No, you really couldn't design a cost-efficient way of generating electricy from family septic.
If you want to save energy, go solar. There are dozens of tried and true ways to supplement your home system with solar-thermal powered appliances. Our water is heated in the roof, in black tubes.
I divert the water that drains from my washing machine to the garden. We use environmentally friendly methods/detergent to wash our clothes, and my garden is nice and green. Also, I don't have a lot of pests... Don't know why, exactly.
We compost all of our food waste, and recycle almost all of our "garbage." We generate less than one full bag of trash per week, in a family of three.
Turn the thermostat down three degrees in the winter, and up three in the summer, from what you're used to. Your body will adjust quickly.
It saves on the utility bills.
If you use a dishwasher, turn off the heated drying feature.
If you use a clothes dryer, smaller loads dry much faster. If you split a large load in half, it will probably dry faster in two batches than one. (If your dryer has an automatic more-dry setting.) Make sure it's vented properly and the lint traps are clean, or you'll waste energy. Clean your lint trap with mild detergent and warm water every other month. Fabric softener builds up and prevents adequate ventilation.
Sorry to write so long...