lower compared to diesels. It is due to difference in production route, feedstocks, technologies used, and methodologies. Therefore, lignocellulosic biomass from agricultural and forest residue has lower GDP compared to biofuel obtained from energy plants. It is due to N2O emitted during energy crop cultivation and with use of fertilizers in energy plants. In lignocellulosic biofuel studies, lignin as residue is used to generate heat and power to meet energy demands [61].
In third generation biofuels, microalgae based biodiesel can reduce or increase greenhouse gas emissions compared to diesel depending on assumptions. The higher emission associated is due to lower algal yield and high use of energy for its production. However, assumptions based on decrease of greenhouse gas emissions may not be feasible for large scale implementation e.g. use of carbon dioxide from cement plants, cane sugar as nutrient and recycling of nutrients from anaerobic digester.
2) Energy use
Energy use in terms of fossil fuel consumption, energy demand and net energy ratio have been studied and a large variation was found out due to differences in feedstock productivity, agricultural practices, conversion technologies and allocation methods. However, various studies reported that algal biofuels are not energetically viable due to high energy requirement for pumping, dewatering, lipid extraction and thermal drying.
3) Water use
In first generation biofuels, water requirement is high for irrigation of feedstocks, increased agricultural biomass production for biofuels and requirement of additional water in water stressed areas.
4) Biodiversity
Biodiversity can be lost through habitat loss and degradation, excessive nutrient load and over-exploitation of land due to biofuel production directly or indirectly. The impact of biofuel on biodiversity mainly depends on feedstock used, its production scale, and management practices. Use of fertilizers, pesticides in cultivation for first generation biofuels can cause loss in biodiversity [62]. Therefore, for plant based biofuels, selection of plants with long growth, low requirement of agrochemicals, and less human interference is required. Biodiversity like bird species, butterflies and flowering plants can be enhanced by large scale cultivation of short rotation coppice. Moreover, degraded land, perennial grasslands can also be used for cultivation which increases biodiversity. Additionally, forest and agricultural residues used as biofuel feedstocks can reduce the negative impact on biodiversity. In algal biofuel, the large scale cultivation of algae could cause risk to coastal biodiversity due to attack of algae on coastal shallow ecosystems like mud flats, salt marshes, sea grass bed and coral reefs. Biodiversity loss can be identified as environmental concern therefore, preserving biodiversity is very important for sustainability of environment.
5) Other environmental impacts
Production and use of biodiesel may result emissions of particulate matter, SO2, CO, NOx, and various volatile organic carbons which results directly or indirectly in respiratory and other cardiovascular diseases [63]. Other environmental impacts depend on assumptions like type of vehicle in which biofuels are used, acidification and eutrophication of land and lakes. These impacts arise due to use of fertilizers and associated emissions of acid gases and nutrients to air and water.
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