Flow properties of vermicompost particle with different moisture contents

The earthworm processing of waste is recognized as a very sustainable approach for management of organic wastes. The vermicompost resulted from this procession has been proved to be practical and useful in farming, substrate nursery, gas absorbent etc. However, this technology has been widely adopted to treat different organic waste material. Most of scientific investigation in vermicompost concentrates on its nutrient characteristics and microorganism rather than physical properties. The flow ability is key factor for the mechanized operation of vermicomposting. The basic parameters like rolling friction coefficient and surface energy are needed for the equipment design, material calculation, and vermicompost product manufacture. The moisture content of vermicompost can reach 40%-50% under natural condition, even with high moisture content, the vermicompost can still keep the granular condition. Flow parameters, such as friction coefficient, cannot simply test though the normal physical method. In order to provide the quantized flow parameters of vermicompost for mechanization, in this study, vermicompost transformed from pure cow dung by the worm procession was used. The experiments were undertaken in condition of different moisture contents, changing from 25%-65%. The friction coefficient and surface energy were investigated by the method of pile angle simulation and calibration using discrete element method (DEM). First, Plackett-Burman (PB) design was conducted to select the impacted physical frictional parameter factors that have significant influence on pile angle. Then, the steep climbing test was conducted to shorten the parameter ranges from the maximum range of PB design. According to the result of steep climbing test, the response surface methodology (RSM) was carried out to build the model which can reflect the relationship between pile angle and friction coefficients. Besides, for explaining the flow ability of vermicompost in further, the direct shearing test was applied to calculate the internal frictional angle of vermicompost with different moisture contents. The results of experiment indicated that three key factors vermicompost-vermicompost rolling friction coefficient, vermicompost-steel rolling friction coefficient and Johnson Kendall Roberts surface bonding energy (JKR surface energy), significantly affected the pile angle (P<0.05). With the rising of moisture content from 25% to 65%, the vermicompost-vermicompost rolling friction coefficient dropped from 0.135 to 0.110, the vermicompost-steel rolling friction coefficient dropped from 0.116 to 0.102. From the result, the surface energy was the most impact factor in three factors, which changed from 0.179 to 0.345 J/m2. The few rise of moisture content can improve the flow ability of vermicompost significantly under low moisture content condition. When the moisture content was over 50%, the surface energy and the cohesion force were increased, which would restrict the flow movement of vermicompost to some extent. The study is expected to provide numerical parameter reference for the practical application and development of mechanization.