As stated, do not put lye into your septic tank.
However, lye can go down the municipal sewer. It is after all "drain cleaner." The amount we use is generally need to dispose of is only one to several cans in a large volume of water. I heard many years ago that municpalities in general never object to lye from local industries, because their biggest pH problem (acidity/akalinity of a water solution) is excess acid.