Effect of Bulking Agents on Dewatered Sludge Biodrying Followed by Thermal Drying

As an exogenous substance, bulking agent plays a considerable role in dewatered sludge (DS) biodrying by affecting the organics degradation and water migration. Meanwhile, biodried product needs to be further dried because its final moisture content (MC) is usually too high for direct incineration. In this study, the influence of adding different bulking agents on organics decomposition and extracellular polymeric substances (EPS) release during DS biodrying, and thermal drying characteristics of the biodrying mixtures was investigated. Results showed that spent coffee ground (SCG), corncob (CC) and wheat husk (WH) improved water removal ratios of DS biodrying. SCG contributed various types of organic matter to the biodrying matrix, thus attaining the longest high-temperature period (4.5 days). Meanwhile, SCG contributed lipids, which have high calorific value, to the biodrying pile, leading to the highest biodrying index (3.70) among all treatments. SCG, CC and WH also accelerated EPS delamination, which was beneficial for microbial activity and bound water release. Besides, the biodried product with SCG was endowed with the highest C content and calorific value. Furthermore, bulking agents improved the thermal-drying rates of the feedstocks, and intensified the positive impacts of biodrying on the thermal-drying characteristics of the mixtures. In particular, the biodried product with SCG had a high drying rate even with low residual water content in the mixture.