Singapore has been doing this for more than twenty years, nothing new. And it works just fine there.
They used to get it the same way from Malaysia but their not so wonderful experience as one country before Singapore and Malaysia went their separate ways in 1965, convinced them to diversify their sources.
Raw water is not sewer water, per se, but can include portions of it. Pricing is discounted according to the amount of foreign matter in the water much as grain prices paid are discounted for things like dust, insects, mouse crap and the like.
It is much easier and cheaper to refine raw water into safe drinking water than it is do do so through methods such as desalination. The only exceptions are where raw water isn't easily available such as the Middle East. IOW, where desalination is widely in place for that very reason.