Rainwater Harvesting Systems

Wahaso Rainwater Harvesting Systems

The practice of harvesting rainwater dates back centuries and is still the most common type of water harvesting system. Wahaso specializes in designing and building rainwater harvesting systems for large-scale applications.

Commercial Rainwater Harvesting Systems

Unlike the simple systems installed by other rainwater harvesting companies for homeowners, our rooftop rainwater harvesting systems are designed with the commercial-grade components and sophisticated controls required in commercial buildings for reliability, automated functions and connectivity to BAS systems.

The Wahaso commercial rainwater harvesting systems produce treated water that is safe and suitable for non-potable applications such as toilet flushing, irrigation and cooling tower make-up.

Water Harvesting System Design

Wahaso commercial rainwater collection system is designed to meet rigorous commercial building and regulatory requirements. The first step in any Commercial Rainwater Harvesting Systems is pre-filtration upstream of the cistern. This helps to keep the cistern clean and reduce the load on downstream processing equipment. Wahaso rainwater harvesting for commercial buildings uses a variety of pre-filters including hydraulic jump cascade filters, NSBB units, vortex filters, downspout filters and more.

Harvesting Rainwater

After pre-filtration, the harvested water is stored in a cistern rainwater harvesting storage. (See Storage for more information.) The Wahaso system includes transfer pumps that convey the water from the cistern rainwater harvesting storage to a skidded processing system where it is filtered and sanitized for reuse. Wahaso uses Ultraviolet (UV) sanitation in our rooftop rainwater harvesting systems, which kills pathogens and bacteria that can present a health risk. Chlorine can also be used for applications such as toilet flushing, where residual sanitation is required.

Rooftop Rainwater Harvesting

Treated water is delivered to the end use via Wahaso booster pumps. These rain water harvesting pumps include Variable Frequency Drives (VFDs) and integrated into the Wahaso Control System. Wahaso employs custom-programmable logic controllers that not only manage the pressurization functions, but also monitor tank levels, differential pressure on filters, UV function and chlorine levels. Our control packages are capable of interfacing with the Building Automation System (BAS) and logging critical data.

System Benefits

  • The Right Size for the Job. Wahaso’s proprietary pre-design analysis helps to determine the proper size for any rainwater system. A system that is too large is not cost-effective, but under sizing a system can lead to lost potential.
  • Low Maintenance. The Wahaso system has been designed for minimal maintenance with few consumables.
  • Integrated System Design. Wahaso’s greywater processing skids are pre-assembled and integrated into comprehensive system designs that include all pumps, storage, filtration, sanitation and controls.
  • Robust Controls. Wahaso’s proprietary control system is custom programmed to monitor and track all system activity.

Standard Systems

Wahaso offers both standardized and custom rainwater harvesting systems. Our standard systems are designed for both small and medium-sized projects and incorporate filtration, sanitation and tank sizing. If our standard systems do not fit the project, Wahaso will work with architects and engineers to design a custom system. For more information on our standard rainwater harvesting systems, download the information sheets.


As rainwater comes into the system from the roof or parking area, there is usually one or more stages of filtration before the storage cistern.
First Flush or Pre-Filtration
The first step is to remove as much debris as possible from the rainwater before it enters the system. A first flush sequence can be programmed into the control system that senses a rain event and diverts the first few minutes of rainfall to the stormwater system. This is an effective way of minimizing debris and contaminants in the water but also wastes many gallons.

A high-capacity vortex filter uses the pressure of water coming off a roof or parking lot to spin it in a centrifugal manner and then force it through a cleaning screen. Debris drops through the center and to the stormwater system while clean water exits out the sides. We like this method because it is 95% efficient when screening large quantities of water and requires minimal cleaning or maintenance. Vortex filters can be located above ground or below ground.

For higher capacity systems, we like to use commercial-grade cascade filters and nutrient separating baffle boxes (NSBB). These systems effectively filter several cubic feet of rainwater and stormwater per second, separating oils and suspended solids before the water is sent to the cistern.

Organic Filtration
For stormwater collected from parking lots and walkways, organic filtration can be both effective and relative inexpensive. Stormwater is diverted to a vegetated swale area where it collects and then drains to a subsurface cistern. The plants, sand and organic microbes living in the soil all serve to filter and break down contaminants in the water before it enters the cistern. Water in the cistern can then be used for irrigation between rain events. The swale must be sized appropriately to prevent flooding and will not work on properties struggling with heavy silt run-off.
Additional Filtration
Depending on how the water will be stored and used, additional filtration may or may not be needed. Water stored in a below-ground cistern can settle out most small material so that no additional filtration is needed – especially for irrigation when absolute clarity is not a requirement

For toilet flushing, cooling tower make-up and other more specialized uses, more filtration may be desired. Bag filters and multimedia filters can be effective for removing sediment and particulates as small as 20 microns or finer. Carbon filtration can resolve any issues with odors or colors.


It is usually necessary to sanitize the water to kill algae, parasites or viruses that could enter the storage system from the source or be introduced in storage; many municipalities require sterilization for public safety. There are several technologies available for this purpose.
Ultra Violet (UV) Sanitation
For many rainwater applications, ultraviolet sanitation (UV) is sufficient for killing pathogens that could present a health risk. These systems expose the water to a specific wavelength of UV light that destroys the DNA of any organisms present. Stored water can be constantly circulated past the UV bulbs to protect the entire water supply, or exposure can take place right before application. The biggest advantage of UV sterilization is that it is chemical-free and requires minimal maintenance. Its biggest disadvantage is that the UV has no residual killing effect – so that algae and other pathogens can still form down stream of the system. Sizing of the UV system is critical to maintain sufficient exposure rates to keep the water sterile.
If residual sterilization capacity is required, added chlorine is the most common approach. For Wahaso projects, our preferred method of chlorination is calcium hypochlorite in the form of solid briquettes. The chemical is similar to that used in a municipal water treatment systems but is in a form that is safer and easier for building maintenance staff to handle.

Wahaso’s dry hypochlorite sanitizing system uses chlorine briquettes that are added to a hopper in the dosing unit where they are combined with water to make a high concentration chlorine liquid. An automatic free chlorine analyzer continuously monitors the parts per million (ppm) level and proportionally controls a chemical injection pump to dose in more chlorine as necessary to maintain the preset level.

Other sterilizing agents that can be used include Ozone and Chlorine Dioxide. These sterilizing agents are very powerful and are typically only used on greywater systems. They are more hazardous to handle and require on-site generation of the Ozone or Chlorine Dioxide gas plus an injection system.

Harvested Water Storage

Storage of the rainwater is determined by the demand and uses for the water, available rainwater and groundwater volume and frequency, and space to locate the tanks.

Because storage capacity, type and location are primary drivers of system design and cost, this is one of the first analyses done on any project. Wahaso works with clients early in the process to think through the supply and demand side of the system and determine the most efficient size, type and location of the storage system.

On the supply side, we calculate total rainwater available to the system by measuring collection surfaces on rooftops and parking areas and applying average monthly rainfall for the building location. On the demand side, we help determine anticipated monthly demand for toilet flushing, irrigation, cooling tower make-up and other uses. Storage options include above-ground tanks that can be located inside or outside the building and subterranean tanks that can have unlimited storage capacity. For a more thorough review of the different storage options for harvested water, see our Harvested Water Storage page.

Most systems have a connection to a municipal water source to provide a constant supply of water during drought periods – especially when the system is being used to flush toilets. A level and control system automatically maintains a minimum level for this purpose.

Applying Harvested Water

A repressurization system is required to move the rainwater to toilets throughout the building. Typically, the system includes duplex pumps in tandem with each pump alternating in operation. If a high demand situation occurs, both pumps can be made available. And if one pump should fail, the system will continue to provide harvested water. It is important to properly size the pumps so that adequate pressure and volume is available at the farthest – and highest altitude – end point in the system.

Water Harvesting Monitors And Controls

Wahaso Commercial Rainwater Collection System as opposed to other Rainwater Harvesting Companies employs programmable logic controllers, (Allen-Bradley as standard), that fully automate and control the entire harvesting process for rainwater harvesting systems.

Our proprietary software is customized for each application and provides the capability of interfacing with building automatic systems and other alarm and condition monitoring.
In addition to monitoring the system mechanicals, the control system can track the amount of water in each tank and track and display the monthly amount of water harvested. An interface allows remote monitoring via a web page for maintenance or educational purposes. Wahaso Commercial Rainwater Collection System unlike other Rainwater Harvesting Companies can use the remote access to help a building maintenance staff diagnose potential system problems.

To learn how a rainwater harvesting system might be incorporated into your building plans, please contact us.

Wahaso Rainwater Harvesting System FAQ’s

To learn how a rainwater harvesting system might be incorporated into your building plans, please contact us.

1. What are the four main components of a rainwater harvesting system?

The four key components of a rainwater harvesting system are:

  • Storage/cistern – This may be above ground, below ground, concrete vault, modular, corrugated, HDPE or fiberglass
  • Filtration – This includes pre-filtration prior to water going in the cistern and the type of filtration depends on the type of water and where the system is located
  • Sanitation – This can be either UV light, chlorine, ozone or cupridyne
  • Controls – Measures tank levels, flow rates, water savings and pump and system statuses.
2. What are the types of rainwater harvesting systems?
The three primary types of rainwater systems are indirect pumped, direct pumped, and indirect gravity. The best system to use will depend on the specific situation. An indirect pumped system is ideal where you cannot install a header tank. On the other hand, an indirect gravity system would be perfect if you could install a high-level header tank.
3. Are rainwater harvesting systems worth it?
While rainwater harvesting systems have some downsides, their potential benefits outweigh the disadvantages. Therefore, rainwater harvesting systems are generally worth it and should be your primary water conservation technique.
4. How long can rainwater be stored in tanks?
You can store rainwater in tanks for over 30 years. However, you will need to take measures to maintain the integrity and cleanliness of the collection system and storage tanks. For example, you should shield the water against UV light to prevent algae growth or sanitize it regularly with chlorine to kill bacteria.
5. What are the limitations of rainwater harvesting?
The major limitations of rainwater harvesting include rainfall unpredictability, storage limits, and regular system maintenance. You also need technical skills to install the system or hire an expert for the job. Incorrect installation may lead to mosquito infestation and various waterborne diseases.
6. How do you keep rainwater from stagnating?

You can take several precautions to prevent your harvested rainwater from stagnating, including:

  • Use the water as often as possible
  • Block sunlight from the storage container to prevent bacteria and algae growth
  • Cover the container with a tight-fitting lid to keep leaves, twigs, and other organic matter away
  • Cycle the water past a UV bulb or chlorine injector
7. What happens when the rainwater tank is empty?
If worried about what happens when your rainwater tank runs dry, consider investing in a rainwater-to-mains changeover system. When your rainwater runs out, the system will automatically switch to the mains supply. In a commercial system this is done automatically with an air gap or back flow preventer whereas in a residential system the harvested water is typically completely separate from domestic water supply.
8. How to maintain a rainwater harvesting system?

You can take several measures to maintain a rainwater harvesting system, such as:

Residential systems

  • Keep the gutters clean and free of leaves and other debris that may cause water discoloration
  • Place cisterns in a shady area to prevent algae growth
  • Paint PVC pipes to protect freezing and UV effects

Commercial systems

  • Each system will have a maintenance manual to be followed
  • Change chlorine and UV bulbs as needed
  • Service pumps and other moving components
  • Check control system for alerts
9. How much does a rainwater harvesting system cost for commercial buildings?
The cost of a rainwater harvesting system varies significantly, depending on size and other factors. You can spend as little as $25,000 for a simple system for a commercial building. However, a complete system for a commercial building could cost over $100,000.
10. What are the health risks of the rainwater harvesting system?
The most significant health risk comes from contamination of the harvested rainwater with bacteria, viruses, and other disease-causing microorganisms from birds and pests. Leaching from copper, lead, and other roof materials also poses a health risk. It is essential to sanitize harvested rainwater in case of accidental ingestion.

Since 2004

Wahaso Mission

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.