Water evaporation Bad Idea Theater style
We don't claim to be experts and you shouldn't consider us to be. We simply want to share what we've learned with you and hopefully help you with your projects or understanding. The info here is for you to use with the exception of commercial use.
We assure you its mostly true.
This page is under construction, pretty pictures, examples and spellchecking to follow.
Evaporation: Evaporation Is the process of turning a liquid (a solid would be sublimation), into a gas (vapor). It requires:
Low humidity, the humidity of the atmosphere needs to be less than that of the surface to be evaporated. At 100% humidity no evaporation takes place. If you have ever been someplace very humid, you know how hot it is on your skin because your sweat can't evaporate to cool you.
Energy (heat),the evaporation of one gram of water requires 600 calories of heat. The energy given to the water molecules is what changes the water from a solid (or liquid state). You can see an active example of this in a pot of boiling water on the stove. ( 3785.4 grams per gallon = 2,271,240 calories or BTU's)
Air movement, Only a small surface layer of the water can evaporate at any time, the water below the surface has to wait it turn. The movement of air removes saturated air and replaces it with drier air ready to absorb water vapor. The molecules of moving air also have more energy which increase the energy of the evaporation system. You can feel an example of this by the cooling effect of a breeze on your skin.
These three factors are the most important in determining how much water will evaporate.
Another factor related to the above is surface area. More water surface that is exposed to air movement and heat, will result in a greater quanitity of water being evaporated. One of the few examples where bigger actually is better. You can test this by noticing that the same quantity of water in a shot glass will evaporate slower than in a shallow pan. You will also notice if you spill an ounce of water on the counter and pour another ounce on a paper towel, that the water will evaporate from the towel first. This is a result of increased surface area, and the use of an absorbtive material to increase the surface area is key to another discussion down the page.
Evap ponds: Evap ponds have been used for years in the Black Rock Desert. They usually consist of a 2x4 frame lined with plastic sheeting. The plastic is usually black to absorb as much sunlight (heat energy) as possible. I have read some claims that one could evaporate as much as 1 quart per square foot per day. For a 4' x8' pond this would be (32 sq ft. / 4) 8 gallons per day. Some of the drawbacks to a pond are smell (see destinkification), contamination with dust (mud), the size of the system (/trip hazard), and most importantly, to me, difficult clean up. However, this isn't to say that many people haven't had success with them
One of the other drawbacks might be the mirror like surface of the water. Full summer sunlight provides about 1000 Watts per sq. Meter (75 Watts/sq. ft.), depending on how you measure it and how sciencey you want to get. When the sunlight hits the surface of the water a substantial amount of energy is reflected from the surface and never has a chance to transfer heat (you can see this when walking beside a swimming pool between you and the sun). Some would argue that the dust makes the water cloudy and obscures the black plastic, reducing heat transfer, although I believe the dust particles would trap heat.
Any open system is subject to dust and the convenience of size is up to ones personal tastes, the most serious drawback that I see is clean up. Unless you stop using the system days before there will always be water left in it. Water in the form of mud or muddy water. This is very difficult to remove on a flexible plastic sheet, It will also contain in a concentrated form, all the impurities and contaminants (we call it Yucchh) that were originally dumped in the system, that have had time to fester in the warm sun.
Destinkification: Any greywater system will contain contaminants as well as water, that's what makes it greywater(graywater). If your greywater is shower water it will contain soap and shampoo, and dirt, hair, skin, sweat, and all the other wonderful things your body produces.
If your greywater contains kitchen waste it gets much worse. Take a look in your refrigerator in the crisper drawer (I've never seen anything get "crisper", it should be called the rotter). Organics like food waste are perfect breeding grounds for bacteria as they decay. Kitchen waste can be dried to stop the decay but can't be evaporated and keeping them in a wet environment like a muddy pool is an invitation to all kinds of wonderful smells and unhealthy things. Toothpaste and oils also fall into the category of things that can't be evaporated.
One way to avoid or lessen the "Yucchh" factor would be to separate the waste products. Drying your kitchen waste on a piece of hardware cloth over some black plastic is one solution. Filtering your greywater through a succession of screens to strain out contaminants is another. If you don't let sewage into your system, it is less likely to be an open sewer later.
One way to deal with kitchen waste and other things that shouldn't go in an evap system would be a solar oven. By surrounding a metal can (new paint cans are sold at Home Depot with lids) that is painted black, with reflective sheets (Mylar, Foil, windshieild shades, etc), you can boil off the worst of your waste and pasteurize it to kill the bacteria. At the end simply put the lid on it to seal it. You can find free plans for solar ovens on the web. Use caution these will get the can hot and there is no on/off switch except for blocking the sun.
An activated charcoal filter will trap organic contaminants, and one could be easily built to clarify, to some extent, you wastewater. The water could also be treated to kill bacteria, using something as simple as household bleach.
EPA Safewater page
(Take care with this, more is not better and bleach is caustic, it only takes drops to be effective. If you can't be bothered to do it correctly, please don't do it at all). Chlorine disintegrates under exposure to sunlight and its effectiveness is modified by the PH of the water. You can read more about this by googling swimming pool chemicals. This is something you wouldn't need to go overboard with, if your water went into a holding tank first you could disenfect it before it ever reached the evap system. This will not turn it into clean water, it will only kill some of the bacteria.
We are currently in the process of building of building and testing our "next Generation" evaporation design. A lean and mean version of our previous design incorporating maximum evaporation in a minimum footprint, lightweight, portable design.We'll post new info as it becomes availible.
This page won't be finished until fall of 2005. We'll be adding more as time allows and boredom returns.Don't say we didn't warn you!
Other sources of evaporation info to check out (good Luck!)
Some questions regarding evaporation
More sciencey stuff
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