Rainwater Harvesting Systems for Commercial Buildings


Published: Nov 26, 2022 | by Wahaso Water Harvesting Solutions

Rainwater Harvesting Systems for Commercial Buildings

With evermore environmental challenges cropping up on an ongoing basis, now is the time to take steps to help reduce environmental impact. As the world population passes 8 billion people, UNICEF expects nearly half the world population to face water scarcity by 2025. It is therefore imperative to make water efficiency a priority on the level of energy efficiency.

Rainwater harvesting systems for commercial buildings play a role in the process of establishing a more sustainable fresh water supply. They are environmentally forward, and often help owners significantly reduce water consumption and associated costs. The following expands on how effective these systems can be.

How Do Commercial Rainwater Harvesting Systems Work?

Are Rainwater Harvesting Systems for Commercial Buildings Economical?

Rainwater harvesting systems are indeed economical when enough rainwater supply is available and may be harvested to reduce significant volumes of municipal water, upwards of a million gallons or more each year. This water is filtered and sanitized to safe levels for use in non-potable applications including irrigation, toilet flushing, cooling tower makeup, boiler makeup, laundry wash, and beyond.

Especially in areas of water scarcity, high water demand, and/or high or rising water costs, rainwater harvesting systems become more feasible. In such cases, harvesting and reusing site water is less costly over time, than ongoing reliance on municipal water, and the first cost expense of a system may be offset by the reduction in water consumption expenses.

How Do Commercial Rainwater Harvesting Systems Work?

The primary approach for rainwater harvesting systems is fairly straightforward. Rooftop rainwater is collected, pre-filtered, then stored in a cistern or tank, and filtered and sanitized over time. Afterwards, the water is then pumped to one or more water demand systems throughout the building. The following provides a more detailed overview.


The first and most crucial step of rainwater harvesting is filtration. After rainwater falls on the roof, designated collection systems are in place to redirect water to the cistern. Pre-filtration of the water upstream of the cistern helps ensure that the cistern remains clear of sediment and particulates, while reducing the load on downstream filtration and sanitization components.

This step uses various pre-filters, such as hydraulic jump cascade filters, NSBB units, vortex filters, downspout filters, etc.


After the initial filtration stage, the water flows to a rainwater storage tank or cistern. These units differ according to site constraints, volume of water, type of water demand, and other factors. Clients often choose a specific cistern type based on that, although a manufacturer can also provide extensive guidance based on experience.

An expert team analyzes project specific data to estimate water supply and demand. This is done by measuring the building’s rooftop surface area and monthly rainfall, in conjunction with analyzing how much water is required to meet occupants’ toilet flushing, irrigation, and other demands on an ongoing basis.

Sometimes there is an imbalance in the supply and demand volume due to decreased rainfall during a dry period or even drought. When this occurs, a backup method is always built into the water harvesting system to revert to the municipal water supply and ensure some source of water is always available to meet demand.


Even though the rainwater has gone through several layers of filtration at this point, it must now be sanitized to eliminate bacteria, microorganisms, fungi, or pathogens that may have existed in the rainwater or surfaced when it flowed into the cistern.

Sanitization is provided through Ultraviolet systems that destroy pathogens that may cause diseases. UV rays can destroy the DNA of the present microorganisms and clean the water. Since constant UV exposure is necessary to sanitize the entire volume of water, the cistern can circulate the water across the tank, or UV light treatment can take place just prior to the treated water flowing to the water demand(s).

The other standard method is chlorination using Calcium Hypochlorite. It is safe, and commonly used in municipal water systems. Sensors inside the tank constantly analyze PPM (Parts Per Million) levels, and pump chlorine into the system if levels increase. These sensors maintain a standard quality of level that is sanitary and safe for use.


The final stage of the rainwater harvesting system is providing the treated water for toilet flushing, irrigation, cooling tower makeup demand, and beyond. Depending on the demand volume, duplex pumps are required to work in tandem to transport water to the non-potable water demand system(s).

Multiple pumps are in place to handle high water demand volume, and to ensure at least one pump is operational if the other fails. The size of these pumps is critical in ensuring adequate water pressure throughout a building and is determined by the type and volume of water demand, project location, and other factors.


What is the Cost of Rainwater Harvesting for Commercial Buildings?

Because each project has distinct characteristics, many different data points must be evaluated before a price estimate is feasible. That said, rainwater harvesting systems for commercial buildings tend to range from about $85,000 to over $700,000, depending on type of water demand, project scale, location, and system requirements. There are additional costs associated with installation and operation, however, integrated sensors and alarms constantly monitor and control all stages of the harvesting, treatment, and reuse process, significantly reducing associated maintenance costs.

While the first cost to integrate such a system may be significant, one can often save money over time, especially true in locations where water reserves are very low or susceptible to future drought and scarcity.


Rainwater harvesting systems for commercial buildings are both environmentally friendly and often cost-efficient. In the end, recycled rainwater may be used to reduce potable water consumption for non-potable demands, while reducing the load on municipal water systems, and operational expenses for the end user.


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Tags: rainwater harvesting systems for commercial buildings, rainwater harvesting systems

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Our mission at Wahaso is to help municipalities and commercial property owners reduce the impact of their buildings on the environment through innovative and sustainable water practices.