The Stormwater System and Sanitary Sewer System
The County’s stormwater system and sanitary system are distinct and separate pipe systems. However, sanitary sewer systems are sometimes inundated with stormwater or groundwater through two phenomena known as “Inflow and Infiltration” (commonly referred to as I&I).
Inflow is the introduction of groundwater or surface water from direct connections to the sanitary sewer system such as residential foundation or area drains, downspouts, or illegal cross connections. Prior to the 1960’s foundation and area drains were permitted to connect to the sanitary sewer – which was typically easier to connect to than storm sewer systems (if they existed).
Infiltration is the introduction of groundwater seeping through cracks or leaks in the sanitary sewer. Infiltration is influenced by seasonal variation in the water table, which increases the external water pressure on the sanitary sewer system and thus leads to higher infiltration.
During significant rain events, I&I may overwhelm the design capacity of the sanitary sewer system, and lead to surcharges from the sanitary sewer system into homes.
Backflow Preventer Valves
Homeowners in flood prone areas that have experienced sanitary sewer surcharges may consider installing a sanitary sewer backflow preventer valve. These devices can be installed on the privately maintained sanitary sewer lateral by a licensed plumber. Backflow preventer valves have a flap that remains flat during normal flow to allow sanitary waste to flow out of the pipe. If water begins to surcharge from the sanitary sewer main pipe towards the house, the valve closes to prevent a backup from the sanitary sewer main. Backflow preventer valves require periodic maintenance and when they fail can lead directly to sanitary sewer surcharges.
Sanitary Sewer Master Plan Update
The County is in the process of updating the County’s Sanitary Sewer Master Plan. Approximately 35 monitoring locations were placed in the field to measure flow in our sanitary sewer system including several rain gauges that will inform what peaking factors to use in the design process. The gauges were installed in April, 2019 and were in the field during the storm on July 8, so this information will inform our process. There is ultimately still the potential for a storm to occur that will exceed the capacity of the sanitary sewer system. Our focus is to be informed by this data to determine if there are areas of the system that need improvement to minimize the occurrence of these overflows. The Sanitary Sewer Master Plan is planned to be finished in late 2020.