Specifying Stormwater Solutions
The essential requirement of stormwater technologies is to disinfect the captured rainwater so that it is safe for its intended applications. Among the simplest, most reliable disinfection technologies -- and one that does not present complications or collateral risks -- is ultraviolet light (UV).
Since UV relies on light waves, efficacy is compromised by the presence of particles in the water. Therefore, the optimal sequence for a stormwater solution is:
- Filtration to clarify the water and assure a high UV Transmittance (UVT) value.
- UV applied to the filtered water to assure adequate disinfection.
The Omicron 2900UV is a unique product that sequences filtration before UV within a single piece of equipment, completely assembled and integrated at the factory level. It is at the core of our customized, modular approach to stormwater treatment.
As with domestic water filtration, there is an inviolable relationship between a filter’s micron degree and its maximum flow rate. The 2900UV is offered with two screen options, 25 or 50 micron.
Maximum flow rates:
2900UV @25 micron max 60 gpm
2900UV @50 micron max 110 gpm
Maximum flow rates, twin parallel versions:
Twin 2900UV @25 micron max 120 gpm
Twin 2900UV @50 micron max 220 gpm
More than two units can also be manifolded to achieve even higher flow rates, but we advise that if your flow requirement is greater than about 220 gpm, it may be more efficient to specify a sequence with separate filtration and UV systems, rather than manifolding more than two 2900UV units. Please inquire.
For most applications, 50 micron is a fine enough micron degree to assure adequate UVT after filtration. A finer filtration degree significantly reduces the maximum flow rate but is generally of limited value for UV efficacy.
The purpose of filtration in a stormwater application is to assure sufficient clarity for required UV Transmittance.
The usual application of the filtration + disinfection sequence is to supply a sidestream recirculation loop in and out of the retention tank. Our skid-assembled systems therefore typically include a circulation pump and pressure sustaining valve. Alternatively the pump(s) can be supplied external to the system (by others). In either case, screen filtration requires adequate pressure at the inlet to sustain the backwash cycles. An efficient use of the pump on our skid is to supply enough pressure both for ongoing recirculation and flush cycles.
Managing the probability factor
Ultraviolet light disinfection is highly effective but confers no downstream residual. The underlying theory of a disinfection sidestream loop is that with sufficient recirculation frequency, nearly all pathogens in the detention tank water will be neutralized over the course of numerous passes of the total volume through the system. Typically, water discharged from the filtration/UV system will be introduced into one side of the tank, and will exit the tank from the opposite side from where it returns for the next pass through the equipment. This creates a robust recirculation loop that over time should ensure that the entire tank volume is disinfected.
The concept is nonetheless more probability than certainty. To achieve virtually 100% assurance that no water-borne pathogens will discharge from the tank to application(s), we offer two alternative methods. One is to supply a dedicated UV (only) on the discharge line between the tank and the supply to application(s). For options, please inquire. The other is to add a three-way valve so that water at the point of discharge is directed to make a final pass through the 2900UV.
Additional Options
Pumps: Specify two pumps (lead/lag) instead of one.
For applications in which pump redundancy is a required value.
Activated Carbon: Adding a secondary stage of activated carbon to reduce VOC’s.
Depending on the application and regulations in effect, VOC reduction may be required. If so, it is essential that the amount -- and type -- of activated carbon included with the solution be sufficient to assure effective VOC reduction at the required flow rate.
Detention Tank: Adding a flush water detention tank.
If drain capacity will not be enough to handle the volume of discharge(s), specify a surge tank. One common tank can be sized to receive the discharge from all equipment included with the system.
Examples
In the referenced documents linked below, you will see examples of some of our solutions in detail. We emphasize that there is no single approach or one solution that should be considered “the best” or “the only”. We recommend that after reading up on the technologies and options, you contact us to discuss your project and requirements.
OMICRON 2900UV engineering specifications: Single, twin, 50 or 25 micron, with or without circulation pump included
Generic submittal package OMICRON SEQUENCE 2900UV-50-2CP-GAC
Single 2900UV (50 micron) with duplex VFD pumps, activated carbon media filtration system, detention tank. Supplied as two skids bolted together on site. See P&ID within. Design enables final pass through system upon discharge.
Drawings
Single with Circulation Pump
Single without Circulation Pump
