How to Filter Rainwater for Drinking

Believe it or not, rainwater can be filtered into the best-tasting, freshest water you’ve ever had.  In our home state of Ohio, there are literally thousands of homes that rely on rainwater for their sole water supply, and thousands more homes that utilize rainwater for the majority of their watering needs.  

But care must be taken.  While rainwater is filtered naturally through solar distillation, some not-so-fresh things happen to the rain on the way back down -- especially once the rain hits our roofs and collects all the organic material (animal feces and all) that collects there.  To return the rain to a potable state once again, there are some time-tested, effective methods we can use for filtration: There’s the short-term fix (great for emergency water situations) and the long-term rainwater filtration method (great for utilizing rainwater for your water needs).  Here’s an overview of both:

THE SMALL-SCALE, SHORT-TERM, SIMPLE METHOD 

If you live in a rural area and rely on a well (and, more importantly, on electricity to provide power to your well pump), or if you are on a municipal water supply and want to have a back-up water source for emergency preparedness, you may want to consider having a short-term filtration solution on hand.  In this case, we recommend ultrafiltration units.  LifeStraw, in our opinion, is the best, most affordable example of this.  Ultrafiltration and/or forward-osmosis technology operates on the principle of reducing a filtration element to such a fine degree that 99.9999% of water-born bacteria cannot pass through, thus making the water that passes through the filter safe for drinking.  In fact, ultrafiltration is so effective that no other filter is needed (see this ABC News report for visual evidence). 

The disadvantage of ultrafiltration, though, is the scale at which this filter can be used.  It is great for emergency situations, but for household water options, this method has its limitations.  

RAINWATER FILTRATION FOR HOUSEHOLD APPLICATIONS

If you want to utilize rainwater for your home and are looking to install larger-scale filtration, there are a few steps to follow to ensure a fresh, clean, efficient system.  (CAUTION: It is tempting to cut out one or two of these steps, but, in so doing, you’ll put more burden on the other steps and will create more work for yourself down the road.  After working on rainwater systems of all types for a decade, we’ve learned that lesson the hard way.)

1. First flush filtration: Because the majority of bacteria enters rainwater from a roof and gutter system (where the water picks up fecal matter from squirrels, birds, etc., as well as other organic matter), pre-filtration is a VITAL step in creating and storing a fresh water supply.  First, you’ll want to consider installing first flush filters.  A first flush filter works under the principle that the most contaminated water is the first bit of water that falls from a roof during a rain event (because this is the water that’s flushing off the fecal matter and organics).  For an illustration of this, click here.  Please note that the downpipe component on first flush filters should be sized according to the type of roof you have (e.g., asphalt shingle roofs will need more first flush diversion -- and therefore a larger downpipe on the first flush filter -- than metal roofs because they are more gritty and it takes longer for fecal matter to be cleaned from the surface from a rain event).  For roofs that in a clean environment (i.e., not many trees/birds around), it is recommended to flush 12.5 gallons/1,000 sq. ft. of roof area.  For roofs that are more susceptible to organic material and/or roofs with asphalt shingles, a flushing of 50 gallons/1,000 sq. ft. of roof is recommended.
2. Pre-tank filtration: Next, you will want to consider a tank pre-filter for your system.  While there are many on the market (and several in our store), a lot of the options available are designed for commercial and industrial applications and are not always cost-effective (or even efficient) for residential-scale systems.  In fact, we generally recommend our precast concrete roofwasher to our Ohio-region customers as an affordable and highly effective pre-tank filter -- and our concrete roofwasher actually has a first flush filter built into it, so it takes care of the first two filtration steps in one unit.  We also have a plastic roofwashing filter available. Even doing something as simple as installing downspout filters can be an effective means of drastically improving your water quality BEFORE it enters your tank.  Click here for pictures of pre-tank filtration options.
3. In-tank filtration:  In-tank filtration is simple to do and can have a big impact on overall water quality.  First rule of thumb is to try to plumb your tank inlets so that they go down inside your tank and empty at the bottom.  Consider putting concrete blocks around the inlet pipe at the bottom of the tank, or install a 90-degree elbow fitting on the pipe.  Doing so act as a “force breaker” and will reduce turbidity in the water, thereby maintaining sediment zones in the tank.  Sedimentation will be most heavily concentrated on the very bottom and on the very top of the water in a given cistern, so if we can reduce turbidity in the water and draw the water from the middle of the water level (using a floating cistern filter), we’ll get the cleanest water from our system.  
4. Activated carbon: All water will carry with it its own taste and odor, and rainwater is no exception.  To get the best water quality possible, as well as the best tasting water, a granulated activated carbon (or “GAC”) filter is a must for any system.  As a homeowner, you would have the option of going with a simple GAC cartridge filter, or a more thorough and larger household GAC unit. Click here to view our options. 
5. Sediment filtration:  Any remaining sedimentation in the water should be filtered out as thoroughly as possible.  Sediment size is measured in microns -- the higher the micron reading, the larger the particulate.  In Ohio, all rainwater systems that are used for drinking water must be filtered down to at least 5 microns (which is small enough to filter out cysts from the water).  When we are putting in drinking water systems from rainwater tanks, we generally install a two- or three-stage sediment filtration system, starting with a 20-30 micron filter, followed by a 5 micron filter.  When installing sediment filters, always put the higher micron element first, followed by the second highest and so on.    
   
   
   
   
   
   
   
6. UV Sterilization: The final step in any rainwater filtration system needs to be disinfection (killing bacteria) or sterilization (sterilizing bacteria so that it cannot reproduce, thereby rendering it harmless).  Even after filtering the water down to 5 microns, bacteria can still be present in the water.  While many rainwater systems use chlorine to disinfect the water, our preferred method for bacteria filtration is ultra-violet sterilization.  We strive for the best possible water quality, and adding chlorine to the water, in our experience, does not lend itself to this end.  UV sterilizers, by contrast, offer a very safe and extremely effective result.  However, UV lamp sleeves need regular (usually every 3 months) cleaning to ensure that the UV light penetrates the water fully.  Cleaning is not difficult and, with all of the above filtration steps in place, can be a very quick process (under 5 minutes).