Stormwater Harvesting Systems


There is a difference between “rainwater” and “stormwater.” They both begin with a rain event. “Rainwater ” is the relatively clean water coming from rooftops and other above ground surfaces. But once that rainwater reaches the ground, it becomes “stormwater” that may be contaminated with silt, hydrocarbons from automobiles, road salt in northern climates, nitrates, other fertilizers from landscaped areas and more. Because of this, additional filtration and cleaning steps are required, but stormwater can still be an excellent harvested water resource.

The Stormwater Liability

Increasingly, communities are requiring developers to manage the run-off from their impermeable rooftops, hardscapes and parking areas to reduce the impact of the run-off on municipal treatment systems. This requires builders to install storage (detention) tanks that slowly release collected stormwater into surrounding waterways to protect the municipal treatment facilities from high stormwater flows. This storage can require an investment of hundreds of thousands – or millions of dollars — to filter the stormwater, remove hydrocarbons and then detain the treated water until the storm event has ended. Normally these detention systems represent a “sunk cost” from which the property owner will receive no direct benefit.

Turning a Liability into an Asset

Wahaso sees this detention stormwater as an asset rather than just a liability, and offers solutions that filter, sanitize and pressurize the water for on-site reuse. Harvested stormwater can be used for any number of purposes. Irrigation is the most common use, but this valuable resource can also be used to flush toilets, make-up water in evaporative cooling towers and more. Not only does this harvesting effort save the building owner thousands of dollars a year in water bills, but the practice retains the stormwater on site thus reducing the burden on the municipal treatment system and saving the energy needed to transport and purify that water at the remote plant. It’s a sustainable practice that benefits property owners and the greater community.

Economics and Return on Investment

For large stormwater detention systems, the incremental cost for converting the storage from detention to retention for harvesting is a small fraction of the cistern cost. For example, a 400,000-gallon detention cistern might cost $500K or more. Adding a harvesting capability to irrigate the property’s landscaping could be $50K or less – a 10% increase in total system cost — that would yield savings year after year in municipal water and sewer charges. And because the detention systems are usually mandated by local codes, they are a sunk cost when calculating the ROI of the incremental harvesting capability. Wahaso can provide a detailed ROI analysis for any project through our Scoping process to estimate total water savings.

Stormwater Harvesting System Overview

NSBB, DSBB, Nutrient Separating Baffle Box, Suntree Technologies, Bioclean


It is key for any storage system that the water entering the cistern be properly pre-treated. This means that a filtration process needs to remove hydrocarbons, debris, sediment and pollutants before the water enters the tank. If the water is coming off rooftops and other non-parking areas, this may be as simple as passing the water through a screen or vortex filter. Contaminants from parking areas need special treatment to remove the additional contaminants that may be present.

For parking stormwater and high-capacity flows from large rooftops, Wahaso likes the Nutrient Separating (NSBB) and Debris Separating (DSBB) Baffle Box systems developed by Suntree Technologies and Bioclean. These systems utilize screening and hydrodynamic separation to capture pollutants in stormwater.

The patented screening system, suspended above the sedimentation chambers, captures and stores trash and debris in a dry state. Dry state storage of trash and debris minimizes nutrient leaching, bacteria growth, bad odors and allows for easier removal. Trash, debris & organics are captured in the filtration screen as sediments settle to the bottom chambers. This separation prevents odor, bacteria growth and nutrient leaching in the water. The NSBB systems are constructed with pre-cast concrete and available in sizes to handle 1-15 CFS and more, removing hydrocarbons and TSS (Total Suspended Solids) to 125 microns.

The Suntree Technologies Nutrient Separating Baffle Box (NSBB) is an advanced stormwater treatment system utilizing screening and hydrodynamic separation to capture pollutants.


There are many systems suitable for storing treated stormwater for reuse. Our most common methods include underground fiberglass tanks, concrete vaults and the Atlantis D-Raintank system.

Precast concrete systems are a popular option for the systems we design. Modules come in a standard length and width of 16’L X 8’W and in varying depths from 2-14 feet. This makes the system easy to scale to store almost any amount of stormwater. With an open bottom over gravel, the system can infiltrate stormwater. Add a liner and the system can retain stormwater for reuse. For information on additional storage methods, visit our Storage Options page.

Oldcastle Pre-Cast, Storm Capture The Storm Capture system from Oldcastle Precast is scalable to any size and installs quickly with site backfill materials.
Wahaso schematic for sustainable irrigation.


Properly treated stormwater can be used for any acceptable non-potable reuse application. Irrigation is the most common use, but reuse for toilet flushing and cooling tower make-up are also common.

Sustainable Irrigation

After detained stormwater has been filtered, we then sanitize the water using ultra violet light—a chemical free, low maintenance approach to sanitation. This process kills any harmful bacteria or pathogens in order to meet health codes and ensure that the water is safe for spray irrigation systems. Submersible pumps are stainless steel with variable frequency drives and are used for pressurization. The sub-surface placement of the system reduces system footprint and allows complete flexibility in the location of the processing skid.  These systems can be scaled up from our standard 20 GPM treatment skid to systems irrigating several acres from cisterns holding over a million gallons of filtered stormwater.  Visit our Projects page for a large municipal system here.

Stormwater to Flush Toilets

Toilet flushing systems are designed differently than irrigation systems. Because water demand for toilets can vary greatly, our systems generally use a two-stage process, transferring stormwater from the cistern and through the treatment skid in the first stage and into a Processed Water Holding Tank or day tank at a relatively slow rate. Treated water is then pressurized to toilets with the second stage of pumps.

270 Brannan, water harvesting for toilets Wahaso system for flushing toilets and irrigating landscaping at 270 Brannan Street in San Francisco.

Opti Technology

Municipalities that require stormwater detention raise a fundamental issue of turning stormwater detention into retention for reuse: If stormwater is retained in the cistern for reuse, there may not be sufficient detention capacity to meet the detention requirement for the next storm event, defeating the purpose of the detention capacity. That could mean that the developer would have to add redundant storage capacity for the retention portion, negating the benefit of leveraging the required detention for reuse. Many communities won’t count cisterns used for retention to qualify for required detention.

To resolve this issue, Wahaso has teamed up with Opti as a key component in our stormwater harvesting solutions. The Opti system uses predictive weather models and complex algorithms to anticipate storm flows to detention in order to determine how much capacity is required to meet the detention requirement before the storm begins. The smart system then measures the retained stormwater and, if necessary, opens a remote valve to release a set amount of stored water before the rain event begins. That ensures that the expensive storage capacity can serve as both detention and retention space.

Opti technology allows on-site systems to drain storage in advance of forecasted weather events.

Wahaso is happy to talk with you about how we might help you convert your stormwater detention to retention and reuse through harvesting. Please contact us.