Designing a High-Tech,
Hands-Off Rainwater System
by Doug Pushard
© 2005 Doug Pushard, All Rights Reserved
Even though our yard is planted with native vegetation adapted to our hot Austin, Texas, climate, it requires some watering to keep it healthy and lush.
We use a drip irrigation system for watering, which is more efficient than using sprinklers, but this small city yard still consumes lots of water during our long, sweltering summers.
Here's a step-by-step account of how I built my rainwater harvesting system to beat those hot Texas summers and kept my garden green.
Rainwater Harvesting: A Solution for the Dry Season
Our solution was to tap the clouds, instead of the city pipes, to keep our yard green. Harvested rainwater has zero hardness, contains no chlorides, and has fewer salts than city water, so it is much better for our plants.
Using rainwater instead of city water also reduces our monthly municipal water bill, while reducing the impact on the local watershed.
Water Harvesting Components
Austin receives about 32 inches (81 cm) of rain a year, and our roof offers plenty of catchment area, so we did not have to worry about rainwater supply or overall system efficiency. On average, we get 2 to 5 inches (5-12.7 cm) of rain each month. The problem is that it comes in buckets-all at once and then nothing for weeks.
From our water bills, I guestimated our outside watering needs to be about 1,000 gallons (3,785 l) each month on average. We had already installed gutters on the house. Now we just needed some way to store and deliver the rainwater to our yard.
Rainwater catchment systems typically consist of three main components:
- the capture system (roof and gutters)
- the storage system (barrels or tanks), and
- the delivery system (pipes, pumps, and valves).
Our rainwater capture system uses gutters that are connected to round downspout adaptors to underground, 4-inch-diameter (10 cm) PVC pipes. These pipes channel water first to a filtration tank and then to the storage tanks.
I chose two aboveground fiberglass tanks for rainwater storage. In-ground tanks would have kept the water cooler in our climate, but would have been significantly more expensive. The durable fiberglass tanks, which are ultraviolet (UV) light-resistant, can be used for both potable and nonpotable water. I placed the tanks on a concrete pad in a corner of the yard, which keeps them level and out of the mud. The two tanks provide 4,000 gallons (15,142 l) of storage capacity, which is not enough water to meet all of our household water needs year 'round, but a good portion for most months every year.
Rain gutter screens remove most of the larger debris from the water. A horizontal roof-washer filtration tank from Tank Town, which is equipped with a sump pump, has a 60-micron filter that screens out the smaller particulates.
When the filter tank fills with water, an auto-on switch on the sump pump activates and pumps the water through a 2-inch-diameter (5 cm) PVC pipe connected to the top of the first storage tank. A check valve on the pipe prevents water from draining back into the filtration tank when the sump pump goes off. This is one of a few small details I added to make the system especially user-friendly.
Near the ground, 2-inch-diameter, flexible PVC pipe ties the two tanks together. A three-way valve tapped into the middle of this section connects the line to a primary outlet line. A pressure-sensitive pump delivers the rainwater to the irrigation system. The pump automatically activates when the sprinkler system valves open or a hose spigot is turned on.
In-line PVC unions are installed on both the tanks' inlet and outlet pipes. This, along with the three-way valve, enables each of the tanks and/or the pumps to be easily disconnected from the system for maintenance or modification.
When I expand the system to provide potable water, I'll need to install a UV filtration system before the second pump. The three-way valve and unions will allow me to do this easily and efficiently.
Because our system is interconnected with a municipal water source, it was required to have a high-hazard pressure relief valve,also known as a backflow preventer. This valve ensures no tank-stored water can enter the city water supply. Regulations vary from city to city, so check with your local water municipality first to see what type of valve they require, if any. . I installed the relief valve near the front of the house, so when I start using rainwater for our household needs I will not have to relocate it.
The sprinkler junction or valve box, in which city water lines join with rainwater system water lines, may also require a permit. I used two standard-sized in-ground sprinkler boxes. One box holds all the sprinkler valves and the other the system interconnections. This is primarily, a three-way electric valve made for swimming pools that connects the tank and city lines with the outgoing line that feeds the irrigation system. I initially used a sprinkler system valve instead of a pool valve, which was a mistake. Sprinkler valves are low-cost and widely available, but only work one way and only when there is pressure in the line. I also installed a double check valve in this box to prevent tank water from entering the house. I will take out this valve when I install the UV filtration system. Screw-in couplers and manual off/on valve are on both sides of these valves enabling easy removal.
Automating the System
Remainder of this article issue #115 of HomePower magazine.
Questions and Answers from the Readers of the Article
The following are questions from Johnny. Thank you for the questions and for the comments on the artice.
Question: Let gravity do some work for you, the system has 2 pumps; the pump to fill the tanks can be eliminated if you discharge the water from the roof gutters directly to the tanks. Some redesigning of the filter system can be done. The benefit is that you eliminate the first pump energy cost and has the tanks can be filled with water even is there is not electric power. Alternatively, you could run water into the bottom of the tank instead of the top.
Answer: I did look at this. Although the tanks are at the lowest point of the yard, there is not enough vertical lift to lift the water into the tanks and of course lift is required to turn on the water since it nows needs to move uphill. I did look at the placement of the tanks to see if I could just drop the water off the roof and into the tanks. The way the house is situated on the lot this was not possible. To minimize electrical use the pumps only run when required. Consequently, they are only on for a very short periof of time. I did not think about running it into the bottom of the tank and this would have worked for some of the fill, but I would still need the pump to fill the tanks.
Question: You should add an overflow pipe to the tanks if the tanks fill with water during a rainstorm the excess can be disposed by gravity automatically to the nearest street gutter. You can eliminate some sensors and electronics here.
Answer: The tanks do have an overflow, I failed to mention this in the article. The electronics are for when/if the filter box near overflows. This might happen very infrequently and in hindsight was a bit of overengineering on my part.
Questions: For safety reasons the backflow preventer should be moved to the nearest part of your rainwater system to preventer cross contamination of your house domestic water supply system. Even with a UV unit in a close system some bacteria or fungus can wreck havoc in your gastric system.
Answer: The city requires the backflow preventor to be before the first house inlet. So I installed two units. The first one is near the street and is a High Hazard unit as required by the city. The second is near the electronics box. Consequently, no cistern water should ever enter the house. This design gave me extra protection, while enabling the system to be expanded in the future for house use if I added a whole house filtration system.
Question: You should think adding a limestone contact chamber to help neutralize acid rain. Limestone is inexpensive but is redundant assurance in case you have plans to use it in your house domestic water system. As an alternative put some limestones CAREFULLY inside the tanks.
Answer: This is an excellent suggestion. I never thought of adding limestone to offset the acid.
Questions: Use other hidden water resources like air conditioning water drippings, you will be amazed by how much water you can collect in a week. You can use or construct easelly a parallel greywater system, you can blend gray water and rainwater for your irrigation system. I only use the rinse water from the delicate cycle of the washing machine because it needs the least treatment, again you will be amazed by the quantity of water you can collect.
Answer: Totally agree on this. It is always best to conserve before installing a system. It will save money in the long run and require a smaller system to be installed.
Question: If you plan to use your system for domestic use, try first using the water for toilet flushing. If you use a septic system you should not have any problems but if you have a connection to sanitary sewer you should check if the permit the discharge of rainwater through the system. For this I use a Kohler Class 5 toilet.
Answer: Agree on this one as well. It would be prohibitively expensive in my house with a concrete foundation. In house that has access under the house this is a very good idea.
Questions from other readers:
Question: Where can I get the transducer you mentioned in the article, the LM31.
Answer: The transducer is made by Measurement Specialties (www.msiusa.com) and can also be bought at Digikey.