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Effectiveness of a Progressive I/I Reduction in the Woodhaven Community in the Region of Halton

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The Region of Halton, located west of the City of Toronto, Ontario, is a suburban region with a population in excess of 500,000 people. The Region is responsible for sanitary sewer systems serving the communities of Oakville, Burlington, Milton, Georgetown, and Acton. Over the past 10 years, the Region has been proactively dealing with infiltration and inflow (I/I) issues within their sanitary sewer systems to manage peak wet weather flows entering their various treatment facilities.

A study in 1998 identified the several sanitary catchment areas within the Town of Oakville that contributed excessive I/I to the sanitary sewer system. Several projects were identified based on the study and the Region did construct a 7,000 m3 storage tank to address high I/I flows entering the SouthWest WWTP from the eastern half of Oakville. One smaller area, Woodhaven, located to the west of the Town was also identified as contributing excessive I/I to the sanitary sewer system. The Woodhaven area also had a history of basement flooding. The Woodhaven area is relatively small and an analysis was completed to determine the most feasible method of reducing peak I/I flows receiving treatment from this area. Because of the small size of the area, a source controls approach was identified as the most cost effective method. Following the 1998 study, the Region embarked on I/I reduction pilot program in Woodhaven. At initiation of the pilot program, baseline flow monitoring data was collected in a sanitary sewer on Trudale Court to characterize I/I in a small three block area. It was anticipated that this data would be used to evaluate the effectiveness of various pilot program actions. Over a six year timeframe, main line sanitary sewers were relined and in some cases replaced to eliminate deficiencies identified in the CCTV inspections. Flow monitoring data was again collected at Trudale Court at the conclusion of the mainline sewer relining activities. Data analysis concluded that significant I/I was still occurring. In 2004, the Region elected to investigate the condition of the house laterals in the Trudale Court area. Lateral CCTV inspections were completed during wet weather and identified clear flow entering the sanitary sewer system from many laterals. Lateral deficiencies were also identified. On the basis of the CCTV inspection, the Region launched lateral lining program and lined a total of 52 laterals. Lining was selected as the preferred method of rehabilitation to avoid any reinstatement of landscaping on private property. Of the 52 laterals lined, 31 were lined to the property line while 21 were lined to within 1m of the house. A public participation program was also launched to provide information to the residents and encourage their participation in the lateral lining program. Ultimately, the public participation program was highly successful with 52 residents agreeing to have their laterals lined. The costs for all lining activities were borne by the Region.

Following the conclusion of the lining program, flow monitoring was again collected at the Trudale Court site and analyzed to assess the effectiveness of the lateral removal program. To assess the effectiveness of the lateral lining program, a number of metrics were developed including the average I/I volume, the peak rainfall intensity needed to achieve a wet weather response in the sewer, the event duration, time to peak, and average and peak I/I rates. In general, flow monitoring data was collected for a series of events with different antecedent and rainfall characteristics. A regression analysis approach was utilized to develop relationships between the rainfall characteristics and measured I/I volumes and peak I/I rates. A further modeling analysis was undertaken to compare I/I volumes and rates for a series of common rainfall events. The results of the modeling data analysis concluded that completion of the main sewer rehabilitation reduced I/I volumes by 25% on average and peak I/I volumes by 35% on average. Peak I/I rates were reduced by 28% for road sewer rehabilitation and by 39% for lateral sewer rehabilitation. These reductions in volumes and peak rates were also combined with the costs to complete the lining programs to develop cost effectiveness ratios. It was determined that 116/ha could be saved in treatment plant operational costs by completing road sewer rehabilitation. A total of 132/ha could be saved in treatment plant operational costs by completing both road sewer rehabilitation in combination with lateral sewer rehabilitation. In contrast, the total project cost to complete lateral sewer rehabilitation was 50,760/ha. The results of this analysis show that treatment plant cost savings alone cannot justify a lateral rehabilitation project for this area. However, if there are other reasons for embarking on an I/I reduction program, such as basement flooding reduction, a lateral rehabilitation program can be justified. Lateral rehabilitation may be particularly attractive in areas with chronic basement flooding.
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Keywords: Sanitary sewer rehabilitation; cost benefit analysis; infiltration and inflow

Document Type: Research Article

Publication date: 2009-01-01

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