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Rainwater Harvesting 101: Pre-Filtration

Pre-filtration...

is perhaps the most important step in designing a rainwater harvesting system as it aims to removes large debris from the supply of rainwater prior to entrance into the rainwater storage tank. 

Removing large debris is an essential step to rainwater harvesting for several reasons. By removing larger particulate from the water supply, only small particles make it to the rainwater storage tank. By using WISY pre-filters, you can expect to eliminate particles ranging from as small as 280 microns to 440 microns (this range varies based on which filter is used). This means that the particles that make it into the tank are smaller than a grain of sand! What makes it into the tank at this point is very small an will either float to the top and be siphoned off through the overflow, or will sink to the bottom and help form a healthy "biofilm" in the tank--more on these two aspects in coming posts.

Why filter my rainwater?

As mentioned in previous posts, rainwater itself is not dirty. Your roof is though. Leaves, sticks, dirt, particles of shingles, and other contaminants all end up on your roof or in your gutters at some point. You may be thinking "so why can't I just let debris go to the storage tank?" Two main reasons come to mind: 1.) Unfiltered biological debris in the tank provides "food" to any bacteria that may be in the water and will eventually create a very unhealthy water environment and 2.) Unfiltered debris builds up at a much quicker rate in the bottom of the tank and tank cleaning becomes mandatory due to this. In addition, this also tends to lead to suspended solids, more water discoloration, and last, but not least: odor. And man, does it smell! This simple step of pre-filtration eliminates or drastically reduces all of these factors and makes system maintenance very minimal. Most of our customers only need to clean their filter screens 2-3 times per year and never have to clean their tanks. Reducing the amount and size of particulate that makes it into the tank also prolongs pumping equipment and post-tank filtration equipment by not clogging it with gunk. 

How does it work?

So let's talk about pre-filtration operation focusing solely on WISY products. This is the only filter that we use as we feel it is the best rainwater filter available in terms of sediment removal and low maintenance. Water enters the filter housing and due to the offset inlet, swirls around the interior of the housing. As it cascades over the edge due to gravity, surface tension keeps the water close to the stainless-steel filter insert. The filter insert has a very, very fine screen that keeps particles larger than 280 microns (or 380-440 microns, depending which filter screen is being used) from passing through. Capillary action properties of water will then pull the water through to the other side of the filter and down to the chamber that leads to the tank, leaving debris on the other side of the filter. As more and more rain comes through the filter, this debris that is left on the inside of filter housing will wash away to the storm drain. See the video below for a demonstration of this.

Sizing Pre-filtration

Sizing your pre-filter correctly is very important as it can lead to a lack of efficiency if improperly sized. The way to size this is based on your roof collection area. For smaller collection areas, downspout filters are a good fit, but for larger collection areas we recommend vortex filters. See the below for the recommended product sizes for pre-tank filtration.

For roof areas up to 1,000 square feet:

  • WISY Garden Rainwater Filter Collector Package

For roof areas up to 1,600 square feet:

  • WISY Downspout Rainwater Filter Package
  • WISY Polypropylene Downspout Rainwater Filter Package
  • WISY Polypropylene Raincatcher Package
  • WISY Standpipe Rainwater Filter Package
  • WISY WFF100 Vortex Filter

For roof areas up to 2,100 square feet:

  • WISY WFF100 Vortex Filter
  • WISY WFF150 Vortex Filter
  • WISY LineAr100 Rainwater Filter Stainless Steel
  • WISY LineAr100 Rainwater Filter Poly

For roof areas up to 5,500 square feet:

  • WISY WFF150 Vortex Filter

For roof areas up to 33,000 square feet:

  • WISY WFF300 Vortex Filter
  • WISY WFF300 Stainless Steel Vortex Filter

So, as you can see above there are some redundancies where a filter in shown in two different roof sizes. You can use filters for roof areas up to their capacity, but exceeding it is where efficiency is decreased. For example: A client I have worked with did not want to use multiple downspout filters to collect from all of her downspouts, so instead we used a WFF100 to collect all of the downspouts into one pipe below ground and then fed the vortex filter with just one pipe. You can always combine multiple filters to find a solution for different scenarios based on roof sizes, elevations, and other factors. For example: just because the WFF300 does not filter more than 33,000 square feet of roof area does not mean that a project's needs cannot be met. The addition of filters allows you to meet the project requirements in this scenario. 


These are the basics of rainwater harvesting pre-filtration. Removing sediment is the top priority to rainwater harvesting as it improves the quality of water while reducing the amount of maintenance of other components. This very simple step can save lots of headaches and improve the quality of water that you are collecting. 

Next post: Smoothing Inlet