Rainwater Harvesting 101: Pre-Filtration

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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:

For roof areas up to 1,600 square feet:

For roof areas up to 2,100 square feet:

For roof areas up to 5,500 square feet:

For roof areas up to 33,000 square feet:

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

Rainwater Harvesting 101: Rainwater Tank Selection

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Continuing on our Rainwater Harvesting 101 Post series, we will look at storage tank selection.

One of the first factors to consider when designing a rainwater harvesting system is your expected usage compared to expected rainwater collection. Tank size is a very important factor for the overall rainwater harvesting system (which we will get into later in the post) and also becomes a big factor in the price of a system. Factors that frequently come into play with selecting a tank are:

  • Storage Capacity: will this tank allow me to collect enough water to achieve the sustainability goal I have set?
  • Location: above or below ground? 
  • Materials of construction: plastic? metal? fiberglass? concrete?
  • Space restrictions: Am I limited by height or footprint?
  • Cost: does the tank fit my budget?

It is key when selecting a tank to make sure that you take into account all of the factors above.

Storage Capacity

This is an important aspect of a rainwater harvesting system because you want to size the tank appropriately to a.) allow overflow and b.) not overflow all the time, while also allowing the tank to serve your needs for your project. Allowing the tank to overflow helps to remove floating particulate from the tank as well as create movement of water and provide excess incoming water a place to go in the event that the tank is full. 

We implement tank sizing software to help determine the proper tank size for your project based on historical rainfall data, expected water usage, and several other factors. If you need assistance sizing your tank, fill out the tank sizing form and one of our team members will contact you!


Rainwater tank location plays a big factor in tank selection as water must be conveyed by gravity to the pre-tank filter and eventually the storage tank. This means that sometimes there simply is not enough elevation for above-ground tanks to work and below-ground tanks must be considered. Other times the elements must be considered and whereas an above-ground tank may be well suited for a warm climate like Florida, it is likely to be subject to freezing in a cold climate like Minnesota. Other physical factors can come into play as well where digging in rocky terrain or a high water table may make below-ground tanks more difficult/impossible to use. 


At times...

...visibility may come into factor as well. I have worked on several projects where Homeowners Associations prohibit visible water tanks, but do not have issues with below-ground tanks. While above ground tanks tend to be less expensive than below ground tank options, below ground tanks can offer hiding conveyance piping and may make elevations much easier to accommodate. 


On the contrary...

some projects are wanting to tout or "show off" the rainwater system and implement large above-ground storage tanks in their design.

While above ground tanks tend to be less expensive than below-ground tank options, below-ground tanks can offer hiding conveyance piping and may make elevations much easier to accommodate. Installation location has several factors, so consider all of them before coming to a decision.

Materials of Construction

This attribute comes into play primarily in terms of price, aesthetics, and potable vs. non-potable installation. As shown above, there are several types of rainwater harvesting cisterns. Examples are:

  • Polyethylene (above or below ground, several different types available)
  • Metal: corrugated or smooth; stainless steel, galvanized, 
  • Fiberglass (above or below ground)
  • Concrete
  • HDPE pipe

The end use should also be considered as not all rainwater tanks are suited for potable water storage by default and may require a specialty liner or finish (NSF 61 is typical).


All materials of construction offer their own advantages and disadvantages, so finding a happy medium is essential. For example, Polyethylene tends to be the most cost-effective option, but doesn't necessarily carry some of the aesthetic values that a metal tank does.

Space Restrictions

Here's an example of this factor:  50,000 gallons of storage is not a small volume to accommodate. So, while your project may be optimized to offset 97% of water demands at 50,000 gallons of storage, you may only have the physical space to accommodate 10,000 gallons of storage. Determining what you can accommodate will 


Financial fit is also important as tanks can account for a sizable portion of a project's budget. Different tanks may also have different cost-associated factors such as shipping, specialty installation, expensive material of construction, and more. However, rainwater cisterns can also come into play and allow quicker returns on investment by allowing you to store and use more water if applicable to your project.

If you have any questions on tanks or how to properly size and select one, please contact us by phone, email or fill out the tank sizing form here! 

Next post: pre-filtration.

Rainwater Harvesting 101: Overview

Rainwater harvesting doesn't have to be hard. By following a few simple steps, you can design a high-quality, low-maintenance rainwater harvesting system for your home or business.

This post will serve as a brief overview of how we at Rainwater Management Solutions design rainwater harvesting systems. As always, we are here to help so if you get stuck along the way, feel free to comment on the post, call us, or email us!

Over the course of the week, we will be posting a few more blog posts going into more detail on each of the steps seen below. So, let's get started!

The core of how RMS approaches system design is the WISY 4-Step System. This 4-Step System greatly improves the quality of the rainwater and is designed to continue reducing the amount of sediment in the water as it is taken into end use. In addition to the 4-Step System, we size tanks to ensure overflow and make sure that they fit the project's needs. Pump and post-tank filtration are then the final steps to a complete system!

When researching and designing a rainwater harvesting system, it is important to consider the factors of:

  • What will I use the water for? 
  • How much water can I collect?
  • What is the impact of my project?
  • Where will I place my storage tanks?
  • Are there local/state restrictions on rainwater harvesting? What permits do I need?
  • What is my budget?

Of course there are more items to consider, but these are the common ones that come into play.

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Pre-filtration is key to creating a high quality of water in your tank. Rainwater by itself is very clean water, but collection from roof areas introduces pollutants/contaminants in the form of debris from leaves, sticks, rocks, or anything that may wind up on the roof. Reducing particulate is important for several reasons, but most importantly it removes larger particulate that may carry pathogens as well as causes cloudiness in the water.

Reducing particulate is important for several reasons, but most importantly it removes larger particulate that may carry pathogens as well as causes cloudiness in the water. WISY pre-filters work by removing larger particles of debris (as small as 0.011" in size) and drawing clean water through the filter screen to send it to the tank. This means that only very small particles make it to the tank after the pre-tank filter. With proper pre-filtration you can have cleaner water and you should not have to clean your tank out. 

Step 2: Smoothing Inlets

Smoothing Inlets are designed to direct the water upwards upon entrance to the tank. This prevents disturbing the healthy "bio-film" that has settled at the bottom of the tank. The rush of water directly into the bottom of a tank without a smoothing inlet would stir up fine sediment (think of quickly pouring water into a fish bowl) that could then be drawn in by the pump. The "bio-film" at the bottom of the tank actually works to eat any bad bacteria that may enter the tank and help create a healthy water environment.

The third step in the WISY 4-Step System is the floating filter, which draws water in to the pump from below the surface level to avoiding taking in any small particulate that may be floating.

This step reduces the amount of particulate that goes to end use which can promote the longevity of the pumps, post-tank filtration components, and reduce build-up in piping (or drip-emitters in irrigation systems).

Tank overflow is essential for a healthy water environment. The WISY overflow devices will siphon small, floating particulate off the surface of the water, while also providing a path for excess incoming rainwater to overflow to. These overflow devices also protect backflow from storm drains.

So there you have it! Four simple steps that help create a very high-quality rainwater harvesting system for your application. In the next few days we will have more posts going into some more detail on each of these steps, explaining the importance of these steps and how to properly size them so stay tuned!


Welcome to the newest addition to our website: our blog!

We're excited to bring this feature to life in the New Year and we look to using it to provide education on rainwater harvesting, stormwater, and graywater re-use. We're also looking forward to interacting with you more and believe this will create a good platform for discussion of sustainability, problem-solving methods, solutions, product spotlights, and so much more.


In the coming weeks, we will begin posting some introductory information on rainwater, stormwater and graywater and hope that you will join us along the way!