Hi there, I know this is extremely old, however I've just started playing again and I ran into this issue. I wanted to reply in case anyone else playing was searching for an answer. I fixed this by "peppering" in a few low income houses near the sewage treatment plant. Maybe only 8 trailers and you can't even see them amongst the trees. They walk to work and keep it running smoothly. There are also two bus stops on either side of the street in front of the building. Hope this helps someone!