Now that you have all this data collected from flow monitoring, interviews and records review, manhole inspections, smoke and dye testing and cleaning and televising the line, just what do we do with it?
First, you should determine why you should get rid of the inflow and infiltration. Are you violating your KPDES permit with hydraulic overloads, sanitary sewer overflows or combined sewer overflows? Is your sewer system under capacity for its current demands? These issues could control your decision more than the economics.
Once you have identified the quantities and sources of infiltration and inflow to your system, you will need to prioritize the defects in order of importance. The highest priority for rehabilitation is to maintain the structural integrity of your sewer system. Your biggest investment is the hole through the ground. If your investigation reveals places where your sewer is failing structurally, either experiencing collapses or where collapses are imminent, use these methods.
A fairly simple cost benefit analysis can be done to determine a cut-off point for fixing infiltration or inflow sources. First, determine the cost of transporting and treating a gallon of sewage in your system. Next, determine the costs for repairing the various defects. Divide the cost of the repair by the amount of I/I the fix would remove from the system. If this cost is less than the cost to transport and treat, it is cost-effective to do the repair; if this cost is higher, continue to allow the I/I from that source into the system.
One method is to assign a unit amount to the I/I, gallons of I/I per minute, gallons of I/I per foot of line or million gallons per day per 1000 feet of line. Define a unit of measure that can be used in all the basins and sub-basins whose flow you monitored. The chart below is an example of how one system showed pertinent data all in one place.
There are many methods and technologies that are available to remove and reduce I/I. Modern techniques allow for sewer lines to be inspected, cleaned, and even replaced without traditional open cut excavation. By utilizing “Trenchless Technologies”, rehabilitation and repair can often be performed without interruption of sewer service or traffic disruption. Specific techniques that are available for fixing Infiltration sources are listed below:
- Manhole-to-Manhole Lining:
- Fold & Form
- Slip Lining
- Pipe Bursting
- Dig and Replace
- Spot Repairs – sectional cured in place liner
- Spot Repairs – dig and replace
- Exterior Coating or Grouting
- Interior Lining
- Inflow Dish
- Raise or Replace MH Covers and Frames
Inflow can also be removed by a variety of techniques, but these usually require a direct disconnection of the sanitary sewer system from the inflow source. Below is a list of methods that may be used to remove inflow:
- Disconnect roof leaders, sump pumps, foundation drains, or other illegal or improper connection from the sanitary sewer system
- Encourage the removal of private inflow sources through enforcement of State, Local Regulations or Incentive Programs
- Raise manhole covers that are located in low areas where stormwater may pond
- Divert stormwater that flows to manhole covers
After all the assessments, monitoring and calculating of information, it all goes back to why you want to remove I&I from your collection system. Is it to stop overflowing manholes and lift stations, either sanitary or combined sewers? Or is it to combat hydraulically overloading the treatment plant which puts you in noncompliance with your KPDES permit. What ever your reasons may be, if the data is collected and interpreted correctly, a plan to remove the inflow and infiltration is well on the way.