Water, Climate Change, and Sustainability. Группа авторов

and conversion to ethanol around 5 to 2138 L of water is required per L of ethanol (Mckenna, 2009); the main reason for this high variation is regional irrigation requirements. In addition to water use for crop production and conversion to biofuels, a large amount of water is needed to remove excess heat in the power plants. For example, in thermoelectric power plants, generation of 1 kWh electricity needs the evaporation of about 7.6 L of water to remove the excess heat. Thus, water is likely to become the most limiting resource in the development of bioenergy systems from bio‐based feedstocks.

Figure 3.5 Energy and water use and supply in bio‐based systems.

3.5. ASSESSING SUSTAINABILITY OF WATER AND ENERGY IN BIO‐BASED SYSTEMS

      Objectively assessing the sustainability of bio‐based systems requires reliable metrics that accurately depict the particularities and complexities of these systems. Reliable assessment tools allow a fair analysis of the bio‐based processes and the comparison with other bioprocesses and non‐bioprocesses, and provide information for strategic decision‐making. Bio‐based systems are expected to be environment‐friendly. However, accurate assessments are necessary to avoid resource overexploitation or unexpected water/energy overuse due to land use change (Martin et al., 2018).

To allow comparison between different systems, standard tools and indicators are required. However, this standardization is yet to be achieved for the bio‐based systems (Bosch et al., 2015). Thus, there is a variety of tools and indicators that can be used for assessing the sustainability of bio‐based systems, including metrics designed originally for other systems, such as chemical processes. Most of the sustainability assessment tools include indicators related to energy use, but very few assess water use directly (Broeren et al., 2017). Even more infrequent are the metrics that evaluate the water–energy nexus. Indicators used to assess the sustainable use of water and energy in bio‐based systems could be classified into the sustainability triple bottom line categories from environmental, economic, and social perspectives. Combining the indicators from all three categories is important to achieve a fair assessment of the sustainability of bio‐based systems; however, most of the tools fail to include all of them (Martin et al., 2018). Most of the metrics used for bio‐based systems focus on efficient resource utilization.

Table 3.4 Selected indicators for water and energy use.^*

Indicators	Unit	Definition
Water use
Total water use	m^3	Total water used by a process
Water productivity (WP)	kg/m^3 or kg/kg
Water intensity (WI)	m^3/$
Fractional water consumption (FWC)	m^3/kg
Specific liquid waste	m^3/kg
Energy use
Total energy use	kJ/h	Total energy consumed by a process
Energy efficiency (ηE)	kJ/kJ
Energy intensity (REI)	kJ/$
Specific energy intensity (RSEI)	kJ/kg
Renewable energy index (RIE)	kJ/kJ
Water–energy nexus
Water and energy productivity	kg/m^3∙kJ
Water energy ratio	kJ/kJ

      ^* Adapted from Ruiz‐Mercado et al., 2012 and Khan et al., 2009.

      3.5.1. Assessment of Water Use in Bio‐Based Systems

As discussed earlier, bio‐based systems consume significant amount of water. Most metrics for assessing water use are based on quantity of water applied; however, sustainable evaluation of water use needs assessment of water quality as well. Water footprint is a methodology for the quantification of water use in a system and its related environmental impacts, usually from a life cycle perspective. Thus, it includes direct and indirect water use (Verones et al., 2015). Water use can be accounted in volume or cost in sustainability assessment. Some water and energy use indicators are presented in Table 3.4.

      Several of the impact categories commonly used in life cycle assessment (LCA) affect water resources. Water quality assessment can be accounted by freshwater eutrophication and acidification, as well as eco‐toxicity and human health measures (Verones et al., 2015). Most analyses fail to account for water scarcity and local water use restrictions due to large quantities of water used in bio‐based systems, which can become a bottleneck for sustainable development of bio‐based systems in some freshwater basins (Martin et al., 2018).

      3.5.2. Analysis of Energy Use in Bio‐Based Systems

Energy use in bio‐based systems could be assessed in terms of material consumption or waste generation (Sheldon, 2018). Energy balance allows quantification of the calorific energy used and produced within the defined boundaries of the bio‐based systems. To account for energy quality, exergy