Rainwater Harvesting for Potable Use
So you ask, "How's that rainwater harvesting thing going?" Just great. Thank you for asking. We built our new 2,500 square foot four star Green home on 21 acres near Jarrell, Texas, in the spring and summer of 2010. Our builder, David Weekley's Custom Build On Your Lot division was very flexible and worked with us to do a lot of advanced high performance home measures. One was plumbing the house to use harvested rainwater as our source for all the potable water inside the house.
We finished the house in August of 2010 and it stopped raining in September of 2010. As any Texan will tell you, it was hot, really hot, even by our standards for the next two years. In fact, as it was in much of America, here in central Texas, 2011 and 2012 are the two hottest and driest years on record! Lake Travis is only 35 percent full, and in 2011, we had 90 days of temperatures over 100 degrees. We average only 12 days of temperatures over 100 in a typical year. Our new home is plumbed to use harvested rainwater as our primary source of all indoor water.
The water quality is top shelf:
It's quite literally like having bottled spring water at every tap and shower head. The water is soft, odor free, and pure. My wife says it makes her hair more manageable, and we both can't help but notice the odor of chlorine in our water when we go to an area restaurant. She also noticed that she only needs about one third of the laundry and dish washing soap that they recommend. There is no hard water buildup or scum anymore either. By the way, our groundwater is really hard, and it also has the distinct odor of sulfur or what is often called "rotten egg" smell.
The Dollars and Sense:
I often wonder how much a "typical" family interested in high quality water spend annually on bottled water? Our rainwater system cost us $21,000. That includes some things like gutters that we would have done anyway. A well would have cost about $12,000. So, we incurred about $9,000 in additional expense that will be amortized over the life of the mortgage. The rainwater system is an additional marginal cost of about $43/month as a part of our mortgage. For that we have no monthly water bill, we don't put salt into our water, we don't pay Culligan, we didn't buy and maintain a reverse osmosis machine, we don't spend a dime on bottled water, and we have a virtually unlimited supply of water. I'll let you figure out how long the payback would be for your family in dollars and quality of life. Some folks don't spend a dollar on better water and some spend a bundle. To each his own I always say.
Let's Do the Water Supply Side Math:
If you want some hard numbers, here you go. Our house and garage have a total roof area of about 3,000 square feet and our roof is a standing seam galvalume (http://www.galvalume.com/ ) roof. The cistern is from an Australian company, Pioneer (http://bluescopewater.com.au/our-brands/pioneer/ ), and it holds 16,500 gallons in storage. The cistern we chose looks like a galvalume tank (the silo look matched our roof and fit our rural setting) that is eight feet high and sixteen feet across, but you can get a wide range of colors if you like, and some people bury their tanks. It's lined with a five layer thick bladder (http://bluescopewater.com.au/products/rural-tanks/tank-liners/ ) comprised of food grade elements. Because it's totally protected from the sun, kind of like being down in an aquifer, no algae grows in it. Now for the big news: the cistern has never been below 8,500 gallons of reserve since our second month in the house when we got a big thunderstorm that almost filled it.
The keys are to have enough roof area relative to the number of people in your household and to buy enough storage to be comfortable. I would also mention that making your home water efficient is important. When we were looking into buying our cistern, the vendor made a recommendation for a 10,000 gallon tank based on our needs. I was going to do it but wondered about the what if of a long drought. So, I asked what the next larger tank was, and he told me it was a 16,500 gallon capacity tank. That extra 6,500 gallon margin of safety sounded good to me, so I asked how much more that would cost. He said it would increase the job cost by 10 percent. I simply said, "Do it." I figured that the marginal cost was so small and for it, we got two-thirds more water in reserve. I'm glad I made this decision. We've never needed that extra storage, but it certainly does make you feel better to see that even in a drought of record, you still had six months of water.
The rule of thumb is that you can harvest 600 gallons of water for every inch of rain falling on a thousand square feet of roof area. In our case, we have 3,000 square feet of roof area. When we get one inch of rain, we harvest: 3 x 600 gallons = 1,800 gallons of water. In our driest year on record, we got sixteen inches of rain. That would mean a harvest of 28,800 gallons. In an average year here just north of Austin, we get thirty-four inches of rain and that is a harvest of 61,200 gallons.
Wise Water Use:
Now all of those numbers mean nothing without a frame of reference. When we built our home we specified that all water fixtures and appliances must be chosen from the U.S. EPA's WaterSense Program (http://www.epa.gov/watersense/ ) and our clothes washer is one of those horizontal axis models. WaterSense is like the Energy Star program but with a focus on water efficiency. There is also one other aspect to WaterSense: the fixtures are tested by consumers, and they have to like them. I swear, our shower head is a low water use fixture, but it beats the heck out of those old water conserving fixtures I've experienced so often in motels. I had to test it myself to be sure it was less than 2.0 gallons a minute. Yes, I stood in the shower with a bucket and a timer. OK, so I'm a bit of a geek. Guilty.
I've watched our water use, and our family of two uses about 1,500 gallons of potable water a month. We don't do anything weird and, to my mind, haven't changed our habits since we moved in here. At our old house, which we built in 1984, we used around 3,000 to 4,000 gallons a month, but with these new WaterSense fixtures and appliances, it has dropped a bunch with no loss of comfort. I still take ten to fifteen minute showers. I admit, at first, with the drought, I was a bit concerned, and I cut back on this luxury. But, after I saw how well the cistern's water balance did, I admit that I backslid to my old sinful and wasteful ways. Why not? Our water use doesn't impact the local aquifer or the lake levels.
We only need 12 inches of rain a year to break even and to keep the tank refilled. A family of three or four would only need 24 inches of rain to do the same. That's not a problem in most of the country. It appears to me that rainwater harvesting would cover somewhere between one-half to all of a typical homeowners indoor needs if they used WaterSense fixtures. If you didn't want to flush with your rainwater (as we do), you could have just the sinks, laundry, and showers plumbed to use it.
Cleanliness is Next to Godliness:
We do a three step treatment regime. To start with, rainwater is a relatively high quality source of water. It is, after all, natures own distilled water. We are also lucky that being fifty miles outside of Austin, our air quality is quite good. First, we filter the water to remove any particles larger than 5 microns or five millionths of an inch. Then we do a second round of filtration with an activated carbon (carbon block) filter that also catches stuff that's larger than 10 microns but, more importantly, it removes any organic chemicals present. Activated carbon is what is used in most commercially available home water filtration systems. The third level of purification is a high intensity UV (ultra-violet) light chamber. Here, the water passes through a stainless steel tube housing a UV light. The UV kills any pathogens that might remain in the water. I change the filters every six months and the UV light annually at a cost of about $160.
Water Pumps and Hot Water On Demand:
We rely on a one hp Grundfos pump (MQ-45) that operates on demand. When it senses a flow of 0.3 gallons a minute or a pressure drop in the line to the house, it immediately comes on to provide a steady water pressure of between 50 and 65 PSI. The whole thing works like a charm.
We also didn't want to have the inconvenience or wasted water associated with having to wait forever for our hot water at the sink or shower while our water flowed down the drain. I've had homes where you could turn on the water, get undressed, come back to the shower and still have to wait before the hot water got there! As an energy guru, I knew that a pump running constantly powering a circulating hot water system could cost up to $80 a month in electricity to run the pump plus the heat lost from the hot water running all over the house and coming back to the water heater cooled by the trip. So, we installed an on-demand hot water system called the D'Mand system from Advanced Conservation Technologies. It's really cool and convenient, too. You can go to the web site if a diagram and more detailed explanation interests you ( http://www.gothotwater.com/ ).
When I'm preparing for a shower, I push the "hot button" (as I call it) on the wall next to the sink in the master bathroom. The little pump charges all of the hot water lines in the house with hot water in just 15 or 20 seconds. It has a thermocouple attached to the hot water pipe farthest from the hot water tank and when it senses that the hot water has made the full circuit, it shuts off. No water has been wasted down the drain. The cold water stored in the hot water loop is simply sent back to the hot water tank! When I turn on my shower, I have hot water right there with no waste and no wait. You would be stunned at how much water this convenient little device saves. I figure the simple payback in electricity and water savings at about one year!
How Do We Heat Our Water?
We also installed a Geo-thermal heat pump system with an integral hot water heating unit. This means that our hot water is almost free. In an all-electric home, that saves us about $35- $40 a month in electricity. I'll write a blog on that system soon, so check my web site for it if you are interested.
That is the story of our two and a half years of living in central Texas with a rainwater harvesting system for our healthy potable water and an on-demand hot water recirculation system for efficient and convenient hot water. Of all of the "green" stuff we did on this house, these two are right at the top of our "we're glad we did that" list.
Just another day in the life of a building science geek...