[1] http://www.gatesnotes.com/Development/Omniprocessor-From-Poo...
NEWater is mainly used for industrial and air-con cooling purposes at wafer fabrication plants, industrial estates and commercial buildings. The biggest users of NEWater are wafer fabrication plants which require water quality that is even more stringent than water for drinking. NEWater is delivered to industrial customers via a dedicated pipe network.
During dry periods, NEWater is added to our reservoirs to blend with raw water. The raw water from the reservoir then goes through treatment at the waterworks before it is supplied to consumers as tap water. [1]
[1] http://www.pub.gov.sg/water/newater/Pages/default.aspx#newat...
"When you flush in Santa Ana, the waste makes its way to the sewage-treatment plant nearby in Fountain Valley, then sluices not to the ocean but to a plant that superfilters the liquid until it is cleaner than rainwater. The “new” water is then pumped 13 miles north and discharged into a small lake, where it percolates into the earth. Local utilities pump water from this aquifer and deliver it to the sinks and showers of 2.3 million customers. It is now drinking water. If you like the idea, you call it indirect potable reuse. If the idea revolts you, you call it toilet to tap."
http://www.nytimes.com/2008/08/10/magazine/10wastewater-t.ht...
While I'm fairly confident that we can eliminate most live pathogenic organisms from sewage with enough work, excreted toxins, hormones, and undigested pharmaceuticals are another matter. Diluting those through a watershed serves a real purpose that sewage treatment facilities are not up to replicating.
And yes, the cost of that would be huge, not just in cities. In the UK the decision the victorians made to not have separate sewage and rain water drainage systems still has a knock-on today and costs a vast amount in water treatment, but the cost of separating rain runoff and sewage is still seen as prohibitive.
edit:
However, on a small local-scale grey water systems can do a lot of good, both for re-use but also rain water capture and re-use would help with flood prevention. If houses captured rainwater for use for lawn watering and other appropriate uses, this is something that could be done without great expense.
I'm not arguing against the idea, especially as it is typically implemented in the UK where a house will generally only have about 500 litres of storage, but scaling it up and making it a requirement could cause some interesting problems.
For example, a house would fitted with a storage tank for shower water, which is then used for flushing the toilet, or even watering a yard. The toilet water is sent to city sewer system.
In arid third word countries people have low tech outdoor arrangements like pit toilets.
http://www.ornl.gov/ornl/news/news-releases/2015/ornl-led-te...
Yes I know I'm ignoring things like biofilm buildup, particulate settling, etc. I'm mainly trying to illustrate the "basic work" required to get drinking water from sea water using the latest technology. Most existing reverse osmosis membranes are only capable of 1 litre per square centimetre per megapascal per DAY. That's not even a remotely practical rate without mechanically applied pressures, and since we're now dealing with hydraulic pressures... a lot more engineering and maintenance.
I am still wondering about the efficiency in terms of filtering toxins. Water molecules are smaller than most other molecules but I am not sure if some toxins would still be able to get through the holes in the graphene? Things like heavy metals