A Fact Sheet Presented at the 1995 Navy Pollution Prevention Conference on the Investigation and Rehabilitation of Sewer Systems

Investigation and Rehabilitation of Sewer Systems

by Ellen K. Brown, NAVFAC EFANW

Navy Pollution Prevention Conference
6 June 1995

Presented by Ellen K. Brown
Engineering Field Activity Northwest
Naval Facilities Engineering Command

I. Introduction

Why Rehabilitate Sewers?

In the Navy, sewers are generally "out of sight, out of mind." They are built during initial construction of the base and added to and connected to over time as new facilities are built. We only think about them when they fail catastrophically, with blockages, backups, or overflowing wet wells. Municipalities, on the other hand, find that it is more economical to have a comprehensive program of on-going investigation and preventive maintenance on their sewers. This has the double benefit of cutting down on expensive and inconvenient emergency repairs and maximizing system performance. Conveyance and treatment of excessive infiltration and inflow is costly and can lead to NPDES permit violations.

What are Infiltration and Inflow (I/I)?

Infiltration is groundwater or stormwater which enters the sanitary sewer through cracks, holes, and open pipe joints. During rainfalls, the water table may rise above the elevation of the sanitary sewer, allowing the water to surround the pipe and find its way in. This is especially common in low-lying areas, areas near wetlands, and areas which use flood irrigation. Also, granular-backfilled pipe trenches can fill up with water, acting like a perched water table.

Inflow is storm water which enters the sanitary sewer system directly from cross-connection with the storm sewer, from storm water collectors such as roof drains or catch basins, or from ponding over manhole covers.

Inflow enters the sanitary sewer very soon after rain begins and stops soon after the rain stops. Infiltration enters the sanitary sewer after the stormwater has seeped through the ground to the level of the sewer. It continues until the groundwater table falls below the level of the sewer line.

How do sewers develop I/I problems?

When a sewer system is brand new, it is fairly tight against extraneous infiltration and inflow, but not perfectly so. Even new systems experience a 5% increase in flows during wet weather. Over time, many biological, chemical and physical forces act on the sewer pipes to reduce their integrity. In active seismic zones, ground shifts can cause pipes to become misaligned, opening gaps at joints or causing breaks. Construction activity above the line can have similar effects. Underground springs can undermine the pipe allowing it to sag and open joints. Hydrogen sulfide produced in the sewer can eat away the crown of concrete pipe. Root intrusion at joints and cracks can wedge open pipelines. Poorly made lateral connections can weaken a pipe or leave a gap around the connection.

[See source document for figure-Typical Infiltration Entry Points]

Inflow sources are sometimes intentionally and sometimes unintentionally added to sewers. Storm sewers or storm water carrying laterals can be tied into sanitary sewers by mistake due to inaccurate or non-existent drawings. Sometimes roof or parking lot drains are tied to the sanitary sewer because a project was developed in an area that didn't have a storm sewer and the project didn't have money to install one. It is even possible to find a potable or fire protection water line mistakenly tied into a sanitary sewer, but, luckily, this is very unusual.

Why do we need to identify infiltration and inflow and keep them out of sewer systems?

Infiltration and inflow can greatly increase the amount of water conveyed by the sanitary sewer system. A system with a big I/I problem could experience wet weather flows twelve times its normal sanitary flows plus a little infiltration, so excessive I/I can overload both the sanitary sewer and the treatment plant. Since storm water runoff generally doesn't need to be treated, it is a waste of conveyance and treatment capacity to design or enlarge plants to accept infiltration and inflow. Also, dilute and variable waste streams are much harder to treat, making I/I highly undesirable from a treatment perspective. Most POTWs (Publicly Owned Treatment Works) require that discharging activities limit the amount of I/I in their sanitary sewer system as much as possible. Activities with NOTWs (Navy Owned Treatment Works) also need to maintain good effluent quality and to keep from exceeding their flow limits on their NPDES permit.

How do we find and get rid of infiltration and inflow?

Because of the economic significance of infiltration and inflow, many investigative and rehabilitation techniques have been developed and used over the years. Sewer systems, rainfall patterns and soil characteristics vary greatly across the country and, accordingly, so do investigative and corrective measures. The right ones for each system should be determined on a case by case basis. We will consider an overview of the techniques here and I will point you to some additional resources.

II. Investigation Techniques - The I/I Study or Sewer System Evaluation Study

Introduction

I/I or SSES studies 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 flow monitoring, interviews of maintenance personnel and review of repair records, visual inspections of lines and manholes, smoke and dye testing, and 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.

Flow monitoring

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.

Flow monitoring can be accomplished using hand-held weirs (cheap equipment, but labor intensive to use) or electronic flow meters. Flow meters invented by Marsh-McBirney Company are easy to install, can take flow measurements every five minutes for a month at a time, and can download data directly into your computer for analysis. These flow meters can be rented or purchased. If you have a large system to monitor, unpredictable weather, or are going to initiate a regular monitoring program, it is most economical to purchase the meters, which cost about $5K each.

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.

To determine how much I/I the system experiences in wet weather, first measure the dry weather flows. Check the measured dry weather flows by calculating what they should be, based on building occupancies and types of usage. If 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.

[See source document for graphic - Yearly Infiltration and Inflow]

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. First perform interviews and a record review.

Interviews and Record Review

Public works maintenance personnel and workers at the various industrial facilities should be interviewed to find out where the known trouble spots are. Information of particular use is: where blockages usually occur, where there is flooding or sewer backups in conjunction with storm events, whether there are areas of the base which have buildings with down spouts going into the ground, but with no nearby storm sewers, which areas have had major repair work, what is the quality of the drawings you are basing your work on--should your first step be to have the drawings field corrected?

The record review should include sewer service requests, repair projects which have been completed or proposed, correspondence with regulators or the POTW about excess flows, internal facility plans, and drawings of the storm and sanitary sewers.

Visual Inspections of Lines and Manholes

A lot of infiltration and inflow enters the sewer through deteriorated manholes. Manhole defects are readily apparent upon visual inspection. The manhole can be physically entered if the steps are in good condition and confined space entry precautions are observed *such as certifying the space as without a team at the surface and wearing a safety harness). The following are common sources of infiltration and inflow through manholes (all are exacerbated if ponding occurs over the manhole): holes in manhole covers, poor fit between manhole cover and rim, cracks and holes in the pavement around the manhole rim, cracks or misalignment between bricks in the manhole, loss or absence of mortar between the bricks, cracks in the invert, and gaps or misalignment of connecting pipes.

The lines can be visually inspected through the manhole, by either lamping the lines and looking up them while in the manhole, or by using a remote halogen light and mirror while standing above the manhole.

[See source document for graphic - Typical Manhole Defects]

[See source document for graphic - Quick Visual Sewer Line Inspection]

An important word of caution: Remember that even while working over an open manhole, you must observe confined space precautions. Sewer gases can render you unconscious before you detect them with your unaided senses, and many people have been killed by falling unconscious into manholes. In addition, methane gas, common in sewers, is very explosive, so sparks and open flames must always be kept away from sewers or manholes.

Smoke and Dye Testing

Smoke and dyed water testing can be used to identify inflow locations and cross connections, where inflow has been implicated as a problem in the results of the flow monitoring.

In smoke testing, a non-toxic "smoke bomb" is used to produce smoke. A blower is fitted over the top of the manhole to 15-20 minutes to purge the sewer of gases before the smoke is introduced. The pipes at the upstream and downstream manholes are blocked off to isolate a section of line. If there are any connections to the sewers, the smoke will travel up them. If there are any untrapped drains, the smoke will continue to travel until it gets to atmosphere. Smoke testing is an effective way to detect storm connections to the sanitary sewer. Roof drains and catch basins connected to the smoked line will emit smoke. If there are significant cracks or holes in the pipeline, the smoke will come up through the ground above the pipe. Top detect this, the smoke testing must be done during dry weather periods. Smoke testing is inexpensive and sewer maintenance crews can easily carry it out. The engineer or technician in charge of the investigation should be on hand during the smoke test to observe and interpret the results. Pictures are usually taken to include in the report. Always inform the fire department and the nearby building occupants when you will be performing the testing, to prevent undue alarm.

Dyed water testing is like the reverse of smoke testing, in that dyed water is flooded on the surface or into potential sewer connections, and the nearest manhole is observed for signs of the dyed water. If the surface is flooded with dyed water and it gets into the pipe, it has entered through holes or cracks. If you suspect that a particular catch basin is connected to the sanitary sewer, you can dye running water from a hydrant or hose into the catch basin and look for it at the nearest downstream manhole. This is a very inexpensive technique and involves no equipment, just a bottle of dye and a garden hose.

Cleaning and Televising the Lines

Before lines are televised, they must be cleaned. This can be accomplished by pigging the lines, which is dragging a large rubber pig or plug through the lines, or jetting, which is sending a high pressure water jet through. Jets can also be used to pull a cable through the pipe, which will then be connected to the television camera to pull it through. While the lines are being cleaned, observe the debris which is removed. The contents of this debris will give you some indication of the condition of the sewer and the possible trouble sources. If there is grease, you may need to install or better maintain your grease traps. If there are roots, root intrusion may have caused cracks or joint separation. You may wish to regularly rod the lines or apply chemical root killers (this would have to be done with the full knowledge and authorization of the POTW or NOTW). If the removed debris includes dirt and pieces of broken pipe, you've probably got missing sections of pipe and may have to replace parts of the line.

Televising must be done during wet weather or dyed water flooding of the surface. Wet weather is most effective, because it will also show sources of inflow. As the camera is drawn through the sewer, the film will record exact locations (in stations) of water entering the pipe. An expert can figure out by watching the tapes how much water is entering in each defect in gallons per minute. The tape can also show the structural condition of the line. Also look for signs of hydrogen sulfide induced corrosion. (If you regularly off load CHT into your sewers, this is of particular concern. Several products and devices are on the market to combat the production of hydrogen sulfide in sewers.)

Televising is an expensive investigation technique--about $2 per linear foot, plus the cost of light or heavy cleaning which will add about another $1/ft. While televising rigs can be purchased for about $100K, most activities find that it is more cost effective to hire a local sewer service company when this work is needed.

[See source document for photo - Water Jet Sewer Cleaning]

[See source document for photo - Television Inspection]

Analyzing the Results

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. So, if your investigation reveals places where your sewer is failing structurally, either experiencing collapses or where collapses are imminent, these prescribed if the investigation showed signs that this was causing deterioration of your system. The next priority is the exclusion of extraneous clear water.

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. If you pay a POTW, you can use that rate, plus something for conveyance through 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.

There are some important exceptions to this simple analysis. You must also take into consideration why you are looking to get rid of the I/I in the first place. Are you being sued by your POTW to reduce I/I? Are you violating your NPDES permit with hydraulic overloads? Is your sewer system under capacity for it's current demands. Is your system picking up groundwater contaminants form infiltration from IR sites? These issues could control your decision more than the economics.

III. Rehabilitation Techniques

General

The rehabilitation methods you choose will depend upon the results of your I/I Study, the projected life of the base, the need to enlarge or decrease system capacity, your available resources, and your motivation for doing the study. Rehabilitation techniques range in price and life span from corrosion control coatings at $10 per linear foot, to complete replacement of lines, which will give that stretch of line a whole new life, but could cost $50-100 per linear foot. Costs presented here are 1991 costs, and are presented primarily for comparison between the methods. There are price variations depending on the area of the country and site characteristics.

Structural repairs may require trenching and installation of new pipe, or may be accomplished with numerous trenchless technologies, such as sliplining or inversion lining. Manholes may be structurally repaired by complete replacement or by varying degrees of rebuilding.

Correcting sources of direct inflow into the sewer system usually gets rid of a great deal of excess flow. Rerouting roof drains and parking lot drains from the sanitary sewer to the storm sewer is generally very cost effective. Sometimes these have been routed to the sanitary sewer because there was no storm sewer in the area. Under this circumstance, extending a storm sewer to the area. However, it may be possible to just direct this storm runoff to a detention pond or discharge it on the land. The main techniques for excluding inflow from the sanitary sewer are correcting cross-connections with the storm sewer, raising manhole rims, and installing solid manhole covers.

The prevailing wisdom for excluding infiltration is to repair every significant defect in a particular segment. If only the larger cracks or holes are fixed, water will find its way in at the small cracks which remain. This will eventually cause them to become large infiltration sources once the water using them as its main infiltration route. The main techniques for excluding infiltration from the sanitary sewer are grouting of joints, sliplining, inversion lining, applied-in-place concrete lining, coatings, lateral repair or replacement, and manhole repair.

Grouting

Chemical grouting of sewers is used to seal leaking joints and small cracks. A typical application technique is to run a small grouting rig through the pipe. The rig has a TV camera on the front so that the operator of the rig can see where there are cracks to be grouted. The rig can be positioned in joints to test their integrity by inflating a seal on each side and pressurizing the joint. If the joint cannot hold the pressure adequately, the rig will grout it. For a 12" line, grouting costs about $36-54 per linear foot. Grouting does not improve structural strength of the pipe and may reduce the carrying capacity. There have been some health and safety concerns associated with the application of acylamide grouts, so check on the status of this before selecting a grout.

[See source document for graphic - Grouting]

Coatings

Coatings can be sprayed or brushed onto the interior of pipes using internal rigs. They do not improve the structural strength of the pipe and usually do not correct leaking cracks, but they do improve resistance to corrosion. If the primary goal is to smooth the pipe interior and protect against continued corrosion, they might be a good choice. The cost is typically less than $10 per linear foot.

Manhole Repair

Manhole rehabilitation techniques include raising the frame and lid above grade to decrease ponding, installing lid inserts to prevent the flow of surface water into the manhole, grouting, mortaring or applying shotcrete to the inside walls of the manhole, and installing a new, precast manhole. The method selected depends on the condition and location of the manhole. Costs vary widely, from around $250 to raise a frame and cover, to around $2000 for manhole replacement.

Slip Lining

If a pipeline has numerous cracks and leaking joints, but is still continuous and not very misaligned, slip lining, or insertion, may be a good rehabilitation option. Slip lining is the pulling of a new pipe through the old one. Usually the new pipe is thinner and made of a polyester or PVC material. A cable is pulled through the pipe from one manhole to an insertion trench, where the cable is attached to an end of the new pipe. After a new pipe is pulled to the next manhole in each direction, the ends of the new pipe are bonded in the middle. If there were any lateral connections to the old pipe, these will have to be recut in the new pipe to allow a connection. Usually a grouting rig is sent through to grout around the newly cut lateral connections and at the connection of the new pipe and the existing manhole.

For a 12" line, sliplining costs around $50-75 per linear foot. It does improve structural strength and prevent infiltration, and, although it slightly reduces the diameter of the line, it could increase capacity due to decreased friction.

[See source document for graphic - "Sliplining" or Inserting Methods]

Inversion Lining

Inversion lining is similar to sliplining in that it results in a new continuous pipe surface inside the existing pipe. It is easier to install, as it does not require an insertion trench, but rather uses the existing manholes for access. It is also useful for applications where significant misalignment is present, which would have prevented the use of a slipliner.

The inversion lining is formed by inserting a soft, resin-impregnated felt tube into the pipe, inverting it by filling it with water or air from one end, and curing it in place, usually by heating and recirculating the water in it. Once it is cured, it is as hard as a lip liner, and it follows all the contours of the pipe. The cost is higher than sliplining: $75-125 per linear foot.

[See source document for graphic - Inversion Lining Installation]

Sewer Replacement

When a line is completely collapsed, severely blocked, or undersized, sewer replacement is the best option for rehabilitating the line. Replacement generally requires extended downtime of the line, open trenching, removal and disposal of the existing pipe, and construction of a new line. This also usually involves repaving the street if the sewer is located under one.

Sewer replacement is, in many instances, less expensive than sliplining or inversion lining, but is generally much more disruptive to base operations. In active industrial areas, the disruption and safety hazard of having open trench work and longer disruption of sewer service may warrant going for the higher cost of other options. Sewer replacement for a 14" line will cost $50 - 100. The higher number is associated with repaving or deeper excavations.

IV. A Regular Sewer Maintenance Program

All well-run municipal public works departments 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. Hardly any Navy public works departments perform preventive maintenance on their sewers, although it is even more important for us, with our mission needs and long waits for funding and contracts, to be able to project our maintenance costs and system downtime well in advance.

A worthwhile maintenance program should include a good record keeping system, quick response to service requests, a cycle of regular televising of revolving part of the sewer system each year, regular cleaning of the system, and regulator 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.

V. Resources for Further Study

EPA Handbook: "Sewer System Infrastructure Analysis and Rehabilitation," EPA/625/6-91/030

American Society of Civil Engineers and Water Environment Federation Manual of Practice FD-6: "Existing Sewer Evaluation and Rehabilitation"

National Association of Sewer Service Companies: "Inspector Handbook for Sewer Collection System Rehabilitation" and "Recommended Specifications for Sewer Collection System Rehabilitation"


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Last Updated: January 25, 1996