What does the grey water footprint in your report tell you?
We explain the idea of dilution volume step by step using a paint plant example.
Grey water is the line item that raises the most questions in water footprint reports. Blue water shows the water you actually withdraw; grey water is the theoretical volume of clean water needed to bring the pollution you discharge down to a level the receiving water body can accept.
How is it calculated?
The core formula is simple: the pollutant load is divided by the difference between the maximum allowed concentration and the natural concentration of the receiving water. The same load discharged into a more sensitive water body therefore creates a larger grey footprint. Each pollutant is calculated separately and the most critical one is used.
A paint plant example
Imagine a plant discharging 300 m³ of wastewater a day. Even if COD at the treatment outlet is just below the limit, the high load can make the grey footprint several times larger than the blue one. Here, savings come not only from using less water but from cutting pollutant load at the source.
What to look for in the report
Check which pollutant is decisive, where the limit values come from and whether the calculation uses measured or estimated data. A grey water figure based on measured data also lets you show the effect of improvement investments with real numbers.
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