Net Water and Sewer Cost of a Cooling Tower

The single most common costing mistake on a cooling tower is billing sewer on the makeup. Water and sewer are two different rates applied to the same meter, and on a tower they should not be applied to the same volume: every gallon of makeup is bought at the water rate, but only the gallons that actually reach the drain — makeup less evaporation, which is blowdown plus drift — belong at the sewer rate. A tower buying 1,000,000 gallons a year and evaporating 800,000 of them, at $3.00 water and $4.50 sewer per 1000 gallons, costs 1000 × $3.00 + 200 × $4.50 = $3900. Bill the sewer on all million gallons and the same tower appears to cost $7500. The difference, $3600, is the evaporation credit, and it is nearly half the true bill.

Sewer is usually the dearer of the two rates, which is why the error is so expensive — treatment plants are more costly to run than distribution systems, and many municipalities also add a stormwater or industrial surcharge on discharged volume. Three practical notes. Get the credit in writing before you count it: some utilities require a dedicated evaporation meter or a bleed meter, some accept a calculated figure, and some do not offer a credit at all. Watch the argument this equation makes about cycles of concentration — raising cycles cuts blowdown, which cuts both the makeup you buy and the volume you sewer, so the saving is worth more than the water rate alone suggests. And note what is not in here: the fixed meter charge, which does not move with volume, and any softener or filter backwash that discharges without being part of the tower balance. Both rates must be entered in the same currency, and the answer comes back in that currency — this site has no exchange rates.