Why does this matter? Please explain.
Two reasons.
1.) Bleach kills the organisms at work in the septic tank. You have to be real careful. If the biologics in the tank are killed solids won't be turned into liquids and in turn drain into your leech (or is it leach) field. Can turn into one huge smelly problem turning your yard into a poo-swamp.
2.) Types of Laundry Detergent. If you don't have a grey-water outlet (laundry water is diverted from y our septic) you have to be careful on the type of laundry soap you use. Phosphate free, and usually liquid is preferred. Laundry detergent turns into a solid slowly and is suspended in the liquid draining into your leech field. The particles from powdered laundry soap grab onto the rocks used for draining and filtering the waste water. They in turn turn to cement, and you'll end up digging up and replacing your entire leech field $$$$$