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The Rainwater Harvesting System Process

This article takes an in-depth look at the system process of rainwater harvesting as the primary source of water in a residential application.

The Process – Simple or Complex?

The basic principles of rainwater harvesting are rather simple, catch rainwater, store it, use it. But there are components, details, and processes that are critical to the success of a rainwater harvesting system.

Rainwater Harvesting System Process

A Successful System

What do I mean by a successful system?

Rainwater harvesting as your main water supply should only bring benefits and improvement to your water, this includes:

Water quality: The water should be clear, odorless, and taste great. The water needs to be bacteria-free through the use of a water disinfection system such as ultraviolet light.

Water pressure: The water should flow at the highest pressure possible without exceeding proper limits.

Water storage: The water being stored should be odor-free, slime-free, and clear.

System design: The system should be designed to meet the supply and demand of the home and its occupants. What is the sq. ft. roof surface that will be collecting rainwater? What is the average annual/monthly rainfall in the area? How much storage gives enough buffer to supply the house with ease, even with rain lapses?

These are 4 points that help in determining a successful rainwater harvesting system, but more aspects are involved even beyond these.

When it comes down to it, rain harvesters should be able to turn on their faucet, start the clothes washer, take a shower, use their water like they normally would, and there be no issue. We like to say perfect water at every faucet all the time. For this to be true, every part of the system process must be properly designed and incorporated.

Where the System Starts (Rooftop)

The first component of every rainwater harvesting system is the catchment area. Rooftops provide a hard surface that is separated from the debris that are found on ground surfaces. For instance, dirt, gravel, animals, cars, oil, and many others are common on the ground. This is why collecting rainwater from a paved driveway requires a much more intense filtration process. Roofs, on the other hand, are generally only exposed to a few contaminants which are easily filtered and removed.

The type of roof does matter. Generally, roofs such as tin, slate, and rubber are excellent for a rainwater harvesting application. Roofs that will release more contaminants into the water can cause problems. What about shingles? There are certain types of shingle roofs that approved to use, but make sure to talk with a rainwater harvesting professional before making your decision.

Diversion (Conveyance)

The next step in the rainwater harvesting process is diverting the rain from the rooftop into the rainwater storage vessel. There are two primary methods of diversion:

Below-Grade Storage: Water flows from the downspouts and into underground conveyance where it is gravity fed into the storage.

Level or Above-Grade Storage: In the case where the water storage is on grade or even a little above where the downspouts end, a wet-conveyance system is needed. Many homes today use a wet-conveyance system. They require the downspouts be replaced with sealed conveyance. Therefore, as the water fills up the conveyance, gravity will push the water into the storage. The conveyance can then be painted or covered to match the exterior design.

After the downspouts, the conveyance is placed underground all the way to the storage.

Pre-Storage Filtration

Before entering the rainwater storage vessel, the water is sent through a robust pre-storage filter. This pre-filter has two primary functions:

Filtration: The major contaminants that could flow from the roof and through the conveyance such as leaves and other large contaminants are easily filtered out through the filter.

Oxygenation: As the water passes through the filter, it is designed in such a way that highly oxygenates the water, which helps keep the water naturally fresh and pure.

The filter is designed to be very efficient, yet effective.

Most pre-storage filters use a metal mesh filter that is either gravity-fed or filtered through centrifugal force. These filters do not need to be replaced, but rather should be cleaned about 2-4 times per year.

Entering Storage

After the water passes through the pre-storage filtration, it is diverted into the water storage. But even this process has been designed to work precisely with the system.

There is a healthy microbiological layer that forms on the bottom of the tank, known as the biofilm, and it helps keep the water clean. And to make sure that this layer is not disrupted, when entering the storage, water goes through a smoothing inlet. This sends the water into the tank in an upward motion, rather than forcing water to the bottom.


There is a lot to know about rainwater storage, here are two articles I wrote that take an in-depth look at rainwater storage. Storage TypeStorage Ecosystem

Here is an overview:

Rainwater storage can be placed below ground, above ground, or partially buried. The benefits of being above ground or partially buried are the cost is usually less, and you can also use the storage as a part of your landscape design. While the benefits of below ground storage are that the storage is completely out of site, but the cost of excavation will generally bring the overall price up.

There are many different types of tanks. The most common are above and below ground polyethylene, and above ground metal tanks. It is important to make have high tank quality, it needs to be U.V. resistant and designed for potable water use.

The amount of storage is completely based on the application. Most residences will fall somewhere between 3,500 gallons and 10,000 gallons of water storage. Often times, storage is placed in series, such as five 1,300 gallon tanks.

Overflow | Groundwater Recharge

When the water in storage is filled to the top, it sent out through an overflow device. This is specially designed to let the water flow out, without compromising the inside of the tank to any outside sources. This water can be diverted into an animal water trough, a pond, or can be sent into groundwater recharge.


Every rainwater harvesting system includes a pump. There is little strain put on the pump compared to a well because the water isn’t directly fighting gravity for hundreds of feet. Rather, the pump is simply mounted inside the water storage tank.

And using a floating mesh water intake, it draws the water from about 4 inches below the surface where it is best. The water is then pumped out of the tank and sent to the filtration panel. The pump should be high-quality and robust, but should the pump ever be damaged, or need to be replaced, it is a quick job to replace the pump since it is very accessible.

Exiting the Tank

It is not only important how the water enters the tank, but also how it exits. Using a floating filter intake, the water passes through yet another filter on its way out of the tank. It is collected just a few inches under the surface, where the water is at it’s best.

Filtration and Controls

Due to the water already being very clean, three simple stages of filtration and disinfection are needed to make certain the water is 100% safe to drink. The water is sent through a sediment filter that removes any remaining particles. And then through a carbon filter that removes other contaminants and makes the water fresh and taste great. And finally, the water is sent through an ultraviolet disinfection system. This removes any possible bacteria from coming through the water. The filters only need to be changed once or twice per year.

Controls such as water storage level indicators can be used. Pressure readings are also common.

The filtration and controls panel can often be placed on a wall using a 3ft x 3ft panel, so it doesn’t take a lot of space.

Automatic Crossover to Secondary Water Supply

This is a great way to calm any nerves of ever running out of water. The system can be tied into a municipal or well water supply that can be automatically switched on in case of a drought. If this is used with municipal water supply, backflow prevention is required.


The water has now made it through the final part of the rainwater harvesting system process. The rainwater should be tied in directly to the house plumbing.

It is ready to use!

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