Rainwater Harvesting 101: Tank Overflow

Continuing our series of posts on Rainwater Harvesting 101, today we will look at using an overflow in a rainwater storage tank. This is the final, crucial step to consider when setting up a rainwater harvesting system. Although overflowing a rainwater tank may initially seem counter intuitive, it is actually a beneficial process for rainwater harvesting as it helps to move water which creates oxygenation and helps prevent stagnation. Using an overflow accomplishes two primary goals: 1.) Creates an exit path for excess incoming water, and 2.) helps to remove floating particulate from the surface of the water. 

Creating an exit path for incoming water is important so that the tank does not overflow through the manway, back-up through your pre-tank filter, or create excess pressure on the storage vessel. This also allows you to control where excess water goes and often excess water is routed to a garden or other areas for supplemental irrigation. Removing floating particulate from the surface of the water is also beneficial as it is part of the low-maintenance approach that we take to improve the quality of water in your tank. 

Overflow can be achieved either through standard PVC piping or by way of the WISY Multifunction Overflow devices. We recommend the WISY overflow devices for installations that have 4" or greater piping entering the tank as they siphon particulate off the surface off the water, use a stainless-steel device to prevent small animals from entering the tank, include a float that prevents back-up from a storm system from entering the tank, and are an easy installation. The picture below shows how the WISY Multifunction Overflow Device works.


As mentioned in previous posts, the small bits of sediment that make it into the tank after pre-filtration will typically either sink to the bottom of the tank to form the bio film or float to the top of the water level. As shown above, using an overflow device can help to remove those floating particles from the surface of the water, while also allowing excess water an exit path.

A few things to consider when installing an overflow:

  1. Match the overflow pipe size to the inlet pipe size. This makes sure the overflow can keep up with the incoming water if your tank is full of water. Example: If you have a 4" inlet to your tank, you'll want your overflow to be 4". 
  2. Ensure that the overflow pipe is installed at a lower elevation than the inlet pipe. In most instances, you want to make sure that the overflow is a minimum of half the pipe diameter below the inlet pipe. This allows for overflow before backing up into the pre-tank filter/inlet pipe.
  3. Make sure the overflow has a lip/bevel. It is recommended to have some sort of angle on the pipe that the water exits through. This creates a siphon effect and helps to draw water into the pipe (think of a pond skimmer). This also helps to siphon the floating particles through the overflow. If PVC piping is used, an angle of 11 or 22 degrees cut across the pipe is recommended. The WISY Multifunction Overflow Devices contain this angle already. See "ITEM A" below for an example.
  4. Prevent backflow. If your tank is tied into a storm drain, you will want to make sure that you either use a WISY Overflow Device (which contains a backflow prevention device) or a backwater valve. Both of these devices prevent water from entering the tank in the event that there is a sewer/storm line back up. This protects the quality of water in the tank. These both will also function as a method to prevent small animals such as mice from crawling into the tank. See "ITEM B" below for an example.
  5. Prevent mosquitoes/other bugs. Creating an air-tight connection on the tank helps to prevent mosquitoes from entering the rainwater storage tank. Using the example of ITEM A below, once enough water is in the tank to overflow it once, there will be water that sits in the bottom of the P-Trap and fills up enough to prevent any air gap to allow entrance into the tank. This isn't guaranteed to prevent all bugs from entering the tank, but it is a very effective method nonetheless.
 item a

item a



That covers the basics of rainwater tank overflow. If you have any questions or comments, please leave them below and we'll be happy to help!

In the final post of the series, we will cover post-tank filtration.

Final post: Post-tank Filtration

Rainwater Harvesting 101: Pump Selection

Continuing our series of posts on Rainwater Harvesting 101, today we will look at pump selection. If you plan to use gravity to force water from your tank to your garden or lawn, this probably isn't the post for you. But, if you need to pump water at a higher flow rate and pressure than gravity will allow, let's get started! 


Above are a few examples of pumps and we'll go into the specifics a little bit more below on what to consider when selecting a pump.

There are a few important things to consider when selecting a pump:

  • Flow rate required
  • Pressure or head required
  • Installation Location/Pump Style
  • Electrical Requirements

Flow rate

Flow rate comes into play as a pretty basic requirement for selecting a pump because an end application will require a certain amount of flow.

Here are a few examples of flow rate requirements

  • Garden Hose: 2-4 gallons per minute
  • Toilets: 1.6 gallons per flush
  • Washing Machine: 2 gallons per minute

Make sure to account for all of your potential combined uses and choose a pump that can accommodate the maximum total flow rate. 

Pressure or Total Dynamic Head

Pressure or total dynamic head required are arguably just as important as flow rate. If you have a below-ground  installation and need to pump to the 7th floor of a building, you need enough Total Dynamic Head (TDH) to push the water to overcome that height difference, otherwise the water will never reach that portion of the building. Likewise, if you need your pump to provide enough pressure to mimic that of a normal residential system, you'll need one that is able to produce 45-60 PSI. 

Pump Curve

So, now that you know your flow rate and your pressure/TDH required, you'll need to make sure your selected pump can accommodate both. Let's take a look at the example pump curve below.


Say for example, you need 25 Gallons per minute at 100' TDH. The 1/2 HP and 3/4 HP options do not quite get there, but the 1 HP pump (1S51E-H) does and would be the pump you need to select. 

Installation Location/Pump Style

This is typically a very project specific factor. The installation location typically refers to whether or not the pump is installed inside the storage tank (submersible) or outside the tank (booster/jet). Each style has it's own benefits. For example, a submersible pump is often quieter as it is submerged in the water tank, it is out of sight/not in the floor space, allows for constant pump access to water (as long as float switch is used for dry run protection). However, on the negative side, it must be disconnected and removed in freezing conditions, is more difficult to maintain, and typically requires stainless steel components for long-term submersion, which could affect cost. A booster/jet pump is easy to maintain, can be disconnected easily during freezing conditions, and can sometimes be more cost effective with lower cost materials since it is not submerged. However, they can sometimes be noisy, they take up floor space, and can lose prime if they are not in flooded suction state.






Electrical Requirements

This is probably the most straight forward of the bunch: what electrical characteristics do you have available? Some flow rates and pressures are only able to be accomplished with higher voltage and three phase power. For most residential applications, 115V/1 phase/60 Hz or 230V/1/60Hz power will suffice as these pumps are providing flow and pressure at rates similar to a city or well line.

Still need help? Just call us! We'll be happy to help you size a pump for your rainwater harvesting project.

Next post: Overflow Protection

Rainwater Harvesting 101: Floating Filters

Continuing our series of posts on Rainwater Harvesting 101, today we will look at the use of floating filters for extracting the rainwater. After taking the proper steps to pre-filter the rainwater and use a smoothing inlet to prevent disturbing the biofilm, you'll eventually want to use that rainwater, right? The floating filter offers the best way to extract the cleanest water from the tank to take it to end use location. 

Let's start by looking at the picture below:

floating filter diagram.jpg

As mentioned in previous posts, the small bits of sediment that make it into the tank after pre-filtration will typically either sink to the bottom of the tank to form the biofilm or float to the top of the water level. Very little particulate remains suspended at this point. Using a floating filter allows you to draw water from this area between the surface level and the bottom as the actual filter sits below the float as shown above. Typically rainwater is drawn from 6-8" below the surface and as such avoids the majority of particulate. 

These floating filters are called "filters" for a reason as they have stainless steel mesh screens to help filter out sediment in an additional step. The coarse floating filter contains a screen that keeps out particles larger than 1.2 mm in size, while the fine floating filter filters out particles larger than .3 mm. This is just an additional step in trying to reduce the amount of sediment that makes it to the end use.

We use food-grade suction hose to connect the end of the floating filter to the pump. We add hose to allow the float to move more as the water level in the tank fluctuates. 

This step reduces the amount of particulate in the water which promotes longevity of pumping systems, is less likely to be discolored, less likely to carry sediment that has pathogens, and reduce the frequency of needing to do maintenance on post-tank filtration components. 

regenwasser-ansaugfilter-fuer-zisterne-wisy-sz9927 small.jpg

Next post: Pump selection

Rainwater Harvesting 101: Calming Water at Entry Point

Continuing our series of posts on Rainwater Harvesting 101, today we will look at the importance of calming the rainwater as it enters your tank. This is a very simple step to include in your system and improve the quality of the water that you end up taking to the end use.

After pre-filtration, the rainwater entering your tank will have very small pieces of sediment that remain. These particles tend to either float to the top of the water surface or sink to the bottom of the storage tank. The ones that sink to the bottom of the tank create a biofilm, which is easiest to describe as a healthy "ecosystem" within the tank in which bad bacteria is typically consumed by the biofilm. So, this biofilm plays a big role in your water's health so it is best to keep it intact right? That is where the smoothing inlet comes into play.


By use of...

the smoothing inlet, the rainwater is directed upwards and away from the bottom of the tank as it enters

Think of a fishbowl for a moment. If you have a fishbowl with a low water level and you suddenly dump a large amount of volume into it, it will typically stir up the water and cloud up the water with whatever has settled on the bottom of the tank. This is what we want to avoid doing when the rainwater enters the storage tank. By directing the water upward and away from the floor of the tank, we are not disturbing the biofilm. This also prevents the sediment from being stirred up and floating around when your pump or hose bib is attempting to draw water out for use. This process also has the added advantage of introducing oxygen into the tank.

For most applications, we use the WISY stainless steel Smoothing Inlets (available in 4" or 8") for the ease of installation, the long-term durability of the component, and the fact that it takes up very minimal space in the tank. For smaller applications (for example: using a downspout filter), we recommend using two 90-degree elbows to make a "candy cane". Both ways are acceptable as long as you direct water up and away from the floor of the tank.

Next post: Floating Filters

Rainwater Harvesting 101: Pre-Filtration

300 with debris.jpg


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

tank header.jpg

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.

wisy 4-step system.jpg

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!