Dewatering from sludge is an important sustainable issue in recent years, in this work, we found the unique behavior: Skeleton builder additions can improve the dewatering performance greatly, which related to the different pore structure of skeleton builder. As compared to the coal ash, sawdust and rice husk char are easier to construct porous channels in the sludge body, which is responsible for the discharge of water. the dewatering efficiency can increased from approximately 30%–65% by pipe network effect and interlayer channel effect, a sufficient amount of skeleton builders establish a complete pipe drainage network in the sludge body, allowing the water to be discharged fluently. Moreover, the skeleton builders can cause the sludge body to form a layered structure. Under the combined action of pipe network effect and interlayer channel effect, the deep-dewatering effect increased largely by the addition of skeleton builders.
Environment and Energy aspects are two main topics in the world (Du et al., 2018; Zhou et al., 2018), but the waste disposal will become an important environmental issue recently, Domestic sewage treatment by bio-methods can produce a large amount of sewage sludge. The key issue of sludge treatment aims at reducing the moisture content efficiently (Ren et al., 2015; Skinner et al., 2015; Zhang et al., 2015). In order to facilitate the subsequent disposal and resource utilization, the general method of mechanical dehydration can reduce the moisture content of sludge to the level of approximately 80%, which is far below the requirements of sludge treatment (Yang et al., 2015), and thus, a deep dehydration is necessary and required. Thermal drying and sludge conditioning as well as mechanical dehydration methods are commonly used in sludge deep dewatering process. However, high energy consumption and serious secondary pollution problems will significantly inhibit its efficient application (Mahmoud et al., 2016; Tang et al., 2018; Zhang et al., 2014); therefore, efficient approach is urgent and raised a wide-spread research interests. As well known, sludge dewatering mainly contains two important procedures, that is solid-liquid separation at first, and the sequent extrusion for separation water from sludge (Christensen et al., 2015). Recent study shows that the surface adsorption water and internal hydration water can be converted into free-water by series sludge pretreatment (To et al., 2016), thence favoring for the solid-liquid separation. However, in fact, it is difficult to achieve the efficient deep dehydration, because the organic matter content of sludge floc will lead to a high compressibility, and the drainage channels in sludge body will be shut down by the high mechanical pressure during the squeeze process (Collard et al., 2017; Li et al., 2014a). Thus, a dense layer will form on the surface of the sludge body, and deep water separation cannot be discharged completely. Therefore, the use of chemical conditioning can only improve the sludge dewatering performance to some extent (Vega et al., 2015; Zhang et al., 2017c) However, because of the supporting effect of skeleton builder, the porous structure still exist in the sludge body under the great mechanical pressure, so the water can flow out of the sludge body easier. The commonly used skeleton builders by researchers are simple and easy available solid materials, such as powdered coal ash (Cieślik et al., 2015), lime (Hu et al., 2017), sawdust (Deng et al., 2017), wheat grits (Guo et al., 2015), gypsum (Nittami et al., 2015), red mud (Zhang et al., 2014), lignite (Hoadley et al., 2015), rice husk char (Wu et al., 2016b) and so on.