**4. Co-substrates**

AcoD is the use of a mixture of biomasses to obtain a relatively higher methane yield [10]. This modified technique is considered economically more viable and easier to control mixed biomass compared to traditional mono-digestion systems. Depending on the species, microalgae contain significantly high or low amounts of protein, carbohydrates, and lipids. To balance the nutritional requirements of microorganisms in anaerobic reactors [13]. In addition, it would be possible to improve the stabilization of the process with well-balanced mixtures. With this, it could be possible to increase the organic load capacity, reduce the concentration of possible inhibitors, and increase the buffering capacity of the digestates. Apart from the nutrient balances and net synergistic effects that would occur when co-digesting microalgae with an efficient substrate. In the revised bibliography, synthetic co-substrates, agri-food residues and slurry, and liquid residues can be found.


*Anaerobic Co-Digestion of Microalgae and Industrial Wastes: A Critical and Bibliometric Review DOI: http://dx.doi.org/10.5772/intechopen.104378*

liquid, deproteinated cheese whey, dewatered ww, food waste leachate, mill residue, municipal, paper sludge, palm oil mill effluent, septic tank sludge (STS), sewage sludge, swine wastewaters, waste activated sludge (WAS), and thickened WAS.

The production and processing of food and feed results in the generation of a large amount of waste. AD stands out as a suitable technology to reduce the environmental impact of agro-industrial waste and increase the energy self-sufficiency of these industries [29]. However, agro-waste is characterized by the lack of nutrients in its composition necessary for AD. In addition, having a high C/N ratio can affect the performance of AD. Another waste used as a co-substrate is animal manure (i.e. pig, cattle, and poultry). Contrary to agri-food residues, it has a relatively low C/N ratio, which increases the risk of ammonia inhibition [102]. Finally, one of the most studied co-substrates is sewage sludge, which was studied for the first time in 1983 [103], when it was co-digested with *Spirulina maxima*. Sludge of various varieties can be used as co-substrate, although like the rest, the interactions between microalgaesubstrate must be studied to obtain a good balance between the different parameters that can affect AD [12].
