A site's water footprint has three volumes: blue water (water abstracted and not returned to the same catchment), green water (rainwater evaporated from the soil) and grey water (the volume needed to dilute the pollution in your effluent to the receiving water's standard, calculated as load / (cmax − cnat)). ISO 14046 adds the local dimension: the water scarcity footprint is consumed blue water multiplied by the AWARE factor for the location. The arithmetic is easy. Results go wrong because of where the data comes from and a few rules that get skipped.
This article follows Hoekstra et al. (2011), The Water Footprint Assessment Manual (the WFN Manual), with page references, the scope of ISO 14046:2014, and WULCA's description of AWARE. The worked example is a textile dyehouse; every input is an assumption and is labelled as such.
Blue, green and grey water at site level
Blue water is consumption, not abstraction. A plant that pumps 100 units from its well and returns 85 units, treated, to a stream in the same catchment has a blue water footprint of 15. The Manual gives two equivalent routes: abstraction minus return flow to the same catchment in the same period, or the sum of evaporation, water incorporated into the product and lost return flow. Water released into a different catchment counts as consumed.
Green water is a small item for industry: evapotranspiration of rainwater from green areas on site, or from agricultural raw materials (Manual pp. 29-30). Roof-harvested rain is not green water. According to p. 26, it would otherwise have become runoff, so when it is used it belongs in the blue water footprint.
Grey water is a dilution volume, not a loss. The water physically stays in the catchment, but that much clean water is considered "occupied" to keep the receiving water within its quality standard.
| Component | Meaning on site | Typical data source |
|---|---|---|
| Blue | Abstraction − return to same catchment; or evaporation + incorporated + lost return flow | Well and mains meters, effluent flowmeter |
| Green | Rain-fed evaporation from vegetated areas | Area (m²), meteorological rainfall and evaporation data |
| Grey | Pollutant load / (cmax − cnat) | Effluent lab reports, receiving water quality class |
The grey water formula and two rules people skip
The basic formula:
Grey WF (m³) = L / (cmax − cnat)
L is the pollutant load per period, cmax the maximum acceptable concentration in the receiving water body and cnat its natural concentration without human influence. Since mg/L equals g/m³, a load in grams divided by a concentration difference in g/m³ gives cubic metres directly.
The load is the mass in the effluent minus the mass the abstracted water already carried (Manual, Equation 4, pp. 34-35):
L = Effluent volume × c_effl − Abstracted volume × c_act
The concentration of the intake water is multiplied by the abstracted volume, not the effluent volume. The difference looks minor until a plant evaporates part of what it takes in. If the water comes from another catchment, the abstraction term is zero (p. 35). A negative load is set to zero.
The second rule is that pollutants are not added (p. 39). At each discharge point, sum the load of each pollutant over the period, then take the largest grey water volume among the pollutants. The water that dilutes COD also dilutes nitrogen. Separate discharge points, on the other hand, are added together. Summing pollutants inflates the result by 54% in the example below.
Example: a textile dyehouse (all inputs assumed)
The plant abstracts 140,000 m³ a year from its own well and discharges 120,000 m³ of treated effluent to a stream in the same catchment. Real cmax values must come from the regulation applying to the receiving water's quality class; the figures below are for illustration only.
| Parameter | COD | Total nitrogen |
|---|---|---|
| Effluent concentration (assumed) | 110 mg/L | 12 mg/L |
| Intake water concentration (assumed) | 8 mg/L | 1 mg/L |
| Effluent load | 120,000 × 110 = 13,200,000 g | 120,000 × 12 = 1,440,000 g |
| Intake load | 140,000 × 8 = 1,120,000 g | 140,000 × 1 = 140,000 g |
| Net load L | 12,080,000 g | 1,300,000 g |
| cmax − cnat (assumed) | 25 − 5 = 20 mg/L | 5 − 1 = 4 mg/L |
| Grey water | 604,000 m³ | 325,000 m³ |
Grey water footprint = max(604,000; 325,000) = 604,000 m³. Adding the two would have given 929,000 m³.
Blue water: 140,000 − 120,000 = 20,000 m³ (evaporation in dyeing and drying, moisture in product, water leaving with sludge). Green water: with 4,000 m² of landscaping and an assumed 300 mm of rain-fed evaporation, 1,200 m³.
Grey water comes out at 30 times blue water. That is not unusual for a dyehouse, but adding the two and saying "the plant consumes 625,000 m³" would be wrong. Grey water is not consumption.
Water scarcity footprint and AWARE
ISO 14046 links volumes to local conditions through characterisation factors. The usual choice is AWARE (Available WAter REmaining), developed by WULCA. In WULCA's words, it measures the relative water remaining per area once the demands of humans and aquatic ecosystems are met, normalised to the world average and inverted.
Water scarcity footprint (m³ world eq.) = Consumed water (m³) × AWARE CF
What to keep in mind:
- The factor is capped between 0.1 and 100. A value of 10 means ten times less water remains per area than the world average.
- Factors are computed per sub-watershed on a monthly time step and aggregated to country and annual values. There are separate factors for agricultural, non-agricultural and unknown use.
- A value of 1 is not the factor for average world consumption. WULCA gives consumption-weighted world averages of 20 for non-agricultural use, 46 for agricultural use and 43 for unknown use.
- The factor applies only to consumed water, i.e. the blue footprint. Grey and green water do not enter this calculation.
If the plant's sub-watershed has an assumed non-agricultural AWARE factor of 40: 20,000 × 40 = 800,000 m³ world equivalent. Where consumption peaks in summer and the monthly factors for the basin vary a lot, multiplying monthly consumption by monthly factors gives a more defensible result than one annual factor.
WULCA notes that AWARE is recommended by the UNEP-SETAC Life Cycle Initiative, the European Commission's PEF/OEF programme and the International EPD System.
WFN Manual vs ISO 14046
The two share vocabulary, which is why they get mixed up. ISO 14046:2014 specifies principles, requirements and guidelines for water footprint assessment of products, processes and organisations based on life cycle assessment. Its result is a single value or a profile of impact indicator results. Of air and soil emissions, only those affecting water quality are included. Communicating results as labels or declarations is outside its scope.
| WFN Manual (2011) | ISO 14046:2014 | |
|---|---|---|
| Approach | Volumetric accounting: blue, green, grey m³ | LCA-based impact assessment |
| Output | Volumes per component | Single value or indicator profile |
| Local scarcity | Volume accounting uses no characterisation factor | Indicators such as water scarcity footprint use factors (e.g. AWARE) |
| Pollution | Grey water volume | Impact indicators for emissions affecting water quality |
In practice a good study uses both: build the inventory with the WFN volume rules, then report the scarcity indicator with AWARE in the ISO 14046 report. If a grey water figure goes into an ISO 14046 report, state the method that produced it.
Collecting the data at a site
The formula takes three lines; the time goes into data. An order that works on site:
- Water balance diagram. Every intake (well, mains, surface water, harvested rain) and every discharge point (stream, sewer, industrial-zone treatment plant, sea) on one diagram, with the receiving catchment noted for each outlet.
- Meter data. Monthly abstraction and discharge volumes. If there is no effluent flowmeter, write down how the figure was derived (for example, abstraction minus estimated evaporation).
- Quality data. Effluent concentrations from lab reports, plus the same parameters for the intake water. The intake analysis is often missing, which inflates the load.
- Receiving water. The quality class of the receiving water and the limit value in the applicable regulation for cmax; a referenced source for cnat. For a plant discharging to sewer, pollution reaches the environment after the municipal treatment plant, so state clearly where the load is measured.
- Location. The sub-watershed for the AWARE factor; read it from the WULCA maps and record the dataset version.
- Green areas and rainfall. Area and meteorological data. The effect is small for most industrial sites, so a rough but documented estimate is enough.
Undocumented assumptions cluster around cnat and intake water quality. Both move grey water directly, so expect a verifier to question them.
How Carbolyt handles it
Carbolyt keeps the water footprint as a separate ISO 14046 inventory. Blue water is entered by one of two routes (abstraction minus return to the same catchment, or the sum of components), with rainwater harvesting as its own blue water method. Grey water is calculated per discharge point and pollutant: loads are summed over the period, the determining pollutant is selected, and the other rows are kept and marked "not determining" instead of being deleted. If an intake concentration is entered without an abstraction volume, the calculation stops with an error rather than assuming abstraction equals discharge.
The AWARE factor is entered by the user as a property of the site, and the 0.1-100 range is enforced. If no factor is defined, the water scarcity footprint is not calculated; the software does not assume a value. See the module screens on the platform page, the source texts on the regulation page, or request a demo with your own site data.
Frequently asked questions
How is the grey water footprint calculated?
Divide the pollutant load by the difference between the maximum acceptable concentration in the receiving water and its natural concentration: grey WF = L / (cmax − cnat). The load is effluent volume times effluent concentration minus abstracted volume times the concentration of the abstracted water. At each discharge point, sum the load per pollutant over the period and take the largest resulting volume; pollutants are not added together.
Which water does the AWARE factor apply to?
Consumed water, i.e. the blue water footprint. The grey and green components do not enter the water scarcity footprint. AWARE factors range from 0.1 to 100 and are calculated at sub-watershed level on a monthly time step.
Are the WFN Manual and ISO 14046 the same method?
No. The Water Footprint Assessment Manual is a volumetric accounting method that reports blue, green and grey water in cubic metres. ISO 14046 is based on life cycle assessment and reports a single value or a profile of impact indicators; indicators such as the water scarcity footprint are calculated with characterisation factors.
Is harvested rainwater green or blue water?
Under the WFN Manual, rain collected from roofs or paved surfaces would otherwise have become runoff, so its consumption counts as blue water footprint. Green water is only rainwater held in the soil and evaporated by vegetation.
Sources
This article is for information and is not legal advice. Dates and rates reflect the official texts at the time of publication; check the current text before acting.