**1. Introduction**

Globally, the severe problem humanity is facing today is the availability of fresh water, wastewater generation and energy supply. As the continuous use of fossil fuel are depleting day by day the natural stock of fossil fuels. This natural energy reserve may end in next 45–50 years. This depletion is posing stress to continue the various anthropogenic activities i.e. industry, agriculture, production of precious chemicals for food, and pharmaceutical which directly affects the global economics due to shortage of energy sources. In these circumstances, this is the high time to identify and develop alternative, cost-effective, efficient renewable energy resources for enhancing the sustainability of anthropogenic activities. Algal biomass could be utilized to generate extensively energy support as algae bear high productivity of biomass [1]. The more concentration for diversification of agro-ecosystem from food to fuel is also fulfilled by the algal biofuel as these living being are also not required agricultural land due to their aquatic nature. As per the report of the Central Pollution Control Board of India [2], 71853 MLD [million liters per day] wastewater (considering both sewage and industrial discharge) is discharged into the water bodies of India and out of which only 37% get treated. In these days, pollution of natural resources especially water is also at alarming point and the climate change making it more serious. Thus, minimum contamination/waste of water and reuse of the contaminated/waste/used water is also the highly required. The presently available technologies of wastewater treatment are not only costly but also generate huge amount of sludge. The generated sludge after wastewater treatment essentially need to be treated and disposed, these two requirements further increases the financial effectiveness of the any technology [3, 4]. The algae mediated wastewater treatment is an environmentally sustainable and efficient approach and can be integrated with secondary wastewater treatment process. Algae are the small, mostly aquatic, photosynthetic (converts sunlight into the oil form stored energy) organisms, currently, getting more attention due to their capabilities to address the different environmental issues including energy. Microalgae have been noted for their enormous potential to remediate waste water i.e. Phyco-remediation. Phycoremediation is the utilization of alga culture to remove/biotransformation of pollutants, nutrients, xenobiotic from waste water. Phycoremediation can handle more than one environmental problem such as pH correction, BOD, COD and TDS removal simultaneously over the chemical methods. Phycoremediation consider highly eco-friendly as did not cause secondary pollution. Presently, biodiesel production utilizing microalga is not economically sound due to its cost. Thus, algal biorefinery concept can serve an important option to minimize the microalgal biofuel cost. Algal biorefinery is the analogous concept to present petroleum refinery as petroleum refinery produces multiples products including fuels from petroleum. Algal biorefinery is having potential to increase the values of the products obtained from the biomass feed stocks. Algal biorefinery can be integrated among biomass conversion process, fuels (low value, but high volume), intermediate compounds (low volume, but high value) and value added chemicals along with electricity generation through advanced technologies such as combined heat and power (CHP) technology. Microalgae are having high capacity to convert the solar energy to chemical energy per unit land than terrestrial phototrophs due to their high productive rate. Thus, microalgae can address the increasing energy demands as well as growing environmental issues such as climate change. Beside this, microalgae having some advantages as feedstock for value added product generation. The microalgae are capable of synthesize huge quantity of lipids (20–50% dry cell weight). The growth of algae is very fast compared to terrestrial plants (double the biomass within 20–25 days), so can be used for bioremediation [4]. Algae do not require arable land and fresh water for the growth. Algal biomass can also contribute significantly to reduce the enhanced atmospheric carbon. Keeping this view, the present chapter is focused towards the utilization of algae in wastewater treatment, biofuel, biofertilizer production, CO2 sequestration, bioremediation and challenges with future perspective through algal biorefinery interventions.

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