Effect of combined cooling and heating methods on low-temperature drying of municipal dewatered sludge
FANG Xing, LI Zhi-hua, YANG Cheng-jian
1. Key Laboratory of Northwest Water Resource, Environment and Ecology, Ministry of Education, Shaanxi Key Laboratory of Environmental Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China
This work was to explore the feasibility of drying municipal dewatered sludge by combined cooling and heating methods, and to reveal the influence of the freezing operation on the drying of dewatered sludge at low temperatures. The experimental sludge samples were collected in a municipal wastewater treatment plant after mechanically dewatering. The influence of freezing operation was studied with scanning electron microscopy (SEM) and flow cytometry. In addition, the influence of different freezing temperatures and time on the characteristics of the drying process for the dewatered sludge were also investigated, as well as the effect of combined cooling and heating methods on the dewatered sludge with different moisture contents and cake thickness. It was found that the freezing operation could increase the micropores inside the sludge samples. When the temperature was reduced to -30℃, the death rate of cells exhibited 15.5%, which was 9.7 times higher than that under unfrozen condition. With the drying temperature set at 60℃, as well as freezing temperatures at -10℃, -20℃ and -30℃, respectively, the drying effect was strengthened and the drying rates were improved by 25.0%, 33.3% and 29.2%, correspondingly. Especially, after freezing for 6 hours, the drying rate of the sludge sample reached the highest value, which had the weight of 5g ±0.1g, the cake thickness of 3mm and the diameter of 50mm. It was also found that the decrease in moisture content might weaken the strengthening effect, which was even lost when the content reduced to 45%. The increase of sludge cake thickness might decline the drying rate as well, which exhibited the values of 33.3%, 31.3% and 30.4% at the thickness of 5mm, 10mm and 15mm, respectively. In a word, this work might provide a valuable reference for the development of the sludge drying process using a heat pump with the combined cooling and heating methods.
方兴, 李志华, 杨成建. 冷热联用对市政脱水污泥低温干燥的影响[J]. 中国环境科学, 2020, 40(6): 2546-2553.
FANG Xing, LI Zhi-hua, YANG Cheng-jian. Effect of combined cooling and heating methods on low-temperature drying of municipal dewatered sludge. CHINA ENVIRONMENTAL SCIENCECE, 2020, 40(6): 2546-2553.
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