We define a water credit as one cubic meter of water treated. In terms of how much impact we also include carbon reductions from reduced transport. 
For example, with septic waste one water credit would be a cubic meter of water treated, but that would also include a carbon reduction based off of:
1/19 (cubic meters per truck)  
120 miles traveled / 5mpg
CO2 emissions per truck

0.052 * 243kg co2 emissions = 12.64 KG CO2 reduction per water credit.