Those involved in the management and design of wastewater treatment collection systems have recognized the benefits of a "tight system" in which extraneous flows (groundwater leakage or infiltration, and stormwater sources or inflow) are held to a minimum.

The obvious benefits of a system with low infiltration and inflow rates include:

few or no surcharged sewers;
efficient operation of wastewater treatment facilities; and
the hydraulic capacity of the collection system is used for wastewater and not infiltration/inflow.

Perhaps the more salient benefit of a "tight system" can be lower overall capital and operations costs. For example, savings may be encountered through:

smaller expansions of hydraulically sized collection system components (e.g., sewer lines, pumping stations) and
lower operating costs realized by a decrease in the energy requirements associated with pumping.

Possibly the most important development that has highlighted the need for controlling infiltration/inflow has been the increasing costs of sewage treatment and resulting flow-based rate setting occurring in some areas. Federal and State programs also encourage Infiltration & Inflow (I/I) reduction by:

providing a systematic approach to discovering and subsequently eliminating extraneous flows; and
providing some funding and low or no interest loan programs.

The State Revolving Loan Fund Program (SRF)

With the passage of the Federal Water Pollution Control Act Amendment of 1972, a structured program for sewer evaluations and rehabilitation became a prerequisite for obtaining funds under the US Environmental Protection Agency's (USEPA) Federal construction grant program. The Federal construction grant program, initiated in 1972, evolved into the current State Revolving Loan Program (SRF).

Prior to 1991 the Federal government provided approximately 90% of the grant money issued under the construction grant program to assist states with the development and implementation of wastewater treatment programs. This practice was changed as the Federal government shifted the responsibility toward the individual states.

The Federal government now provides "seed" money to the states to help fund low interest loans for environmental projects. The money provided by the Federal government and State match now make up the current funding for loans through the SRF program.

The following is a brief summary of DEP's publication of "The Guidelines For Performing Infiltration/Inflow Analyses and Sewer System Evaluation Surveys", January 1993.

Infiltration & Inflow Analysis

The I/I analysis should be considered a study of limited scope to determine if infiltration/inflow is excessive within a given sewer collection system. The investigation should identify the presence, flow rate, and type of infiltration/inflow conditions which exist in the sewer system. The following steps are usually part of an I/I analysis:

Inventory of Existing Conditions

The purpose of the inventory of existing conditions is to provide a better understanding of the sewer system. The following inventory sources should be reviewed:

Past engineering studies;
Detailed maps of the wastewater and storm sewer systems;
Maintenance records kept by the town;
Flow records for the treatment facility and pumping station(s); and,
Historical USGS groundwater data (groundwater levels).

Based on the review of above noted sources, it is possible to establish:

The type of collection system (combined or separate);
How often overflows, bypasses, and surcharges occur within the system;
Type and age of sewer lines and manholes;
Condition and age of the existing facilities; and,
Identification of problem areas.

The inventory of conditions should also include a physical survey of the sewer collection system. The purpose the physical survey is to isolate I/I problem areas, select flow gauging areas, and provide a general assessment of the physical condition of the sewer collection system. At a minimum, the following tasks are to be completed during the inventory of existing conditions:

The limited inspection of a representative number of manholes (10% of the total number of manholes within the system). This will help verify sewer system configurations.
Selection of continuous and instantaneous gauging manholes.
Determination of gauging area boundaries for flow continuous and instantaneous gauging areas.
Measure groundwater levels (evidenced by wet rings and/or leaks noted during the limited manhole inspections).

Initial Flow Monitoring

The purpose of the initial flow monitoring is to determine the overall system flow quantities and infiltration trends during high and low groundwater periods and for rainfall related inflow during wet weather. At least 70% of the total flow is monitored with the least number of flow monitoring devices (one monitor for every 20,000 linear ft (LF) of sanitary sewer line is recommended).

The information provided by the flow monitoring data will aid in locating those areas with excessive I/I. Continuous metering must be conducted for a minimum of 10 consecutive weeks, typically between March 1 and June 30 to determine "peak" infiltration rates (infiltration rates during high groundwater periods). For "minimum" infiltration rates continuous monitoring should be conducted until August 30. Other time periods may be used but need DEP approval.

To determine infiltration rates, wastewater flow metering data should be examined to identify periods of dry weather (generally three to five days without a rain event). During a dry weather period, nighttime minimum flows represent a period of minimal sanitary flow; therefore, a high percentage of the nighttime flow may be attributed to groundwater infiltration. The portion of the nighttime minimum flow which can be attributed to sanitary flow should be subtracted from the observed flows (engineering judgment should be used to determine sanitary flows).

If tidal infiltration is suspected, a separate analysis must be performed during dry weather periods using nighttime minimum flows corresponding to the tidal cycle.

Rainfall Monitoring

The purpose of the rainfall monitoring is to obtain data to compare variations in gauged flow rates to rainfall intensity, total volume, and duration per event. This information is required to identify inflow rates and its components (direct inflow, delayed inflow, peak inflow rate, and total inflow volume).

Rainfall data must be collected within the area of study. This site specific data is used to develop a rainfall/inflow volume relationship to estimate the inflow volume expected to be produced from the design storm (one year, six hour storm event) in a given subsection of a sewer collection system.

Recommendations for Further Study

Infiltration
After the flow data analysis portion of an I/I analysis has been completed, extensive manhole inspections and flow isolation programs can be recommended for all subsystems exhibiting an infiltration rate equal to or greater than 4,000 gpd/idm (gallon per day per inch diameter per mile of sewer). Further work on subsystems with a lesser rate can be justified on a case by case basis.

Inflow
After the completion of the flow data analysis, the inflow estimated to be produced from the standard one year, six hour design storm must be used to rank all subsystems. The ranking system takes into consideration the following items:

volume of total inflow;
volume of direct inflow; and,
volume of delayed inflow.

The purpose of the ranking system is to evaluate the highest priority subsystems for further inflow studies (i.e., extensive manhole studies, house to house inspections, smoke testing, dye water testing and dye water tracing).

The recommended subsystems must account for at least 80% of the total inflow volume. The 80% rule of thumb should be used as a minimum threshold; however, it is recommended that all subsystems influenced by inflow should be considered for further study.

Phase I Sewer System Evaluation Survey

The SSES is performed to determine the specific location, estimated flow rate, method of rehabilitation and cost of rehabilitation. The SSES is intended to confirm the general overall findings of the I/I analysis. The SSES is performed in two phases. The key tasks associated with Phase I are presented below.

Phase I SSES: Flow Isolation

Flow isolation is conducted on a manhole by manhole reach during dry weather and high groundwater periods between the hours of 12:00 A.M. and 6:00 A.M. Flow isolation results will be addressed in groups totaling approximately 1,000 LF for analysis purposes in justifying follow-up television inspections. Flow isolation gauging incorporates the use of portable weirs, or flow depth measurement in conjunction with velocity measurements. The upstream manhole is plugged when possible.

A 1,000 LF continuous sewer section with an infiltration rate above 4,000 gpd/idm is considered to contribute excessive amounts of infiltration into the system. Therefore, line groupings with infiltration rates equal to or greater than 4,000 gpd/idm are eligible for follow-up television inspection.

Phase I SSES: Extensive Manhole Inspection

This is a more detailed manhole inspection than the one conducted during the I/I analysis phase. Extensive manhole inspections must be performed in subsystems with infiltration rates above 4,000 gpd/idm, and in the inflow areas which account for 80% of the total inflow volume based on the I/I analysis. This will help determine the physical condition of the manholes and the connected sewer pipes.

Phase I SSES: Smoke Testing, House to House Inspections

All are methods to be used in the identification of inflow sources in the selected subsystems (80% inflow area). These field investigations are aimed at locating sump pumps, cross connections between storm lines and sanitary lines, and other sources of inflow. Recommendations are based on the results of the field studies and estimated inflow rates.

Phase II Sewer System Evaluation Survey

Phase II SSES: Television Inspection for Infiltration

Television inspections are utilized to pinpoint the exact location(s) of extraneous water entering the sewers. These types of inspection must be conducted during dry weather and high groundwater conditions between March 1 and June 30. Nighttime flow gauging shall be conducted immediately prior to the television inspection. The infiltration rates measured during the TV work and the flow gauging shall be compared and discussed.

Phase II SSES: Final Cost Effective Analysis

If infiltration and inflow rates are excessive, the contributing sewer lines are eligible for rehabilitation work. Infiltration/Inflow is excessive if the cost for the correction of the I/I conditions are less than the cost for transportation and treatment of these flows. The cost effective analysis must consider a life cycle analysis of the rehabilitation methods. It must also be performed on 1,000 LF sections to avoid "shotgun" infiltration and groundwater migration. Rehabilitation of infiltration sources can raise in trench groundwater elevations which can cause unrehabilitated defects to leak. The 1,000 LF section was selected because groundwater migration occurs over a finite distance, and dissipates as the in trench groundwater dissipates to the soil outside the trench.

SSES Summary

With the passage of the Federal Water Pollution Control Act Amendments of 1972, a structured program of sewer systems evaluations and rehabilitation have become a prerequisite for obtaining funds for water pollution abatement projects which involve infiltration and inflow. The SSES process involves the:

identification of the extraneous flow sources;
preparation of cost effective analyses to assess the feasibility of making the needed repairs; and
subsequent rehabilitation of the sewers which contribute excess infiltration and inflow.

Based on the current levels of funding available through the SRF, there are sufficient funds available for towns seeking financial assistance to alleviate much of the extraneous flows into their sewer collection systems.

The obvious benefits for towns striving to achieve a "tight" sewer collection system include:

few or no surcharged sewers;
efficient operation of wastewater treatment facilities; and,
the hydraulic capacity of the collection system is used for wastewater and not infiltration/inflow.

Perhaps the more salient benefit of a "tight system" can be lower overall capital and operations costs. For example, savings may be encountered through smaller expansions of hydraulically sized collection system components (e.g., sewer lines, pumping stations) and (2) lower operating costs realized by a decrease in the energy requirements associated with pumping.

For more information on how CEI can assist your municipality with infiltration/inflow issues, please contact us at 1-800-725-2550.