This is part 2 of my design for my greenhouse. Part 1 is here.
When I had a nice big garden at my last house, water was a serious source of contention between my wife and me. Though we never did the math, her contention (probably rightly so) was that we spent more money on water than we got in benefit from food. It is just too dry here in Arizona. So in my design for my greenhouse the design of water was crucial. Now it is not to say that I am going to have a low water greenhouse. If that were the case, I would only be growing cactus and such and I really don’t want to do that. No, in this case, the concepts we are going for are wise use of water and a distribution system that reuses water wherever possible. For this, I have a two-pronged approach to water. Yes, I will have a spigot in the greenhouse, but it is my sincere hope that I will almost never have to use it. The vast majority of my water will come from either greywater or rain water.
1) Greywater
Here in Arizona, we have a blanket permit for using greywater. That means that, as an individual homeowner, I have the right to use my greywater without getting an individual permit. I am expecting some friction from the local municipality since they like to use the treated water to recharge the aquifer, but I am a strong believer that it is better to not pull it out and treat it in the first place than it is to pull out twice as much as you need and then put half of it back. Most people I have talked to seem to think I can squeak it through.
There is one problem, though, that becomes a major design concern. The blanket permit has a few stipulations. The one that really affects my design is that you cannot surface irrigate food crops with greywater. Honestly, this just makes sense. Whatever dirt you just washed off your hands shouldn’t end up back on your food. So I am taking a two-pronged approach to this problem. The first prong is to filter the water to get as much out as possible before it waters my plants. That should get rid of the hair and dirt and a lot of the soap before it gets distributed to the plants. To do this, I am going to use compost to filter it. The exit of the greywater system will be right next to the back door. The line will run under the sidewalk and dump straight into the compost. The compost bins will be fairly shallow and despite not being shown this way in the drawing (one level of detail I didn’t feel like doing), will be a little above the rest of the beds.
There will be two compost bins with a filter cloth of some sort between them. The water will flow into the first bin, flooding it, and then trickle into the second bin. From there it will trickle into a filtered tub in the corner where it will enter the distribution system. Since I have a dog and don’t want the greenhouse to be smelly anyway, the compost used for filtration will be strictly a repository of wood chips and plant waste from the greenhouse, with no food waste in it. I’ll try to put living mushroom mycelium in there to help with filtration, but I anticipate difficulties keeping it alive for very long. This will mean a non-smelly compost that has lots of microorganisms in it to help grab the nutrients out of the water. It will also mean that the water leaving the compost bins will be a sort of weak compost tea that will be composed of very little leachate.
The distribution bin will have two outlets, one that goes to the central bed and one that goes to the outer beds. From there, the pipe will run along the inside of the wall of the bed that is next to the path, about 6” below the surface of the dirt. There will be an outlet with a valve every two feet or so along the edge. That should give me a fairly even distribution of the water. If I have an area that is not planted or getting too much water, I can turn off zones as I need to.
One of the problems I have struggled with is the even distribution of water. If each outlet was at the same elevation, the dirt at the beginning of the system would get most of the water and the end of the line would get almost none. I plan to remedy this problem by having an inch or two rise between the bottom of the distribution pipe and the outlet of each outlet pipe. That way the pipe fills up before it starts to distribute water, so it should distribute fairly evenly.
The distribution system for the central bed will be similar to the one for the outer bed.
2) Rain Water
I am going to do my best to capture as much rain water as I can for the greenhouse. I am hoping to bury some sort of tank on the northwest side of the house that captures rain from at least half of the roof of the house. 300 to 400 gallons would be good, though I am going to try for as much as I can. I am also planning on putting some sort of water feature on the northwest corner of the greenhouse, though I haven’t decided what form that will take yet (just a big tank or a living system?). The overflow of the large house tank will go to the greenhouse tank to make sure it is full. From there, the overflow of the greenhouse tank will dump into a terracotta channel mounted along the edge of the outer raised bed wall. This terracotta channel will be grouted to the wall and tilted towards the soil. Assuming I can get it all level, it will fill up and then spill over into the dirt, acting sort of like a trench irrigation system without using any of my precious growing space.
A rainwater source and distribution system will be crucial to this working. Since I can’t surface irrigate with greywater, I need a way to water seedlings and other plants whose roots don’t go deep enough to take advantage of the greywater. The water feature will also have a spigot on the side for filling a watering tank. Then I can just top off the greenhouse tank with water from the house tank.
Ideally the combination of the two systems will serve to keep the greenhouse watered without too much effort from me and almost no use of additional city water. In fact, I did some research on how much the average household produces a day, and they say about 35 gallons of greywater per day per adult. Now we are pretty water conscious, so I’d say we are under that. However, with two adults and two kids, we’ll probably still generate about 50 gallons of greywater a day. That is probably more than I need. I might just have to turn the valve off half of the time and give the City its recharge water back.
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