03 April 2012

Rainwater Harvesting Update

We're still waiting on our rainwater cistern, but that's okay because we're behind schedule on the prep work.

As shown in the drawings we submitted with our rebate application, we're putting it at the back corner of our lot in the gravel area dedicated to vegetable gardening.  (It's also semi-mocked up here, although the cistern will go farther back on the lot, and we have decided to tie in the last downspout on the back of the house, too.)

Here's the abbreviated list of what we need to get done (in addition to actually getting the cistern on-site and into its spot) to start collecting rainwater:
  • build a solid, level base of decomposed granite
  • trench across the big back bed, future patio area, and gravel zone (which we affectionately call "the quarry")
  • run PVC lines in the trenches to connect the gutters to the cistern
  • swap out our galvanized downspouts and rainchains for PVC to tie into the underground lines
  • hire a plumber to install an RPZ (reduced pressure zone -- a type of backflow prevention device that's required for a pressurized rainwater harvesting system)
So far we're midway through the first item.  (Sigh.)  Unfortunately, the perfect spot for the cistern was once the perfect spot for compost...and since the bunnies are eager contributors to the compost pile, that area was looking like this:

So the first step was to move all of that compost closer to the back fence...

...and then, since the yard gently slopes from left to right, the area needed to be leveled.

Fortunately, I know of a trench nearby that needed filling (long story...but it was all totally legit), so I was able to take the soil I excavated there, bit by bit, instead of having to find a spot for it somewhere else on our lot.

Here's where I would like to describe all about how we lined the area with leftover house stone and then filled in the base with decomposed granite, but we haven't gotten that far.  (Double sigh.)  So when I received an e-mail from the cistern manufacturer letting me know that he's finishing it up this week and asking where we wanted the various fittings, I knew I had to get right on it.

The three fittings that we will need to place are the inlet, the overflow drain, and the spigot.  I have a rough idea of where they will go -- the inlet should face the house, the overflow should face toward the back but still discharging onto the gravel area, and the spigot should face sideways across the lot, toward the (future) vegetable garden.  But the exact location, and the size of each opening, is going to take some thinking.  I plan to spend some time working on the base tomorrow, so while I'm doing that, I'm going to visualize the cistern and stone vegetable garden beds in place and finalize the placement.

I also took a research field trip this morning to study an existing cistern and figure out how everything ties together.  Here's the cistern:

I think it's about 1,500 gallons; ours is 2,500 but otherwise very similar.

Here's the inlet:

Water from the gutters comes up the left part of the pipe.  The first bit, which often contains little pieces of debris that you don't want in your cistern, goes back down the right part of the pipe, which is the "first flush" mechanism.  Here, again, are the inlet (on the left, coming up from underground) and the first flush (on the right).

The first flush slowly releases that first bit of water (through a nozzle that looks like it belongs on a sports bottle)...

...while the rest of the water coming from the gutters travels right over the now-full first flush pipe and into the cistern.

That rubber gasket thing surprised me...and didn't help me figure out how big the opening should be into the cistern, so I have a call in to a friend who is an expert in rainwater collection (in fact, his company actually installed this one) for some guidance.  I thought I had picked his brain enough that it would be smooth sailing, but apparently not.

The overflow drain has the same gasket situation...

 ...as does the pipe to actually use water from the cistern (although that doesn't surprise me, since it's at the bottom of the cistern and needs to be watertight).  But that complicated mass of pipes have me stumped.

I thought there would be a regular old hose bib, possibly leading to a pump and maybe tying into the irrigation system (as we might want to do...we're still undecided on that).

So that's where we are on the cistern.  The beginnings of a base, delivery expected in the next couple of weeks, a lot of work ahead of us, and a lot of questions left to answer.


  1. Those folks have what's called a "wet system" (which I reckon y'all will have given the location of the tank relative to the house). It's "wet" since the pipes will typically be full of water in non-raining times. You want those inlet/outlet pipes to be pretty big to take the flow from god-sized storms. Interesting that their first flush system is at the tank rather than at the downspout: Seems like there could be an issue with gunking up a wet system (i.e. pipes underground) that way (not that I have experience with wet systems: our system was dry, but we had the first flush at the house, not at the tank). Some installers suggest not worrying about a first flush system if the water will just be used for outdoor purposes (still thinking about that one...). Those are good-looking tanks!

    1. Yup, ours will be wet -- very similar to this one (which is why it's so convenient that it's on display in the 'hood. We're using 3" PVC for the downspouts and 4" for the underground lines and inlet, etc., but I'm not sure how that translates into an actual hole in the cistern (not sure how the connection is actually made).

      Exciting times!