**5. Biomass harvesting and dewatering**

Development of biorefinery for production and conversion of algal biomass will be the unified solution to meet the day-by-day increasing energy demand and to reduce risks associated with global warming due to tailpipe emissions. The microalgal-based production chain is classified into three series of steps, namely biomass inoculation and cultivation, harvesting and dewatering, and extraction of concentrated biomass for desired applications. The systems for algal cultivation can be tanks, trays, open ponds, closed or hybrid photobioreactors. It has been suggested that these systems deliver a very dilute biomass concentration ranging from 0.05 to 0.075% dry matter for open pond systems and 0.3–0.4% for closed reactors [57]. Hence, there is an immediate requirement to develop an efficient algal dewatering process to reach the biomass up to 30% in total dry product. Concentrating algal biomass and purifying it into products from broth occur in two stages: a single step of harvesting followed by one- or two-step separate biomass dewatering, which is then fractionated and extracted to extend the "shelf-life of biomass" and to make the product accessible for further application [58, 59].

Recent advance and novel high-tech research in bioprospecting new strains, breakthrough innovations in culture cultivations and complete process optimization are certainly increasing our hope about the forthcoming achievements by microalgal biorefinery. However, the potential of successful commercial deployment is associated with simple and indigenous innovations in downstream operations, specifically cell harvesting, cell disruption and extraction, which can actually cut down the costs at a biorefinery level, along with process integration. During algal biomass cultivation, the harvesting process is the main constraint, representing more than 20% of the total production cost due to low biomass density (0.2–2 g L<sup>−</sup><sup>1</sup> ) and small size (10–30 μm). The methods for harvesting either used independently or in combination. However, most of these methods still involve economic or technological drawbacks, such as a high-energy cost (centrifugation), algal biomass contamination (chemical flocculation), or nonfeasibility of scalingup. Besides the operational cost, concerning selection of the adequate harvesting method, several aspects such as the following should be considered: (i) harvesting speed, (ii) harvesting efficiency, and (iii) density and quality of biomass in the resultant concentrate [60, 61]. Among different polymers, the chitosan prepared from the waste of white shrimp is reported as a good cost-effective and efficient flocculant for algal biomass because of its properties such as faster deposition rate [57]. However, the optimal pH, chitosan dosage, chitosan physiochemical characteristics and flocculation time to achieve ~100% of algal biomass harvesting efficiency and optimization of storage condition for harvested biomass should be investigated more clearly for further applications.

Because of high protein content and biomass yield, microalgal biomass offers a potential alternative for bioplastic and biofertilizer production, either directly or in secondary metabolites form. Dewatered algal biomass can be modified into bioplastic and biofertilizer. Bio-based plastics help to "decarbonise" the economy. However, unlike soy protein isolate or feather meal protein, it is not economical or technically feasible to extract the protein from the algal biomass [11]. Consequently, more research must be developed aiming to optimize the extraction of secondary metabolites to create a sustainable and biodegradable alternative to fossil fuel–based plastics. After secondary metabolite extraction, the residual algal biomass can be reused as feed for biogas production via anaerobic digestion and biofertilizer. Several researchers have developed an indigenous assembly of macroalgae, which was installed and grown in CO2- infused wastewater effluent (**Figure 2**) [62–64]. Later, algal biomass was harvested and co-digested with sewage sludge to enhance

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

bio-methane production. Several techno-economic constraints have to be solved for the generation of biomethane from algal biomass is economically feasible [63, 65]. For instance, potential issues to be focussed further to enhance biomass conversion to biomethane are high sensitivity of methanogenic microorganisms, unbalanced C/N ratio of algal biomass, and high lipid contents, and cost associated with biomass recovery [65].

In addition to different solutions to huge environmental problems like deficiency of nitrogen content in the soil composting causing pollution must work in parallel with other action. Algae can serve the purpose by fixing atmospheric nitrogen and synthesizing plant growth promoters as nitrogen content of the soil is the second major factor affecting plant growth after water [66, 67]. Biofertilizers made from algae will be an effective replacement for chemical fertilizers by means of circular bioeconomy. Thus, the application of microalgal biorefinery concept to produce renewable energy will enhance the economics of bioenergy production by means of circular bioeconomy.
