**3. Microalgae and growth mediums**

Since the first study on biogas production with microalgae by Golueke [101], a wide variety of microalgae genera have been studied. Due to the composition of the microalgae, it has been seen that each strain has a very specific biogas production, and has very diverse productions. One of the factors that most affect the AD of microalgae is the structure of its cell wall, which is why the selection of the type of microalgae is important. One of the main characteristics for microalgae to have a good methane production potential is to have a thin or null cell wall, large cytoplasmic components, a high growth rate, and a high tolerance to stress [102]. Other important aspects are whether it has a low content of hollocellulose in the cell walls, metabolic, and growth


**Table 1.**

*Synthetic culture medium for the growth of microalgae.*

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

conditions are favorable, the morphological traits of the microalgae strains [27]. In addition, the selected strains offer a feasible genetic manipulation to control metabolic activities and improve tolerance to nutrient and ecological stress [102].

Nineteen genera of microorganisms have been investigated in the 92 articles found on the AcoD of microalgae. Of these 92 articles, 51 of these works have been studied with the genus *Chlorella* and 24 of these works with the genus *Scenedesmus*. The rest of the work carried out was with Chlorophyta as *Nannochloropsis* (5 studies), *Micractinium* (3 studies), *Dunaliella* (2 studies), *Dictyosphaerium*, *Closteriopsis*, *Desmodesmus*, *Chlamydomonas*, *Stigeoclonium*, *Botryococcus* and *Tetraselmis* genus. Other genus as *Tribonema* (Ochrophyta), *Phaeodactylum* (Bacillariophyta) and *Tisochrysis* (Haptophyta) have been also studied as co-substrate. In addition, the following cyanobacteria have been studied: *Arthrospira* (4 studies), *Spirulina* (2 studies), *Merismopedia*, *Oscillatoria*.

Another factor that should be considered is the culture medium for microalgae growth. Depending on the medium, it could favor the production of biogas in a later step due to the nutrient requirement of the microorganisms in the AD process [96]. According to the reviewed bibliography, the microalgae used for AcoD are obtained through other research groups or are cultivated using three different types of medium for growing it.

1.Synthetic medium: BG11, Bold's basal (BBM), Conway enriched medium, Jaworki's medium, modified Zarrouk medium, BlueBIOTech Ltf. (**Table 1**).


2.Digestate or effluent anaerobic: Anaerobic sludge, continuous stirred tank reactor (CSTR) digestate, up-flow anaerobic sludge blanket (UASB) digestate, pri-

#### **Table 2.**

*Removal of N-NH4 and P-PO4 in microalgae culture.*

mary effluent and sludge, chicken manure digestate, swine digestate, Anaerobic membrane bioreactor (AnMBR) digestate.

3.Wastewater: Tannery effluents, piggery WW, industry, domestic, municipal, fresh waters, lake waters, natural seawater enriched, soft drink. WW, Winery WW.

As shown in **Table 1**, microalgae are microorganisms that need certain nutrients to perform vital functions. Therefore, synthetic culture media contain macronutrients such as calcium, sodium, potassium, magnesium, and chloride. In addition to adding micronutrients such as iron, cobalt, molybdenum, manganese, copper, zinc, and vitamins. Finally, ethylenediaminetetraacetic acid (EDTA) is also added to form a complex ring (a chelate) with the trace elements, which, when used in low concentrations, stimulates the growth of microalgae by making this element available in low quantities. These nutrients are found in nature and are bioavailable in the other two natural culture media such as anaerobic digestate and wastewater, but may be found in lower concentrations than necessary [10].

In addition, using wastewater for the growth of microalgae could prevent eutrophication of the water due to the consumption of excess nutrient of these type of waters. In this sense, microalgae have been succesfully used for removing nitrogen and phosphorous from various wastes as shown in **Table 2**. Significant amounts of removal of nutrients and biomass production achieved in these studies demonstrate the feasibility of coupled wastewater treatment and microalgae cultivation processes.
