Inflow and Infiltration is a major concern with most collection systems. Inflow is the excess rainwater that enters the system very soon after the rain begins and can normally be traced to unsealed manholes and an illegal connection such as roof downspouts, parking lot and yard drains. Infiltration is the excess water that continues to enter the system for three or four days after the rain has stopped and is the result of groundwater seeping into the system through breaks in the line and unsealed pipe joints. Inflow is usually more controllable and more easily eliminated.
Construction requirements limit the loss of waste from (or entrance of ground water into) a sewer system to 200 gallons per inch diameter per mile per day. This limitation is inclusive of manholes, sewer lines, and appurtenances. At least 30” of ground cover shall be provided for additional protection. As part of the construction, the integrity of a new system has to be verified by means of either the infiltration/exfiltration, or low-pressure air testing methods. An infiltration or exfiltration test shall be performed with a minimum positive head of two feet.
The infiltration test is generally preferred when the groundwater level is above the crown of the sewer. The upstream end of the section to be tested is plugged, and a flow measuring device (weir, etc.) is installed in the manhole at the lower end. The rate of leakage can then be measured.
In the exfiltration test when ground water levels are too low to use the infiltration test, both ends of the section of sewer to be tested, including a manhole at each end, are plugged, and all stoppers and plugs are braced or otherwise secured to resist the internal pressure resulting from the test. The section is then filled with water to a predetermined level above the crown of the sewer, and the rate of leakage is computed on the basis of the observed drop in water level over a reasonably long period of time or by metering the volume of water to be supplied to the system to maintain the original water level.
In the air pressure test, a section between manholes is plugged and the plugs secured to withstand the expected internal pressure. Air is then introduced at a pressure above the maximum pressure exerted by any groundwater that may be present outside the pipe. After the air is shut off, the time it takes the pressure in the pipe section to drop by a predesignated amount is determined. Manholes should be tested separately.
Sewer lines, when flowing full, should have a mean velocity of not less than 2.0 fps (feet per second) to reduce the possibility of solids deposition in the collection system. A mean velocity of 10.0 fps or more may cause serious damage to manholes. The velocity may be calculated using the following equation:
Velocity (feet per second) = (Distance in feet) / (Time in seconds)
The numbers needed for the equation above may be obtained by measuring the distance between two manholes, in feet and then inserting a ping pong ball in the upstream manhole and measuring the amount of time, in minutes and seconds, it takes to reach the second manhole. Normally this requires at least two people with two way communication devices.
All well-run municipal public works department recognize the importance of having a preventive maintenance program for their sewers. Not only does it cut down on the number of customer service complaints, it reduces maintenance costs in the long term. A worthwhile maintenance program should include a good record keeping system, quick response to service requests, a cycle of regular televising of revolving parts of the sewer system each year, regular cleaning of the system, and regular attention to corrosion protection against hydrogen sulfide. Regular wet and dry weather flow monitoring can be used to see problems emerging, so corrections can be programmed in advance. In conjunction with water metering, this can be used to detect water main leakage and help with that system’s maintenance.
Inflow and Infiltration or Sewer System Evaluation Studies (SSES) are performed to identify the specific causes and quantify the amounts of I/I entering the sewer system. This information allows the public works department to prescribe the most beneficial corrective actions and estimate their costs.
The techniques usually employed in the I/I or SSES study are:
Interviews of maintenance personnel and review of repair records
Visual inspections of lines and manholes
Smoke and dye testing
Televising of the lines, usually with dyed water flooding of the surface
All have their place in the investigation process, and information from the least expensive techniques should be analyzed before going on to televising of the lines. Usually an adequate diagnosis can be made based on appropriate application of the cheaper techniques plus televising of about 20% of the system. We will now consider each of the major investigative techniques and their applications.