Let's see what I can find . . . here's a site from West Africa that has great information on sewage treatment that is easy to understand.
North Dakota is a state in the US that has a lot of farms and rural homes. Here is a reliable source of information for home sewage treatment systems by ND State University.
YouTube is always a good resource. Here's a fun video you could use from Australia, if your project is an online one.
Here's a video for small, on-site sewage treatment.
And, of course, there's always Wikipedia.
Hope this helps.