Recent laboratory observations from research groups in California provide new insight into the shielding of pathogens from ultraviolet (UV) radiation in wastewater disinfection systems. These observations are of two types: particle size distributions, obtained with a commercial particle
counter at the Hyperion Treatment Plant (HTP) in Los Angeles; and direct determinations of UV light attenuation in wastewater solids from several plants, obtained by a novel fiber optic microprobe. They show that particles small enough to pass through a typical microfiltration unit with a
pore size of a few hundred nanometers are much too small to be detected by a turbidimeter or a particle counter, and the particles of sludge from most types of secondary treatment are also too small to provide a high degree of shielding to viruses, the only pathogens small enough to be present
in the filtrate of intact microporous membranes. Hence, this is another step toward quantitatively understanding why UV doses much lower than those mandated in, for example, California water reclamation standards, are sufficient to disinfect water that has been microfiltered. Hence, combined
microfiltration-UV disinfection systems can be designed to consume less electricity for the lamps than UV disinfection systems that operate alone. Since the cost of microfiltration is also going down, and since the bacterial pathogens removed by microfiltration eventually go into primary or
secondary sludge, to be killed by heat at a much lower cost if the sludge receives pasteurization or thermophilic digestion, these results appear to be a potentially significant contribution to the rapidly improving overall economics of pathogen reduction in reclaimed wastewater.
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