Last week we left you with some ways to figure out how much water you need to store and what sorts of gutter considerations you need to think about. This week we will be going through our roof washer, first-flush system, and our barrel inlet and outflow design considerations. This will effectively wrap up the design portion of our water collection system.
We are going to attempt to create a large rainwater harvesting system for our home. This system is temporary and removable, so it does not apply to Portland city regulations. In most cities, harvested rain water can only be used for irrigation. You must check with your city laws to determine if this is the case for you. It is not, under any circumstances, legal or safe to connect a harvested rain water collection system to a municipal water system. This is because a contaminant could be brought into the city water through the rain water harvesting system and endanger other peoples’ lives. Please check with local government agencies before using water for uses other than irrigation. It is also common to only allow collection of water from roof tops, so if you are planning collection from somewhere else, it will probably need to be checked out.
Downspout Flow requirements
Often in water related projects, you need to calculate your precise flow requirements for all parts of the system otherwise havoc may ensue or damage can be done to facilities. In a small rainwater collection system, this just isn’t the case. The reason is because it is a gravity fed system with zero pressure. All we have to do is ensure that our flow in all parts of the system equals or exceeds the downspout flow. For most of the system this means we just need to equal the cross-sectional area of the downspout. The only part of the system that will need to be checked out is the roof washer, first-flush system, and barrel outlet flow.
There are two ways to find the flow through the three parts: 1.) we can calculate it–or, rather, Pam can calculate it as this is a civil, dirt-pusher sort of thing to do, 2.) we can just build the thing and measure it in some sort of test setup–a slacker sort of mechanical, grease monkey thing to do. We will be doing both for the sand filter. The calculations are given here, but the test measurements will occur in the build process. This just sort of seems fun to us. We’re nerds…sorry. Check out our links page for references in sand filters here. You can find first-flush information in the rainwater harvesting site links on the same page.
Roof Washer
A sand filter is the type of roof washer that we are going to make for our rain water collection system. These kinds of filters work by being under constant flow and allowing organisms to eat the organic content in the rain water and by preventing passage of particles larger than the pore space of the sand. They can eliminate many diseases this way and reduce the organic content in the water drastically. However, our filter is not going to be working under constant flow because it does not rain all the time. With that being said, we cannot expect to reduce diseases very much with our filter, but we can expect to filter out any particle that is larger than the space between sand particles.
We plan to make a sand filter out of a five gallon bucket. We will place gravel at the base of the bucket, place trench wrap on top of that to prevent sand passage, and finally the sand on top of that. We have completed some calculations to estimate the flow through the sand filter. For brevity, please see the spreadsheet at the bottom of the blog for calculations. This spreadsheet includes basic instructions on use and allows you to enter your own values.
First-Flush system
When we first came across this idea of a first-flush system, it seemed kind of stupid and wasteful. Why waste the water you are working so hard to collect? The reason soon became obvious. Dirt and debris build up on a roof in between rain events and if you don’t let it get washed away, you will just have to filter it out of your water stream later. This is what a first-flush system accomplishes. This will help keep the overall contents of the barrel collectors much cleaner.
Our first-flush system will be of a very simple design discussed in some of the reference material we collected. It consists of a vertical pipe coming off of a horizontal section of the downspout pipe. The volume of this pipe needs to equal or exceed the amount of water you want to discard at the beginning of each rain event. Generally, here in Portland, this volume is 10 gallons. The bottom of this vertical pipe should be capped and have a small drain hole drilled in it to allow the water to slowly drain away. See the diagram at the end of the article for details on what this whole setup will looks like. Our drain hole diameter will be 0.068in for a total drain time of 100 minutes.
The eaves on our house are not high enough to accommodate very tall vertical pipes so we plan to use two 5 gallon buckets stacked together to form one 10 gallon reservoir. One of the buckets will have the bottom cut out of it and be placed inside the other one. At the bottom, we plan to place a wire frame around the outlet with trench wrap on it. This will prevent particles from clogging the small outlet. You want the first-flush system to drain as slowly as possible. If it drains too quickly, clean water will be wasted during long storm events. Our drain hole size was a compromise between long drain times and clog prevention.
Barrel inlet and outflow
For our water collection system, it was decided to use standard food-grade barrels and a garden hose faucet. This sort of attachment allows us to easily water our lawn and also allows a line into the house for a future filtration unit without committing to a rigid PVC line or holes in walls. Also, it is best to have an elevated platform for the barrels sit on so that there is pressure in the line when draining the tanks.
We want to know some basic information about how our system will operate:
How long does it take one barrel to empty, assuming it is full, from a hole discharging freely?
This calculation can be used to calculate the flow out of the barrel and the flow going out of the first-flush system tube. The only difference is the water level difference and diameter difference. We have provided another spreadsheet at the end of this blog so you can just input values for yourself. The spreadsheet includes a calculation for the max pressure at the bottom of the tank, but we did not show that here because it is not necessary in designing the system. Now that we have without a doubt classified ourselves as nerds, you get a bonus calculation, with love 
Let’s calculate the flow (Q) out of the barrel through a hose. This will tell us how much water is going to come out of the barrel in a given amount of time. It is most usually measured in gallons per minute.
Controlling equation:
Q = C*A0* (2*g*h)^0.5
“This is an image to show the drainage system that we are trying to calculate as well as a table to show the appropriate C coefficients.”
Description of variables:
Q = Flow
A0 = Area of opening (where the water is coming out of the barrel)
C = Coefficient of friction due to opening edge sharpness and faucet
g = Acceleration due to gravity
h = Height from top of water surface in the barrel to the center of the water outflow from the barrel
We know:
h = 2.5ft = 30in
g = 32ft/sec^2 = 384in/sec^2
A0 = (0.5in)^2*Pi /(4) = 0.1963in^2
C = 0.8 for short tube
Which gives us:
Q = 0.8*0.1963*(2*384*30)^0.5 = 23.8in^3/sec
Q = 23.8in^3/sec = 0.013 ft^3/sec = 0.826 ft^3/min = 6.182 gal/min
Say we have a container that is 55gal and volume divided by flow will give us time: therefore, 55gal/6.182gal/min = 8.89 minutes to empty a single barrel.
Obviously, the flow is going to change with whatever dimensions your water storage containers are. If you want to know the flow coming out of your barrel, then you need to complete the calculations for your specific circumstances. As a design aid for you, we have put together an Excel spread sheet so that you can simply enter your values for h, d, C, and your containers volume in gallons and it will give you A0, Q, and your time to empty a container. See the bottom of the post for a link.
Barrel Inlet
Our barrel inlet needs to satisfy a few requirements: 1.) it needs to be at the bottom, not the top, of the storage barrel; 2.) it needs to be about 3 to 6 inches above the bottom. Both of these reasons have similar motivations. After a rain event, the water in the barrel will settle and the particulates will move to the bottom of the barrel leaving clean water on top. When we put more water in an already partially full and settled barrel, we don’t want to dump it down through the clean water. Thus, we want the inlet at the bottom. Similarly, we don’t want the incoming water stirring up the sediments on the bottom of the barrel so we put the inlet 3 to 6 inches above the bottom and not pointed directly at the sediment laden bottom.
We will be using a barbed pipe fitting and some silicone sealer to provide a connection to the outside of the tank. From this, we will run a PVC line to the top of the barrel where the roof washer will connect. You will notice that the first-flush system is located before the sand filter so that the majority of the large debris will collect in it rather than the sand filter.
Barrel Outflow
Similar to the inlet issues, we have the outflow. We don’t want to take out the sediments at the bottom of the barrel so we will be using a flexible hose connected to the outlet and a flotation device (see the diagram bellow). This will keep the entrance to the outflow just bellow the surface where the water is the cleanest yet also affords us the most pressure in the tank.
Rain Water Collection System Diagram
The diagram below shows you how all the components that we have been talking about work together. We find that pictures explain much more than words on many occasions.
Note:
We have seen some other sites propose stacking the rain barrels on top of one another. We do not recommend this unless the barrels are on a support frame that can handle the overturning forces of the 400+lb barrels full of water. Stacking the barrels with out a support frame could cause them to tip over and injure people or things in a seismic or wind event. This is highly unsafe.
That’s it for this time. We find too much math bores people to tears and causes us to do unspeakably private things to each other. Next time we will post build pictures, steps, and details of how to install screens on your gutter, the sillcock in the barrel and divert your downspout to the barrel. Don’t forget to drop us any questions, corrections, ideas or comments you have. We love to hear from you so don’t be shy!
Links:
Barrel outlet flow & pressure calc Spreadsheet
Flow through sand filter calc spreadsheet
P&S
