## 3 Anaerobic Digestion of Food Waste
### AD Process
#### 3.1.1 Hydrolysis as Stage-Limiting and Rate-Limiting Reaction in AD  
When a process develops as a result of various chain reactions, as in the case of AD, generally one step has a slower development and, consequently, is considered as the limiting stage of the process [91; 92]. In the AD process, hydrolysis is considered to be the stage-limiting process, being closely linked to substrate nature, its particle size, temperature, pH, and organic load [93], as it is the first stage of the process. A good contact or interaction between substrate and inoculum is required for a correct hydrolysis development [45] to enable enzymes in developing the extracellular reaction through adsorption on the surface of particle substrate [94]. Moreover, a correct evolution of hydrolysis, if surface area that is available for bacteria is sufficient, results in a better and significant development of further reactions (acidogenesis, acetogenesis, and methanogenesis) and subsequently in more biogas production [95; 96; 97]. This results in hydrolysis as stage-limiting and rate-limiting, as it is responsible for developing the subsequent reactions and enhancing biogas production [98; 99; 100]. Most organic matter is present in the form of particulate organic matter (POM), especially FW. During hydrolysis, particulate compounds are transformed into monomeric or dimeric soluble substrates, which can be incorporated into the cells for its methanation. Thus, in the case of organic matter, hydrolysis not only comprises classic hydrolysis of Organic Matter (OM) polymeric material, in the shape of lipids, proteins and carbohydrates (LPCH). Other transformation processes should also be reported (enzymatic or not) to transform the particulate matter into accessible polymeric compounds and expose the maximum contact surface for enzymes to act upon. Therefore, hydrolysis can be disaggregated into two different processes, (a.1) \"disintegration\" of POM into lipids, proteins, and carbohydrates, and (a.2) the proper enzymatic hydrolysis of LPCH [101]. As these two stages take place simultaneously and they are impossible to decouple, they are commonly included within a unique kinetics reaction. Meaning that, on a practical level, hydrolysis of POM and its rate is defined as the rate at which the POM is converted into assimilable substrate for anaerobic archaea. Parameters influencing hydrolysis and hydrolysis rate are mainly (a) temperature, (b) pH, (c) substrate structure, and (d) particle size. Temperature and pH  
Temperature's global effect on hydrolysis is originated by the combination of enzymatic kinetics, bacterial growth, and substrate solubility effect on improved activity [94]. There has been a lot of effort to determine the kinetic constants of anaerobic treatment [102; 103; 104], which are typically considered a first order kinetics that increases with temperature [89]. First order kinetics variations are commonly estimated by Arrhenius equation:  
\[k=A\cdot e^{-\frac{\Delta G^{*}}{k+l}} \tag{1}\]  
where \"k\" represents kinetic constants (hydrolysis constant in this case), \"\(A\)\" Arrhenius constant, \"\(G^{**}\)\" and \"\(R\)\" standard activation energy and perfect gases constant, expressed in \(\frac{J}{mol}\) and \(\frac{J}{mol-K}\), respectively, and finally, \"\(T\)\" represents the process temperature measured in absolute temperature [K]. For its part, pH has a complex feature, similar to the global AD process, as many different enzymes intervene in hydrolysis, each one with a different optimum pH for its growth. Net pH effect in hydrolysis is determined by the optimum pH for the different enrolled enzymes [99]. Substrate structure and its accessibility for hydrolytic enzymes is a key parameter, as a dissolved substrate is easier to hydrolyse, as compared with a particle substrate [87]. Substrate accessibility can be modified by forming complexes with other compounds. As an example, proteins can be affected by humic acids [105].