Hot-air drying characteristics and optimization of the process parameters for the solid fraction of dairy manure

A large amount of dairy manure has posed a great threat to the environmental pollution (air, water, and soil) in the large-scale intensive dairy farming in China. Taking the dairy manure as a bedding material, some new types of manure treatment have been implemented to improve the internal resources recycling in the dairy husbandry industry at present. The safe moisture content of dairy manure is required normally 40%-50% or less for the dairy bedding material. But the high moisture content (about 75%) has been confined to the extending of dairy manure after the solid-liquid separation. Furthermore, the dairy manure is characterized by a kind of high-humidity porous material with a high free water content and dry heat conductivity. Therefore, a drying process of dairy manure can be a necessary step to fully meet the safety standards of dairy bedding materials. This study aims to optimize the process parameters and hot-air drying characteristics of the dairy manure solids under different drying conditions. Some factors were selected, including the drying temperature (60, 75, 90 and 105 ℃), the thickness of the manure layer (15, 30, 45 and 60 mm), and the duration of mixing frequency (2, 4, 6 and 8 min). The moisture content and drying rate were also measured for the dry matter in the dairy manure. The drying and weighing procedures were carried out as follows. In drying, after deforesting with at the room temperature for 12 hours, the dairy manure was put into a square stainless-steel tray with a specified thickness, where the hot-air drying test was carried out under different conditions. In weighing, the dairy manure was taken out of the drying oven every 30 minutes, and then weighed with an electronic balance, where the mass of the dairy manure was recorded. The drying test was stopped until the moisture content of the dry matter basis in the dairy manure was under 1 g/g (the moisture content of the wet matter basis was 50%). After that, six representative thin-layer drying models were applied to simulate the effective diffusion coefficient and activation energy of the drying process. An orthogonal test was also designed for the process parameters of rapid drying with the highest drying efficiency. The results showed that the higher the drying temperature was, the smaller the dairy manure thickness was, and the shorter the mixing frequency interval was, the faster the dry matter basis moisture content decreased. Moreover, the drying process of dairy manure was divided into three periods: an accelerated drying, an approximate constant-rate drying, and a falling rate period, indicating a better consistence with the drying characteristics of high-humidity porous materials. The Wang and Singh model was more suitable for the drying process of dairy manure, where the minimum effective diffusion coefficient was 7.31×10-5 m2/h, the minimum activation energy was 14.596 kJ/mol, and the orthogonal test results showed that 0.017 h/g, when the drying temperature was 105 ℃, the manure thickness was 6 cm, and the mixing frequency interval was 4 min. The finding can provide a theoretical data support to optimize the rapid drying process of dairy manure.