An approach to pathogens flux simulation in a combined sewer system Une approche de la simulation des pathogènes dans un réseau unitaire

Combined sewer systems convey organic matters and pathogens coming from human and animal faeces. They are mixed with rainfall runoff during storm event and can overflows to natural aquatic systems. Preliminary genetic analyses of pathogens were conducted in an experimental small water course and its lateral combined sewer system (OTHU site). Results indicate that natural system pathogens are an evolution of sewer system pathogens. However their presumed survival should not be confounded with their frequent delivery to the water course. Then a first step is to check for concentrations and frequencies that are delivered to receiving waters. First data on three pathogen species concentrations were sampled from 2009 to 2011 into the combined sewer system. Data exhibit variations that can be related to dry and wet weather sequences. However variations do not fully correlate between pathogen species. This would indicate growing rate and/or differing locations in the sewer pipe. The main hypothesis to explain concentration increase with storm events is the flushing of organic biofilms lining the immerged part of sewer conduit. This hypothesis is to be confirmed with complementary data more focussed on the water column during storm events and lining biofilms during dry weather.