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Text of Document: Managing Infiltration Inflow in Wastewater Collection Systems
By Sharon Rollins, P.E.
Technical Consulting Program Manager
Revised 2006
What Is I/I?
INFILTRATION is water that enters the sewage collection system from the soil through foundation drains, defective pipes or joints, and faulty connections (i.e., ground water).
INFLOW is water that enters the sewage collection system from sources such as roof leaders, basement and yard drains, and cross connections with drainage lines (i.e., surface or rainwater).
If you have a wastewater collection system, chances are you have infiltration and inflow (I/I) problems. Many systems have recognized I/I problems that are not addressed because the problems seem overwhelming—both in terms of money and manpower.
Is It Really I/I?
When an overflow occurs in the system, you may think it is I/I. But, it could be
· debris, roots, or grease in the system, restricting flow;
· sags in lines;
· collapsed lines due to corrosion;
· design bottlenecks; or
· pump station problems—leaking check valves, improperly sized impeller, improperly sized motor, impeller wear, etc.
The Traditional Way: Bring in the Outside Experts
There are two ways to manage infiltration and inflow problems—the traditional way or the operator's way. The traditional way is usually an expensive and slow approach that may or may not prove effective. The operator's way is generally less expensive and often produces quicker results. However, like the traditional way, it may or may not prove effective. The best approach may be a combination of the two.
Bring in the Outside Experts
1. Map the sewage collection system.
2. Get flowcharts from the treatment plant.
3. Divide the collection system into subregions.
4. Flow monitor the subregions using flow meters at key manholes.
5. Based on flow meter readings, prioritize which subregions to rehabilitate first.
6. Televise (TV) the subregions high on the priority list.
7. Smoke test the subregions with the highest priority.
8. Start at the dead ends of the high priority subregions and make repairs and/or replacements.
9. Determine the amount of flow reduced using flow meters at the same key manholes described in Step 4.
10. Extrapolate the potential flow reductions to the other subregions on the priority list.
11. Determine which areas are cost-effective to repair and proceed with repairs.
The Operator’s Way: Use In-House Resources
Use In-House Resources
1. Map the sewage collection system.
2. Get flowcharts from the treatment plant and pump station capacities. Also get run times. (Note: this requires run time meters at all pump stations.) Pump capacities can be obtained from the manufacturer's literature or the wastewater system's equipment specifications.
3. Staff members who know the collection system best should study the map and list the most likely areas for I/I. Use pump run times (you will need data through a wet and dry season), records of overflows, rainfall records, and knowledge of the collection system.
4. Locate and document all overflows at pump stations, including complaints of sewage backups, surcharging manholes, and so forth.
5. Start with the worst areas (based on information from Steps 3 and 4) and physically walk the sewer lines to look for problems.
6. Do nighttime flow isolations. If there's water in the line at night, it is probably coming from an I/I source.
7. Clean the line.
8. If the line is lower than the ground water table, televise it. If it is above the ground water table, smoke test.
9. Fix the line and go on to the second-worst section.
If the problem is mostly system-wide infiltration, the traditional way may be the best approach. If the problem is mostly inflow, the operator's way may be the best approach. A study of the traditional way and the operator's way shows that they have many elements in common. Therefore, it is easy to switch to the traditional way if the operator's way does not yield results.
What’s the Problem?
To help decide whether you should use the traditional way or the operator’s way to solve I/I problems, you must first define the problem.
Document and be as specific as possible. Use complaint records. Does sewage back up in Mr. Johnson's basement every time it rains? Use influent biochemical oxygen demand (BOD) and suspended solids (SS) data. Weak (or low) BOD and SS influent readings usually indicate dilution of sewage by extraneous water. Use rainfall records. It is easier to see the impacts if the data is plotted. At the wastewater plant, post a running record of these parameters.
[Insert graphic here.]
Identify and document problem areas. Operators generally know the manholes that surcharge, pump stations that overflow, and lines that run full during rainfall events. Document the information.
Discuss the information. Using the map, operator’s knowledge of the system, and data you’ve collected, ask the following:
· Do influent flows spike up during a rainfall event then go down quickly? If so, suspect roof leader, storm sewer, or surface drain cross-connections. If flow jumps up and declines slowly, suspect stream inflow and/or groundwater infiltration.
· Where do lines cross creeks? Do the creeks stay up about the same length of time as the flow in the lines?
· Where are likely cross-connections with drainage lines or roof leaders?
· Do lines pass through swampy areas or near springs?
· What color is the plant influent during a heavy rainfall? Normal raw wastewater is gray. If the influent is muddy, you may have an inflow source.
· Are grinder pumps located in depressions or near down spouts?
Look, listen, document, and analyze. Take a close look at the system. Walk the lines, open manholes, and make notes of the following:
· surface depressions, cave-ins, or road collapses;
· debris in inverts;
· broken manhole lids;
· brick manholes showing active I/I;
· clear flow in lines;
· unusual flows; and
· sluggish flow.
What Are You Trying to Solve?
Not every problem has a cost-effective solution. What is the benefit-to-cost ratio? It should be 1.0 or greater.
· Is there environmental damage? Are there health problems? Are regulatory agencies pressuring the town to solve its I/I problems?
· Are you trying to eliminate lift station overflows or excessive pumping costs?
· Are you trying to reduce treatment costs?
· Are you trying to solve a specific problem—solids washout of the treatment plant, a backup in Mr. Smith's basement, etc?
Develop an I/I Correction Plan
At this point, you have in mind what you want to accomplish and why. You have accumulated data to help identify where the worst problems are, and you have ruled out other possible causes of the high water problems. During this process, you will be able to assess how much time and expertise you have in-house for solving I/I problems. Now you are ready to tackle solutions. If you have decided your staff just does not have the time, equipment, and/or expertise to identify, analyze, and fix the problems, it is time to hire consultants and contractors.
If you can use in-house resources to further identify, analyze, and fix I/I problems, here are some tips:
· Look at the geologic and topographic conditions for the part of the system you will be working on first. Is the collection system under the ground water table? If yes, use nighttime flow isolation, televising the lines, and flow monitoring to get more information. If no, use smoke testing, rainfall simulation, dye testing, and flow monitoring to gain more information. In either case, use physical inspection of manholes. They can be big sources of I/I. Brick manholes are especially subject to I/I. Manhole covers that are in depressions, near streams, or in streets or other areas subject to sheet runoff are subject to I/I.
· Focus on interceptors (large sewers), especially ones near creeks. They can carry more I/I on a per-inch basis than smaller lines. For this reason, many sewer line rehabilitation experts advocate working from the sewer plant back up the system and not from the dead ends down.
· Size up the magnitude of the problem and what it is going to cost. Does the town need to borrow money and make an all-out effort to improve the system? Or does a phased approach interest the town's governing body? First, you will have to answer why you have decided to fix I/I problems. Once you get the present I/I crisis solved by either the traditional way or the operator's way, MTAS strongly advocates annual budgeting for collection system maintenance. The goal is to strengthen the collection system infrastructure in order to minimize future I/I problems.
Rehabilitate the System
Use in-house and/or outside expertise to fix the problems. Until your operators have experience with repair methods, outside contractors will be needed to make many of these repairs.
Rehabilitation methods include
· point repairs;
· cured in-place pipe lining;
· grouting;
· manhole liners, coatings, and specialty seals;
· slip lining;
· root control; and
· line replacement.
This information is only intended to outline approaches to managing inflow and infiltration in wastewater collection systems and to list some tips. The user needs to consult other sources of information for repair methods. You can always call your MTAS public works consultant for more information. |