Sewer systems do not magically remove waste and many of them just reduce the biologic oxygen demand and let the solids settle out. These solids often contain substantial amounts of chemical toxins that are then just dumped in a land fill. I (like 10s of millions of Americans) live in a large city with a combined sewer system so all of the waste I put down the sink on Thursday just flowed right into the Ohio river with NO treatment at all. A septic system would have been better on that day.
So what you can to limit the waste but do not think it is OK to put whatever you feel like down the sink and the sewage treatment plant will clean it up.