1 Introduction
Water degradation: negative change in water quality.Water availability: extent to which humans and ecosystems have sufficient water resources for their needs. If water availability only considers water quantity, it is called water scarcity.Water scarcity: extent to which demand for water compares to the replenishment of water in an area, e.g. a drainage basin, without taking into account the water quality (ISO 2014).
2 Methods to address water quality in water footprinting
2.1 Methods based on the WAF
2.1.1 Grey water footprint (Hoekstra et al. 2011)
2.1.2 Pollution-induced water scarcity (Zeng et al. 2013)
2.1.3 Water Impact Index (Bayart et al. 2014)
2.1.4 The method of Boulay et al. (2011a, b)
2.2 Methods based on the WDF or combined approaches
2.2.1 Impact assessment models for water quality degradation
2.2.2 The method of Ridoutt and Pfister (2013)
2.2.3 Pollution Water Indicator (Lovarelli et al. 2018)
3 Case study
Scenario 1 | Scenario 2 | NEQS | |
---|---|---|---|
Water use data | |||
Water withdrawal [l/kg] | 128 | 128 | - |
Water discharge [l/kg] | 101 | 101 | - |
Effluent data | |||
Chemical oxygen demand (COD) [mg/l] | 738 | 150 | 150 |
Biological oxygen demand (BOD5) [mg/l] | 297 | 30 | 80 |
Total suspended solids (TSS) [mg/l] | 152 | 50 | 200 |
Total dissolved solids [mg/l] | 4980 | 3500* | 3500 |
Total nitrogen (Total N) [mg/l] | 30 | 20 | no threshold |
Oil and grease [mg/l] | 14 | 10 | 10 |
Total chromium [mg/l] | 0.06 | 0.20 | 1 |
Copper [mg/l] | 0.19 | 1 | 1 |
4 Results
4.1 Methods overview
Type of WF | Impact assessment approach | Considered pollutants | Thresholds provided? | Quality of withdrawn water considered? | Region-specific impact assessment? | Covered AoPs at the endpoint level | Stand-alone indicator | Comparison to the WSF possible? | |
---|---|---|---|---|---|---|---|---|---|
GWF (Hoekstra et al. 2011) | WAF | DtT | One, most penalizing | No | Yes | No | No | Yes | Yes (Blue Water Footprint) |
Pollution induced water scarcity (Zheng et al. 2013) | WAF | DtT | One, most penalizing | No | No | Yes, based on water scarcity | No | Yes | Yes (WSF is calculated as WAF related to only water consumption) |
WII (Bayart et al. 2014) | WAF | DtT | One, most penalizing | No | Yes | Yes, based on water scarcity | No | Yes | Yes (WSF is calculated as WAF related to only water consumption) |
WAF | Functionality | All for which thresholds are available and in concentrations higher than the thresholds | Yes, for 136 water quality parameters | Yes | Yes, based on water scarcity | Human health | Yes | Yes (WSF is calculated as WAF related to only water consumption) | |
Impact assessment models for water quality degradation | WDF | Environmental mechanism | All which contribute to selected impact categories | Not applicable | No | Yes, based on the cause-effect chains | Human health, Natural environment, Resources | No (can be applied as a stand-alone indicator at the mid- or endpoint level, if only one impact category (e.g. eutrophication or human health damage) is evaluated) | No (midpoint) Yes (endpoint) |
Method of Ridoutt and Pfister (2013) | Combines WDF and WSF | Environmental mechanism | All which contribute to selected impact categories | Not applicable | No | Yes, based on the cause-effect chains | Human health, Natural environment | Yes | Yes |
PWI (Lovarelli et al. 2018) | Combines WDF and WAF | Environmental mechanism and DtT | All which contribute to selected impact categories and one most penalizing for the GWF calculation | No | No | Yes, based on the cause-effect chains | No | Yes | No |
4.2 Case study results
Method | Unit | Scenario 1 | Scenario 2 | WSF | ||||
---|---|---|---|---|---|---|---|---|
Pollutants [inventory] | Pollutants [considered] | WDF/WAF | Pollutants [inventory] | Pollutants [considered] | WDF/WAF | |||
GWF (Hoekstra et al. 2011) | m3 | COD BOD5 TSS TDS Oil and grease Total N Cr Cu | COD | 499 | COD BOD5 TSS TDS Oil and grease Total N Cr Cu | none | 0 | 21 |
Pollution induced water scarcity (Zheng et al. 2013) | dimensionless | COD | 2.09E-09 | none | 0 | 0.5E-09 | ||
WII (Bayart et al. 2014) | m3 impact index eq. | COD | 103.8 | none | 25.7 | 25.7 | ||
m3 eq. of water (midpoint) | BOD, TSS, oil and grease, Total N, Cr, Cu | 128 | BOD, TSS, oil and grease, Total N, Cr, Cu | 128 | 21 | |||
DALYs (endpoint) | 9.64E-03 | 9.64E-03 | 1.58E-03 | |||||
Impact assessment models for water quality degradation | Midpoint: FETP (kg 1.4-dB-eq.) | Cr, Cu | 3.31 | Cr, Cu | 17.10 | - | ||
Midpoint: HTP (kg 1.4-dB-eq.) | Cr, Cu | 0.228 | Cr, Cu | 0.860 | - | |||
Midpoint: ME (kg N-eq.) | Total N | 0.912 | Total N | 0.609 | - | |||
Endpoint: Human toxicity (DALY) | Cr, Cu | 5.20E-08 | Cr, Cu | 1.96E-07 | - | |||
Endpoint: Natural environment (species.yr) | Cr, Cu, Total N | 3.84E-09 | Cr, Cu, Total N | 1.29E-08 | - | |||
Method of Ridoutt and Pfister (2013) | m3 H2O-eq. | Cr, Cu | 2.72 | Cr, Cu | 14.62 | 34 | ||
PWI (Lovarelli et al. 2018) | dimensionless | COD, Cr, Cu, Total N | 0.303 | COD, Cr, Cu, Total N | 0.126 | - |
5 Discussion and recommendations
5.1 GWF
5.2 Pollution-induced water scarcity
5.3 WII
5.4 The method of Boulay et al. (2011a, b)
5.5 Impact assessment models for water quality degradation
5.6 The method of Ridoutt and Pfister (2013)
5.7 PWI
5.8 Applicability and limitations
Method | Type of WF | Inventory requirements | Completeness of scope (pollutant coverage) | Regionalization | Main methodological aspects influencing the result |
---|---|---|---|---|---|
GWF (Hoekstra et al. 2011) | WAF | Emissions of pollutants to water | Low | Not regionalized | Selected threshold |
Pollution induced water scarcity (Zeng et al. 2013) | WAF | Emissions of pollutants to water | Low | Yes, based on water scarcity | Selected threshold Water resources in the study area |
WII (Bayart et al. 2014) | WAF | Emissions of pollutants to water and water flows | Low | Yes, based on water scarcity | Selected threshold Water scarcity factor |
WAF | Emissions of pollutants to water and water flows | High | Yes, based on water scarcity | Pollutants included in the inventory Water scarcity factor | |
Impact assessment models for water quality degradation | WDF | Emissions of pollutants to water | High | Yes, based on cause-effect chains | Pollutants included in the inventory Selected impact categories |
Method of Ridoutt and Pfister (2013) | Combines WDF and WSF | Emissions of pollutants to water and water flows | High | Yes, based on cause-effect chains | Pollutants included in the inventory |
PWI (Lovarelli et al. 2018) | Combines WDF and WAF | Emissions of pollutants to water | High | Yes, based on cause-effect chains | Pollutants included in the inventory Selected impact categories Results’ plotting on the graph |
5.9 Recommendations
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Most probable impact pathway is the intake of or contact to the emitted contaminants; a comprehensive inventory is available (i.e. the inventory data includes the emissions of all contaminants relevant for the study); the inventory can be classified to an impact category (e.g. acidification, eutrophication etc.). We recommend to quantify WDF by means of the impact assessment models for water quality degradation. This allows to determine the full range of the impacts associated with water pollution and to identify potential trade-offs between different impact categories (e.g. eutrophication vs. human toxicity as demonstrated in the case study)
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Most probable impact pathway is water deprivation due to water pollution; a comprehensive inventory is available. We recommend to quantify WAF by means of the method of Boulay et al. (2011a, b), which allows to consider all pollutants included in the inventory (in contrast to other WAF methods, which are based on the DtT approach and therefore consider only one most penalizing pollutant)
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Most probable impact pathway is water deprivation due to water pollution; only one or few water quality parameters are available in the inventory. We recommend to quantify WAF by means of the WII. It should be taken into account that the WF result might be significantly underestimated, since only one pollutant is considered. We do not recommend using the GWF, since the method does not allow to conduct a regionalized impact assessment. We also do not recommend using the pollution-induced water scarcity method, since as demonstrated in the case study, it does not fit well for the impact assessment on a product level, which is typical for WF studies.