This is written information on the shower loop system I designed for my mini bus conversion. I want everything we design to be open source and available to anyone who wants to give it a shot. Just remember, you build it, you're responsible for it.
Our bathtub is a 2'x3' galvanized horse trough. It will sit on supported base that is 3.5" high on the drain end and 4.5" high at the other. The trough is flat and water needs to move toward the drain. On the shower / drain end of the tub is a water closet containing the loop system.
Water drains from the tub (we do not use surfactants or lauryl sulfates or any non organic, non environmentally friendly products on our hair or body or the dog for that matter). The drain contains a "sock filter" our is likely gonna be pantyhose to catch hair as we have a lot of it. The filter can easily be removed and replace or cleaned from inside the tub. So the water goes through the drain filter and first hits a bypass. Water can go off the bus and water a tree from here or enter the loop system at Pump #1 inside the water closet. Self priming 12v, 1.4gpm, can run dry safely.
Pump #1 pushes the water through a very cheap granulated carbon filter generally used for garden hoses and such which removes most of the shampoo and gunk. This filter is installed in order to lessen the demand on our more expensive filtration system later in the loop. The water goes through the GAC filter and up into a "faucet" which is really a spout for the hose made from bamboo which pours the water out into a sink.
Back inside the shower: The wall you are facing has a square cubby hole 16"D x 19"W x 16"H which contains the bamboo spout and sink and the water temporarily pools here. Why? because we don't have enough power to have a water heater. Inside this hole are two digital fish tank thermometers. They can be seen inside a plastic box above the cubby hole. One says "coming in" the other says "going out". As they read the temperature of the water leaving the shower loop and coming into the shower loop.
Still with me? It is also here where the shower "turned on". 1.3 gallons of water are held in storage in the water closet or in the cubby sink. 1qt of water is heated on an electric burner or wood stove so that when combined with the room temperature water, it reaches 105 degrees (a good hot shower). There's a forumla for this. To start the shower, unplug the cubby sink. But for now, consider the loop is running and water is flowing into the cubby sink from Pump #1 but now the water is cooler. When water leaves a shower head at 105 degrees, it will go down the drain at about 95! As your body warms up, the difference is dramatically less.
So, back while you were heating up your water for your shower, you also fill the bottom of the pan with rocks (NEVER HEAT THESE ROCKS OVER A FIRE. rocks heated this way can boil water! heat your rocks IN the water you are heating). Also, do not use rocks you find around creekbeds or other porous rocks as if they heat up enough, they will crack apart. round, flat, egg-shaped landscaping rocks work well. You pour your water into the cubby sink to bring the temperature up to 105 and place the rocks into a ceramic container that sits inside the shower. When you see that the "coming in" temperature is below, say 101, you can drop a rock or two into the cubby sink and reheat the water. Two rocks will raise the temperature ~3 degrees! (we've had numerous kitchen experiment days with thermometers and calculators, rocks and buckets of all material). Now, that you've reheated your water the water the "going out" thermometer should read your new, heated, temperature.
This newly heated water, drains from the cubby sink and primes Pump #2 which pumps the water through a Pure Water Systems brand water filtration system. It's an under the sink model with a ceramic, a carbon block, and UV sterilization filter. The water goes from the filter into a shower head that increases the pressure slightly if needed.
SO: Down the drain, Pump #1, GAC filter, cubby sink, reheated, down the drain, Pump #2, filter system, shower.
This system will cycle approximately 1.5-2 gallons of water in a continual loop. Reheating the water with stones is no more inconvenient than reaching down to adjust the water in your own shower. One just has to be more aware of actual water temperature. You will go in with 2gallon, but you will come out minus about a liter that was absorbed by your body or left as steam so some water replacement will be needed after each shower. Although it reuses water, the filter system is STILL filtering 40 gallons as if you took a real shower so the filters will still only last 6months to a year depending on how much you use them. a 30 minute shower uses approximately 337 watts of power (250 of which is from heating the shower water on the burner, if you used the wood stove instead, total power usage for a 30 min shower would be 87 watts)
Some Notes: You could reheat your water using more hot water instead of rocks however, adding too much water will flood the loop and cause water to pool in the tub. Also, that hot water will cool down faster than the rocks in a ceramic container. Also, we feel the rocks gives the shower more of a sauna, zen-like atmosphere. We like it <3
Rain catch can be integrated in order to have a bath. Getting 40gal of water up to 105 would be more of a challenge, however. Also, you'll be pulling an additional ~400 pounds of rainwater.
It will be a couple of months into the build before the water system is installed but at that time, we will post pics and hopefully video of the operating system. In the meanwhile, feel free to ask questions.