There has been considerable work over the last few years on developing methodologies for monitoring for Covid-19 virus in wastewater. One rationale is that detections in sewage can precede the public health manifestations by a week or two, providing an early warning system. Often, sampling of influent at a wastewater treatment plant is monitored, but some approaches involve monitoring withing the sewer collection network to provide opportunities for isolating the source of an outbreak or to take advantage of potential increased sensitivity in monitoring for a localized outbreak by avoiding the dilution effect of sampling at the wastewater treatment plant. In addition, it has been established that the virus is primarily associated with human stool and that there is more sensitivity by sampling the sewer solids as opposed to the liquid phase. This study examined the study of a small portion of a sewershed associated with student dormitories and academic buildings on the University of Michigan campus with the objective of monitoring tracers released at individual toilets as they moved through the collection system and past a downstream measurement site. The tracers included rhodamine fluorescent dye as a passive tracer in the flow and cat food spiked with de-activated virus as a surrogate for human stool. The objectives were to gain some understanding of the flow and transport of both the liquid and solids within a collection system which is assumed to not have a detailed understanding of sewer flows and the implications on the development of a monitoring protocol. Tracer injections were made at three different locations at varying distances from the single monitoring location where the flow is also monitored (but not at other locations in the system). One of the injection points was essentially at the upstream end of the collection and the two others involved injections that joined the main sewer line from short side branches further along the system. It is found that the hydraulic transport times and dispersion of both the liquid and solids are generally similar. Since the distribution of flow along the sewer was unknown and somewhat variable over the course of an experiment, movement of the tracers are sensitive to the assumed inflow distribution, while the results of the downstream tracer injections are more sensitive to the lower flow conditions within the short inflow branches. Methods for estimating the longitudinal dispersion coefficients in the partially full sewer flows and measurements under similar flow conditions in previous studies provide results consistent with the observed spread of the dye slugs. The results are translated into practical considerations for applications to similar monitoring programs in smaller scale sewer systems.
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