**3. Algae mediated wastewater treatment**

Algae are autotrophic organism however; there are some other algae, which are heterotrophy or mixotrophy in nature. The dominant mode of microalgae metabolism in wastewater is heterotrophic (approximately 50%) in nature. The heterotrophic microalgae metabolize organic components in wastes, and convert them into organic biomass along with inorganic components. Microalgae contribute approximately 50% of global primary production (GPP) i.e. most efficient convertor of solar energy to chemical energy and act as producer of aquatic food chain [6]. The quantity and quality of bioactive compounds of microalgae is based on the ambient environmental, ecological factors and taxonomic position. The removal of this generated algal biomass results in the purification of wastewater as their removal decreases the biological oxygen demand especially in case of their death to minimizing the chance of back release of nutrients in the ecosystem.

In 1960s Oswald and Gotta, [7] reported the potential role of microalgae for the removal of pollution load from the tertiary wastewater treatment by algae. Phycoremediation is the removal/biotransformation of pollutants such as nutrients, xenobiotics from wastewater and CO2 from air. Thus, phycoremediation can be used for to extract nutrient from municipal wastewater/effluents which are rich in organic matter; to complete removal/transformation and degradation of xenobiotic compounds utilizing as biosorbent; to treatment of acidic wastewaters; to sequestrate CO2; and to detect toxic compounds using algae-based biosensors. There are various studies which recommends the removal of nitrogen and phosphorous from wastewater to protect the waterbodies from eutrophication [5, 8–12]. The controlled growth of algae in wastewater leads to reduction of contamination load on natural resources and can also be enhance reuse efficiency. The utilization of algae in the treatment of different waste such as agro-based industrial wastes, sewage, industrial wastes (metal finishing, paper, and textile) and even landfill leachate [10, 12–15]. Waste mitigation potential of an algal species entirely depends on the algal productivity, nutrient and pollutant removal efficiency, and cost of biomass harvest [16–18]. In addition to removal of pollutant load from wastewater, algae make available oxygen (O2) to bacteria (heterotrophic aerobic) for mineralization of pollutants and CO2 produces by bacterial catabolism is subsequently consumed by the photosynthetic activity of algae (**Figure 2**). The photosynthetic process of algae is reduces the pollutant volatilization through mechanical aeration and contribute to reduce the cost of operation directly. Thus, the dual purpose utilization of microalgae in biorefinery approaches is

**Figure 2.** *Principle of photosynthetic oxygenation in BOD removal.*


*Algal Biorefinery: A Synergetic Sustainable Solution to Wastewater Treatment and Biofuel… DOI: http://dx.doi.org/10.5772/intechopen.104762*

