Evaluating natural resources use for potable water production

Several assessment methods have been proposed to evaluate the use of natural resources in the life cycle of industrial processes. However, these methods are not adapted to all kinds of resources (especially to renewable resources) and can lead to very different results. This study performed a comparison of methods for the case of potable water treatment, considering both the construction and operation phases of the plant. The results show that the contribution of the construction phase is less important than the operation, although being not negligible. A paradigm shift toward a nature centred viewpoint is suggested a step forward to proper consideration of renewable and non renewable resources and related research challenges are outlined. 1 Methods to assess natural resources use: state of the art 1.1 Life cycle impact assessment (LCIA) approaches Existing Life Cycle Impact Assessment (LCIA) approaches show clear shortfalls in the evaluation of natural resource use, i.e. in quantifying the value of natural resources which support production activities in the Technosphere. To begin with, the main focus of the Area of Protection (AoP) "Resources" is on the depletion of natural resources, mainly fossil and mineral. The AoP is built on the notion of scarcity of resources, therefore clearly adopts an anthropocentric viewpoint. Higher is the abundance of a resource lower is its value up to valueless. Targeted resources are the ones which are intentionally requested by production activities (via market mechanisms) and are scarce. The value of resources is therefore limited to the function they provide to production systems (see e.g. notion of "resource functionality" in [1]). In practice, operational approaches do not strictly follow the AoP definition and are usually grouped in three categories (see [2] for a short presentation of methods and further references): 1) approaches evaluating an intrinsic property of resources, namely the Cumulated Exergy Demand CexD (a.k.a. Industrial Cumulated Exergy Demand); CExD represents the potential work that resources could provide as compared to a reference state (baseline) which is the standard environment. As it is, CExD (and slightly modified methods, like CEENE) represents the exergetic cost of a product, but does not assess scarcity and therefore is not recommended in LCA. 2) Methods based on the "use to availability ratio" (e.g. EDIP) or "use to availability current rate of extraction ratio" (e.g. CML2001), which are fully compliant to the AoP Resources but still subject to large uncertainty, e.g. because of the estimation of reserves. 3) Methods evaluating the (marginal) consequences of resource use, i.e. the marginal additional effort (in energy MJ or monetary $ unit) the mankind will have to provide in the future to extract the same quantity of resources which is currently extracted in the product system investigated (and therefore is getting scarcer). Marginal effort is calculated by identifying the marginal technology and estimating the (cost or energy) difference to extract resources as compared to the current (conventional) technology. Methods like ReCiPe and Stepwise provide such endpoint values for a wide range of resources but are based on very different assumptions, regarding e.g. future technological developments and discounting ratios. Finally, they usually end up in very different evaluation of natural resource use. The uncertainty on such estimation is quite high as well. 1.2 Consideration of water and renewable resources Potentially (locally) scarce resources, like water, are mostly ignored by the aforementioned methods. While plethora of methodological developments are ongoing to assess the effects of water use to the other areas of protection (Ecosystem quality and Human health), few attempts have been (and are being) done to evaluate water within the AoP Resources. The reason is that water depletion is very site specific, i.e. there is no global market for water, as e.g. for fossil resources. Within the method category group 1) CExD consider the same characterization factors (50 MJ/m or 0.05 MJ/kg) for all the water resources, irrespective of the type of source (from river, lake, ground etc). Basically, average water properties are considered and chemical exergy is then calculated. In group 3), [3] calculated endpoint characterization factors of consumptive water use per watershed for the entire world. First, water stress, defined by the ratio (WTA) of total annual freshwater withdrawals to hydrological availability, is calculated per watershed. Afterwards fractions of freshwater consumptive use that contribute to depletion (Fdepletion) are calculated for each watershed and further aggregated using total annual withdrawal within the watershed as a weighting factor to obtain an average Fdepletion per country. Finally, the surplus energy is calculated by considering the actual consumptive water use of the studied system, multiplied by the average Fdepletion of the country of consumption times the surplus required by desalination of seawater (11 MJ/m), considered as a backup technology to compensate for water resource depletion. All the other methods propose a common evaluation unit (e.g. marginal cost or MJ energy) for a wide range of natural resources, in order to compare their relative importance and then to aggregate all the results into a single score which should describe the total magnitude of resource use. For group 1), the problem of substitutability of resources arises [4]. Aggregation implies the assumption that resources are substitutable, which does not hold true for renewable and non renewable CExDs. From the standpoint of including water resources, and more generally locally scarce or renewable resources, this problem becomes even more severe, because of the different originating processes of renewable and non renewable. 1.3 Paradigm shift: nature centred viewpoint Accounting for more and more renewable resources which are significantly supporting human production activities is urgent in today's assessments, and goes clearly beyond the current LCIA framework. The outcomes of the Millennium Assessment [5] showed that ecosystem services (not only ecosystem quality! ) are being degraded by human activities. Ecosystem services are intimately related to natural resources, which could be seen as sort of "outputs" of natural processes which are actively supporting human activities despite they are not explicitly 1 Consumptive use (water consumption) represents freshwater withdrawals which are evaporated, incorporated in products and waste, transferred into different watersheds, or disposed into the sea after usage required (through market mechanisms) by production processes. Therefore the value ecosystem services (via natural resources) provide in supporting human activities shall be accounted for in the environmental assessments. This value has nothing to do with the notion of scarcity or utility for mankind, but is instead related to the efforts put into place by natural processes to make natural resources available at a given level of quality. The value of natural resources, combined with a quality differentiation, has therefore to be quantified from a nature-centred viewpoint. To this aim, alternative approaches based on the concept of Emergy have recently emerged. Emergy is defined as the content of equivalent solar energy (MJse) required by every kind of natural resource (at a given quality state), i.e. the equivalent solar energy that was used by natural processes to make that resource available. Basically emergy quantifies the energy flows behind natural resources and ecosystem services which are typically not considered in conventional life cycle approaches, whose inventory boundaries are defined at the entrance of resources into the Technosphere. When calculating the solar energy equivalence for obtaining a unit exergy, the method informs about the quality of resources. This represents a major change of paradigm with respect to LCA: emergy provides the value of natural resources in terms of the effort spent by natural processes to make them available, which is fully complementary to the LCA perspective based on scarcity and human utility (Fig. 1). Fig. 1. Paradigm shift from human-centred to nature-centred evaluation perspective. The emergy framework presents however known limitations, in terms of transparency of calculations and data availability. Work is in progress to overcome these limitations. A couple of attempts to implement emergy-based approach indicators into life cycle database have already been done [6,7]. In [7] the indicator of Solar Energy Demand (SED) has been developed within Ecoinvent database (v2.2), and is implemented as a single score LCIA indicator. SED represents the total solar energy required to sustain production systems, as it is for emergy. In general, the higher is the SED, the higher the use of resources of a production system. SEFs are the Solar-Energy Factors applied to accumulated Ecoinvent resources to characterize the SED impact (SEFs in MJse/kg, MJse/Nm, MJse/m, MJse/myr, MJse/MJ). The SEF is calculated by allocating the emergy baseline 9.26E18 MJse/yr [8] according to the annual flow of a given resource (e.g. kg/yr), estimated by multiplying the stored quantity by its turnover time [9]. The baseline represents the annual budget of energy that flows in the geobiosphere, i.e. sum of energy in sun, tide, and crustal heat. A SEF can be calculated for each resource (land, water, mineral, energy carrier, and so forth) of the Earth, by assuming the baseline as ‘free’ energy that feeds and sustains each of the resource flows. 2 Objective of this study This study aimed at applying and testing the methods of natural resource evaluation to the specific case of potable water production against two objectives: 1) evaluate the relative contribution of (renewable and non renewable) resources, with special focus on w