Drying characteristics of sewage sludge pre-conditioned by CaO and sawdust under low-temperature drying conditions

Heat pump drying is a low-carbon method of sludge drying. The operating temperature of a heat pump is generally not more than 70℃. To improve the drying efficiency of heat pump dryers, the effects of air parameters and additives on sludge drying characteristics at low temperatures were studied. The sludge drying experiments were conducted at an air temperature 50-70℃ and an air velocity of 0.5-1.7 m/s. The experimental results showed that the increase of air temperature, velocity and the addition ratio of additives can accelerate the sludge drying process. The average and maximum drying rates of sludge pre-conditioned by CaO and sawdust increased by 14.23% and 25.71%, respectively, compared with those of pure sludge. The two-way analysis of variance (ANOVA) revealed that the influence of air temperature on the sludge drying was higher than that of air velocity. Five reference models were fitted by the drying experiment data. The Page model has the highest R², so it is the most suitable model to predict the drying time of sludge at low temperatures.