In Mesa, AZ sewer rates are based on water usage in the first two or three months of each year. If one were to drain/refill their pool during that time, contacting the city and having them review your water bill will usually get you an adjustment on the sewer rate - not the quantity of water used.
In Chandler, AZ sewer rates are a flat fee, regardless of water usage in any specific month(s). Last time I drained/refilled my 12k pool, the additional water cost was less than $50.
$500 for 16k gallons, especially in Seattle where water is more plentiful, seems a bit excessive.
Have you called them to discuss?