## 2 Common Substrates Used in Anaerobic Digestion
### Carbohydrate-Rich Substrates  
Food wastes, wastes from the food processing industry, catering wastes and source-separated wastes from residential homes are characterized by a high carbohydrate content. Saccharides and disaccharides are the main components of fruit and vegetable wastes. These compounds are easily converted into fatty acid intermediaries by the anaerobic microflora, giving rise to pH changes if the accumulation of these acids overcomes the buffer capacity of the fermentation media [23]. The VFA imbalance may adversely affect the production rate of biogas. Accumulation of these intermediaries is commonly observed during the anaerobic conversion of easily degradable wastes. Wastes from the food processing industry, such as cheese whey or fruit wastes, also have a low nitrogen content leading to poor buffering characteristics of the fermentation liquor. The summation of these features results in inhibitory levels of acetic and propionic acids. When severe digestion imbalances are present, higher carbon chain (C\({}_{4}\)-C\({}_{5}\)) acids and iso-forms can be measured in the fermenting slurry.  
The anaerobic digestion of cheese whey has been studied under different reactor configurations such as up-flow anaerobic sludge blanket (UASB) reactors and sequencing-batch anaerobic reactors (SBR) [34,35]. Cheese whey is a high organic content stream with soluble sugars, which are derived from cheese manufacturing. The digestion of this substrate has proven challenging due to the lack of sufficient nutrients to keep a balanced  
Figure 1: Schematic representation of different substrates suitable for anaerobic co-digestion and valorization of main process products (biogas and digestate).  
microforlora and the low buffering capacity of the digestion system. In addition, the presence of soluble sugars aggravates the reaction imbalance, with acidification outcompeting the subsequent degradation stages. The application of high organic loading is attained by retaining anaerobic biomass inside the system. Mesophilic and thermophilic digestion of this single substrate have been studied by [PERSON] et al. [36] and [PERSON] et al. [37], indicating the accumulation of VFA and proposing two-stage systems as a way of overcoming the acidification problems. Another solution proposed for stabilizing the fermentation is the addition of different nitrogen-containing substrates such as manures and sewage sludge [38; 39].  
Another high sugar-containing substrate is sugar molasses. The digestion of this material shows similar behavior to that of cheese whey. Therefore, two-stage configurations where acidification and methanogenesis, or hydrogen production and methanogenesis, have been proposed to overcome the problems associated with the rapid evolution of VFA and slow degradation of the acid intermediate stream [40; 41; 42]. The use of rich-carbohydrate substrates may be interesting in co-digestion systems, but the availability of these substrates is usually determined by their use in animal feeding. Therefore, an increase in the demand for these by-products will ultimately affect market prices and probably create adverse effects on the economy. Market distortions should be avoided either by the use of specific energy crops or by the application of specific policies intended to attenuate market deviations.