Sewer Credit for Evaporated Water

Sewer is billed on the assumption that everything you buy goes down the drain. On a cooling tower that assumption is simply false: 75–85% of the makeup leaves the stack as vapour and never reaches the sewer at all. Most North American municipalities will therefore credit the sewer charge on evaporated water if you can measure or defensibly calculate it — a tower evaporating 3,600,000 gallons a year on a $4.50 per 1000 gallon sewer rate is being over-billed $16,200 annually, and the credit is worth roughly a third of the tower's total water spend. The credit is not automatic; it is claimed, usually by installing a second meter on the makeup line or a meter on the bleed, filing the utility's evaporation-credit form, and then reporting quarterly.

Where the volume comes from matters more than the arithmetic. If the municipality accepts a calculated figure, it is the evaporation rate from the range-and-flow rule multiplied by the operating hours — and a tower that is drained for the winter must be costed on its real run hours, not 8760. Where the utility insists on measurement, the usual arrangement is a bleed meter: evaporation is then makeup minus bleed minus drift, and the credit follows from the difference of two meters rather than a model. Note that the answer is in whatever currency the sewer rate was entered in, since this site carries no exchange rates. And check which way your utility charges before promising the saving: some bill sewer on a fixed percentage of water, some bill it on a separately metered discharge, and a handful in dry regions bill neither but meter consumption against an allocation instead — in which case the water saving is real and the sewer credit does not exist.