This is part 3 of Mike Homenuke’s series on Inflow and Infiltration. For part 1 click here and part 2 click here.
Why care about I&I?
Before we get too much further into this, we should probably discuss why we should even care about inflow and infiltration into our sanitary sewers. Whether you’re an engineer or an urban planner, inflow and infiltration (I&I)—surface runoff and groundwater that has entered the sanitary sewer system—should be on your radar.
Sanitary sewer systems are designed to carry a specific volume of wastewater, usually from systems including toilets and sinks in homes and businesses. Meanwhile, stormwater sewers are designed to carry rainwater and groundwater away. However, when this water flows into sanitary sewers, it taxes the load on the overall system, including wastewater treatment plants.
The result—public health risk and extra costs to your community. And if that doesn’t get your attention, consider that sanitary sewer overflows can kill fish and will violate state and federal environmental regulations.
Extra Load and Dangerous Consequences
Wet weather and storm events can quickly fill sanitary sewer systems beyond capacity. At that point, wastewater flows at higher-than-usual water levels and can begin to flood basements and homes and spill onto streets. These wastewaters and potential pathogens are a significant public health risk. And, as these flows mix into water bodies like streams and lakes, contamination becomes a major issue for all those interconnected waterways—harming the flora and fauna that reside there and can prompt officials to issue beach advisories and closures.
The Environmental Protection Agency (EPA) requires regulated agencies possessing a National Pollutant Discharge Elimination System (NPDES) permit to stop all wastewater overflows from reaching United States waters. It’s a daunting and nearly impossible task because I&I can’t be stopped altogether, and will continue to increase over time as the infrastructure ages. Since the late 1980s public awareness and interest has grown in upgrading sanitary sewer infrastructure and municipalities are continuing to respond.
Costs can be high when it comes to dealing with I&I, and encompass fees associated with overflows, wastewater treatment and transportation facilities and funding opportunities. Also bear in mind that sewer backups that spill water into households can result in litigation for which the city or local agency may be responsible.
The best thing municipalities can do is to understand the I&I within their systems, and determine if larger responses could result in a problem. I&I events are based on climatic events and as such their return periods can be calculated. A return period I&I response that causes a sewer backup or overflow can be calculated for every pipe in a city. If the return period is less than the expected service level promised to the rate payers or results in an overflow that harms the aquatic environment, action is required. Since I&I increases with sewer age, it’s important for utilities to understand the current response and calculate the corresponding return period. Information such as this helps to avoid overflows and resulting fines—and starts with effective monitoring tools.
Stay tuned for our next blog about how to quantify I&I and how FlowWorks can help.