## 1 Introduction  
A large amount of surplus sludge, as by-product, is generated from the primary and secondary sedimentation tanks in a wastewater treatment plant every day [1]. The sludge is mainly composed of organic materials, water, nitrogen, and phosphorous nutrients, heavy metals, and pathogens [2]. Resources would be wasted, and harmful substances would be discharged into the environment if the sludge were not treated properly. Anaerobic digestion (AD) is proved to be an environmental-friendly and cost-effective treatment compared with other methods in realizing sludge reduction, stabilization, and harmlessness. The biogas produced in AD process could be used as a type of biofuel to generate energy, replacing above 86% of the energy derived from traditional fossil fuels [3]. Nutrients, such as nitrogen, phosphorous, and potassium, could also be recovered and used in AD process [4]. Basically, AD is a serious of interrelated complex biochemical reactions participated by various microorganisms, interacting with each other, to degrade organic compounds [5]. AD is usually divided into four procedures: hydrolysis, acidification, acetogenesis, and methanogenesis [6]. Complex organic materials are first hydrolyzed to soluble organic molecules, which then are fermented into short-chain fatty acids by hydrolytic and acidogenic bacteria in hydrolysis and acidification. Afterwards, the fatty acids are oxidized to H\({}_{2}\) and acetate by acetogenic bacteria during acetogenesis. In the final methanogenesis, two metabolic paths exist. One is H\({}_{2}\) used by hydrogenotrophic methanogens as the electron donor to reduce CO\({}_{2}\) to methane [7]. The other is acetate decomposed by acetonitrile methanogens to methane [8]. As most of the organic compounds of the sludge are distributed in microbial cells in solid form, cell walls and membranes form firm barriers prohibiting the release of the organic compounds, and the hydrolysis rates of the hydrolytic enzymes outside microbial cells towards complex organic materials are relatively low, hydrolysis is considered to be the control step in AD process [9]. Physical, chemical, and biological pretreatments have been adopted to accelerate the speed of hydrolysis and alleviate the burden on AD process [10]. Enzymes are usually applied in the biological method as biocatalysts to degrade organic compounds [11], but they are less effective in treating surplus sludge as they easily degrade themselves before hydrolysis starts. Alkalization and acidification are commonly adopted chemical pretreatments to solubilize microbial cell walls and release intracellular organic matter [12]; however, the quality of the produced biogas pretreated by these methods needs to be further guaranteed. Physical pretreatments, such as ultrasonic and microwave, demand much more energy than other methods although the biogas yields can be greatly improved. Taking comprehensive consideration, thermal hydrolysis has been considered to be the prior selection for the surplus sludge pretreatment, because it is a high potential pretreatment in enhancing both the degradability of organic compounds and the yields of biogas with no extra energy input [13; 14]. For a wastewater treatment plant, a commonly adopted sludge-treatment process consists of pretreatments, AD and posttreatments aiming to compress the sludge volume for final disposal [1]. Various types of microorganisms are involved in different biochemical reactions in both pretreatments and AD process, and the microbial metabolic viability could affect the efficiency and stability of the sludge-treatment process [15; 16]. Methanogens have more strict growing requirements and longer doubling times than other microorganisms participating in the sludge-treatment process, which is widely concerned as the control microbial community in AD process [17].