A common manhole allows access for sewer and drain pipe inspection and maintenance within the same structure. Typically, the drainage pipe runs above the sanitary sewer pipe, separated by a steel plate. Over time, the steel plate or the manhole structure deteriorates, causing sewage to mix with much cleaner stormwater runoff. This can lead to two problems. First, during a storm, it can cause clean storm runoff to enter the sewer lines causing sewage to overflow and back-up on to the street or into basements. Second, this can cause untreated sewage to be released into the Charles River along with stormwater runoff. By separating these common manholes, we will be reducing sewer backups and the discharge of untreated sewage into the Charles River.