Sewer System Master Plan (SSMP)

The Sewer System Master Plan (SSMP) is required by the EPA to be implemented by the City by per the conditions of the City’s Wastewater Treatment Facility discharge permit. The purpose of a SSMP is to locate and minimize infiltration and inflow  (I/I) into the public sewer system. Infiltration is the introduction of clean groundwater into the sewer system through aged and defective pipes and manholes, and cross connections between the sewer system and  closed street and parking drainage. Inflow is the introduction  of clean water into the public sewer system by basement sump pumps, foundation drains and roof drains. Inflow is illegal per City Ordinance Chapter 200.

I/I entering the public sewer system costs sewer rate payers extra. Additional electrical and chemical costs are required to pump, convey and  treat this clean water. Additionally, I/I in the public sewer system reduces its capacity and limits the City’s growth. It is estimated that as much as 4 million gallons per day enters the public sewer system and treatment facilities during large storms.

I/I reduction begins with investigation of the sewer system. This is done by breaking the system down into about 20 catchment areas, each of which is metered during dry periods and storm periods. Next, close circuit TV probes are sent down pipes to evaluate the condition of the pipes and to see where infiltration (through cracks for example) and inflow (clean water coming out of a sewer service) is entering. Concurrently, infiltration is also evaluated by quantifying the amount of groundwater entering pipes during periods of low sewer flow such as midnight to 6 AM. Next over 200,000 feet of sewer main is smoke tested with a non-toxic smoke. The smoke gives indications of what drain systems may be connected to the sewer system. Smoke from floor drains and roof drains would indicate connections. Smoke from a parking lot catch basin would indicate that it is connected to the sewer and rainwater can enter the sewer system.  

The SSMP will require the City to invest greatly in the reduction of I/I over the next decade.  Areas of I/I will be prioritized through evaluation. Those with higher flows will be addressed first. Correcting infiltration is usually done by apply an epoxy linter to the inside of an aged, cracked pipe. In some cases, if the pipe is in very poor condition it may need to be replaced.

The need to minimize I/I  is not unique to Rochester and many other NH communities are working to reduce it. Correcting inflow is usually done to replumbing the drain or sump pump to road or yard drainage.