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.
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.
ABOUT OUR APPROACH
The underlying premise of our storm water design is to prioritize simplicity, efficiency and reliability -- which leads to a reduced footprint and lower cost. When the core technologies are best in class, having fewer moving parts is a plus. There are in fact just two essential elements in play: Filtration and disinfection. Along with one or more pumps as needed and a pressure sustaining valve, our approach enables highly modular solutions to accommodate a broad range of flow rates and retention tank volumes.
To understand why not, note that filtration degrees are about probabilities, not absolutes. People often think that a 30 micron particle will sail through a 50 micron screen and be stopped by a 10. Not exactly. It is rather a matter of probability: More 30 micron particles will be stopped by a 10 micron filter than by a 50.
Therefore, in most cases, it is far more effective to filter directly at the final filtration degree by adequately sizing the overall surface area of the filtration element to accommodate the required flow rate. Note that a 25 micron filter will have a lower maximum flow rate than a 50 micron filter, where both have the same surface area. Or to put it another way: At any given flow rate, the finer the filtration degree, the greater the filter surface area is required.
For rainwater harvesting applications, the most important purpose of filtration is to ensure sufficient clarification before UV disinfection. In our experience, the difference between 25 and 50 micron in this application is not that consequential, and achieving finer filtration than 25 micron is of little value. Remember, it's really not about the absolute micron degree -- it's about the effect of filtration on overall Turbidity (NTU). Therefore, in most cases it is simpler, more effective and less costly to increase the UV power rather than applying a filtration degree smaller than about 25 micron.
For these reasons we argue against solutions we often find offered by various system integrators that include a multi-stage sequence of filters along with other steps of limited value. (We also often see the technologies used by these integrators to be sourced from manufacturers who offer the cheapest products, not the most reliable.) We advise engineers specifying stormwater solutions to examine the actual components a system integrator is providing, in order to ensure an optimal solution ends up on the job. Our Omicron 2900UV series, with filtration + UV entirely built and integrated at the factory level in a single consolidated system, offers various options to accommodate the essential factors referenced above. Single, twin (or more) parallel configurations are supplied with either 25 or 50 micron screens, and fully skidded at the factory. As shown here, we include a pressure sustaining valve and (unless provided external to the system) a circulation pump to drive the sidestream loop. All systems employ the same optimal high performance screen filtration technologies used with our Omicron domestic water filtration systems.