Occasional use, perhaps once a month, of a 15% caustic soda solution in amounts of a gallon or less, is not likely to harm the septic system, based on this calculation using Powell's online caustic soda dilution calculator [ https://www.powellfab.com/ ] ...
Septic Tank Capacities Septic Tank VolumeMaximum Capacity gallonsm3Single family units (number of bedrooms)Multiple family units - one bedroom each (number of units)DFU 7502.81 - 215 10003.8320 12004.54225 15005.75 - 6333 20007.6445 25009.5660