Though the second-generation biofuels overcome the disadvantages of using edible food crops as feedstock, the cultivation of non-edible crops—which serves as the feedstock for the second-generation biofuels—still require land and other resources which could otherwise be used for cultivation of food crops. Hence, the

Processes common to both current and future waste biorefinery

third- generation of biofuels offers an excellent alternative to the first — and second — generation biofuels in that they do not use arable land for their generation but use algae and seaweeds, which can be cultivated on completely nonproductive land and use significantly less water than terrestrial crops. Sea weeds/algae thrive on sea water using merely sunlight and some simple nutrients present in the sea water. About 75% of the earth’s surface is covered by water, and seawater comprises about 97% of total water present on the earth. Hence, there is an immense potential for cultivation of algae/seaweeds. Algae as feedstock for production of biofuels include all unicellular and simple multicellular organisms such as prokaryotic microalgae (e. g. cyanobacteria), eukaryotic microalgae (e. g., green and red algae), and diatoms. Millions of years of evolution have enabled algae to develop an efficient system which is capable of capturing unlimited amounts of solar energy continuously via photosynthesis and converting simple inorganic molecules to complex organic compounds such as carbohydrates, fats, and proteins. The pho­tosynthetic efficiencies of algae are much higher than most terrestrial plants, hence algae can absorb higher amounts of CO2 from atmosphere and as a result, provide higher amounts of these complex molecules, which can be converted to biofuels (bioethanol) and other molecules. Figure 1.28 shows the variety of compounds that can be obtained from algae.

Singh et al. [51] have comprehensively reviewed all the aspects of using algae as a potential feedstock for the generation of third — generation biofuels. Other than the advantages of higher photosynthetic efficiencies and nonrequirement of arable land, use of algae as feedstock for biorefineries offer many other advantages

compared to plant biomass feedstock. Algae have an almost exponential growth potential-doubling of biomass in as short a time as 3.5 h can be possible. In addition, more than five harvests can be obtained in a year [52]. Another major advantage of algae is that they thrive on nutrients such as nitrogen and phos­phorous, which can be obtained from wastewater, and on organic effluent from agro-food industry, thus serving a dual advantage of utilization of waste water and enhanced cultivation of algal feedstock. Algae also do not require fertilizers, herbicides, and pesticides like their plant counterparts for their sustained cultivation.

Microalgae are rich in carbohydrates. These can be fermented to produce bioethanol. Chung Sheng et al. [52] have reviewed the potential of such a biore­finery in playing the role of a sustainable energy provider for efficient production of bioethanol. A number of flow charts have been proposed for efficient production of biofuel from algae.

The common steps in all processes involve collection of algae, extraction, purification, and separation of polysaccharides, hydrolysis, fermentation, and final purification (Fig. 1.29).

In order to successfully compete with a petroleum refinery, the efficiency of biofuel production of an algal biorefinery can be enhanced significantly by developing good strains of algae with increased carbohydrate content which will give a high yield on fermentation. Simultaneous production of biogas using methane fermentation technique is also possible. The residual biomass can be reprocessed to make fertilizers.

Algae also contain a lipid fraction which can be used for production of bio­diesel. Microalgal lipids are neutral lipids having a lower degree of unsaturation (similar to fossil fuels). By integrating processes such as transesterification, cracking, etc., into an algal biorefinery, a range of products other than bioethanol, which is normally obtained by hydrolysis and fermentation of carbohydrates, can be obtained [51]. Such an integrated algal biorefinery is shown in Fig. 1.28.

In addition to the biofuels viz. bioethanol, biodiesel, and biogas, other products such as food supplements, pigments, etc. can also be obtained from an algal biorefinery. There are a number of algae-based biorefineries in different regions of the world, producing the products stated above. Mussgnug et al. [53] have investigated six species of freshwater and saltwater algae and cyanobacteria for their suitability as substrates for production of biogas. They showed that the methane content of biogas from microalgae was 7-13% higher compared to that obtained out of maize fermentation. They also reported that drying as pretreatment step decreased the amount of biogas production to approximately 80%. Chla — mydomonas reinhardtii has the ability to produce hydrogen via hydrolysis of water during illumination. The hydrogen production cycle induces an increase in the amount of starch and lipids within the cells which increase the fermentative potential of the algal biomass. Thus, a two-step biorefinery process where hydrogen is produced in the first step by sulfur deprivation method, and subse­quently, the remaining biomass, after production of hydrogen is used as substrate for anaerobic fermentation was found to increase the biogas production to 123%, compared to the use of fresh algal biomass. This synergistic effect gives a dual advantage of providing an environment friendly gaseous fuel hydrogen, and an increased amount of biogas.