I&I Rates and Sewer Infrastructure Age: Is There a Strong Correlation?

Introduction and Background The Capital Regional District (CRD) is a partnership of 13 municipalities (including Victoria, the capital city of British Columbia) and 3 electoral areas with a total land area of about 2,400 square kilometers, located at the southern tip of Vancouver Island. The CRD provides services that are regional in nature including the sewerage system that serves some 350,000 people. Within the Core Area of the region, municipal sewers feed into the major CRD trunk sewers, which collect and convey sewage to two discharge points. The outskirts of Greater Victoria is experiencing significant growth, (particularly in its Western Communities), and that along with high inflow and infiltration in the older downstream infrastructure overloads the systems capacity during rainstorms, often resulting in sewage overflows.The CRD's first sewers were installed in the early 1900s, and most are 70 to 100+ years old. The older sewers were primarily constructed using vitrified clay (VC) pipe. In the 1950s, 60s, and early 70s asbestos cement (AC) pipe was predominately used. Since the late 1970's, almost all sewers have been constructed using polyvinyl chloride (PVC) pipe. The Western Communities were serviced and connected to the CRD trunk sewers in 1996 using PVC pipe.The Western Communities were originally characterized as having free-draining soils. As a result, a relatively low inflow and infiltration (I&I) rate of 7,500 litres/ha/day (800 U.S. gallons/acre/day) was selected for design. For less permeable soils, a rate of 11,200 litres/ha/day (1,200 U.S. gallons/acre/day) is typically used throughout the region. Capacity upgrades to the Western Communities Trunk system, due to growth, were scheduled for 2015.In 2002, during a major storm event, the six-year-old infrastructure reached 80% of its design capacity eleven years ahead of the planned upgrades. A subsequent analysis of the flows made it clear that population growth and I&I rates were far greater than originally anticipated. More specifically, I&I rates in the newer areas were more than twice the design rate, and appeared to be similar to those of less porous soils. I&I rates in the older areas appeared to have increased as well. Fearing a continuing trend, the CRD commissioned a study to investigate the relationship between I&I rates and sewer infrastructure age, using sewer flow monitoring databases in Greater Victoria and Vancouver. This relationship could then be used to more accurately determine future I&I rates and evaluate various funding levels by municipalities on the rehabilitation of the sewage collection systems. The relationship can also be used for benchmarking by comparing an individual basin to others of similar age.