How to Filter Rainwater from the Roof

Getting your water supply dialed in without a municipal line can be quite challenging. That’s why many people going off grid out of choose to remain hooked up to the municipal’s water supply and sewerage system.

If this isn’t an option in your setting, you will have to look for your own water supply, process it and plumb it to your off grid property. Since finding one supply to cater for all your needs might be too expensive or even impossible, I found that taking all the water you can (including rainwater) works best.

Is Harvesting Rainwater Legal?

Harvesting and using your own rainwater is legal in most parts of the world. However, some regions and states in the USA have some limitations or regulations regarding what you can do with rainwater. Be sure to go through your local regulations to see the necessary compliance regulations you need to observe when building your rainwater harvesting system.

The filtration process breaks down into:

  • Filtering before storage (pre-filtration)
  • Keeping water clean and aerated in the tank
  • Filtering for drinking, cooking and other direct consumption purposes

How to Process Your Rainwater

In this piece, I will be assuming that you have already figured out how to harvest water from your roof and channel it to a clean water tank. If you haven’t, be sure to first go through my guide on how to set up a rain water catchment system.

With that out of the way…

The level to which you process your rainwater depends on how you want to use it. With basic pre-collection filtering, rainwater is good for irrigation, flushing toilets, laundry and even bathing.

Your System Must Filter the Water Before Storing it

Ensuring that the water going into your storage tank is as clean as possible is a great starting point. Twigs, leaves, and other pieces of debris will spoil your stored water fast. It will also clog pipes and jam pumps reducing your system’s lifespan.

Here are some simple tips to ensure that you collect water that is clean enough to pass the physical eye inspection.

keep roof and gutters clean for clean rain water

Keep Your Roof and Gutters Clean

Cleaning your roof and gutters just before the rain season kicks in will go a long way to making your gathered water easier to process. Get rid of any twigs, leaves, stray toys and other pieces of debris that might have ended up there one way or another.

While this might not be a necessary step if you live out in the open, it is mandatory if your collection roof is next to trees. Also ensure that there is no moss or anything growing on your roof if you intent to use the water for any other purpose but irrigation.

Use Sieves and Mesh Filters

A sieve or a mesh filter is a good way to prevent any visible rubble from flowing into your tank. Most tanks out there come with a sieve bundled in.

The mesh filter is often good enough for most debris but you could get better filtration by substituting it with an aftermarket filter.

Sometimes, a fine net or even a piece of nylon cloth might do the trick. These sieves will keep small pebbles, insects and even some dust or mud off your tank.

Don’t Collect the First Batch of Rain Water

Your roof is bound to gather some murk and dust no matter how clean you keep it. Rain washes down all this dust the very first time it falls after a while. The result is dirty water that you shouldn’t bother collecting at first.

Creating a system that gathers at least the first three gallons ( or more depending on your roof size) of water before letting the rest flow to your main storage tank is a good way to ‘filter’ out this dirt.

You can use a simple system like a couple of dropper pipes (or small containers) on an horizontal running pipe. The idea is the water won’t flow past the reservoir the dropping valves until they are full. This means that they will gather the first flush of water and on cleaner rainwater will flow through to your tanks.

Keep Your Collection Tanks Clean and Sheltered

It doesn’t matter how much effort you put into cleaning your rainwater if your tanks are dirty and neglected. A dirty tank will immediately reverse this.

Tanks need frequent washing. This varies depending on your tank type. Don’t go for more than a year without cleaning yours. Doing it twice a year or immediately after winter is a good idea.

Fine particles of dirt and debris still make their way to your first storage tank. They will settle down to the bottom forming a murky dirt line that can even encourage the growth of algae or moss that turns the water green.

Sheltering the tank protects it from falling debris and the weather giving it a longer lifespan.

The Initial Filtering is Enough for Irrigation and Laundry

If you just want to use your water for irrigation during dryer months, there is no need to filter it any further. By now, the water will be good enough for your lawn, garden or greenhouse. You can also use it to flush your toilets or shower if you don’t mind.

Laundry is also possible but this will vary depending on the clarity of the water. In my experience, I found that the water is always clean enough to wash even white clothes without discoloration. You get the bonus of using less soap since rainwater is naturally soft and lathers very easily.

How to Prevent Your Water from Going Anaerobic

Untreated water will start smelling foul after a couple of weeks in storage due to microbial growth and other organic infestation. You can avoid this by adding some chlorine to the water. mix non-scented NSF-approved household bleach (5.25% chlorine) in the reservoir at the ratio of 1 gallon of bleach for every 1,000 gallons

Another huge problem you have to deal with especially if you will store the water for a couple of weeks is when the water starts smelling. Water will go anaerobic if it stays still for long. You can avoid this by treating the water with chlorine before storage.

If you won’t store your water for long and don’t want to add chlorine so early in the collection stages, you can use alternatives that keep the stored water circulating. A calmed inlet that delivers the water to the bottom of the tank instead of just dumping it through the inlet positioned at the top.

Push the pipe delivering water to the bottom of the tank and use a U joint to angle it upwards. The incoming water will induce a flow in the water hence stirring it up without disturbing any sediments at the bottom of the tank.

You can get rid of any residual smell in the stored rainwater by adding an activated carbon filter down the line.

Always have a plan to biologically treat your water. Chlorination is the easiest way to kill bacteria and other biological contaminants. If you don’t want chlorine in your water, explore other options like boiling, Ozone treatment or super fine bacteria capturing filters.

How to Filter Rainwater from the Roof for Household Use

You can purify your water further using progressively finer filters that get rid of all solid particles and microbes to make it safe for all consumption. An extra disinfection step kills all organic components making it as safe as a municipal water supply.

Filters are rated by the smallest size particle they can capture. This is often down to microns. The finer the filter the more it can capture.

When dealing with rain water, you should go for filters as large as 50 microns at the very start to eliminate all visible large particle that make your water dirty and gross to the eye.

After this, you can decide on which approach to use to get rid of microbes.

  • You can use chlorination, ozonization, UV light to kill the microbes
  • Go for finer filters
  • Try membrane filtration
  • Boil the water
  • Solar pasteurization

Filter to the Required Level

Filters are expensive and require replacement the more they work. A good way to keep your rainwater harvesting costs low is filtering only as much as you need to.

For instance, combining a 50 micron filter (or finer) with a solar pasteurizer or another disinfection method should be good enough for your dishwashing and showering water.

Using a dedicated filter and disinfection system for your drinking and cooking water will ensure that you don’t using the expensive and finer system to filter gallons of water that you will end up flushing down the drain.

The Bottom Line

Harvesting rainwater is one of the ways to go off-grid on water or reduce your reliance on the paid municipal supply. Knowing how to filter the water well enough to suit your purpose will ensure that you use clean water without the monthly bills. You can cap it up by ensuring that all your pumps run on solar power and you will be 100% off the grid.

Oscar

In his spare time, Oscar loves tinkering with electronics. Solar panels, wiring, old TVs and sometimes DIY powerwalls. When he is not busy trying not to electrocute himself, you can find him in the garden tending to his vegetables and chickens.

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