Treated wastewater effluents contain high concentrations of
particles; many of these particles are large (with diameters
greater than 100 m) and consist of densely-packed bacterial cells.
Microorganisms occluded in wastewater particles can be difficult to
inactivate in chlorine disinfection systems, as the chlorine must
first diffuse through the macro- and microscopic pore spaces prior
to inactivating the occluded microorganisms. The impact of
microorganisms occluded in particles is evident in disinfection,
where reduced inactivation rates occur even with increasing doses
of the disinfectant. Reduction of occluded microorganisms in plant
effluents can be accomplished using filtration to remove the total
number of particles, and disinfection to ensure that intra-particle
chlorine concentrations are sufficient to inactivate the occluded
microorganisms. In addition to addressing inactivation of dispersed
microorganisms, treatment systems design and operation should
include consideration of the removal of microorganisms in
wastewater particles that may pose a health risk in post-treated
waters. In this project, a systematic approach was developed to
co-optimize filtration and chemical disinfection systems to
collectively reduce the concentration of occluded viable
microorganisms in treated effluents to acceptable levels. The
optimization process was successfully applied to wastewater samples
collected from seven facilities, each with different treatment
trains. A range of operating conditions was identified that
resulted in acceptable treatment based on particle guidelines
developed using the existent regulatory framework for indicator
organisms. Extension of the current approach to a pathogen basis
was considered, but current data are insufficient to adopt such a
procedure although preliminary results suggest that intra-particle
chlorine concentrations that are sufficient to inactivate indicator
organisms may not be adequate to sufficiently reduce concentrations
of occluded pathogens.
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