Home dehumidifier

Aside from uses in things like rescuing rare books from water damage, reducing the water content of hygroscopic plastics prior to molding, and reducing the water content of natural gas to discourage clathrate formation, dehumidifiers and desiccators are potentially useful human household items — indeed, in raw vegan circles food dehydrators are already ubiquitous, as low-temperature dehydration is a crucial preparation step in many raw vegan dishes. In developed countries, clothes dryers are also ubiquitous, to the point that hanging laundry to dry is prohibited in some neighborhoods, and of course a hair dryer is precisely a personal keratin desiccator.

Perhaps a more general desiccation appliance would be useful.

Household applications for desiccation include the following:

• Human food, as mentioned above, both for preservation and to enhance flavor and alter texture. This has limitations on the temperature it can tolerate — raw vegans normally use a temperature limit of 40.5°, but different foods have different optimal ranges.
• Garbage — human food waste is commonly 80% to 95% water by weight. So the food waste stream can be reduced by a factor of 5 to 20 by desiccation. Desiccation also stops the food waste from decaying before further handling. Furthermore, aerobic composting requires a humidity level within a certain optimal range, a range food waste normally exceeds. Incineration suffers even more from excess water than composting does. The usual solution to this problem is to add additional dry matter to the compost, but probably a better solution is to remove the excess water. At Burning Man, we use a very simple solution to this: we hang the garbage outside in net bags, where the sun and dry wind desiccate it within hours.
• Poop — much the same set of concerns applies as those described above for pre-eating food waste, but the desiccation process is more demanding, as the air will pick up shitty odors as well as water, and thus needs postprocessing before release into the environment.
• Dishes — drying dishes with a towel is prohibited by many human health codes, because the towel can both spread pathogens from one dish to another and provide a suitable medium for the proliferation of certain airborne pathogens. Active drying, as performed by dishwashers, can solve this problem.
• Laundry — of course a clothes dryer is precisely a desiccator. The modern unvented clothes dryer uses a heat exchanger to condense the water and pump it into a holding tank or drain.
• Moisture harvesting — humans typically die some 32 to 64 hours if they cannot ingest water, but can survive 1024–4096 hours without food. But they do not need very much water, typically only 2–4 ℓ/day (50 μℓ/s). When drinking water supplies are disrupted, they could survive much longer with a system that can recycle drinking water from breathed air, from evaporated sweat, from garbage, and from passing air.
• Cleaning tools such as mops — these are wet when in use, pick up a wide variety of micronutrients, are often composed of carbohydrates or protein, and can sometimes remain wet for a long time after use. This makes them good environments for the growth of microorganisms and fungi, which can be pathogenic to humans, smell bad to them, or both.
• People after bathing; hair dryers are one example, but an “air shower” of high-speed warm wind could eliminate the need for towels and the labor of washing them. This could even be installed in the same physical place as the regular shower.
• People after dying: the decomposition of human corpses involves many microorganisms that produce smells that humans find very disagreeable, and they can even be pathogenic to them. Also, they can host macroscopic organisms, such as flies, which also create health hazards for humans. Desiccated corpses do not have these problems. (In fiction such as Dune, they are also an important source of drinking water, but this seems unlikely to ever happen on Earth.)
• Coolth: evaporating water cools the system by the water’s enthalpy of vaporization, and this is an important way to cool human habitations in dry places like the Southwestern USA and Iran — and of course humans’ voracious appetite for water results from their biologically programmed use of water as sweat for the same purposes, but in other places the available water and the pre-existing humidity of the air limit the possibilities. Most of the uses mentioned above for a desiccator could additionally be taken advantage of to produce a lower-than-ambient temperature.
• Making aerogel, which requires supercritical drying to prevent the hydrogel to a xerogel.

These uses seem like they might require multiple different desiccation appliances. Poop will probably contaminate the poop desiccator in such a way as to make it unsuitable for desiccating laundry, dishes, food, and possibly even food waste. Poop, human corpses, and food waste should probably be shredded before desiccation in order to speed the process; this is never desirable for living people, mops, laundry, or dishes, and only sometimes desirable for food. Vacuum, as discussed below, may be a useful way to speed up desiccation for some things, but it is fatal to living humans within seconds, and thus its routine use after bathing could be counterproductive.

However, it seems like a general-purpose desiccation appliance could perhaps serve a substantial range of uses.

Physically, evaporation is a complex thermodynamic process, but the main determinants are temperature, airflow rate and turbulence, air humidity, and air pressure. Higher temperature, faster and more turbulent airflow, drier air, and lower pressure all speed up evaporation. The interaction among these factors is complex. The usual simplified model is that a thin boundary layer of air next to the moist thing is 100% saturated with humidity, and the air above that has a linear humidity gradient down from 100% down to the ambient humidity, driven by diffusion and advection. But the thickness of the saturated boundary layer also varies with airflow.

The exponential rise of water’s vapor pressure with temperature means that even a small temperature difference can make a big difference in the air’s moisture capacity, and the amount of water that can evaporate is driven by the difference between the air’s moisture capacity and its pre-existing moisture content.

I don’t have a good handle on the energy costs of using these different factors to accelerate desiccation.

Oven desiccation experience

I’ve been using my apartment’s electric oven over the last few months to desiccate food waste before discarding. It has a thermostat, a timer, and a circulating fan; I set the thermostat for 70° (160 archaic degrees) and the timer to an hour or two. 70° is hot enough to nearly sterilize foods, while not being hot enough to set common foods on fire in any reasonable period of time. The oven has the major disadvantage that the timer is disabled by power outages — following a power outage, the oven turns back on with no timer — so it isn’t safe to leave it unattended. Commonly I put the food on a glass plate or a sheet of aluminum foil before starting the oven, since I can clean or discard those easily if food gets stuck to them.

Vegetable wastes, even onions, generally smell pleasant during this procedure, which also reduces them greatly in volume and perishability. I try to ensure that they are sliced to a thickness of 10mm or less to accelerate the process. Animal cadaver bones lose less mass and smell less pleasant — they smell like cooking meat — but the smell is far less objectionable than rotting meat, which is what they would become in the garbage after a couple of days without pre-desiccation. Eggshells have no noticeable smell and become brittle quickly.

I’m currently trying to compost some of the results from this process, without any desiccated animal remains, but adding my own hair. It seems to be working somewhat — at least the compost has a reasonably earthy odor rather than smelling like rotten fruit, and it had only a few flies at first, nothing since. Most of its mass comes from yerba mate and eggshells. I suspect that the eggshells, which are made of calcium carbonate rather than calcium phosphate like bones, provide a pretty robust carbonate buffer against acidification, but I haven’t actually measured the pH of the compost.

In any case, desiccating food waste before composting it allows you to delay composting until you’re ready, which can be valuable when you’re getting a compost heap started and makes it easy to avoid anaerobic conditions in the compost.

Presumably, oily food could trap water inside a layer of oil, once water was removed from the outer layers of the food. I’m not sure I’ve observed this; even meat bones seem pretty dry if you crack them open after this process. Maybe it doesn’t happen; maybe water molecules can diffuse through the oily layer anyway, or maybe water has lower surface tension at higher temperatures and so it doesn’t push and pull the oil into a coherent layer, or maybe the water doesn’t have sufficiently coherent surface tension when it’s just occupying the pores of the foodstuff.

For a while, I was using a generic tupperware lid as a plate, since it was even easier to clean than glass plates. After lasting through dozens of cycles, one day half of it melted down onto the plate on the wire rack below it. The melted and resolidified plastic was stiff and brittle; the unmelted part remained flexible, resilient, and ductile, despite otherwise being similar in appearance. I’m not sure what happened; three possibilities occurred to me:

1. Maybe this time the oven got hotter than on previous occasions, but only part of it — the circulating fan was not as effective as usual. So the hotter part of the lid melted and degraded, perhaps through hydrolysis from moisture absorbed into the plastic. In this possibility, different levels of heat on this one occasion caused one part of the plastic to both melt and chemically degrade.

2. Part of the lid had come under chemical attack from some food I’d placed on it, either at that moment or in the past; perhaps it absorbed some oil, plasticizing it further and lowering its melting point. Oil by itself wouldn’t explain the brittleness, but maybe other factors could. Acid, for example. In this possibility, different levels of chemical degradation in different parts of the plastic caused the more degraded part of the plastic to melt.

3. Perhaps the lid had lost plasticizers to evaporation over time in the oven. If this were the case, though, you’d expect the part that had lost more plasticizers (and was thus more brittle) to be the last to melt, not the first.

Regardless, I don’t plan to use plastic trays in a desiccator in the future without thoroughly qualifying the plastic for handling the relevant temperatures.