## 4 Anaerobic Co-Digestion of Food Waste  
Anaerobic co-digestion can be defined as the simultaneous treatment of various substrates, at the same time, in the same anaerobic digester with the same inoculum. Digesting FW solely is a suitable solution for FW generation and management. However, not all biogas yield is released, and, in addition, inhibition may occur. The main reason might be a nutrient imbalance between organic compounds (LPCH) in the anaerobic digester, a lack of trace elements (Fe, Zn... ) combined with a macronutrient excess (Na, K, Ca... ), and a non-optimum C/N ratio [24; 46; 47; 194]. Problems and inhibitions caused by each organic element were formerly reviewed in previous sections. Anaerobic co-digestion (CoAD) means mixing substrates, for example, FW and sewage sludge [36], FW and manure [24], or even straw [82] and slurry [195], to balance nutrients in the digester providing a more stable environment for anaerobic bacteria [190; 196; 197]. The main advantage of CoAD, is the synergy that is created between substrates in terms of biogas yield, biogas composition and methane enrichment. This means that biogas created whilst digesting two substrates simultaneously, is higher than the sum of each mono-digestion (AB > A + B). Actually, CoAD can enhance biogas production from 35% to 400% over the mono-digestion of each substrate [198; 199]. Other important advantages are achieved with CoAD [199; 200; 201; 202; 203; 204; 205; 206], such as, (a) improvement of process stabilization, (b) dilution of inhibitory substances, (c) higher buffer capacity due to higher ammonia from organic wastes, (d) nutrient balance, (e) optimum C/N ratio achievement, (f) trace element supplement, (g) increasing the total amount of OM loaded, (h) synergy between substrates in biogas yield, (i) improving moisture content, (j) methane enrichment and its subsequent greenhouse emissions reduction, and (k) economic feasibility. FW can be used in CoAD as co-substrate or main substrate, according to its compositions, the other substrate should benefit the balance. When FW is used as a co-substrate, the main substrate is, typically, sewage sludge (SS) and animal manure (AM). These two (FW and AM) are characterized by a low C/N ratio resulting in a high accumulation of ammonia in the digester. Adding FW increases the C/N ratio and it improves digestion as a consequence of ammonia reduction [183; 204; 207]. FW also dilutes inhibitors and toxicity caused by heavy metals and pathogens, clearly existing in SS and AM. However not only is the main substrate benefited by co-digestion of FW, but also SS and AM act with a high buffer capacity, which are able to withdraw pH-low problems caused in FW mono-digestion by VFA and LCFA accumulation during the degradation of OM, especially the lipidic content [208; 209]. The use of FW as a main substrate requires employing a co-substrate capable of reducing inhibition problems of AD of FW and enhancing biogas yield. As stated in previous sections, methanogenic activity while digesting FW is often inhibited due to the accumulation of VFA and LCFA from lipids and carbohydrates biodegradation, to the lack of nutrients, such as N or P, and trace metals. FW has been traditionally digested along with straw and other lignocellulosic substrates, solid waste leachate, wastewater (industrial and agroindustrial, such as piggery wastewater), slaughterhouse waste, manure, and sewage sludge [4]. These co-substrates create a synergy in biogas yield due to several reasons: Lignocellulosic substrates (i.e., straw) increase the C/N ratio and biogas production; moreover, it decreases hydrolysis rate because of its low degradability, and consequently reduces VFA accumulation and inhibition, providing stability [210; 211]. Wastewater and OFMSW leachate dilute FW lipidic content and feed it with nutrients and metal elements that are not present, or in small amounts, in FW.