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Comment by mapt | Hacker News Reader

I'm a bit anxious about sewage to drinking water efforts.

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.

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Not sure what size these toxins are but nanoporous graphene works for filtering out anything bigger than water molecules -

http://www.ornl.gov/ornl/news/news-releases/2015/ornl-led-te...

it's always an issue of volume flow and membrane effectivenes with these things. many things 'work' that don't work to resupply the population on a daily basis.

Nanoporous graphene is pretty awesome stuff. The numbers are up in the range of 4 litres per square centimetre per hour per megapascal. Or more practically, just a one centimetre by one centimetre "hole" at the bottom of a 100m tall "tank" (it can actually just be a pipe, the key is the water depth not the reservoir capacity) would produce 4 litres per hour powered by nothing but gravity. Of course there's the requisite effort of approximately 100N of work per litre to lift it up that hundred meters, but being otherwise passive is a good illustration of how much more efficient these materials are.

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.

Thank you, was going to mention the flow rate vs current reverse osmosis tech.

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

What about a grey water system? neither your toilet nor your lawn needs human consumption grade water.

Wouldn't it be an absolutely huge cost to lay down another set of pipes for non-human drinkable water? Especially in cities.

Non-potable is the industry term for "non-human drinkable".

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.

Be careful of the law of unintended consequences. Rainwater capture will be fine so long as only a minority do it (how small I don't know) or only a fraction is captured but if everyone does it then it will change the economics of water supply and give people incentives to use the captured water for other than lawn watering. For instance they might use it for flushing toilets or even bathing. the problem that then happens is that the local water table will fall because it is not getting the water that it used to because that water is now flushed into the sewage system.

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.

Some US states have laws preventing rainwater capture. Water rights being a weird and complicated thing.

I thought that the idea of rainwater capture was to prevent the water from going to "waste" as runoff. That is, the water that hits the roof of your house ends up in a torrent in the downspouts, which runs into the streets and the sewer system. Whereas the water that hits your yard has more of a chance to sink into the water table.

It's unlikely that a given residential area is drawing water from the area directly beneath it, or that its water supply is fed by residential run-off. That's not going to be clean water that you'd want to drink. Most residential water comes from resovoirs fed by mostly unpopulated catchment areas which are often 10s or 100s of miles away. Also bare in mind that around 50% of rainwater evaporates or ends up in the ocean, so there's plenty to go around.

I believe most gray-water systems are internal to individual buildings.

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.

Interestingly, growing up, I thought a second set of pipes for "non-potable" or reclaimed water was standard. Where I grew up in Florida, every lawn was watered with reclaimed water. According to their website[1], Florida is currently reusing 660 million gallons of water per day. Really makes me wonder why states that are in apparent perpetual drought condition haven't adopted similar techniques.

[1] http://www.dep.state.fl.us/water/reuse/

Water based toilets and watered lawns aren't used in places with shortage of drinking water. (Except in places with high inequality where an upper class can afford to use water in toilet but lower class can't afford to drink it)

You mean like California? :-)

Afraid I don't know enough about California to get the joke, do you mean tap water is too expensive for the poor there?

I mean that the way California is going that water will be in short supply there.

Is there a toilet that isn't water based? Or are you talking about using tree/hole/... as toilet?

Composting or incinerating toilets.

Yes, UDDT toilets work so well that environmentally conscious people install them in homes even in places with good access to water. https://en.wikipedia.org/wiki/Urine-diverting_dry_toilet

In arid third word countries people have low tech outdoor arrangements like pit toilets.

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