I often use spent starsan as a rinse to eliminate alkaline residues on my equipment to keep my fresh Starsan longer.
I’ve taken to boiling water on the stove during the vortex to add some humidity and a little warmth to the kitchen. (The builders of my house in 1916 apparently figured the stove was going to be on a lot so they figured a small radiator would be quite sufficient.) I discovered that adding a splash of Starsan to the water prevents scale. Instead calcium phosphate precipitates which is very soft and easily removed with a soft sponge.
My 1925 bungalow doesn’t have a radiator in the kitchen at all. Kitchens weren’t built to be a place where people hung out aside from cooking or eating a meal so there would generally be heat from the stove/oven anytime people were in the kitchen. The chimney for the originl coal boiler in the basement also runs adjacent to the kitchen on the main floor so between the chimney and the stove the kitchen would normally be reasonably warm when in use. Subsequent remodels swapped coal for gas in the boiler and opened up the kitchen to the dining room so it now isn’t such a warm room but I still refuse to use it as a place to entertain.
Anymore, I only use Star San to sanitize bottles for packaging beer in for competitions. For general sanitizing of my fermenters and plate chiller assembly, I use Saniclean because it doesn’t foam like Star San. I can keep it around for a couple months in a Gott cooler. If the temp gets down to the 50’s in my brewery, there will be some of the solution that tends to precipitate out. I asked Five Star about this and they said it would not degrade the effectiveness of the solution. Just make sure if you store either that you check the pH from time to time to insure that it stays below 3.0.
I just dump down teh drain when finished with them or they lose their effectiveness. It doesn’t seem to bother my septic system.