**1. Introduction**

Microalgae are a wide family of photosynthetic organisms able to increase their biomass by using CO2 and sunlight as energy sources by a rate 100 times faster than plants [1]. Moreover, the water and nutrients consumption is lesser than the needed for the same amount of biomass of terrestrial crops and it does not compete with other biomass from land areas [1, 2].

For the above-mentioned reasons, microalgae have been studied for decades for their potential conversion to energy. However, it was not until recently that microalgae have been considered feasible biomass to be used as a feedstock to produce

biofuels (e.g. biohydrogen, biodiesel, biomethane, bio-oil, bioethanol, etc.) [1–4]. This is mainly due to several drawbacks (e.g. the need of solvents to produce biodiesel contributing to greenhouse gas emission, the use of expensive enzymes for bioethanol production, etc.) that have been overpassed thanks to technology and research efforts over the years focusing on the understanding and optimizing those factors that affect the different systems along with a better understanding of the biomass itself (e.g. the effect of the cell wall, the algae growth requirements, etc.) [1–5].

Among these technologies, anaerobic digestion (AD) has shown promising results. AD is a biological process where the organic compounds from certain biomass are degraded in the absence of oxygen (O2) by a microbial consortium. The main effluents of this process are biogas (i.e. a gas composed primarily of methane and CO2) and a nutrient-rich digestate [3]. While the produced biogas is considered a renewable energy source to produce electricity and heat in cogeneration plants, the nutrient-rich digestate could be used as a fertilizer [3, 4].

Microalgae are not only a viable AD feedstock, but can also serve as a means of biogas upgrading and its cultivation in the digestate can reduce the excess of nutrients and mitigate its potential toxicity [5]. However, the mono-digestion of microalgae has shown some concerns regarding its viability at industrial scales. Briefly, these concerns are related to the presence of long-chain organic compounds, mainly in the cell wall, the low carbon to nitrogen (C/N) ratio, and the high retention time needed in the reactors, which led to low methane yields, undigested organic matter in the digestate and more importantly to the inhibition of the AD process [2, 3].

In order to overpass these problems, anaerobic co-digestion (AcoD) of microalgae along with a wide range of co-substrates has been the focus of several research groups recently [1–7]. AcoD has several benefits due to its capacity to enhance the C/N ratio, the buffer capacity, the nutrient balance, and to dilute inhibitory compounds [4]. These improvements produce higher methane yields which in most cases are higher than the theoretical values obtained from the sole digestion of each co-substrate showing a synergetic effect [1–5].

Nevertheless, AcoD presents some drawbacks such as a higher organic load in the digestate, the usual need of pretreatments, or the difficulties to maintain a stable feedstock along the seasons [3]. This chapter aims to summarize the state of the art of microalgae used as co-substrate in AcoD processes and to highlight knowledge gaps and potential future developments. Moreover, a comprehensive analysis of the parameters affecting AcoD of microalgae in order to enhance methane yield is included in detail. Lastly, the energetic viability of several scenarios is discussed and future trends proposed.
