Waste water treatment fee, based on your water usage. Almost all water usa
ge during the winter goes into the sewer system. Some of the water used in
the summer (gardening, etc) does not go into the sewer system, so the sewe
r fee is adjusted, during the summer, for that (gardening, etc) allowance.
As stated, in general, it's about double (mine's a bit more) than your wate
r usage fee.