# Application and Evaluation of Energy Conservation Technologies in Wastewater Treatment Plants
## 2 Green Energy Utilized in WWTPs
### Energy Recovery of WW&S
#### 2.3.3 Thermochemical Treatment and Organic Fuel Production  
WW&S contains enormous amounts of chemical energy. Traditional sludge compost and landfill will not only cause secondary pollution but also waste a lot of energy[85]. Research about measuring the chemical energy of municipal wastewater shows that chemical energy in municipal wastewater can reach 26.4 kJ/g COD[86]. These chemical energies can be utilized by thermochemical treatment. According to the environment of the redox reaction, the common thermochemical treatment technologies of sludge can be divided into three categories: aerobic (incineration, wet air oxidation), anoxic (gasification) and anaerobic (liquefaction, pyrolysis) treatment[87]. A comparison of several thermochemical reactions is shown in Table 4.  
Sludge is mainly composed of organic matter with the calorific value, so it can be incinerated with auxiliary fuel to reduce the number of bacteria and oxidize toxic pollutants. The calorific value of dry sewage sludge is about 12-20 MJ/kg, equivalent to that of lignite (11.7-15.8 MJ/kg) [78; 93]. The process of incineration can be used for thermal power generation, and the ash can be used as building materials or adsorption materials. Some WWTPs with sludge incineration facilities are expected to generate 2.0 MW of electricity, which can provide 20% of their energy needs [62]. Sludge incineration has been widely used for power generation in Portland [87]. However, sludge incineration tail gas will also bring secondary pollution. Wet air oxidation is useful for the treatment of hazardous, toxic, and nonbiodegradable waste streams [88]. It is a liquid-phase reaction between organic material in H\({}_{2}\)O and O\({}_{2}\), which does not produce NO\({}_{x}\), SO\({}_{2}\), dioxins, furans, and fly ash [94]. Under these conditions, organic compounds are mainly oxidized into CO\({}_{2}\), H\({}_{2}\)O, and simple biodegradable substances. However, wet air oxidation has many shortcomings, such as corrosion, odor problems and high energy consumption [62; 95]. More than 400 wet air oxidation plants are operating around the word to treat wastewater from petrochemical, chemical and pharmaceutical industries and residual sludge from biological treatment plants [96]. Compared with pyrolysis, gasification requires the participation of water and oxygen. The dry sludge is decomposed into bio-gas and bio-char in reducing atmosphere. Data from the Mannheim demonstration plant shows that CO, H\({}_{2}\) and CH\({}_{4}\) account for 13.8%, 13.3% and 4.2% of gasification gas, respectively. The calorific value of gasification gas is 4.7 MJ/m\({}^{3}\), which is close to the result of other research (about 4 MJ/m\({}^{3}\)) [93; 97]. Gasification can generate high-quality fuel gas, but it is limited by high energy consumption [89]. [PERSON] et al. summarized the literature pertained to hydrothermal liquefaction of biomass and found that the bio-oil produced by hydrothermal liquefaction has a high calorific value of 20-40 MJ/kg [91]. Hydrothermal liquefaction can break down cell structures, as well as destroy metastable systems and change their sedimentation equilibrium in sludge. Compared with the traditional method of using dry sludge, using wet sludge can reduce energy consumption by 30% [98; 99]. However, the quality of the oil produced by this technology is not high and the waste hydrothermal liquefaction wastewater also needs to be treated. The research of [PERSON] et al. shows that the proportion of gas, liquid, and solid in sludge pyrolysis products is 10.7-26.6%, 23.5-40.7%, and 46.1-60.3% [100]. The main components of pyrolysis gas are H\({}_{2}\), CH\({}_{4}\), CO, CO\({}_{2}\), N\({}_{2}\), etc. Pyrolysis conditions, such as heating rate and final pyrolysis temperature, affected the composition of pyrolysis products [101]. The average calorific value of pyrolysis gas is about 12-13 MJ/m\({}^{3}\) and the maximum calorific value can reach 25 MJ/m\({}^{3}\) in [PERSON] et al. 's research [101].