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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.
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