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Comment by dsfyu404ed | Hacker News Reader

Even if it's just extra salty it's still a local problem. You can't just dump it all in the ocean because it would still kill everything. You'd need to dump it out slowly or over a much wider area for the concentration not to be harmful. It's like a polluted river killing everything where it dumps out even if the amount of pollution is inconsequential overall.

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Someone correct me if I'm wrong (not the best at math), but the ocean is massive. The entire amount of brine they mention (31.5 million cubic meters) fits in one 300m square cube. Even if you drop it in an enclosed 1km-wide area of seawater, it will barely increase concentration by 0.01%. It would take millenia for us to increase sea salinity by this method, and all that treated water is eventually coming back.

Even if you took the total daily water consumption of the entire human race - 150l per person * 7 billion, or 1^12 liters, and made it extra salty, it is dwarfed by the total of 1.26^21 [mostly sea]water on earth.

This would be harmless assuming we are not adding anything to the byproduct, which unfortunately seems to be the case according to the other comments.

The concern is not ocean-level damage through the salt directly but in localized ecological system destruction. The mangrove forest encompases a tiny portion of the ocean overall but removing this forest affects a large portion of sealife. The creation of 'deadzones' through salinity will have knockoff effects elsewhere that cannot necessarily be predicted.

At worst these dead zones would be tiny. They could dilute the waste stream by dumping it into the local sewage system, but I don’t think that’s going to change much.

It’s on the order of building a mall parking lot. Sure bad for the local ecosystem, but we have vastly larger issues.

It's going to be dumped near the shoreline, where sea life is very different from what is supported in the open ocean. If the shoreline is made uninhabitable, the marine life there might not have anywhere else to go.

Near in this case would need to basicly be directly into. Actual desalination plants don’t do this.

Further by parking lot I mean an actual parking lot. Dump it even 1000 feet from the shoreline and you would have trouble detecting the extra salt in the surf.

There are over 7500km of shoreline in Saudi Arabia, so marine life can move along the coast away from the salt water outlets.

Shoreline is also highly variable and localized, and effects to the shoreline have staggering effects even if the region affected is small. I'd recommend looking at mangrove forests, coral reefs, and the mouths of rivers.

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.

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.

Dissolution takes time. If we’re to dump the extra salty water off the coast, it would be hard to create an area of higher salinity.

Of course over time it would dissipate and barely change the salinity is the ocean.

How about dumping it into a shallow pit and letting the water evaporate, creating new salt flats.

At 35 degrees Celsius (308 K, 95 F), it takes about 44 kilojoules to evaporate 1 mole of water [1]; at 55.6 moles/liter, that's 2.4 megajoules per liter.

Averaged over a year, the energy available to evaporate water over a given area is roughly equal to insolation from the sun. Saudi Arabia has excellent solar resources averaging 2200 kWh/m^2/year [2], e.g. 6 kWh/m^2/day, e.g. 21.7 MJ/m^2/day. With perfect sunlight absorption you could evaporate 9 liters per m^2 of evaporation-pit per day.

They're currently discharging 31.5 billion liters of liquid per day, per the article. That translates to about 3,500,000,000 m^2 (3500 square kilometers) of pit area. So that's probably one reason why nobody deals with desalination brine this way. It would require very large evaporation pits even if you have excellent sun levels to evaporate the waste. It seems like it would be technically possible in SA, as they have a low population density and lots of unused land, but it sounds expensive. Some countries that rely on desalination, like Israel, don't have the spare land area even in theory.

[1] https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#/medi...

[2] https://www.sciencedirect.com/science/article/pii/S101836391...

Nice analysis! Three things to consider:

I don’t think you can just look at insolation over a given area. Wind and even the cooling itself will cause the air over the area to redistribute with warmer dryer air that can continue picking up water.

Besides that it’s more important to consider how saturated the air in the region is. If it can hold more water it will.

Other things to consider

Some of the water would seep into the ground instead of waiting around to evaporate. Maybe there are ways to speed that up too.

Finally Another option is the spay the water into the air to speed up evaporation a bit.

I considered mentioning wind from warmer neighboring regions but at this scale you are basically remaking geography, so I wouldn't count too much on said winds reaching the middle of your evaporation ponds. ("Manmade shallow hypersaline lakes", rather than ponds, at this scale.)

Seepage into the ground -- good point. If there is existing groundwater, seepage would taint it. If there is no existing groundwater, but ground is permeable, that helps.

Spray water into air? That costs more energy. Just waiting for natural evaporation is nice because it doesn't require any more human-added energy.

Of course you'd want to analyze more than the One Big Factor if you were pondering implementing this in practice. But I think that the one-factor approach illustrates why this hasn't been an "obvious" disposal solution for waste brine.

>Saudi Arabia emits 31.5 million cubic meters of liquid effluent a day

You'd need some massive pits and you'd have to displace way more than 31.5 million cubic meters of soil as it isn't all going to evaporate in a day. Ideally you'd want really shallow pools so you'd eat up an insane surface area.

For some reference an Olympic sized swimming pool is about 2,500 cubic meters so you'd need 12,600 (pools) just to hold one day worth of the contaminated water.

For a fun little compassion (that amuses me and might someone else) it is estimated that all of the silver ever mined adds up to about 166,375 cubic meters Source: https://www.usgs.gov/faqs/how-much-silver-has-been-found-wor...

And only about 8000 cubic meters of gold has been mined. Source: https://www.jmbullion.com/investing-guide/james/gold-supply/

Edit: derpity-derp-derp fix

Wait. Do you mean a cube of silver|gold that's 55|20 meters on one side, as in the width of the cube and not its volume? As in 55^3 = 166,375 m^3 of silver, and 20^3 = 8000 m^3 of gold? That 5.36m diameter cube illustration representing a single year's gold production in the source is already 154 cubic meters.

Couldn't they just run a big pipe out into the middle of the desert and pump it there? Maybe they could create giant salt flats and extract rare earth elements from them, even...

A cube 55 meters on a side, not 55 cubic meters. That's a huge difference: 166K+ cubic meters

See the nice photos here:

https://en.wikipedia.org/wiki/Sabkha

the copper will be an issue.

So call it a copper mine instead of a salt flat?

it's not a copper mine except in the sense that it has contaminated, zero-value output as a side effect.

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