Unlike municipal water, rainwater is captured in a raw state, neither disinfected nor filtered. Collected from surfaces exposed year-round to outdoor conditions, rainwater will contain high levels of particles and even debris. If it has been collected from hard surfaces with significant vehicular presence (driving or parking), expect petroleum hydrocarbons too.
The most common reuse applications for captured rainwater are cooling towers and irrigation. In response to the 2015 Legionella outbreak, New York City enacted Local Law 77 requiring registration, testing and cleaning of cooling towers. 1 This law impacts management practices for captured stormwater directed to supply these towers. Additionally, New York State and City regulations, as well as EPA and CDC determinations, provide useful guidelines for the selection of water technologies and best practices.
About Stormwater Technologies
UV light is a reliable, safe and entirely non-toxic disinfection method that uses light waves of a specific wavelength. The CDC states that UV disinfection has “very high effectiveness” against bacteria.2 Moreover, EPA testing determines that a “dose of 30 mJ/cm2 achieved 9.999-percent (5-log) reduction in 20 minutes in a recirculating model”.3 Note: New York State mandates 40 mJ/cm2 in reuse applications.4
In order for UV to be effective, the water medium must be highly transparent, since particles can block or scatter the UV light waves meant to penetrate pathogens. The EPA notes that “[e]xcessive turbidity...inhibit[s] the effectiveness of UV disinfection”.3 New York City requires that the turbidity of reuse water be below 2.0 NTU.5 Therefore, stormwater must be filtered before being exposed to UV.
According to the CDC, stagnant water can encourage pathogen buildup,6 which is why both the CDC and EPA recommend ongoing recirculation. UV light is highly effective at the moment of contact but provides no disinfection residual. The optimal strategy for applying filtration and UV disinfection in a retention tank is repeated passes through a sidestream recirculation loop.
The more rapidly the tank volume is “turned over”, the greater the probability that all pathogens present will be captured and neutralized during multiple passes. For example, at a 100 gpm flow rate, a 30,000 gallon tank would be fully re-circulated every 300 minutes. At 300 gpm, the same tank would “turn over” every 100 minutes.
When selecting a filtration / UV solution, keep in mind the tradeoffs. Finer filtration reduces water turbidity, with resulting higher UV efficacy. At the same time, the maximum flow rate of a system is reduced as micron size becomes finer. Which means a slower recirculation rate, or fewer tank turnovers.
There is no right or wrong answer. Available footprint, budget, anticipated water quality and the overall capacity of the retention tank are valid considerations when balancing micron rating against recirculation rate.
What is the risk that even if circulation inside the retention tank is robust, some pathogens might never be captured during multiple passes? This is difficult to predict, but easy to manage. Adding a second UV at the point of discharge will ensure that any surviving pathogens will be dispatched at the point water is discharged.
Our Omicron Sequence 2900UV series, with filtration + UV entirely built and integrated at the factory level in a single consolidated system, offers various options to accommodate the above factors: Single or twin parallel configurations are supplied with either 25 or 50 micron screens. All systems employ the same optimal high performance screen filtration technologies used with our Omicron domestic water filtration systems.