All you need to do (to measure the outgoing sewer water temperature) is to buy one of the inexpensive digital temperature recording devices (see below), put it in a plastic freezer bag and weight it down in the sewer outlet with a string attached. Then retrieve it in a few days.
It will show you the maximum, minimum and average temperature of the water over the data collection period.
Here is a simple RC-5 recorder that works well for me.
RC 5 USB Temperature Data Logger Temp Recorder Internal Sensor 32000 Points New | eBay
When you speak of "freezing cold" winter water - is this a temperature or a subjective assessment? For example, if I get into the shower and it seems "freezing cold" it might be 80 F.
Metrics make all the difference in planning . . . .
Steve