# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing
### Treatments of the Solid Fraction
#### 5.2.3 Thermochemical Treatment  
In a typical AD process, the energy conversion efficiency is about 33 to 50% which means that more than half of the energy remains in the DG [118]. Therefore, thermochemical treatments of DG could be coupled with AD to improve energy efficiency and biomass utilization [119]. However, the application of thermal decomposition treatments of DGs is relatively limited [116]. The three most commonly applied treatments are gasification, hydrothermal carbonization, and pyrolysis [119]. The main advantage of thermochemical processing is the production of value-added products (such as biochar, bio-oil, and syngas) [120] in a short period without worrying about fluctuations in the microbial community. The key limiting factor in these processes is the moisture content, which should be less than 30%. Gasification is the partial oxidation of OM that occurs in a temperature range from 800 to 1200 \({}^{\circ}\)C [121]. The main product of the process is syngas, while other products are solid carbonaceous biochar and bio-oil (a mixture of different polycyclic aromatic hydrocarbons). Several authors have shown that gasification of the dried solid fraction of the DG could be a promising way to produce a gaseous product that can be used as fuel in an internal combustion engine, while the by-products (biochar, bio-oil, and ash) can be further converted into value-added products and used for different purposes [118,121]. Hydrothermal carbonization (HTC) is a process that converts OM into high C content under varying temperatures (190-250 \({}^{\circ}\)C) and pressure (2 to 10 MPa) for several hours. Through relevant studies conducted over the last decade, HTC has emerged as a promising technology due to its many advantages such as the conversion of biomass into numerous products, e.g., solid fuel, bio-oil, soil amendment, activated C material that can be used as an adsorbent, C catalyst [122]. The HTC process may be classified as either a direct or catalytic HTC process. In the direct HTC process, only water and feed are heated in a reactor at different temperature ranges, while the catalytic HTC process uses a catalyst [123]. Pyolysis is a process that converts high solids content substrates into value-added products such as biochar, bio-oil, and syngas by heating in the absence or low concentration of oxygen [119]. It is usually conducted in an inert gas environment at atmospheric or slightly high pressure, although vacuum conditions or pressurized hydrogen (H) are sometimes employed [124]. Biochar is a C-rich solid produced by the decomposition of biomass in the presence of little or no oxygen [125]. Biochar exhibits various properties such as highly-porous structure, functional groups, large specific surface area [119], high cation exchange capacity, and stable structure. Therefore, it has attracted increasing attention, which is evident from the increasing number of published studies related to biochar [126]. Most of the attention on biochar applications has focused on its use to increase crop yields [127,128], improve nutrient availability in soil [129,130], as a soil amendment [131,132], and on environment protection application, such as wastewater treatment [133,134], water purification [135,136], C sequestration [137,138], detoxification [139,140], etc. Bio-oil is considered to be of great importance due to its high content (up to 50% or more) in thermochemical treatment products [141]. Therefore, researchers are increasingly focusing on recovering bio-oil for purposes such as clean and renewable fuel [142] and various chemicals [143]. However, currently, bio-oil is considered an unsuitable drop-in fuel due to its poor properties, such as oxygen content, low C and H content, acidic pH, high instability, and low higher heating value (HHV) [141]. Syngas, or synthesis gas, is a mixture of gases (mainly CO, CO2, and H2). The proportion of each gas depends on the type of biomass used as a feedstock, the configuration of the gasifier, and the process parameters of the thermochemical process.