On the windward side of this island, most homes use catchment water, a rainwater harvesting system that collects rain, usually from a rooftop, and stores it for future use. Unlike most of our neighbors, we are connected to the local municipal water supply, but we have several challenges:
- The water agency recommends that people store enough water to get them through a month of no municipal water in the case of an emergency.
- The new garage is not plumbed, and getting a water pipe to it would involve trenching through our newly-rocked driveway. That would have been unpleasant even before we re-rocked, as many tons of rock have been laid over that drive in the past century and compacted down into the earth. It would have been like trenching through concrete, and it would probably take several years for the drive to recover to its pre-trenching condition. We need water in the garage for the washer and dryer and I’d like to have a laundry sink there as well.
- Even if we got a line to the garage, it’s at the highest point on the property, and the water pressure from the municipal line is already marginal at best.
- I would like to be able to survive a long-term failure of the municipal water supply, as we are at a distant, tenuous edge of their service area.
storage
So how does this all work? Well, we have a nice clean new metal roof on the new garage, with a footprint of about 250 square feet if we just use half of it. According to this reference, “a square foot of horizontal surface receives .625 gallons with each inch of rainfall.” We average about eighty inches of rainfall per year here. That’s 12,500 gallons per year, which is about half of what we’re actually buying from the water district right now. Of course, it’s more complicated than that, because that water doesn’t come conveniently evenly spaced throughout the year.
We bought this 950 gallon tank:
The size of the tank was dictated by the limitations of our terrain, ability to transport the tank, and our budget. The tank itself is sustainably manufactured using an array of mirrors that focus sunlight onto the mold so the heating is done entirely with sunlight.
The reality is that the tank will often overflow with serious rains and waste some of the water (we got 19 inches of rain in the three days prior to my arrival last August), and then run empty during dry spells if we’re actually using it as our main water supply. (I can’t imagine emptying it with just laundry unless we start a commercial laundry business.) Of course, we can easily double our catchment volume just by guttering and connecting the other half of the roof, but the reality is that it’s an inadequate system for full-time use. We would probably need 10-20 times as much storage if we wanted to use water like a typical middle-class couple and never worry about it. We’ll limit it to laundry and emergencies, but we’ll have everything in place to expand it if we lose access to our municipal water.
Next we have to add a gutter to the roof, with a downspout that feeds the tank, preferably with a first-flush system that dumps the initial flow of water after a dry spell so the dust from the dry roof doesn’t enter the water supply.
Of note, the CDC recommends having 1 gallon of water per day for each person for emergency use. If we include Luna the Big Dog™ as a “person,” that means our minimum survival requirement would be 3 gallons per day. We’d survive most of a year with no rain at all, which is not at all likely even with rather extreme climate change. Of course, that wouldn’t give us enough to use our beloved outdoor shower.
Here’s the limited space where we can put the tank without doing any additional earthwork:
We made an extra-thick rock bed when we graveled the drive, and I’ll add a hefty layer of sand between the rock and the tank.
pressurization
The water from the tank will only be under about 2 PSI when the tank is full; less as it drains. That’s not enough to run the washer reliably or the sink conveniently (residential water pressure is usually over 50 PSI, though we’re living with about 30 right now). The usual way that catchment systems simulate municipal water supplies is to use a pressure tank that has an air bladder in it. An electric pump pumps the water into the tank, compressing the air bladder, until it reaches a reasonable working pressure. As usage draws down the pressure, the pump periodically activates and restores pressure.
We’ll probably end up with a pressure tank eventually, but for now I want to try a much simpler diaphragm demand pump designed for low usage scenarios such as RVs. The pumps are small, light, simple, and cheap enough that we can keep one in reserve against the probability of pump failure. We’re starting with a pump we found on Amazon; and we’ll see how it goes.
purification
One advantage of aiming this toward use for laundry is I don’t have to include a system to guarantee that the water is potable. Treating it with occasional doses of sodium hypochlorite (chlorine bleach) should be more than adequate to keep it clean enough for laundry use. If we have to use it as emergency drinking water, we’ll proceed on the assumption that it’s a contaminated source and purify it offline for drinking.
I’ve designed and prototyped a radical system for online purification that involves salting the water, running a salt-water chlorinator, then desalinating it with a flow-by reverse-osmosis filter to render it pure. It’s a system that trades energy use for materials — remember, we have excess energy. We’ll keep that idea in reserve and hope we never need to use it.
maintenance
There’s a big advantage to using this system for laundry water: it will get used. Systems reserved purely for backup have a tendency to develop faults that go undetected until an emergency requires that you use them. We’ll know, whenever we do laundry, that our system is up and running.
Tomorrow, we’ll get the sand and some plumbing hardware. Next week (I hope) we’ll get the electric feeder run to the garage. The tank is ready for pick-up and Timmy is stripped down and ready to do the job.
Soon all we’ll need is some rain.
—2p