Aerobic composting has been one of the most effective ways to dispose the livestock and poultry manure. Resource utilization can also be realized to decompose the organic matter in the large-scale piles. Microorganisms can then be transformed into the stable humus. Among them, bottom aeration can serve as a large-scale aerobic composting project. There is a high demand for the effective design of a bottom aeration system for the high efficiency of oxygen transfer. The local anaerobic formation can be inhibited to improve the composting environment. This study aims to simulate and test the flow field in the pipeline of the large-scale aerobic composting bottom aeration system. The basic oxygen consumption was required for four sizes of piles (48, 90, 180, and 270 m³), in order to obtain the parameters of fans and pipelines; The actual pipelines existed in a certain pile. According to the air and hole distribution of pipelines, the flow field model was reconstructed in the bottom aeration system using Solid Works (USA) software; Different schemes of the pipeline (three air distribution patterns and two hole-layout) were simulated using Fluent fluid simulation software. The distribution of flow velocity was explored in the pipeline and at the vents. According to the case simulation of the 100 m³ reactor in the factory, there was no significant difference between the flow velocity after simulation and the actual measured value. The results indicated that: 1) Different sizes of piles were required the different air volumes and pressures. In the 48 m3 reactor, the length, width, and height of the aeration system were 8, 4, and 1.5 m, respectively. The inner diameter of the pipeline was 95 mm, and the ventilation air volume and partial pressure were 254.6 m³/h and 897.5 Pa, respectively. In the 90 m³ reactor, the length, width, and height of the aeration system were 10, 6, and 1.5 m, respectively, the inner diameter of the pipeline was 123 mm, and the ventilation air volume and partial pressure were 424.3 m³/h and 563.8 Pa, respectively. In the 180 m³ reactor, the length, width, and height of the aeration system were 20, 6, and 1.5 m, respectively, the inner diameter of the pipeline was 151 mm, and the ventilation air volume and partial pressure were 636.9 m³/h and 416.1 Pa, respectively. In the 270 m³ reactor, the length, width, and height of the aeration system were 30, 6, and 1.5 m, respectively, the inner diameter of the pipeline was 213 mm, and the ventilation air volume and partial pressure were 1 273 m³/h and 280 Pa, respectively. The fan models in the 48, 90, 180, and 270 m³ reactors were selected as 2 A (1.1 kW), 1.5 A (0.37 kW), 2 A (0.37 kW), and 2 A (1.1 kW), respectively. 2) A systematic investigation was made to explore the effects of pile sizes, gas distribution, and hole distribution on the gas flow and uniformity distribution at the vent holes in the pipeline. In the small-scale reactors (48 and 90 m³), there was little effect of air distribution on the uniformity of the flow velocity distribution in the ducts and at the ventilation holes, whereas the uniformity was improved with progressively denser holes, compared with uniform holes. In the large-scale reactors (180 and 270 m³), the uniformity of the reactors was improved by the multiple rows of pipelines, where the kinetic energy loss was reduced by 10 %. In addition to the size of the reactor, the particle size of the reactor, the shape of the holes, the angle of the pipes, and the number of holes were subsequently considered as variable parameters in the design of the ventilation system. 3) According to the case simulation of the 100 m³ reactor in the factory, there was no difference from the flow velocity after simulation with the actual measured value. The findings can provide a theoretical basis and data support for the subsequent engineering construction. But there are many influencing factors under the actual working conditions, such as intake air speed, wind pressure, and pipeline processing, leading to the ventilation effect. Field experiments are needed for further analysis on the ventilation.

The cost of dairy manure treatment and bedding material purchase increases the operating cost of the dairy farm. Membrane-covered aerobic fermentation system has been widely used for dealing with dairy manure and recycling the final product as bedding material. However, the microbial safety in each processing step is still uncertain. To better understand the bacterial community dynamics during the whole bedding conversion process, a full-chain and large-scale experiment including 16-day membrane-covered aerobic fermentation and 11-day bedding material application was conducted. The results showed that the pile temperatures in the fermentation stage rapidly increased to 80°C and maintained >50°C for more than 11 d and the use of fermentation product as bedding material provided cows with a stable and comfortable bedding environment. The Chao1 and Shannon index decreased at the end of the fermentation stage and remained stable in the application stage, indicating that membrane-covered aerobic fermentation effectively killed some pathogenic bacteria and guaranteed both the maturity and stability of the final product. The dominant bacteria in the fermentation stage were Acinetobacter, Thermus, and Rhodothermus at genus level. Seven common potential pathogens of mastitis (Staphylococcus, Enterococcus, Serratia, Pseudomonas, Corynebacterium, Mycobacterium, and Bacillus) were found at the end of fermentation stage but the relative abundance was low (0.0025%-0.2727%). The dominant bacteria in the application stage mainly included Acinetobacter, Pseudomonas, and Flavobacterium at the genus level. The relative abundance of Pseudomonas increased in the application stage, which was a reminder to the dairy farm to pay attention to the disinfection and timely replacement of bedding material to prevent the occurrence of dairy mastitis. The results of this study contributed deep understanding of the microorganism-driven bedding conversion process and provide practical guidance and cautions for the bedding materials application.