Rainwater collection for micro houses
Previously I explored improvements to standard micro house water systems for water systems where there is no permanent pressurized water hookup or sewage line. In many cases there may be no source of water but from the heavens. So in this post I’d like to share a year of experience designing (and redesigning) rainwater collection and treatment systems- the water that feeds the Minim House at the Micro Showcase. The good news here is that in most climates it is entirely realistic to collect, treat, and use as much potable water as you will need for happy micro house living. (1)
Off-grid water collection systems typically consist of the following elements:
a) Rain catchment surface. Typically this is the micro house roof, but could equally be a different structure. While rainwater will be filtered, ideally a rainwater will not flow over petrochemical products that leach contaminants into the water- metal is perhaps the best choice. Also be sure to avoid wood shingles, metal flashing or roof treatments that contain lead. Calculate the amount of water the roof may harvest by getting your local monthly rainfall, and using the following formula:
Harvested water= catchment area (ft2) x rainfall depth (inches) x .623
For example, for a 11×22 micro house in Washington DC (avg 3” rain/month), collection could be up to approximately 500 gallons for an average month.
b) Rainwater transport. When rain falls on the roof, it should flow through gutters and piping that allow a high waterflow to the collection tank- I recommend at least 3-4” pipes for a 250 ft2 roof. As we know when it rains it often really pours, but if the pipe system can’t handle the times the rain really lets loose, you’ll be losing a high % of your monthly water collection to spillage. Also, ideally water should simply gravity fall through pipes directly into your cistern (collection tank), without the need for pumping, switches, or active maintenance- having tried an automatic rain-activated pumping system, I’ve found it is far simpler to have a direct gravity ‘roof-to-tank’ system. Early settlers who built rain collection systems heartily agreed.
c) Collection tank/cistern. The tank is where the bulk of the rainwater is stored. In cold climates it must be insulated or placed underground to prevent water from freezing. In slightly more temperate climates, a bit of electric heat tape under the tank and around the pipes can keep enough water flowing during the coldest months. Sizing should be based on both rainfall patterns and expected water use. Just as for electricity, it is best to first minimize water use (rather than invest in larger collection/storage systems) through low flow faucets, shower heads, and foot pedal faucets. Just as a rule of thumb, with efficient fixtures I estimate 30 gal/week/person with regular washing, showers, etc. When the collection tank is full, there should be a basic overflow mechanism that gutters the water far away from the house/tires. Tanks can be rigid plastic or flexible- the key design trait being a large enough inlet to accept high water flow. I personally favor the flexible water pillow tanks (far cheaper to ship than the large rigid tanks, easy to unroll and hide under a trailer). (2)
d) Water pumping. From the collection tank water is then pressurized to move through a potable water hose to the garden, or through the same hose to an onboard holding tank and water filter. A simple 12 or 120 volt Shurflo pump and switch will do the trick (I hide mine in the trailer hitch compartment, and make sure the pump is disconnected during freezing weather to avoid cracking- it could also live inside). For potable water, water could be pumped directly from the collection tank through an on-board filter if freezing is never an issue. I prefer an on-board RV water tank within the micro house that stays warm, then pump/filter from this tank with a secondary water pump. (3)
e) Water filtration. For potable shower and sink water, a quality water filter is essential. The filter takes water from the collection tank/cistern or on-board tank and makes it drinkable. There are many kinds available. The requirements here are water efficiency (no reverse-osmosis), energy efficiency (no UV filters that require electric UV lights on 24/7), maintenance simplicity, filtration efficacy (on bacteria/virus/chemicals), and cost. After extensive research the ceramic Doulton RIF-10 with the additional sediment pre-filter was the tool of choice- I’ve used for over a year, with infrequent cleanings, and excellent water quality. This model is originally designed for freshwater boaters, so taking water from your cistern ‘rainwater lake’ also works well. The ceramic filter has been proven for hundreds of years across the former British Empire, and is easily cleaned up to 50x with a scouring pad. Note the filter is plumbed in after the second (on-board) water pump, so water is pushed through it. Once filtered, water flows to the plumbed micro house water system, very simple.(4)
So to recap a now thoroughly tested/proven off grid water collection and treatment system, version 3.0:
rooftop –> large gutters/pipes –> 250 gallon flexible cistern –> water pump #1 –> potable water hose –> garden or 40 gal onboard tank –> water pump #2 –> RIF-10 filter –> sinks/shower
(1) For foundation built houses, be sure to check local codes on rainwater harvesting for potable use.
(2) Note that rainwater collection experts will argue a pre-filter on the collection tank is necessary to filter out pollen, dust, etc from roof water. This is clearly ideal. Though to me this seems more necessary when using under ground storage- above ground storage tanks can be more easily emptied of any sediments as needed. I simply use a basic leaf screen in line in the gutter.
(3) Note that when filling an on-board sealed water tank, air must be exhausted as water rises. While there are a number of simple RV air release products, I settled on this Hayward air release valve plumbed at the top of the water tank. It has the benefit of releasing air, but sealing automatically to prevent water overflow when the tank is full.
(4) Note RV’ers may find it unpalatable to store ‘unfiltered’ water in an on board storage tank, but even potable water RV water tanks have to be sanitized regularly. I find it preferable to actively filter water at the time of use, though this does mean that water flow rates are a bit less than pumping directly from a clean water tank- the water filter slows the water flow rate down a bit (more if dirty). For this reason I advise not attempting to use on-demand (‘tankless’) water heaters if actively filtering water (have tried and failed). These water heaters just demand too high a water flow rate to remain activated.
(c) 2014 Brian Levy