I'd rather have pollution from a septic tank seeping into my garden than industrial pollution contaminating the environment. It could be carcinogenic, cause birth defects, serious respiratory disease etc etc. I'm lucky to have mains sewage here in our village, but many places in Norfolk still have septic tanks. They don't seem to cause problems, a Council tanker comes when you phone, and pumps it out for you. Poo is better than toxic and dangerous chemicals!