Ultraviolet Disinfection

Sand Filtration and UV Disinfection
Sand Filtration
The Mt. View Sanitary District operates a Dynasand® filter system. The system removes solids from the secondary effluent achieving a clarity that allows downstream disinfection with ultraviolet light. The Dynasand® filter is a continuous-backwash, upflow, deep-bed, granular-media filter system. The filter media is continuously cleaned by recycling the sand internally through an airlift pipe and sand washing. The cleansed sand is redistributed on top of the sand bed, allowing for continuous, uninterrupted flow of filtrate and reject (backwash) water.
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The Mt. View Sanitary District utilizes the Trojan UV 3000™disinfection system. The first POTW (Public Owned Treatment Works) operated in Northern California to use ultraviolet light for disinfection on a full scale operation, MVSD has been able to totally eliminate the use of gaseous chlorine, sulphur dioxide and ammonia, all Acutely Hazardous Materials (AHMs), which can be toxic to both humans and wildlife. The Trojan system uses the same UV found in sunlight, but in a concentrated dose, to destroy harmful microorganisms present in the wastewater. The elimination of AHM’s from the plant premises has increased staff and public safety while decreasing District liability.
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What is ultraviolet light?
UV light is defined as electromagnetic radiation having a wavelength less than that of visible light (400 nm) and greater than that of X-Rays (100 nm). The unit of wavelength is a nanometer (nm), equal to 10-9 meters. The optimum UV wavelength for germicidal effect is 253.7nm, which is found only in small amounts in the solar radiation because energy at these wavelengths is absorbed by the atmosphere.

The source of UV energy that MVSD utilizes is low pressure mercury arc lamps. The radiation is generated by striking an electric arc through mercury vapor. Discharge of the energy generated by excitation of the mercury results in the emission of UV light.