## 3 Results and Discussion  
Under microwave radiation, the sludge could be rapidly heated up, promoting the hydrolysis of carbohydrates into polysaccharides and monosaccharides with low molecular weights, and facilitating the hydrolysis of proteins into polypeptides, dipeptides, amino acids, and other substances that are further hydrolyzed into low molecular organic acids, ammonia, and carbon dioxide [12]. Meanwhile fat would be hydrolyzed into stearic acid, palmitic acid, and so on. Phosphorus and nitrogen in cells are also released due to the hydrolysis of nucleic acid. The mechanism illustration of microwave treatment for the municipal dewatered sludge is shown in Figure 1. The mixtures of farthing nanomaterials, green extraction agent, sludge-based microwave absorbent, curing binder, activator, and other low-cost materials are irradiated by microwave together, resulting in the degradation of the organic matters, and the dehydration and solidification of the residual materials. The above-mentioned hydrolyses could be extracted by suitable extractants [13]. Thus, the choice of extractants plays a significant role in determining the efficiency of the microwave treatment. As reported, dimethyl carbonate represents a new generation of nontoxic solvent, exhibiting the characteristics of safety, convenience, less pollution, and diverse dissolution to organic compounds [14; 15]. Thus, in this work, we investigated the effect of dimethyl carbonate as an extraction agent on the reduction rate of the organic matters under microwave irradiation. The microwave treatment was conducted under the power of 6300 W for 2 min (an optimized condition). The generated temperature is approximately 93.4 \({}^{\circ}\)C. The added mass ratios of the dimethyl carbonate are 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt%, respectively. Figure 2 displays the variation tendencies of the sludge mass loss and the organic matter content as the content of the dimethyl carbonate increased. The plot shows that the quality loss of the sludge mixture is initially increased  
Figure 1: Illustration of sludge mixed with various additives (**a**) and municipal dewatered sludge after microwave treatment (**b**). and then decreased, and the mass loss rate of the mixture reached a maximum of 79.90% with dimethyl carbonate content of 2 wt%. Accordingly, the content of organic matter decreased from 42.17% in the initial stage to a minimum of 38.20%. As calculated, the 2 wt% dimethyl carbonate extracts only 3.97% of the organic matter in the sludge under microwave radiation. There is still a lot of residual organic matter in the sludge. It is considered that the extraction efficiency of the organic matter is hardly improved by microwave radiation depending on dimethyl carbonate at a relatively low temperature of 93.4 \({}^{\circ}\)C, and the organic matters with high molecular weight were tightly attached to the sludge, so a higher energy is required to improve the extraction efficiency. As is well known, nanomaterials have a large specific surface area and many defects, thus, they exhibit high surface adsorption and microwave absorption [15; 16; 17; 18; 19], which leads to the increase in temperature around the nanomaterials. To investigate the effect of ZnO nanoparticles on the degradation efficiency of the organic matters, we mixed 0.2-1 wt% ZnO pretreated powder and 2 wt% dimethyl carbonate with the municipal dewatered sludge under microwave treatment. Figure 3a presents the relationship of the mass loss and organic matter content with the added content of ZnO. The plot reveals that the mass loss of the mixture initially increased as the ZnO content increased, and maintains a saturated value. Correspondingly, the organic matter content is greatly reduced when the added ZnO is increased to 0.1%, and then displays a little increase. Thus, considering the nanomaterial cost, the amount of ZnO powder added in the sludge was selected as 0.1 wt%.