**4. Microalgae**

Microalgae are phototrophic microorganisms that generate biomass with simple nutritional and low light energy and CO2 requests. These are photosynthetically highly efficient (~10–20%) in comparison with terrestrial plants (1–2%) to fix CO2. It was reported that more than 100,000 species exist. Advantages of being sustainable at high flue gas concentration and cogeneration of top-value products put these as the preferential and potential organisms (**Figure 1**). Microalgae have the ability to synthesize high amounts of proteins nearly 51–71% of dry matter compared to meat, 43%, soybeans 37%, milk 26% and rice 8%, which are essential for use in human and animal food supplements. Not only proteins, microalgae carbohydrates, ~25% of its dry matter, are made in the forms of simple mono- and polysaccharides, which are easy to digest. Algae are the best candidates for the production of biodiesel as they do not compete with edible crops and can produce up to 80,000 L of oil per acre per year, which is almost 31 times higher than biodiesel produced by the best terrestrial crop, namely palm tree. Moreover, biomass harvested and dewatered from microalgae belongs to the groups of *Spirulina*, *Chlorella*, *Dunaliella*, *Nostoc*

### **Figure 1.**

*Schematic algal biorefinery showing different products that can be obtained from microalgal biomass.*

### *Contribution of Anaerobic Digestion Coupled with Algal System towards Zero Waste DOI: http://dx.doi.org/10.5772/intechopen.91349*

and *Aphanizomenon* and is available in human healthy foods industry in the form of powders, capsules, tablets, pastilles and liquids [46–48].

Some algae can increase their biomass with double growth rate within 3.5 h in their exponential phase. Algae growth yield is three to four times higher in the presence of soluble gases, namely CO2 and H2S [49, 50]. As the biogas passes through algal reactor, methane, which is not soluble, flashes off, whereas CO2 and H2S essentially infuse and completely dissolve in liquid stream. By allowing biogas stream which is typically composed of 60% methane, 39% CO2 and less than 1% H2S passes through coupled algal reactor, will transform to a biogas that is over 90% methane and the CO2 and H2S being reduced by 85–95% by algae biomass [49, 50].

Biomass cultivated in photobioreactors can be utilized for several applications, including substrate for bioenergy such as biogas, biofuels, biofertilizers, biosorbents and biopolymers [50, 51]. For instance, biopolymers recovered from algae can be adapted into packaging materials and have the advantage of being renewable [52].

What really makes the use of algae a thriving technology is that these microorganisms have the potential to efficiently remove nutrients from wastewater and provide high-value biomass energy with low cost. Enclosed bioreactors and open ponds are the two predominant methods for microalgal cultivation (**Figure 2**) [53]. Interestingly, closed photoreactors provide sterility and allow for much greater control over culture parameters such as light intensity, CO2, nutrient levels and temperature, and thus higher biomass productivities can be reached [11]. In parallel to CO2 mitigation, algal biomass has applications in human nutritional supplements such as vitamins, Omega-3 fatty acids, biotin, production of antiaging creams, antiirritant creams, skin regenerate creams, biogas, biofertilizer, aqua and animal feed, and treatment of waste water [46].

Recently algae-based strategies for the removal of toxic minerals such as arsenic (As), bismuth (Bi), bromium (Br), cadmium (Cd), chromium (Cr), mercury (Hg) and lead (Pb) have also been reported individually or in a mixture, and some commercial applications have been initiated [54–56]. Therefore, a sustainable closed loop microalgae-mediated CO2 sequestration system could be integrated with biogas generation infrastructure after optimization of algal cultivation system and key process parameters, and recovery of novel bioproducts from harvested microalgal biomass.

### **Figure 2.**

*Two different methods of microalgal cultivation.*
