AN EVALUATION OF VENTILATION PATTERNS IN A REGIONAL COLLECTOR SEWER

A field scale tracer test was conducted to evaluate in-situ ventilation rates in a major collector sewer. The sewer under study was approximately 11 km long and ranged from 0.61 to 2.1 m in diameter. For the purposes of the tracer testing the collector was divided into 4 reaches with each of the reaches tested individually. The tracer test involved injection of a measured volume of carbon-monoxide gas into a manhole over a short time period. Carbon monoxide concentrations were then measured in the collector headspace at selected manholes along the length of the reach. The technique employed was able to successfully measure average headspace velocities over extended lengths of the collector. In a section that had a relatively stagnant headspace approximately 1.1 km of sewer could be evaluated with substantial loss of tracer that attributed to losses of tracer to manholes. In a section of the sewer with elevated headspace velocities a section that was approximately 7.0 km long was successfully tested with one injection of tracer gas. The velocities observed in the collector varied substantially with time and location in the collector. The lowest velocities that were measured were in the upstream sections with a minimum observed value of 3.8 m/min. The highest velocities were observed in the downstream sections with a maximum value of 31.5 m/min. In general, there was an increasing trend in gas phase flows with distance along the length of the collector. Flows at the discharge end of the collector were almost two orders of magnitude greater than those at the beginning. In several of the tests the headspace volumetric flow rate decreased along the length of the collector suggesting that there was out-gassing from the collector from these reaches. Testing of selected manhole headspaces concurrently with the sewer headspace indicated that there was migration of tracer into the manholes however, differing patterns of exchange were apparent.