If I only have a small or occasional amount of used, non-fixer chemicals, I just flush them. If I'm working in the darkroom a lot or in the days I did color I collect used chemicals in 5 gallon buckets and take them to the local wastewater plant where they let me dump it in the untreated ponds free of charge. A small amount is not going to hurt your septic. The problem comes in determining when a small amount becomes a big amount. I like to err on the side of caution. If I'm producing gallons of used chemical waste per week, I take it to the plant. If its just a few liters, I flush. What you also want to watch is your water usage, because overuse will flush solids into your field and you'll be buying a new septic system. My wash water goes directly outside so I don't worry about that.