Extraneous flow in a city of Winnipeg separate sanitary sewer system
The variations in sewage flow rates in a separate sanitary sewer system in the City of Winnipeg were studied during the dry and wet weather conditions. It was found that massive amounts of stormwater or extraneous flow enter the sanitary sewer system during even moderate rainfall. Peak extraneous flow rates in excess of seven times the average dry weather flow were monitored. The thesis contends that the chief source of this extraneous flow is the foundation drains around the basements of houses and that poor surface drainage of house lots is a major contributing factor It was found that the volume of extraneous flow was relatively small, being about three percent of the total annual sewage flow, but that the flow pattern is characterized by high short-term peaks. Good statistical correlation was found between the volumes and peaks of extraneous flow and the probable frequency of occurrence of rainstorms. The traditional design procedure for determining the flow capacity requirements of sanitary sewers is based on the projection of water consumption in the service area. It is shown that the critical factor governing the hydraulic adequacy of the sanitary sewers is the magnitude of the extraneous flow occurring in the sewers. The study concludes that this flow component must be given priority in future designs. The thesis studies various methods of dealing with this extraneous flow problem. It is not considered likely that this flow can be completely eliminated at the source. The separate connection of foundation drains to storm sewers is shown to be costly. The simple overflow of sewage to the receiving stream is not recommended and probable annual overflow pollution levels are calculated. The local collection sewer system must be capable of carrying the sewage flows produced while providing a reasonable degree of property protection and an allowance for extraneous flow is suggested for design purposes. It is not economical to provide flow capacity in the interceptor and treatment plant facilities for these short peak flow rates. The recommended solution is to reduce extraneous flow at the source and to provide suitable storage and/or treatment at the interface of collection and interceptor sewer systems for the unavoidable extraneous flow.