Perhaps I’m just not understanding you, but how does composting treated sewage fix the problem of dumping raw sewage into the water before it ever reaches the treatment plant?
The reason for the dumping of raw sewage is because these cities have older infrastructure which combines wastewater and storm water collection into one system. Heavy rains can increase the flow rate in such systems by as much as ten times their usual rate, which is far too much for treatment plants to handle without massively oversizing them and it also could make them lose the microbiology that treats the wastewater for them. Diverting this sewage directly into the receiving waters actually prevents even worse public health problems from occurring.
Another problem with what you are proposing is that, while composting can be good enough to get rid of pathogens, it’s not good enough to remove things like pharmaceuticals and heavy metals. Incineration would be a more effective solution, but it requires even higher upfront costs.
Of course it is, but what’s your method to achieving that goal and what does composting human waste have to do with it?