[philipkglass]

The local sewage systems can't deal with volumes this large. The sewage systems of the entire UK, population 66 million, deal with about 11 billion liters per day:

https://assets.publishing.service.gov.uk/government/uploads/...

Saudi Arabia, with a population of only 33 million, isn't going to have sewage systems sized to accommodate 31.5 billion liters of waste brine.

[Retric]

You would mix after the sewage treatment, simply placing the pipes next to each other would work.

As to impact the quality of brine is less important than the amount of surplus salt. Sea water is ~3.5% salt. You need 1:1 fresh water produced to get back to the default at discharge. While a significant fraction of the water produced would be lost as evaporation etc, desalination is not their only source of water.

Edit: Digging into the numbers it looks like Saudi Arabia is not limiting things to cities which have a closer relationship between water and sewage production.