Disinfecting Peak Wet Weather Flows: Maximizing the Effectiveness of Biological Processes

Providing adequate treatment of peak wet weather wastewater flows continues to present significant challenges for publicly-owned treatment works (POTWs). Protection of public health through effective disinfection has received considerable attention, particularly due to recent controversies over the interpretation of regulatory requirements concerning treatment schemes that fall under the general category of “blending”. Critics of blending assert that biological treatment is necessary if disinfection is to be effective. This paper examines the basis for that assertion, and seeks alternative processes that can work in parallel with biological treatment, on an as-needed basis, to provide an effluent that can be disinfected effectively.The literature is surveyed to summarize data on the pathogen removal and disinfection efficiency of primary, secondary, and physical/chemical effluents. Note that these data typically represent systems operated under steady state, dry weather conditions. A drop-off in efficiency for biological systems under the stress of higher flows can be predicted, but is not usually quantified. Difficulties with disinfecting poorer quality effluents are commonly attributed to particle-associated microorganisms. While these effects have been studied, the dynamics of particle association during different types of treatment processes are not fully understood. Microorganisms that are embedded within raw wastewater solids, and thus shielded from disinfection, are presumably released to some extent during biological treatment. Simulation of this process in laboratory studies, however, has relied on vigorous mechanical agitation accompanied by addition of surfactants. In an activated sludge system, disaggregation of particles undoubtedly occurs, followed by further reduction of dispersed microorganisms through predation and endogenous decay. All of these processes, however, become compromised as hydraulic retention time is decreased, and it is suggested that physical removal of solids, prior to disaggregation, can play a role of increased importance during peak flow conditions.This paper will present approaches for considering particle association dynamics in the context of understanding disinfection effectiveness during peak wet weather flows. Simple process models will be presented that consider microorganisms as aggregated (that is, embedded or clumped), particle-associated (that is, adsorbed), and dispersed. These models will be used to demonstrate how, in some situations, parallel treatment processes may better serve the goals of disinfection than over-designed and/or overburdened biological processes.