By John Barton, PE
One of the thorniest issues about investigating sanitary sewer inflow and infiltration (I/I) is the unavoidable need to access private property, whether the method is as innocuous as asking residents about their gutter and sump pump hook-ups or as extreme as physically getting into their basements.
In answer to public response, many utilities have struggled to develop ordinances that strike the right balance between individual property rights and the municipality's responsibility to correct excessive inflow and infiltration that threatens the entire community's wastewater infrastructure.
A new methodology is being deployed to zero in on problem areas while greatly reducing the need to implement widespread and more expensive traditional sanitary system evaluation survey (SSES) services. Based on a device called a Micromonitor, the four-step process outlined below has the potential to drive down project costs and the number of private-property investigations by more efficiently locating potential sources of leaks and other system deficiencies.Step one: Regional flow monitoring
Inflow and infiltration investigations typically begin by monitoring sanitary sewer flow in larger regional basins over a series of months or even years. It's rather expensive — monitoring a single location for a six-month wet season can range from $6,000 to $10,000. Baseline flow data from dry and wet seasons is collected to develop predicted base rates and rank the basins for the quantity of inflow and infiltration per house or per length of pipe.
While this identifies basins that contain problem areas, it generally doesn't provide the discrete level of information needed to pinpoint the location, or the actual cause, of the high infiltration rates. Any investigations would have to be basinwide, essentially affecting all residents equally, not just those with I/I sources. The new approach adds two more steps to focus the impact and reduce the costs.Step two: Micromonitoring
Micromonitoring follows the basinwide blanket approach of regional flow monitoring: It identifies sewer segments with very high inflow and infiltration to be investigated via highly focused wet-weather technologies. The process defines areas within the collection system where conventional flow meters typically don't perform well, such as small, upstream collection-system areas contributing low base flows.