As stated before, like a refinery, it is still possible to obtain other products in the cultivation of microalgae, such as methane, biohydrogen, and ethanol. Some examples of these possibilities are presented as follows.

Methane. Since early studies on microalgae biofuels, the production of meth­ane biogas by anaerobic digestion of biomass was a main focus (Benemann 2012). This microbial conversion (of organic matter into biogas) produces a mix­ture of methane, CO2, water vapor, small amounts hydrogen sulfide, and some­times hydrogen (Gunaseelan 1997 in Huesemann et. al. 2010). This process has been successfully and economically viable despite the recalcitrance of some algal species to biodegradation and inhibition of the conversion process by ammo­nia released from the biomass. (Benemann 2012; Huesemann et al. 2010). For Huesemann et al. (2010),

Methane generation by anaerobic digestion can be considered to be the default energy conversion process for microalgal biomass, including algal biomass produced during wastewater treatment and for the conversion of residuals remaining after oil extraction or fermentation to produce more valuable liquid fuels.

Hydrogen. There are three main processes to produce hydrogen from microalgae: dark fermentation; photo-fermentation, and biophotolysis. The first involves anaer­obic conversion of reduced substrates from algae, such as starch, glycogen, or glycerol into hydrogen, solvents, and mixed acids. The second, these organic acids “can be converted into hydrogen using nitrogen-fixing photosynthetic bacteria in a process called photofermentation.” The latter, a biophotolysis process uses micro­algae to catalyze the conversion of solar energy and water into hydrogen fuel, with oxygen as a byproduct (Huesemann et al. 2010). Although these mechanisms were successfully proven in laboratory scale, they have not yet been developed as a practical commercial process to produce hydrogen from algae (Huesemann et al. 2010; Ferreira et al. 2013).

Ethanol. On the other hand, ethanol can be generated from two alternative processes: storage carbohydrates (fermented with yeast) and endogenous algal enzymes (Benemann 2012; Huesemann et al. 2010). The main process is “yeast fer­mentation of carbohydrate storage products, such as starch in green algae, glycogen in cyanobacteria, or even glycerol accumulated at high salinities by Dunaliella.” A self-fermentation by endogenous algal enzymes induced in the absence of oxy­gen has been reported for Chlamydomonas. Against the very low ethanol yield from both fermentation, several private companies are now reported to be developing ethanol fermentations.

Electricity and Gasification. The microalgae biomass can be dried and com­busted to generate electricity, but the drying process is fairly expensive even if solar drying is employed. The combustion and thermal process can destroy the nitrogen fertilizer content of the biomass and generate elevated emissions of NOx. In addition, the combustion process competes with coal and wood biomass that are cheaper than microalgae biomass (Huesemann et al. 2010). Although expen­sive, this can be a key factor for algae to achieve energetic balance and improve its sustainability. A lot of research is being carried in new and more effective drying techniques in order to reduce costs.

Oil. The significant quantities of neutral lipids, primarily as triacylglycerols, can be extracted from the biomass (green algae and diatoms) and converted into biodiesel or green diesel as substitutes for petroleum-derived transportation fuels. “Lipid biosynthesis is typically triggered under conditions when cellular growth is limited, such as by a nutrient deficiency, but metabolic energy supply via pho­tosynthesis is not” (Roessler 1990 in Huesemann et al. 2010). Further information on algae biodiesel is presented in the next chapter.

Wastewater Treatment. The nutrients for the cultivation of microalgae can be obtained from liquid-effluent wastewater (sewer); therefore, besides providing its growth environment, there is the potential possibility of waste effluents treatment (Cantrell et al. 2008). This could be explored by microalgae farms as a source of income in a way that they could provide the treatment of public wastewater and obtain the nutrients the algae need.

In particular, algae has a potential for recycling nutrients recovered from the wastewater (removing N and P), achieving higher level of treatment and gener­ating biomass. Compared to the conventional water treatment, these processes reduce overall greenhouse gas emissions, burning of digester gas derived from anaerobic digestion.

Biomitigation of CO2 emissions. In the majority of microalgae cultivation, carbon dioxide must be fed constantly during daylight hours. Algae biofuel pro­duction can potentially use CO2 in the majority of microalgae cultivation as car­bon dioxide must be fed constantly during daylight hours. Algae facilities can potentially use some of the carbon dioxide that is released in power plants by burning fossil fuels. This CO2 is often available at little or no cost (Chisti 2007). Thus, the fixation of the waste CO2 of other sorts of business could represent another source of income to the algae industry. This sort of fixation is already being made in some large algae companies in a trial basis though there is a lack of public data of the results yet. Although this is a very promising future possibility, and some species have proven capable of using the flue gas as nutrients, there are few species that survive at high concentrations of NOx and SOx present in these gases (Brown 1996). Public policies could also perform a great boost in this area depending on future CO2 cap and trade emissions or sustainability standards as shown in Chap. “Governance of Biodiesel Production Chain: An Analysis of Palm Oil Social Arrangements”.