Controlling Inflow and Infiltration

In Wastewater Collection Systems

Mark G. Wade, P.E.¹

Every sanitary sewer system, regardless of its age, size, or location, contributes inflow and infiltration (I/I) to the municipal wastewater collection and treatment facilities. The cause of excessive I/I is due to an aging infrastructure that has not been adequately maintained. This paper presents current methodologies to identify, evaluate, and rehabilitate the municipal wastewater collection system.

The need to reduce, control, and manage wet-weather induced I/I in wastewater collection systems is becoming an increasing priority to many cities, municipalities and wastewater agencies across the United States. Problems associated with I/I often include one or more of the following:

• Uncontrolled overflows and bypasses
• Basement backups
• Hydraulic impacts to the treatment facilities
• System deterioration

These problems are particularly difficult to address because of the enormity of the infrastructure in place. Currently it is estimated that there are 4.2 billion feet of sanitary sewer in the U.S. This does not include "combined sewers" which serve as both storm and sanitary sewer. If this inventory were laid end-to-end, it would represent 286 parallel pipelines that would stretch from New York City to Los Angeles (or 3 round trips to the moon). For older cities, most pipe inventory pre-dates World War II, and represents materials and methods of construction that are well beyond their reasonable service life.

Since the 1970s, the EPA has required all regulated agencies with NPDES permits to eliminate all wastewater overflows that reach the waters of the United States. Of course, the ability to achieve such a goal is virtually impossible for most cities and agencies, since I/I cannot be completely eliminated. The initial efforts to reduce I/I in collection systems were, for the most part, unsuccessful, despite substantial funding through the EPA’s Construction Grants Program. During the mid-to-late 1980s, most I/I control programs were reduced to emergency programs that attempted to address problems in isolated sections of the collection system. However, major sanitary sewer evaluation surveys (SSES) in the late 1980s in cities such as Houston, Atlanta, Nashville, and Miami (Dade County) renewed public opinion for the renewal of the sanitary sewer infrastructure. Also, newer and better technologies were improving techniques to eliminate sources of I/I.

Recent studies show that the asset value of this utility is $1.0 trillion, or 16% of the total public works infrastructure. It is an invaluable utility that is deteriorating at a faster pace than it can be fixed. In fact, anticipated rehabilitation needs to upgrade wastewater collection systems in the U.S. now exceed $34² billion. Some estimates have pegged this value as high as $80 billion. Currently, annual spending for sanitary sewer improvements (not including new or expanded systems) is $1.0 billion. Why are we in this jam? Herwig³ stated it better than most:

• Infrastructure deterioration has been gradual
• Structural problems develop slowly
• Equipment wears out gradually
• Utility customers have become accustomed to the low costs associated with new systems
• We have forgotten the notions of design life and replacement costs
• Collection systems have difficulty competing with other community infrastructure for citizens’ dollars.

The dilemma of the sanitary sewer has always been the same…undersized, underestimated, and underground!

The reduction and control of I/I in wastewater collection systems must be considered in the context of a disciplined, long-term program. Assessing the problem should always be the first step. For I/I assessment, the most common practice is a sanitary sewer evaluation survey (SSES). The purpose of an SSES is three-fold:

1. Quantify the I/I problem
2. Identify the I/I sources
3. Evaluate the cost-effective correction plan

We will present a brief discussion of each step before moving onto actual rehabilitation methods.

1. Quantify the I/I Problem

It is often said of I/I in collection systems that "…you can’t management what you can’t measure". Quantifying an I/I problem often begins by assessing (or measuring) the extent of the problem. This means a serious attempt to locate and record information that relates to a variety of problems including observed overflows, measured or observed surcharges, reported bypasses, customer backup complaints, and chronic maintenance activities. This information can (and should) be gathered from a variety of places including maintenance records, work orders, past studies and engineering reports, sewer maps, complaint records, various department files, and interviews with personnel who are responsible for maintenance and management. It is amazing, if not remarkable, to see how much information can be gathered if one is willing to "dig a little". Once this has been done, the data should be recorded and displayed in a manner that will provide possible correlation between overflows and bypasses and other factors such as capacity models, rainfall records, maintenance activities, and reported backups. If electronic maps of the collection system are available, GIS is an extremely useful tool to evaluate these results. The next step in quantifying the problem is to monitor wastewater flows at various key points in the system. Normally, the collection system can be separated into watersheds. Watersheds can be further separated into basins. Depending on the size the system, basins should be further separated into sub-basins. An example of this is shown in Figure A.

Example Sewer System Layout.

The placement of the appropriate flow monitoring equipment is a critical step and one that needs to reflect the type of data desired. In order to measure wastewater flows and their response to rainfall, it’s important to select a flow meter that will record both depth and velocity of flow. There are a number of models available. They can either be purchased or leased. The following "rules-of-thumb" can be followed in order to measure and evaluate the amount of I/I in a collection system. Of course, these parameters can vary depending on the overall program goals.

• One meter for every 30,000 – 50,000 feet of sanitary sewer
• Flow meter recording set at 15-minute intervals
• Flow meter capable of measuring surcharge and flow reversal
• One rain gauge for every 2-4 flow meters
• Minimum monitoring period – 42 days (60 days, optimal)
• Measurement of 6-8 separate rainfall events
• Monitoring period during high seasonal groundwater

The resulting data needs to be carefully evaluated. This includes adjustment of the data to account for periodic velocity profiling at the monitoring site, anomalies associated with grease and deposition, drift of recorded depth or velocity, and downtimes associated with meter malfunction. Examples of a typical diurnal flow cycle and a flow cycle influenced by a storm event is shown in Figure B.

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Once it’s reasonably certain that the data is accurate, then results can be evaluated for several flow parameters including average dry-day flow, maximum and minimum diurnal flow, inflow, rainfall-induced infiltration, seasonal infiltration, etc. An example flow data summary sheet for several monitoring sites is shown in Table 1. Note that flows are shown in rates and should be expressed in mgd or gpm, depending on the flow regimes that are recorded.

Once the data has been tabulated, a linear regression analysis can be used to make comparisons between the measured I/I and the corresponding rainfall intensity. This regression analysis will provide two key parameters that will be used in quantifying the I/I problem. First, a regression analysis allows us to make comparisons between each basin in order to identify the top priority basins for further study and possible I/I reduction. Secondly, the analysis will provide useful design information if subsequent relief or replacement sewers are required to reduce or eliminate an overflow or bypass. Results of a typical linear regression analysis for inflow is shown in Figure C. Note that each data point represents measured inflow for a single rainfall event. A crucial step in the regression analysis is to try and avoid using data under surcharge conditions.

The basins can then be ranked in a variety of ways. These could include unit inflow or infiltration rates such as gallons/day/foot, mgd/1,000’, gpd/inch-mile of pipe, mgd/acre, etc. By reducing the raw flow data into a measured unit rate, comparisons can be made between basins as well as comparisons with regard to other factors such as general age of the system, frequency of reported overflows, etc. Such a ranking is shown in the following example table.

1. Identify the I/I Sources

Once the basins or sub-basins have been selected as priority areas for I/I reduction, the next step is to implement a plan to locate and analyze the various sources of I/I in the collection system. This is commonly referred to as a sanitary sewer evaluation survey (SSES) and represents a wide range of field inspections and testing procedures. For most collection systems, I/I is contributed from various defects in the pipelines and manhole structures. I/I can also enter into the system from directly or indirectly connected storm sewers. This area is often referred to as the "public-sector" or that part of the collection system that is owned and maintained by a particular city, utility, or public agency.

1. Manhole Inspections

Many communities that have implemented a successful I/I reduction program report that as much as 50% of measured I/I can originate from deteriorated and leaking manhole structures⁴. With more than 18 million manholes in the United States and an estimated renewal cost estimate of $8.5 billion, this makes manhole renewal a growing focal point of the collection system. Prior to the 1960s, most manhole structures were constructed of brick and mortar. Since then, the common materials of construction have been segmented precast concrete with gasketed joints.

2. 

   Therefore, a proper manhole inspection should include an inspection of every component of each manhole structure. These components and possible causes of I/I intrusion are shown in Figure D. Proper safety procedures, following OSHA regulations for confined-space entry, must be followed. During the inspection, a quick check of the pipe conditions entering and exiting the manholes can be achieved by simply lamping these pipes. A standard inspection format should be followed, supplemented with photographs or video recordings, if there are particular defects or rehabilitation requirements that merit further analysis.

3. Smoke Testing

Smoke testing is, perhaps, the most effective and economical method of locating major sources of I/I such as storm drainage connections, curb inlets, and area drains. Typical I/I connections are shown in Figure E. When implementing a smoke testing program, the following procedures should be considered:

• Use high-capacity smoke blowers (>3,500 cfm)
• Isolate individual sewer lines, if possible
• Develop and follow through on a well planned public relations program
• Test only during periods of dry weather
• Target daily production rates of 5,000 – 7,000 feet
• Carefully document all identified defects (photographs or video recording)

Smoke testing will not only identify sources of I/I, but it is an effective technique for locating structural defects such as collapsed, broken or cracked pipe and offsetting, separated, or deteriorated pipe joints. Cities should conduct a once-through smoke testing program every 10 years.

In addition to I/I sources on the "public" side, smoke testing will also locate I/I connections in the "private-sector". This includes all possible ways that rainfall runoff and ground water can enter into the municipal sewer system from private property. Most connections violate current local plumbing codes. However, for older properties, the connections were likely permissible and allowable. Examples of typical private-sector I/I sources are also shown in Figure E.

1. Cleaning and CCTV Inspections

Most cities own and maintain equipment to properly clean their collection system and remove deposition, debris, grease, and other impediments in the flow line. Effective equipment such as high-pressure jetters and jetter/vactor systems have replaced older style mechanical cleaning systems such as rodders, buckets, kites, corkscrews, augers, porcupines, spring blade cutter chucks, and various pick-up tools. However, these same cities limit their use to emergency and non-scheduled responses. By adding a remote CCTV inspection unit and moving to a preventative maintenance platform, many cities would be better equipped to tackle rehabilitation on a more pro-active basis. In addition to data collected from the initial database, and supplemented with the information from the diagnosis program, internal pipe inspections using advanced cleaning and CCTV inspection equipment can confirm and pin-point specific problem areas within the collection system in a very effective manner. As an alternative to purchasing the equipment, smaller communities might consider securing the services of an outside specialty contractor to perform the work, or share equipment with other cities.

2. Dyed-Water Testing (flooding)

During the CCTV inspection, it is not uncommon to miss a potential I/I connection or substantial leak that may have been discovered during smoke testing. Therefore, concurrent dyed-water testing is mandatory in order to observe an active leak. Normally, the adjacent storm sewer, drainage ditch, or creek crossing is flooded while the TV inspection is underway. Using a color TV unit (mandatory), an active leak or I/I connection can be confirmed and quantified.

3. Evaluate the Cost-Effective Correction Plan

Results from the inspection and testing activities should be recorded on standardized data-entry forms (an example is shown in Figure F). Since the information gathered from a conventional SSES or I/I study may result in thousands of potential sources of I/I, it is recommended that a database be utilized to evaluate the information and compile the results in a variety of formats, depending on the program objectives. Several management software programs are commercially available. Customized databases can also be developed by the user, depending on the complexity of the program. For I/I reduction, the most common format is to list all identified sources of I/I on the basis of cost versus the amount of I/I removed ($/gpm). An example listing is shown in the following table. Note the ascending unit costs in the final column. Regardless of the final format of the data, it’s crucial that the information is readily available and useful in identifying the cost-effective rehabilitation plan.

Enhancements to the data collected during the SSES can include a file of digital photographs of the observed defects. Videotapes of the CCTV inspection records can also be converted into an electronic format such as AVI files. Finally, if the system map is available in GIS, study results, specific rehabilitation needs, and various alternatives to correct the defects can be displayed. For example, GIS, which includes ortho-photo base sheets, is an effective resource when combined with a very SSES database to locate special rehabilitation applications such as trenchless pipe renewal.

Before the 1980s, sanitary sewer rehabilitation programs were primarily limited to dig up-and-replace. If an existing pipeline needed to be replaced because of deterioration or leaking joints, the pipe was either replaced or a parallel sewer constructed. This made the repair cost-prohibitive. So the I/I problem remained. Not only were I/I removal programs expensive, but open-cut construction projects were disruptive to traffic, detrimental to other above- and below-ground utilities, and a real source of aggravation to adjacent property owners and public offices. Also, additional easements, rights-of-way, and rights-of-entry were also required.

Today, however, alternative and innovative solutions are being used to successfully reduce I/I and eliminate hydraulic bottlenecks in wastewater collection systems. These solutions feature "no-dig" or "trenchless" construction practices. An example of these technologies is listed in the following table.

During the past 20 years, the expectations of achieving successful rehabilitation programs have risen dramatically. These expectations have been legitimized through the evolution of technology and improved field techniques. For an example, recent developments in CCTV equipment give cities opportunities to remotely reach segments of the wastewater collection system that were not possible before. Other developments in both software and hardware provide tools to sort, analyze, and model huge volumes of data. Information can be transformed from complicated and meticulous spreadsheets to maps and interactive graphical platforms (GIS) that make decision making more logical and consistent. GIS is truly changing the face of many sewer system renewal programs.

If the objective of the program is to reduce wet-weather overflows and bypasses, most cities can realistically expect a reduction in peak flows of 30-50%. It is rare that I/I reduction rates exceed 50%.

If the objective is to minimize the number and frequency of emergency complaints and unscheduled maintenance activities, a program following closely prescribed cleaning and inspection will reduce these events by 70-90%. This will subsequently reduce costs associated with labor overtime and payment of claims filed by property owners who experience real property damage.

Although a pro-active assessment program will require some up-front capital investments such as equipment and hardware, cities can expect cost-recovery within a 3-5 year period (or less) depending on the reduction level of emergency repairs.

Perhaps the question that is most frequently asked is "How much will it cost to rehabilitate our sanitary sewer system". In order to answer this question, several decisions have to be made.

• How much can be budgeted?
• Will a rate increase be required?
• Are there other revenue sources available?
• What are the program's priorities?
• Is it crucial to abate I/I from private property?
• Can the study phase be conducted in-house?
• Can some of the rehab work be done in-house?
• Can the program be phased? How long?
• Are there O&M savings?

For a system of average age and size, the following guidelines can be used to forecast the long-term costs that can be anticipated.

Sewer System Evaluation Survey

- Flow Monitoring/Analysis $0.10 - $0.20/lf

- Physical Inspection $0.50 - $1.00/lf

- Cleaning/CCTV Inspection $1.00 - $2.50/lf

- Hydraulic Modeling $0.05 - $0.15/lf

Plans and Specifications 5.0 - 8.0% of Const. Est

Legal/Administration 1.0 - 3.0% of Const. Est

Construction

The above cost estimating guideline can vary depending on the size of the collection system and the actual rehabilitation requirements. Figure G provides a cost comparison between three levels of renewal programs. "Comprehensive" represents a plan that includes all elements of inspection and testing and rehabilitation to reduce I/I, eliminate structural deficiencies, and implement a preventive maintenance program. On the other end of the spectrum is the "selective" plan which would include minimum program elements such as manhole inspections, very limited smoke testing, and a rehabilitation program that is targeted (such as manhole rehabilitation). Most cities will probably fall somewhere between these plans.

The distribution of how program costs can be allocated will also vary widely.

 Copyright ©2002 Wade & Associates, Inc.