Most of these exisiting systems are meant more to service commercial/industrial/agricultural customers, it is unfeaseable to deliver grey water to every home or business

I’m not sure what you mean here. Even if you limit the great water system to large scale operations that would use the grey water, you’re still looking at billions of dollars to run the pipes plus all the pumps, sampling points, water valves, new water towers/resevoirs etc.

It would be incredibly dangerous to have the wastewater system directly hooked up to a potable water system. A variety of issues could disrupt the flow/treatment/monitoring of wastewater which could contaminate the drinking water system and its reservoirs/water towers and pose a serious threat to public health.

So can any surface treatment system that is currently used. Any city that uses rivers or lakes as their treatment centre already need to purify to a high standard and closely monitor quality of in and outflows. Additionally, most of those areas have their treated sewage outflow to the same body of water as they draw from. The whole point of designing a system is to build in backups and fail-safes to ensure those issues are identified and accounted for. It’s significantly cheaper than creating a whole secondary great water utility system, not to mention the additional costs for all those businesses that need to add another internal plumbing system