Wastewater collection systems are vulnerable to inflow and infiltration (I/I) during wet weather. To determine how much I/I is coming into the system you must monitor the flow in the collection system.
Flow monitoring is the least costly investigative technique for the amount of information gained. Generally one should start an investigation by monitoring flows throughout the system to identify which drainage basins have the most excess wet weather flows. Gravity sewer flows can be directly monitored. For force mains, you will have to calculate flows based on metering of pumping rates and times. Pressurized sewer cannot have I/I problems, but their flows may need to be known for the system-wide analysis.
To set up your monitoring locations, divide the system into drainage basins and locate the meters at the manholes where the drainage basin joins into a larger flow. If possible, establish drainage basins that have similar materials or age, even if this leads to big differences in flows among basins. The goal is to be able to determine from the flow data which basins have the most extraneous flow and whether inflow or infiltration predominates. This will allow you to plan out the rest of your investigation more economically.
The following guidelines may be used to determine a monitoring and evaluation strategy to adequately measure amount of inflow and infiltration in a sanitary sewer system:
- Use one flow meter for every 30,000 – 50,000 feet of sanitary sewer pipe (you may have to lease portable meters).
- The flow meter recording should be set at 15-minute intervals and should be capable of measuring surcharges.
- There should be one rain gauge for every two to four flow meters.
- The minimum monitoring period should be 45 days with 60 days being optimal.
- The monitoring period should be during a period of high seasonal groundwater and encompass six to eight rain events.
To determine how much I/I the system experiences in wet weather measure the dry weather flows by calculating what they should be, based on building occupancies and types of usage. If the measured dry weather flows are significantly higher than calculated flows, there may be a cross connection with a potable or fire protection water line, a leak in one of these lines which is causing dry weather infiltration, an underground spring causing infiltration, or a perched or permanently high water table. Groundwater wells at a few key locations will help you determine if this is the case.
Wet weather monitoring data should be graphed as flow versus time on top of a rainfall versus time graph for the same period. Inflow will show up as elevated flows starting relatively close to the start of rain and dropping off soon after the rain stops. Infiltration may not show up right away, but will continue steadily after the rain stops and until the ground or trench around the sewer is no longer saturated.
Also, compare the total wet weather flows to the dry weather flow to see the magnitude of the problem for the various drainage basins. This way you will know in which basins to concentrate your efforts. Basins with high inflow should be investigated further using smoke and dye testing. Basins where infiltration is the predominant cause can be investigated using joint testing, visual inspections, and televising but first perform interviews and a record review.
Learning how to monitor inflow and infiltration is critical to maintaining safe water supplies and can help to prevent an even costlier disaster if done correctly.