Los Angeles County implemented ultraviolet disinfection systems to treat wastewater effluent from tertiary filters. The county found that low-pressure, high-intensity lamps offer a significant energy savings.
The County Sanitation Districts of Los Angeles County (the districts) had previously conducted two UV pilot studies using low-pressure low-intensity UV lamps. The results of these studies indicated the reclaimed water irradiated with UV meets the stringent Title 22 disinfection criteria for water reuses in the state of California. However, a large number of UV lamps are required, rendering the process expensive and labor intensive.
Low-pressure, high-intensity lamps are reportedly more efficient than the commonly used, low-pressure low-intensity and medium-pressure high-intensity lamps. To evaluate this new technology, the districts conducted a five-month pilot-scale study at the Pomona Water Reclamation Plant using the high-intensity lamps.
The Pilot Plant
Naturally occurring total coliforms were used as the indicator bacteria and the MPN method was used for enumeration. Heterotrophic plate counts (HPC) were performed on selected samples. MS2, a male specific bacteriophage, was used in the seed study, and natural F-specific phages also were analyzed. The samples were analyzed for total and fecal coliforms, as well as MS2.
Seed vaccine polio virus and natural enteric viruses were analyzed to evaluate effectiveness of UV disinfection on viruses. A photoreactivation study was conducted on UV reactor effluent.
Results
UV irradiation effectively inactivated total coliforms in the Pomona tertiary filter effluent. A UV dose of approximately 50 mW-s/cm2 achieves the level of 2.2 MPN/100 mL or less, as required by Title 22. For the Pomona tertiary filter effluent, UV doses of approximately 60 mW-s/cm2 ensured a four-log inactivation of MS2 coliphages.
A substantial savings in energy use of 12.7 percent was achieved by using the low-pressure, high-intensity lamps, when the same safety factors are applied to both lamps.
Editors note:
This article was adapted from a paper presented at the 1999 Energy Efficiency Forum by C. Chen, Z. El Jack, W. Horvath, J.F. Stahl, and J.F. Kuo of the Los Angeles County Sanitation Districts.
