## 6 Strategies to Improve the Efficiency of the AD Process  
### Co-Digestion  
The combination of substrates affects both microbial activity and biogas composition of AD. Co-digestion of different substrates allows the overcoming of many problems related to AD of the single organic materials. As for manure, although it represents one of the most promising biomass wastes for biogas production by AD, it provides low biogas yield, due to the low carbon content. Integration of manure with other biomass substrates for co-digestion is a valid solution to increase the economic benefits of this process. According to many studies, anaerobic co-digestion of cattle manure produces synergic effects such as buffering capacities on inhibition due to low pH, also supplying trace elements and supporting necessary methanogenic bacteria [128]. Another advantage of co-digestion of cattle manure with other organic materials is the improvement of the stability of the anaerobic process because of a better C/N balance [129], while when cattle manure is used as mono-substrate, AD is unstable due to the low C/N ratio (5-8) [130]. In addition, for FVW, co-digestion of these matrices together with livestock manure seems to be the best strategy to improve the AD process and biogas yield. [PERSON] et al. [131] yielded significant methane increases of 41%, 44%, 28% and 12% by co-digestion of horse manure with tomato, pepper, peach and persimmon, respectively. Tomato residues are a valid matrix for the production of biogas [132], although imbalanced nutrients, such as carbon and nitrogen, limit the use of this biomass as mono-substrate in AD [133]. [PERSON] et al. [133] mixed tomato residues with dairy manure and corn stover and gained the highest methane yields with a mixture of 33% corn stover, 54% dairy manure and 13% tomato residue. Macroalgae (MA) represent valuable substrates for co-digestion with many wastes, such as market place wastes, in terms of biogas yield, due to the high carbohydrates content, high growth rates and ability to absorb carbon dioxide [134; 135]. Comparing the production of biogas and methane by co-digestion with that yielded by mono-digestion of a set of five biomass materials (vegetable food waste, cow dung, pig manure, grass clippings, and chicken manure), [PERSON] and [PERSON] [136] found higher yields with co-digestion than with mono-digestion of the same substrates. In another study [137], cow manure was processed with food collected from restaurants (including rice, salad, fish, meat, vegetable soup and beans flour) and fruit (orange, banana, plantain and pineapple peel) waste. The highest biogas yield was obtained in combination with food waste followed by the co-digestion with fruit and food waste and by the co-digestion with fruit waste only (16482.5, 9096.5, and 8390 mL of biogas, respectively). Anaerobic co-digestion of food waste (FW) and cattle manure (CM) enhanced the methane yield by 41.1%, with a methane yield of 388 mL/g-VS, as a consequence of a C/N ratio of 15.8, corresponding to a FW/CM ratio of 2 [138]. [PERSON] et al. [10] by processing a mixture containing 50% food and vegetable waste and 50% residual meat, achieved a yield of methane twice that gained from processing food and vegetable waste alone. In addition, [PERSON] et al. [139] by mixing garbage from cafeterias, restaurant, houses, fruit and vegetable markets and bread stores with civil sludge for anaerobic digestion, yielded 18-47% more methane than that obtained digesting any single waste type. When FWW was co-digested with artichokes, an increased biogas yield (65-70%) was obtained as compared with FVW alone [11]. Moreover, the co-digestion allows the decrease of the costs of processing several wastes at the same time [140] and the treatment of two or more matrices at the same time has a lower impact than their separate treatment [141]. Many studies focused on the assessment of the suitable proportion of the different substrates co-digested in order to obtain the highest biogas yield. [PERSON] et al.