Life with a rainwater cistern

“What about the tank?”

It was 4 PM the day before we needed the bank to approve the final appraisal for our permanent mortgage.  Our contractor was rushing to finish everything that the bank needed and I was on the phone with the guy from the bank.

Me:  “It’s a rainwater cistern.  We flush toilets and water the yard with it.”

Bank guy:  “But what’s happening with it?”

Me:  “It’s connected and working.”

Bank guy:  “But what are they going to do with it?”

Me:  “It already has rain in it, it’s been raining.  So they don’t need to fill it or anything…”

Bank guy:  “I mean, are they going to bury it or something?”

Oh.  We have an above ground rainwater cistern.  It’s on the back of our house, so I always assumed that no one would care, but some people have a hard time with the concept.  We actually DO plan to put up a lattice and grow some vines or something around it, but we’ve lived here for almost 2 years and that hasn’t happened yet.  Clearly, we had no intention of growing vines overnight so we could close on our mortgage.  In the end, the bank relented and we have a mortgage.  But I find that there’s a lot about life with cisterns that most people have trouble getting their heads around.

Rear Exterior LOW_RES
Above ground rainwater cistern on the back of our house

The biggest irony about a rainwater cistern in WNC is that you spend all of your design energy focusing on the situation where it runs out of water, and you spend all your energy as the owner of the cistern managing the overflow when it’s completely full.

The obvious design problem is trying to figure out how big the cistern needs to be.  This involves estimating the longest drought you’re likely to experience and calculating how much water you’re likely to use during that time.  We use rainwater to flush toilets and to water our landscaping.  Toilet flushing is relatively easy to estimate, but we found landscaping usage to be more difficult.  The methods used to calculate water usage for the LEED rating system dramatically over-predicted our expected water usage, even in a drought.  The back third of our property is actually a wetland that we let grow wild.  We wouldn’t water it at all, even if we had a drought.  The rest of  our landscape, including a little grass, won’t need to be watered at all under normal conditions once it’s established.  During a drought we might want to water it once or twice a week.  We finally settled on a 3,000 gallon cistern, which would be enough water to flush toilets and do minimal subsistence watering for a month with no rain.

Readers of this blog who aren’t local might be interested to know that in the 9 years I’ve lived here, the longest period I can remember without rain was about 3 weeks.  We average about 45 inches of rain a year, and it’s more usual to have rain once or twice a week, pretty much year-round.  Based on the size of our roof, it takes about 3 inches of rain to fill the cistern completely.  We’ve had many rain events of that magnitude since I’ve lived here.

Unfortunately, the first of these events happened right after we installed the cistern.  We didn’t have the overflow on yet, so when we saw the forecast my husband hurried to get it installed.  At that point we discovered that the specific tank we have comes with a 2-inch overflow.  Since we have two 6-inch gutters dumping into the cistern, we were a little worried about this.  If the overflow couldn’t handle the water, it would spill out the top and collect right next to our house.  We fixed the problem by putting a siphon pipe about 2 feet into the cistern.  Now, when the cistern fills entirely, the overflow will continue to siphon the top 2 feet of water out of the tank, giving us a nice buffer against additional rain.  The added bonus of doing this is that we get some additional pressure on the 2-inch line and water shoots out of it at a fairly high velocity.  If I saw that a gulf hurricane was planning to dump several inches of rain on us, I’d go out and empty the tank in advance, but so far this has worked fine and we haven’t had any spill-overs.

cistern overflow
Two 6-inch gutters feed into our cistern, and just below them you can see the 2-inch overflow that came with the tank.

There are a lot of rules in the building code related to using rainwater that can add to the cost of the system.  These relate mostly to how the system is “backed up” by city water and making sure that non-potable (non-drinkable) water from the cistern doesn’t contaminate the drinking water.  You might be thinking that rainwater is probably pretty clean and this shouldn’t be a big deal.  But birds poop on roofs, rainwater washes it off, and then it goes into a big tank and hangs out for a while.  The tank is black because that keeps stuff that needs sunlight from growing in there (and it does a remarkably good job), but there’s other stuff, and you really don’t want to drink it.  If for some reason it DID contaminate the drinking water it would be MY drinking water, so I was totally on board.  Also, you need a pump to get the rainwater to your toilets and hose bibbs.  I didn’t really feel like using buckets of water to flush the toilet if my pump died, so I was willing to invest in an easy switchover.

Depending how you manage the switchover to city water, I figure this aspect of the system adds about $1500-$2000 in cost.  Based on our experience so far, we’re more likely to need to do this because our pump has malfunctioned rather than the cistern going dry.  This means that throwing a hose into the cistern is NOT a good solution.  The code requirements for preventing backflow to the potable water supply influence cost and vary by state and locality.  The code in North Carolina isn’t super-clear.  They define two types of non-potable water.  The first one is basically toxic industrial waste.  The second has “taste and odor” issues, but won’t make people sick.  Rainwater is kind of in the middle – it could make people sick, but calling it toxic is probably going a little too far.  I’ve seen different code jurisdictions come to different conclusions about the proper category for rainwater, and this influences the type of backup supply you can use and the cost.

A buried tank is also a little more expensive to do.  The tank itself needs to be stronger.  You also need to pay more attention to keeping crud and critters out of it and the backup water supply can be a little more complicated.  You also need to decide whether to use a submersible pump or to have the pump inside your house.  I personally prefer having it inside for ease of maintenance.

You also have to label the water pipes that carry non-potable water in your house.  If we ever decided to renovate, we wouldn’t want a plumber to accidentally tie into the water supply going to our toilets and sending it to a sink.  You also need to label your hose bibbs pretty clearly.  This seemed odd to me, but you’d be surprised how many people drink out of hose bibbs!  Even though ours are labeled, I’ve stopped people who were doing work in our yard just in the nick of time.  In the US, we are so accustomed to water being safe to drink that people don’t even stop to think that water coming out of a garden hose might not be drinkable.

People ask me a lot if our toilet water gets funky.  We might need to clean the toilets a little more often than other people, but in general I don’t notice it being off-color and it definitely has no odor.  It’s a toilet – rainwater is just fine.  Whenever someone asks me this, I think of an engineer friend of mine who was in a meeting with some code officials about a cistern and finally blurted out, “It’s toilets!  Do you have any idea what people DO in toilets?”

And speaking of toilets, perhaps the best part of living with a rainwater cistern is this:  you can go ahead and flush while someone’s in the shower.  It’s not going to bother them at all.