**6. Biomethane potential (BMP) performance**

**Table 4** summarizes all the BMP assay results up to date complying with the following prerequisites:


The above prerequisites have been selected as they are crucial for comparison purposes. The authors of this review acknowledge the lack of homogenization regarding BMP performance, which difficult the task to evaluate scientific data and provide reliable conclusions. The authors of this review also acknowledge the lack of a widely approved and used standard methodology for BMP tests, although, some are published and proved to be reliable enough [111, 112]. The authors of this


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

> **Table 4.**

*Methane yield of co-digested microalgae with different substrates at different ratios in BMP assays.*

review would also like to highlight that only 20 out of 120 reports complies with the above prerequisites, and if other crucial factors were included within the prerequisites (e.g. use of positive control or details on inoculum acclimation) no reports could have been included in this review. Additionally, when more than one cosubstrate ratio was measured in the same study, only the one producing the highest methane yield was included in **Table 4**.

As can be observed in **Table 4**, methane yield ranged from 237.1 to 4 <sup>1</sup> 639.8 *NL kg CH VS* <sup>−</sup> . AcoD of microalgae with industrial wastes showed positive synergetic effects in most cases, being the improvement against the theoretical values up to 37%. However, some studies had reported negative effects. This could be due to the limits of batch methodologies and could lead to higher methane productions during continuous tests as pointed out by several authors [16, 33] or could be related to the low C/N ratio [45].

The optimum C/N ratio produced higher synergetic effects than those studies where the C/N ratio was above 30 or below 20. The most common and successful temperature is within the mesophilic range with an ISR of 2–3, although, some studies have reported improvements when applying thermophilic temperature at a higher ISR (4–5) [40, 77].

Regarding the co-substrates ratio, when microalgae were used as a nitrogen source in order to balance the C/N ratio, it was added commonly as a fourth part of the whole influent, or even less. When microalgae were co-digested with other low C/N substrates, it was added at higher concentrations. Nevertheless, the optimum ratio between microalgae and other co-substrates is unique and needs to be assessed through experimentation.
