Ultraviolet Radiation

It has been shown that:

• Ultraviolet radiation around 254 mm renders bacteria incapable of reproduction by photochemically altering the DNA of the cells

• A fairly low dose of ultraviolet light can kill 99 percent of the fecal coliform and fecal streptococcus.

• Bacterial kill is independent of the intensity of the light but depends on the total dose.

• Simultaneous treatment of water with UV and ozone results in higher microorganism kill than independent treatment with both UV and ozone.

• When ultrasonic treatment was applied before treating with the UV light, a higher bacteria kill was obtained.

• The UV dose required to reduce the survival fraction of total coliform and fecal streptococcus to 102 (99 percent removal) is approximately 4 x 10 ff Einsteins/ml.

Some limitations are associated with UV radiation for disinfection. These include: (1) The process performance is highly dependent on the efficacy of upstream devices that remove suspended solids; (2) Another key factor is that the UV lamps must be kept clean in order to maintain their peak radiation output; (3) A further drawback is associated with the fact that a thin layer of water (< 0.5 cm) must pass within 5 cm of the lamps.

One way of implementing the UV disinfection process at existing activated sludge plants involves suspending the UV lights (in the form of low-pressure mercury arc UV lamps with associated reflectors) above the secondary clarifiers. The effluent is exposed to the UV radiation as it rises over the wire in a thin film.

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