So again... looking at sewage treatment, this was something that we had to figure out to deal with once populations began to grow exponentially during the industrial revolution. We found that the impact of more humans, living closely together in cities, created some rather serious problems with disease and water quality. These problems still exist in third world countries where sanitation is not what we have come to expect here. We can see the results in mortality rates (especially child mortality), life expectancy and related diseases. Human impact is readily observable on our environment simply by the waste we excrete into our environment. Left untreated in areas where we are exposed to it affects us in a negative way.
We have a direct impact on the environment we live in on the most basic levels. It took leaps of human knowledge to begin to understand the connections between things as simple and now taken for granted, as properly disposing of own feces. That didn't just come naturally. It took quite a long time before we even recognized it was a problem. It was just "natural" for those people to get sick and die. It was God or the Earth just doing what the Earth does.
For people to know, accept and take for granted things like micro biology and at the same time dismiss, wholly, the possibility that man can cause large scale problems for his health and survival on Earth, is only one thing: Sheer, willful, ignorance.