Commonwealth Interceptor Study


Commonwealth Interceptor Study

AlexRenew is studying a portion of the Commonwealth Interceptor to identify sources of excess infiltration and inflow that cause capacity issues during periods of intense rain.

What is the Commonwealth Interceptor?

The Commonwealth Interceptor is a separate sanitary sewer — owned and operated by AlexRenew — that collects sanitary sewage from the City of Alexandria’s separate sanitary sewer system within the neighborhoods of Del Ray, Rosemont, and North Ridge (known as the Commonwealth Separate Sanitary Sewer Area). The interceptor acts as a sewer "highway" that delivers sanitary sewage to AlexRenew's wastewater treatment plant.

Location of Commonwealth Study

Commonwealth Interceptor Map

The map above highlights the Commonwealth Interceptor and the area of study.  

What is infiltration and inflow?

The Commonwealth Interceptor is adequately sized per Virginia regulations to carry sanitary 
sewer flows, also known as dry weather flow, which is based on population and water usage. However, it is common for wet weather flows to enter separate sanitary sewers – this is called infiltration and inflow

  • Inflow is caused by stormwater sources flowing into separate sanitary sewers
  • Infiltration is caused by groundwater leaking into separate sanitary sewers through pipe joints and/or cracks

In Virginia, separate sanitary sewers are required to carry 2.5 times dry weather flow to handle typical inflow and infiltration - the Commonwealth Interceptor is adequately sized to 2.5 times dry weather to meet these regulations.

Excess infiltration and inflow affect the capacity of the Commonwealth Interceptor

During periods of intense rain, the Commonwealth Separate Sanitary Sewer Area (owned by the City) is overwhelmed by excess infiltration and inflow - primarily attributed to unauthorized rain leader connections, stormwater inflow through manhole covers, and pipe cracks/joint failures. This excess infiltration and inflow can exceed the Commonwealth Interceptor’s capacity, causing sewer flooding and basement backups - see the illustrated graphic below.

Inflow and Infiltration CWI
Inflow and Infiltration Graphic


Commonwealth Interceptor Study

The Commonwealth Interceptor inflow and infiltration study will evaluate alternatives to mitigate capacity issues in the interceptor due to excess infiltration and inflow. 

Alternatives being evaluated include:

  • Improving the existing stormwater system capacity to minimize flooding (City project) 
  • Disconnecting rain leaders from the separate sanitary sewer system (City project) 
  • Eliminating discharges from foundation drains and sump pumps into the separate sanitary sewer system (City project) 
  • Removing cross-connections (City project) 
  • Sealing manhole covers on separate sanitary sewers in low-lying areas (City project) 
  • Rehabilitating existing separate sanitary sewers, manholes, and laterals (City project)
  • Increasing the capacity of Commonwealth Interceptor (AlexRenew implemented project)

The study is anticipated to be completed in late 2024.

VIDEO: How Excess Infiltration and Inflow Causes Capacity Issues in the Commonwealth Interceptor