application of slurry in farmlands.