1. What is I&I?
I&I stands for inflow and infiltration. Inflow is the flow of stormwater into the sanitary sewer system through connections like roof drains, foundation drains, and basement sump pumps. Infiltration is groundwater seeping into sewer pipes, including private sewer laterals, through cracks and broken pipe joints.
2. Why is I&I a problem?
I&I in sewer pipes takes up space that is needed for sanitary sewage, basically reducing capacity in sewer pipes. This means that during rainstorms, the volume of I&I in sanitary sewer pipes increases causing sewage overflows onto streets and into local streams. I&I also increases the cost of treating sewage because it increases the overall amount of sewage being treated.
3. Why is I&I a problem in southwest Portland?
When these areas of Portland were originally developed, the sewer system that was installed half a century ago was designed to handle sanitary sewage but not stormwater. Due to southwest Portland’s unique geography and geology, as development continued, properties often managed stormwater by connecting roof drains, basement sumps, and foundation drains into the sewer system either with or without permission from the county or city. Since the area was annexed into the City of Portland many years ago, the city has been looking at the best ways to manage stormwater. Data collected from flow monitors in the sanitary sewer pipes show spikes or high flow volumes when it rains. Hydraulic modeling also shows that the sanitary sewer pipes don’t have enough capacity to handle sudden high volumes of stormwater or groundwater. I&I can increase the volume in sewer pipes up to 100 times during a large storm. Removing I&I from sanitary sewer pipes is more cost-effective than installing larger sewer pipes or installing a separate stormwater collection system.
4. Why implement a pilot project now?
Environmental Services’ investigations of sewer flows in certain areas of southwest Portland found a significant amount of I&I entering the sanitary sewer system. I&I uses up capacity in the sanitary sewer pipes, sometimes causing sewage releases into city streets, private property, and local streams. There have been several sanitary sewer overflows (SSOs) from a manhole near SW Dewitt and SW 25th Avenue over the past few winters. The Oregon Department of Environmental Quality (DEQ) has ordered Environmental Services to eliminate SSOs from the manhole at SW Dewitt within a specific timeframe or face significant fines.
5. Why repair or replace private sewer laterals?
Many cities and counties across the country have I&I problems and have pursued a variety of methods to reduce I&I. According to the research, repairing or replacing private sewer laterals is the quickest and most cost-effective way to reduce I&I. Environmental Services is pursuing this option for a pilot project because of the agreement with DEQ to reduce overflows at SW Dewitt and 25th and the benefits that reducing I&I can have for the overall sewer system. Once the work is completed, Environmental Services will monitor the flows in the sanitary sewer pipes and continue hydraulic monitoring to determine whether additional projects are needed to reduce I&I. Participation in the pilot project is entirely voluntary.
6. How does that increase treatment costs?
Because of I&I, the sanitary sewer pipes transport relatively clean stormwater and groundwater to the city’s treatment plants during wet weather. The sewage collection system and the treatment process can operate more efficiently if excessive stormwater and groundwater inflow and infiltration are eliminated. This is a more cost-effective approach than increasing the size of sewer pipes and pumps to manage the extra wet weather flows.
7. What is the end goal?
The goal is to keep I&I out of sanitary sewer pipes. That will involve replacing deteriorated pipes, fixing cracks and bad pipe joints, fixing cracks in manholes, and disconnecting street stormwater inlets from the sanitary sewer. If it includes re-directing private downspouts, sump pumps and area drains, or fixing private sanitary sewer laterals, Environmental Services will work with property owners to find safe alternatives to manage the stormwater flows.
For More Information
Contact Joe Annett at 503-823-2934. See the project overview.