Sunday, January 1, 2012

The Sustainability Review – Water Storage

By Mike Forbes, Co-op Volunteer Writer

Last month we talked about calculations with the goal of obtaining realistic numbers of our water consumption.  In talking with a friend he mentioned that looking at your water bill is a simple way for those who live in a city to determine this.  On your bill you’ll see your consumption, typically in cubic feet which is easily converted to gallons by multiplying that number by 7.48.  I thought this would be good to pass on.

Now on to the most challenging aspect of rainwater use, storage. People use various methods such as plastic, wood, and concrete tanks, old dairy trucks, wooden wine barrels, and above ground swimming pools.

Our house water supply which includes drinking water consists of four, dark green 1600 gallon HDPE plastic tanks.  This is probably one of the most foolproof, proven and affordable methods of storing water.  Dark colors are important because algae will grow rapidly in a light colored tank if exposed to light.  Having more than one tank is nice on several fronts. They are easier to handle, readily available, comparably inexpensive (our tanks cost approximately $600 each delivered) and provide redundancy in the event a leak.  The HDPE plastic is the same material that milk jugs are made from and current testing shows that it doesn’t leach chemicals into the water.

Above or below ground?  Above is generally easier but not always practical as water freezes.  Our tanks are located above ground in a room with 2” of foam insulation around them.  I don’t fret about the tanks themselves freezing as it would take a really big cold spell to freeze that much water.  However, I do worry about the plumbing and watch it closely when the temperatures dip below freezing.  In 6 years we have never seen pipe temps below 37 deg F. 

If you decide to go with a below ground tank make sure it is rated for below ground use as they are structurally very different.  On the converse, below ground tanks typically rely on the surrounding dirt for stability and can collapse in on themselves if used above ground (ask me how I know this?).

For most people it’s not going to be realistic to store all the water that falls on their roof as this would take a very large tank(s).  This is where calculating usage is essential to balance what you use with having an adequate supply, even through the dry months of summer.  I analyzed the rainfall data by month and estimated when we would run low.  We go into the summer with full tanks from the spring rains and when July comes we don’t see much rain until September.  The lowest our tanks have been was 750 gallons total at which point we started hoping for rain.  An additional tank would give us a more comfortable buffer.

Many people are going to use rainwater for irrigation only.  In this case many problems are eliminated since you can have storage located outside that is seasonal and drained during the winter months.  I’ve seen many people use wine barrels at the base of their downspouts for this and others place large tanks out in the yard.  As long as you are diligent in draining your system in the fall this can be a very effective setup.  My current plan for larger scale irrigation system is to fill a 24’ above ground swimming pool providing over 13,500 gallons of water.

Water tanks can be sourced locally through Hahn Supply and occasionally craigslist.  A word of caution on used tanks. Many have been used for chemical and fuel storage.  These liquids may have absorbed into the tank and would not be a good choice for drinking water.  I see very few used tanks out there that are suitable for drinking water, not to say one might turn up tomorrow. 

If you are looking for more information, our library has the definitive resource on water storage is Water Storage by Art Ludwig.

Mike can be reached at and welcomes questions/comments.

The Sustainability Review – Rainwater Calculations

By Mike Forbes, Co-op Volunteer Writer

Break out your thinking caps, actually just your middle school math skills for this month’s article. We are going to figure out your actual water usage and see if rainwater can supply your household needs (or agricultural needs).

One realistic caveat to supplying your house with rainwater, you must be committed to using water wisely and installing efficient water using appliances in your home.  This doesn’t mean living like you are on a boat but it does mean buying low flush toilets (dual flush preferably), low flow showerheads and faucets, a modern dishwasher, and a front loading clothes washer.  It also entails using those appliances wisely. No expectation of 2 minute lather/rinse shower is being made just not the 30 minute shower experience or daily 60 gallon bath.

Our goal is to find out how much water we use per day on average (we’ll put a fudge factor in at the end to cover unexpected extra use). First we need to make a list of all the water-using devices in your home and how much water they use measured by use or time.  I created a spreadsheet for this that helped dramatically with the calculations.  Let’s look at your shower as an example.  On the showerhead there is most likely going to be a marking that shows the flow rate in gallons per minute or gpm.  Ours is 1.5 gpm.  For every minute our shower is on 1.5 gallons are coming out of it.  If I take a 10 minute shower I can conclude that 15 gallons of water were used (10 min x 1.5 gpm = 15 gallons).  Many appliances it isn’t that simple.  Let’s take the dishwasher for example.  In the owner’s manual there is probably a table that shows water consumption per load for each different cycle.  This is the number you are looking for.  Some dishwashers adjust the water based on how dirty the dishes are so I’d use the higher number to be safe.  If there isn’t a number you can test the usage by running the appliance and capturing the water in buckets and measuring the discharge.  Not the most convenient of methods but it does work and I’ve found the published numbers to be remarkably close to my tests.

Once you know the water usages for all devices you’ll need to make some estimates as to how often you use them.  I would recommend erring on the high side but also keep it realistic.  You could even track your usage before making this calculation on a notepad for several days or weeks.  The longer you track your usage the more accurate the number.  For example, I estimated that each person in our household will flush the toilet 4 times on the #1 flush (.8 gpm) and 2 times on the #2 flush (1.6 gpm).  Doing the math I get (4 flushes x .8 gpm) + (2 flushes x 1.6 gpm) = 3.2 gal + 3.2 gal which totals to 6.4 gal/person/day.  For the four of us our total toilet water use equals 25.6 gal/day.  This is probably high and over the years I’ve found it to be but for estimations sake it is a safe figure to use.  Do this for each device and you’ll get a total water use per day for your household. To account for waste, guests, and unexpected water use I add 10-20% to this number to get my daily household water use figure.  Hopefully you’ll find that your daily usage is much lower than the US average at 80-100 gallons per person. 

All of these calculations above refer to using rainwater for indoor domestic water use.  There is no reason why these calculations can’t be used in the garden.  Drip systems are generally rated in gpm per dripper and water meters can be purchased affordably to measure total water flow through a sprinkler system.

Once we have obtained our daily water usage we can then come together with our expected annual rainfall figure we estimated in the September article.  With these two numbers in hand we’ll start the discussion on water storage.  It however will have to wait until January.

Mike is can be reached at and would be happy to share his spreadsheet with you if you wish to start the journey to rainwater usage.