## 5 Pre-Treatments for Anaerobic Digestion of Food Waste  
Pre-treatments are usually employed to increase substrate solubility and accelerate biodegradation rate when digesting particulate organic matter [110; 232]. Hydrolysis, including the \"disintegration stage\", is considered the stage and rate-limiting step of AD, as its target is to prepare OM for a feasible digestion. If the accessibility to substrate is increased, the conversion yield will be significantly increased as well, and the process develops with no interrupptions [233]. Pre-treatments are, therefore, any previous action to substrate in order to weaken its cell wall and structure, facilitating hydrolysis by terms of allowing for enzymes and methanogens to consume organic compounds inside the cell [95; 234]. Pre-treatments can be classified into various categories, such as (a) mechanical and physical, (b) chemical, (c) thermal, and (d) biological pre-treatments. All of them aim for the same target, improving hydrolysis and facilitate solubilization, through different mechanisms. The basis of these pre-treatments is described below, provided together with experiences and results in Table 4. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Pre-Treatment** & **Substrate** & \multicolumn{2}{c}{**Factors Affected**} & **Results and Improvements** & **Reference** \\ \hline \hline \multicolumn{6}{c}{**Mechanical and Physical Pre-Treatments**} \\ \hline Screw press and screening & OFMSW & - & Separation of unwanted objects and size classification. & - & Biogas yield increases by 15\%. & [4] \\ \hline Grinding & FW & - & Size reduction (2-5-8 mm). & - & CH yield increased by 9-34\%. & [45] \\ \hline Milling & FW & - & Excessive size reduction. & - & VFA accumulation due to & [95] \\ \hline Communication & FW + SS & - & Size reduction & - & Biogas yield increases by 10-25\%. & [92] \\ \hline Pressure (10 bar) + Depressure (1 bar) & FW & - & Break of cell walls. & - & Biogas production increased by 3\%. & [23] \\ \hline Sonication & FW & - & Increased surface area and pore size. & - & CH\({}_{4}\) production increased by 94\%. & [236] \\ \hline Ultrasonic (US) & SS & - & Enhanced solubilization due to cell lysis & - & Increased biogas yield. & [237] \\ \hline \multicolumn{6}{c}{**Chemical Pre-Treatments**} \\ \hline Alkaline pre-treatment & OFMSW & - & Cell wall disruption & - & Unduted particle swelling and increased surface area. & - & CH\({}_{4}\) production increased by 180\%. & [239] \\ \hline Add pre-treatment & FW & - & Biomass destruction & - & CH\({}_{4}\) production decreased by 60\%. & [235] \\ (HCl until pH + 2) & - & Cell wall disruption. & - & due to inhibitors formation (flow pH) & [235] \\ \hline H\({}_{2}\)SO\({}_{4}\) pre-treatment & Lippocellular waste & - & Hydydyne cellulose. & - & CH\({}_{4}\) production increased by 57\%. & [46] \\ \hline \multicolumn{6}{c}{**Thermal Pre-Treatments**} \\ \hline Heat (90-120 \({}^{\circ}\)C) & FW & - & Disintegrate cell walls. & - & CH\({}_{4}\) production increased by 30–40\%. & [241] \\ \hline Heat (80 \({}^{\circ}\)C) & FW & - & Dwaitering, pasteurization and disinfection of organic wastes & - & CH\({}_{4}\) production increased by 50\%. & [241] \\ \hline Heating & & - & Increased solubilization & - & Biogas production increased by 11\%. & [233] \\ (120 \({}^{\circ}\)C-30 min) & - & - & Dswitched cell wall and increased solubilization & - & Biogas production increased & [242] \\ \hline Microwave (145 \({}^{\circ}\)C) & FW & - & Dswitched cell wall and increased solubilization & - & Biogas yield increased by 30\%. & [113] \\ \hline Freezing + thawing & & - & Biogas yield increased by 30\%. & [113] \\ (–80-55 \({}^{\circ}\)C) & FW & - & Cell disruption because of temperature shock & - & Biogas production increased by 23\%. & [235] \\ \hline \end{tabular}
\end{table} 
Table 4: Pre-treatments for anaerobic digestion and co-digestion of food waste: Basis and influence.