Soiled rivers and seas, and huge dividends for shareholders: this is how water companies massage their figures So that’s how they do it. I’d been wondering how, when more sewage has been entering our rivers than ever before, some of the water companies have managed to improve the ratio of the sewage they treat v the sewage that pours untreated from their storm overflows into our rivers and the sea. Now we know.It’s called “flow trimming”. Sounds innocuous, doesn’t it? What it means is that sewage is diverted into rivers and ditches upstream of the water treatment works.