How dangerous are microwaves?

[dragonstar57]

If I heat a sample of water with a dangerous microbial contamination (whatever common, dangerous waterborne illness) until it boils the microbes die.
My question is why do they die?
Is it the microwaves that kill them via radiological effects or is it the heat that does it? (ie the microwaves interacting with the microbes vs the microwaves interacting with the water and the water interacting with the microbes)
Could a microwave be built to kill bacteria with low intensity microwaves?
Low intensity microwaves which could be generated by power from a hand crank or pedal style generator?
If so could microwaves used to sanitize water in developing nations with simple gear like magnets and broken microwaves?

[Delbert]

I understand that the microwave frequency is designed to be compatible with the resonant frequency of water molecules. Presumably the microbe's water content will boil as well as the sample of water. So, they will doubtless be boiled both from the outside and in.

 

As for radiological effects, I don't think microwaves are ionising. And as for any other such consequence (like buggering the chemistry), I think the exposure time to be too short before the double boiling process!

[dimreepr]

“microwave frequency is designed to be compatible...”

 

The invention of the microwave oven was an accident; Percy Spencer stepped in front of a magnetron and noticed a bar of chocolate in his pocket had melted and very soon after, he filed a patent.

 

In answer to the OP I’m not sure if the microwaves directly kill the bacteria but boiling water will kill all but the hardiest bacteria.

 

“could microwaves be used to sanitize water”

 

Why bother when UVGI is an already developed method of disinfection?

[swansont]

Microwaves are not ionizing; AFAIK there has been no conclusive evidence of microwaves killing anything by other than thermal means. i.e. because they get hot.

[John Cuthber]

“microwave frequency is designed to be compatible...”

 

The invention of the microwave oven was an accident; Percy Spencer stepped in front of a magnetron and noticed a bar of chocolate in his pocket had melted and very soon after, he filed a patent.

 

In answer to the OP I’m not sure if the microwaves directly kill the bacteria but boiling water will kill all but the hardiest bacteria.

 

“could microwaves be used to sanitize water”

 

Why bother when UVGI is an already developed method of disinfection?

Because UV only works with clean water.

[dimreepr]

Could you clarify what you mean by “clean water” please John? I ask simply because UV is used in applications, such as, ponds and salt water aquariums, where dirty water is inevitable, are my friends just throwing their money away?

[EdEarl]

Microwave ovens operate at low microwave frequencies (e.g., 2.45 GHz).

 

Wikipedia: Dielectric Heating

Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric. At lower frequencies in conductive fluids, other mechanisms such as ion-drag are more important in generating thermal energy.

 

Wavelength of low frequency microwaves is 1-10 cm, which is much larger than a bacteria. Thus, a bacteria cannot directly absorb microwave energy as an antenna does. Since water, except distilled water, is conductive and can act as an antenna and absorb microwave radiation. IMO, the bacteria are killed from boiling water, not radiation.

[swansont]

Microwave ovens operate at low microwave frequencies (e.g., 2.45 GHz).

 

 

Wavelength of low frequency microwaves is 1-10 cm, which is much larger than a bacteria. Thus, a bacteria cannot directly absorb microwave energy as an antenna does. Since water, except distilled water, is conductive and can act as an antenna and absorb microwave radiation. IMO, the bacteria are killed from boiling water, not radiation.

Atoms absorb microwaves, if they are on resonance. Size is not necessarily a limit.

[EdEarl]

Atoms absorb microwaves, if they are on resonance. Size is not necessarily a limit.

True, size isn't everything, but I have seen small ants survive being zapped with no apparent effect.

 

 

Wikipedia: Ionizing radiation

Ionizing radiation includes both subatomic particles of matter moving at relativistic speeds and electromagnetic waves on the short wavelength end of the electromagnetic spectrum, which act like energetic particles.

X-rays are 1nm and smaller, but 1/4 wave antennas work well. 1nm/4 is 250 pm, which is the size of some atoms. Cells are large enough to be affected by x-rays.

 

I do not know for sure, but suspect atoms can absorb photons with longer wavelengths than 4x their size, but with reduced probability.

Edited July 12, 2013 by EdEarl

[swansont]

True, size isn't everything, but I have seen small ants survive being zapped with no apparent effect.

 

 

X-rays are 1nm and smaller, but 1/4 wave antennas work well. 1nm/4 is 250 pm, which is the size of some atoms. Cells are large enough to be affected by x-rays.

 

I do not know for sure, but suspect atoms can absorb photons with longer wavelengths than 4x their size, but with reduced probability.

That's classical analysis. If it's a resonant transition you really need to look at QM, and know what the coupling strength is, which depends on the lifetime. Cs-133 strongly absorbs at 9.2 GHz, which is around 3 cm. Much, much bigger than a Cs atom because this is not a classical system like an antenna.

 

Edit: this isn't a big issue, though, because once a photon is absorbed, that atom can't absorb again until the system de-excites.

[EdEarl]

ok tyvm

[dragonstar57]

Could you clarify what you mean by “clean water” please John? I ask simply because UV is used in applications, such as, ponds and salt water aquariums, where dirty water is inevitable, are my friends just throwing their money away?

Wouldn’t particulate matter (like if the water was muddy) block the uv whereas the microwave frequency would be able to get through?

 

so you would have to add microwave energy faster than heat leaves the system?

How would the power requirements compare to UVGI?

Would it be possible to use microwaves to kill pathogens with a modest power supply?

Edited July 12, 2013 by dragonstar57

[EdEarl]

Wouldn’t particulate matter (like if the water was muddy) block the uv whereas the microwave frequency would be able to get through?

 

so you would have to add microwave energy faster than heat leaves the system?

How would the power requirements compare to UVGI?

Would it be possible to use microwaves to kill pathogens with a modest power supply?

Very little UV gets through the atmosphere, so muddy water would block it.

 

"add microwave...heat leaves..." I don't know, you haven't given enough information.

Microwave power vs UVGI, UV is generally more lethal to microbes than microwave.

"modest power supply?" not in a swimming pool, but yes in an eye dropper depending on what you mean by modest. A microwave oven has specific power requirements and will not run if it does not have enough power.

[Klaynos]

What's the skin depth of water at both of these frequencies?

[dimreepr]

Wouldn’t particulate matter (like if the water was muddy) block the uv whereas the microwave frequency would be able to get through?

 

so you would have to add microwave energy faster than heat leaves the system?

How would the power requirements compare to UVGI?

Would it be possible to use microwaves to kill pathogens with a modest power supply?

 

That’s the point I was trying to clarify with John because the effect of particulate matter in water can be overcome by reducing the depth of water being treated. By measuring the effective depth of the UV, in said water, once established the flow of water can be optimised through a sluice system. Even with a depth of a millimetre or less, a large flow can be accommodated, by varying the width of the sluice. I very much doubt a microwave based system would be more efficient.

[krash661]

you can stick an ant in the micro wave for at lease 30 seconds and the ant will live

 

reason for this is the ants size and how the micro wave uses point places of heat.

the ant can move out of these areas

and not become heated, and also ants can stand a lot of heat.

[John Cuthber]

If, as is entirely reasonable, the bacteria are hiding inside the particles of stuff in the water they are safe from UV.

(at least, they are if the particles are opaque to the UV, and that's not a bad bet if they look dark.)

 

This

http://en.wikipedia.org/wiki/Solar_water_disinfection

essentially cooks them- just like the microwave.

 

Also, what happens to microwaves in water isn't the same as the skin depth in electronics- it's a resonance phenomenon. Water is pretty nearly an insulator. so the "skin depth" would be near infinite.

 

So the term "skin depth" isn't really applicable.

http://en.wikipedia.org/wiki/Penetration_depth

is better

[dimreepr]

In this case, microwave is just a source of energy, the “point places of heat” are simply points at which micro’wave’ energy coincides positively.

 

Sorry cross posted, however, when did I mention the term 'skin depth' in my post? Not to mention the efficiency saving, using solar energy rather than man made.

Edited July 13, 2013 by dimreepr

[John Cuthber]

 

 

 

Sorry cross posted, however, when did I mention the term 'skin depth' in my post?

You didn't: Klaynos did

[dimreepr]

 

You didn't: Klaynos did

Sorry.

 

 

 

 

“If, as is entirely reasonable, the bacteria are hiding inside the particles of stuff in the water they are safe from UV. (at least, they are if the particles are opaque to the UV, and that's not a bad bet if they look dark.)”

 

True, however, turbulence can be induced in the flow and along with prolonged exposure, would significantly reduce the probability of bacterial survival.

 

 

Edit/ besides which UVGI is far more likely to destroy the endospore than a solar still.

 

Edit/ I’ll leave this edit in place as a stark reminder of just how much alcohol reduces capacity.

Edited July 14, 2013 by dimreepr

[Externet]

...Would it be possible to use microwaves to kill pathogens with a modest power supply?

If it is about killing, yes, if their evironment or cell temperature is raised beyond their survival.

 

For an even more modest power supply, just give the pathogens filled water a zap of 120VAC with a pair of submerged electrodes. In less than a second, ruptures all their cells.

 

Both ways kills them, but their cadavers and fluids stay in a questionable water. I suppose is the same case when boiling water, the cemetery and cell fluids are still in the considered potable and safe to drink water.

[microwaveguru]

Gentlemen: I'm a microwave scientist and the killing of bacteria using microwave energy has been demonstrated many times as due to thermal heat; there is no "microwave effect" (the quantum energy content of microwaves is lower than that of light!) - this appears over and over in peer-reviewed publications.

I need to correct a misunderstanding that appears in the first reply: "I understand that the microwave frequency is designed to be compatible with the resonant frequency of water molecules." This is not true. The resonant frequency of water at 25 C is 22 gigahertz (GHz); the microwave oven frequency is 2.45 GHz - so the efficiency of energy conversion is poor - in fact water is majorly a reflector. Further, as water gets hotter it's resonant frequency rises, while that of the microwave oven remains the same. Therefore, the water absorbs and coverts microwave energy into heat less efficiently as water becomes hotter This is a good thing since it acts as a moderator to control the rate of heating. Try a simple experiment: take three 600 ml beakers, add exactly 500 grams of water to each where the water is either 10, 20 or 30 C (measure and record this starting temperature). Microwave one at a time for exactly 60 seconds and measure the final temperature. Compare it to that sample's starting temperature and you'll see that the coldest water showed the greatest temperature rise.

Another important thing: when salt is present another heating mechanism, ionic conduction, dominates - in this case, the salted water or food (ham for example) heats faster the hotter it becomes - a phenomenon known as thermal runaway - extremely difficlut to control - it may lead to fires in some material.