Rainwater Harvesting Systems


The practice of rainwater harvesting dates back to the earliest days of civilization yet was nearly lost in the age of inexpensive and readily available municipal water supplies.

With the cost and supply of municipal water becoming an issue, there has been a growing interest in rainwater harvesting by home and building owners. There are special challenges for the more complicated systems installed in commercial and institutional buildings. The vastly larger quantities of rainwater collected require more sophisticated processing and storage, and the engineering standards and approval requirements are significantly more rigorous

Wahaso focuses entirely on the commercial and institutional building market – and has developed proven, large-scale rainwater harvesting systems that meet rigorous building and regulatory requirements. You can see examples of installed rainwater harvesting systems that we designed and built on our Projects Page



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 “hydraulic jump”pre-filter has a unique design that captures 100% of light rainfall events. At the start of a large rainfall event, the high flow flushes and cleans the filter screen automatically restoring its filtration capacity.

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.

UV Sanitation Skid
Ultra Violet Sanitation System Designed and Built By Wahaso for our Bolivar Park, CA Project.

Wahaso’s new chlorine dosing technology uses dry calcium hypochlorite briquettes to produce fresh liquid chlorine solution as needed.The technology is scalable to meet any system demand.


If residual sterilization capacity is required, added chlorine is the most common approach. For Wahaso projects, our preferred method of chlorination is to utilize a concentrated chlorine solution that is automatically added to the treatment system. 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.

Oldcastle Pre-Cast, Pre-Cast Concrete Cistern


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.


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.


Wahaso 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 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.