Bearisgray has a good method.
No, the starch will not hurt your septic tank. It is a natural substance.
When I was young I had to hank the clothes on the line. I put a big lump of Argo starch into the enamel wash basin (held about 5 or 6 quarts), added a little cold water to soften, and then boiling water to fill the bowl about half full. Then it went outside under the wash line. Dad's shirts went in first, then our dresses and a few other things, and last was Mom's aprons. It was very difficult to wring out the shirts because they were so hot. They were then hung on the line.
When they were dry, they were brought into the house, sprinkled with water, rolled up and put into a bushel basket that had an oil cloth liner. The next day, they were ironed.
I do remember that Dad's shirts were the stiffest, and Mom's aprons were not very stiff at all. So saving the left over starch doesn't seem like a good idea. I dumped it onto the grass in the summer time. In the winter, the clothes were hung on lines in the basement, or draped over newspaper covered radiators. (I used to love to sit on those radiators to get warm. LOL)