[Lewis Thomson (OP)]

Bob has presented this question to The Naked Scientists.

"Is a compost toilet, enclosed in a shed, safe in regard to hydrogen sulphide gas? My daughter has recently purchased a compost toilet/shed for her new camping site located on a farm field. Basically the toilet consists of a 3000 litre container (IBC) with a loo seat on top that diverts urine to a soak away and allows human waste to enter the container (which is vented) for eventual decomposition. As the loo/seat is enclosed within a wooden structure (like a shed) for privacy and you can see into the container via the loo seat, I am concerned that hydrogen sulphide could be generated within the container that could be lethal to the loo user in the enclosed environment? Am I over concerned?"

Discuss in the comments below...

[alancalverd]

You will want to drill some holes in the shed (if it doesn't already have them) to dilute the general pong anyway. H2S being denser than air, it will vent though the lowest holes.

The good news is that the human nose is excruciatingly sensitive to H2S and gets offended at levels well below toxic.

[Bored chemist]

The good news is that the human nose is excruciatingly sensitive to H2S and gets offended at levels well below toxic.

Ay high concentrations, it deadens the nerves in your nose and you can't smell it.
Make sure you have good ventilation.

[Bored chemist]

H2S being denser than air, it will vent though the lowest holes.

Gases mix.

[alancalverd]

Which is why you need a hole at the top and a hole at the bottom! Diffusion in a closed system would eventually produce a weak concentration gradient but as long as there is fresh air available outside the room the heavier gases will preferentially leave at the bottom.

There are plenty of composting toilets around and fatalities seem rare.  Opening the door occasionally probably does the job.

[Bored chemist]

I'm intrigued by your suggestion that the fresh air enters a house via the chimney and leaves through the door.

In reality, the chimney effect will produce an updraft of air and it will carry the gases with it.

Even if your repeated fallacy that mixed gases separate significantly under gravity were true, much of the off gas from a latrine is methane and that's lighter than air.

On a practical basis, I think outhouses were always ventilated.

[evan_au]

I was interested to see a composting toilet system in a national park, which had a solar-powered fan.
- The fan created enough negative pressure in the pit that it created a downdraft when you opened the toilet lid.
- That kept any noxious gases (or even irritating smells) from reaching the sittee.
- It did feel a bit odd to feel a breeze while sitting, but the lack of odour was worth it!
- It also means that any methane buildup would be eliminated (you don't want someone smoking when they open the lid!)
- It was a long time ago, so I don't recall whether it was sophisticated enough to have battery storage, or a sensor that triggered the fan when you opened the door...

[alancalverd]

I'm intrigued by your suggestion that the fresh air enters a house via the chimney and leaves through the door.

And I'm intrigued by your idea of setting fire to the toilet. Why else would you need a chimney in an outhouse?

On my planet, gravitation separates gases which is why radon sumps work, and centrifuges even separate isotopes.

 

off gas from a latrine is methane and that's lighter than air. 

True, but the OP asked about H2S. However you have stirred an interesting idea. I used to fly hot air balloons. We burned propane or butane to heat the air in the bag to about 100°C at the top, generating about 30% lift at safe maximum (balloon fabric becomes porous above about 120 °C). Now if we had simply filled the bag with methane at ambient temperature, we could have got about 40% lift  without having to carry the fuel cylinders or set fire to anything, and the gas would be a lot cheaper than helium and less prone to diffuse than hydrogen. This could be the beginning of a whole new sport , using sewage waste to fly!

[Bored chemist]

gravitation separates gases which is why radon sumps work,

No.
Radon sumps work because the radon is produced under the ground.

That's why the radon levels are higher in caves and cellars.

On my planet, gravitation separates gases which is why radon sumps work, and centrifuges even separate isotopes.

If gravity worked, why would they spend money on centrifuges?

Here's a quick question for you; roughly how high above sea level do you need to get for atmospheric pressure to drop to half the value at ground level?

[alancalverd]

Radon sumps work because the radon is produced under the ground.

and doesn't diffuse into the house before it is removed by ventilation.

If gravity worked, why would they spend money on centrifuges?

because more g works better.

how high above sea level do you need to get for atmospheric pressure to drop to half the value at ground level?

about 16,000 ft

[Bored chemist]

And, at that altitude it would be something like -15 or -20 C
The average surface temperature is about +10C
So it's a temperature change of 30C
Or, in absolute terms it's only a roughly 10% reduction in temperature.

So, do you accept that, if I'm making very rough approximations, I can consider the temperature to be roughly constant at about 270K?

[alancalverd]

In the troposphere we usually estimate 3 degrees per 1000 ft as the dry adiabatic lapse rate. I won't confuse you with water vapor as you don't think it is important!

[Bored chemist]

and doesn't diffuse into the house before it is removed by ventilation.

So, not because it's a dense gas and gravity holds it down...?

[Bored chemist]

I won't confuse you with water vapor as you don't think it is important!

I live in England and spend a fair fraction of my time talking about water in the air.

[Bored chemist]

In the troposphere we usually estimate 3 degrees per 1000 ft

OK, and half the atmosphere is below 16000 feet.
So, if it starts near 10C and falls by 3*16 = 48 C then it's about -38C on average
That's about 235 K +/- 24 K
So most of the air is within about 10% of 235K

I don't know what you didn't just say "yes"  but... whatever.
Do you understand that the pressure varies pretty much exponentially with altitude?
Here's a plot

p vs alt.png (19.96 kB . 888x535 - viewed 2291 times)
of the data from here
https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

Part of the deviation from the curve is due to temperature changes.
Without those, the exponential model would be an even better fit.

[alancalverd]

Yes, dear, it was all in the Meteorology and Navigation syllabuses. Interestingly, Human Performance (exam 5) also dealt with the varying composition of the atmosphere with altitude. Not much, but measurable.

The reason I didn't just say "yes" is because the DALR and SALR are different.

[Bored chemist]

Not much

So, when ever it was that you did

Human Performance (exam 5)

you presumably learned that I was correct to say

Even if your repeated fallacy that mixed gases separate significantly under gravity were true,...

Because, the changes are small, even over the range of altitudes that pilots worry about and so, for the height of an outhouse, they are not significant.

If you want to know how tiny the effect is, you need to calculate the exponential fall in air density vs altitude and then compare it to the fall for H2S which is roughly twice as fast. (Because it's about twice as dense).

I presume you know it's a tiny effect because you refused to answer about it previously.

https://www.thenakedscientists.com/forum/index.php?topic=84047.msg669462#msg669462

[alancalverd]

No need for me to answer when it is public knowledge

https://earthscience.stackexchange.com/questions/16364/what-is-the-co2-concentration-by-altitude-and-why

Remember that outdoor atmospheric gases are mixed by turbulence and convection.There's not much of either inside a well-built dunnee, nor in the Grotta del Cane. The density of H2S is between those of CO2 and air so I'd expect the equilibrium gradient to be shallower than for CO2.

Anyway that's just on my planet, where newtonian gravitation applies.

[Bored chemist]

No need for me to answer when it is public knowledge

And yet...  you did.

[Bored chemist]

Remember that outdoor atmospheric gases are...

irrelevant.