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Figure 3.2 Water use in ethanol production from corn, and biodiesel production from soybean.

      3.2.4. Existing and Emerging Methods for Water Conservation

      Feedstock production consumes the largest amount of freshwater in bio‐based systems, and hence efforts should be concentrated on minimizing water use in agricultural production systems. Water‐efficient and targeted irrigation systems are possible solutions to minimizing freshwater use in feedstock production (Levidow et al., 2014). Recent advancements in irrigation technologies provide various water saving irrigation options to the farmers over the conventional irrigation methods. Water application efficiency of a conventional furrow irrigation system is around 35–60%, while sprinkle and drip irrigation systems have higher application efficiencies of 60–70% and 80–98%, respectively (Evans, 2019). These techniques minimize water losses through evaporation, surface run‐off and percolation; and hence, they maximize the water application efficiency. In addition, irrigation management practices should be directed to maximization of water use efficiency through proper irrigation timing, supply of appropriate amount of water, and minimization of water losses through evaporation and percolation. For example, corn requires the largest amount of water during the tasseling, silking, and dent forming stages compared to the other stages (Kranz et al., 2008). Therefore, it is important to recognize the required quantity of water during different stages of the crop growth, to maximize the water use efficiency.

      Natural solutions for water conservation are also effective; they include wetland creation and rainwater harvesting in the wetlands, which supply water for irrigation during the dry season. Genetic modification of crops toward development of more drought tolerant varieties also helps water conservation. Application of compost and mulch, planting cover crops, and using conservation tillage practices are other pathways to reduce the water use in agriculture production phase. They improve the water retention quality of the soil and reduce the evaporation during the plant growth (Texas Water Development Board, 2019).

      Water use efficiency can be improved in bio‐based industries by using water recycling and reuse systems. For example, dilution of the feedstock, cooling, and steam generation were found to be the most water intensive processes in a distillery in India (Saha et al., 2005). Water savings were achieved by using treated wastewater instead of freshwater to replace the cooling water evaporation losses (Saha et al., 2005). Total water use for corn ethanol production in the US is 45 billion gallons per year. Water use in ethanol production industries in the US is minimized by recycling the cooling water rather than discharging it, using reclaimed water from the wastewater treatment plants, and using reverse osmosis to reuse the wastewater from the ethanol plants (Jessen, 2012).

3.3. ENERGY IN BIO‐BASED SYSTEMS

      3.3.1. Energy Use and Supply

      Bio‐based systems are both energy users and energy suppliers in the form of bioenergy. Energy supplied from bio‐based systems includes the calorific value of agricultural products, which can be converted to biogas, liquid biofuels, or solid energy materials, and finally to electricity. The life cycle of energy in bio‐based systems can be divided into three phases: background phase, agricultural crop production phase, and industrial phase. The background phase includes extraction and transportation of raw materials, production of agricultural supplies, such as fuel, machinery, fertilizers, pesticides, seed, as well as transportation of supplies to the local markets and farms. The agricultural crop production phase includes all the field operations, such as tillage, planting, fertilizer and pesticides application, harvest and post‐harvest logistics. Inputs for the agricultural crop production phase include seeds, chemical fertilizers, pesticides (including herbicides, fungicides and insecticides), fuel, lubricants, electricity, manure, water for irrigation, labor, and machinery. Farm operations require farm machines and equipment, such as tractors, cultivators, fertilizer spreaders, pesticide sprayers, harvesters, irrigation systems, and short‐distance transportation equipment. The industrial phase comprises processes in which agricultural products are converted to biofuels, which include transportation, storage, drying, grinding, cooling, heating, maintaining reaction conditions, and product separation.

      Most of the energy used in bio‐based systems is derived from fossil fuels. Although part of the modern‐day electricity is produced from renewable sources, such as water, wind and solar, more than 73% of the world’s electricity is still produced from non‐renewable sources (REN21, 2018). The shares of renewable energy sources in heating and transportation sectors are at 27% and 3%, respectively. Since some of the main goals of bio‐based systems are to substitute petroleum‐based fuels and chemicals and to reduce the environmental impacts, it is important to evaluate the sustainability of a bio‐based system by considering the energy use and supply in the system.

      3.3.2. Energy Analysis

Energy analysis of bio‐based systems refers to the quantification of the balance between energy supply and energy use in three phases of energy cycle, as described in section 3.3.1. Energy analysis in agricultural production has become important because of the increase in the amount of inputs. For example, an increase in nitrogen fertilizer used in corn production has driven an increase in corn yield (Kraatz et al., 2009), but it has given rise to other issues such as eutrophication in nearby water bodies, increasing dependency of soil fertility on fertilizers, and negative impacts on human health. Similarly, energy analysis in a bio‐based industry can help quantify energy use in each processing step, so that the processes can be optimized to improve the energy efficiency. In energy analysis of bio‐based systems, all materials involved in the processing path are back‐tracked to their initial fossil energy input.

      Energy inputs and outputs in agricultural fields, fuel, equipment, and consumables are quantified based on a functional unit within the defined system boundary. The functional unit for energy analysis in bio‐based systems can be expressed in terms of mass (e.g. J/ton), area (e.g. J/hectare), or economic value of the products (e.g. J/$1000) (Mousavi‐avval et al., 2018). This allows quantification of the energy used for production, harvest and post‐harvest logistics.

      Direct energy use during the agricultural crop production phase includes energy content of diesel fuel used in agricultural operations, from field preparation to harvest, as well as electrical energy used in irrigation systems. Energy used in background processes is accounted for as indirect energy. Indirect energy associated with physical inputs, e.g. fertilizers, pesticides, seeds, machineries, is defined as the sum of energy consumed during the production of the input and the energy used for transportation of the input from the plant to the end user or local market. For example, indirect energy associated with agricultural machineries is estimated based on energy used during the manufacture of machineries in the factory as well as energy use for transportation of agricultural machinery from the factory to the local market. To estimate indirect energy associated with the depreciation of an agricultural machinery during specific field operation, it is assumed that total energy consumption during the manufacturing and transportation of the machine is depreciated during its economic lifetime. Lack of data availability makes the energy use estimation in the background processes more complicated. However, there are databases, e.g. Greenhouse, Regulated Emissions, and Energy Use in Transportation (GREET) model and Ecoinvent database, that provide an estimation of energy use for these background processes.

      In the industrial phase, liquid fuel (mainly diesel) is used for long‐distance

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