How Big a Problem is I/I ?
Municipal and federal agencies in Canada and the USA have long recognized the problems associated with I/I (inflow and infiltration) in separate sewage collection systems. I/I results from clear water (from rainfall or groundwater) entering a sewer system. This problem has been studied in detail in some municipalities – mostly in systems with overflows (SSOs – Sanitary Sewer Overflows) where cities have received an Administrative Order or Consent Order. However, little information is available (only as indirect estimates) on the extent of I/I as a national problem in the USA or Canada. Such information should be useful to managers and regulators for formulating national control strategies, and to municipalities for benchmarking the performance of their systems.
A current study to determine the quantity, scope, and characteristics of I/I for the entire state of Tennessee (USA) may significantly add to the understanding of this problem. This study is evaluating daily influent flow, influent organic load, and rainfall for one year for each of the 227 municipal wastewater systems in Tennessee. Early results and a detailed description of the methodology were presented at the 2014 WEF Collection Systems Specialty Conference. 126 systems (more than half) in the state have been completed. The results show that those systems are treating ~44% of clear water on an annual basis. I/I represented more than half the annual flow in two-thirds of those systems. Projecting this leakage rate to the total sewage flow in Tennessee results in an annual I/I estimate of 104,720 million gallons (396 million m3) for the state.
This study used data routinely recorded by plant operators in MORs (Monthly Operations Reports) required by Tennessee and other states. Those reports form the basis of the DMRs (Discharge Monitoring Reports – also known as NPDES reports) required to be submitted to the US EPA under each treatment plant’s operating permit. An earlier study (Kurz & Qualls, 2001) used the data in DMRs to estimate I/I in EPA Region 4. However, that approach was limited because the flows and organic levels (usually measured as BOD – Biochemical Oxygen Demand) were only reported as monthly averages which masked shorter periods of low flow needed to estimate base flow from users. The advantage of using DMRs in the 2001 study was that the data was available electronically in EPA’s PCS (Permit Compliance System) database. The current study is much more labor intensive since it required transcription of ~74,000 data values from paper records into Excel spreadsheets.
In addition to annual I/I, the effects of RDI/I (rainfall derived I/I) and long-term dry weather infiltration are also estimated for each system. RDI/I is estimated by linear regression of I/I quantity compared to rainfall depth for discrete rainfall events. Linear regression is a highly reliable method for this purpose (Kurz & Hamilton, 2010 and Kurz et al, 2003). In this case, the regression process was modified slightly and incorporates a simple data transformation procedure (Kurz et al, 2009a & 2009b) that greatly improves the level of confidence (shown as a 95% confidence interval) for the projected values. The results showed that 89% of the plants may exceed the nominal capacity listed in their permit when the RDI/I flow increases for a 2-year storm (about 3.4 inches or 8.6 cm in 24 hours for middle Tennessee). While that figure is greater than expected, it is not necessarily cause for alarm since most plants have additional capacity to absorb peak flows. However, that additional hydraulic capacity threshold was not recorded in the permit summary (as shown on the state website). Therefore, it was not possible to determine which plants may be nearing a critical hydraulic overload condition.
Recent guidance documents from EPA (2014a & 2014b) and The Federation of Canadian Municipalities (2003) describe an approach for estimating I/I which relies on hourly flow monitoring. However, most treatment plants in Tennessee record flows from chart recorders or totalizers on a daily basis. The point of the present study was to demonstrate a simple approach that uses data already routinely collected daily by plant operators that did not require installation of additional equipment or costly engineering studies. Additionally, analysis of flow alone – without considering dilution of BOD (or CBOD – Carbonaceous BOD), may not be sufficient to detect dry weather infiltration.
Average BOD concentrations in the influent to plants in Tennessee varied from 32 to 893 mg/l (minimum concentrations were much lower). The instances of high concentrations of BOD were usually associated with known industrial discharges. Other investigators (Bounds, 1997 and Carcich et al, 1972) found that undiluted residential BOD concentrations averaged around 350 mg/l. This figure was also used by EPA (1977 & 2002) as an average concentration for undiluted residential BOD in its guidance for design of pressure systems and small alternative treatment systems. When the Tennessee influent BOD levels were analyzed to estimate the amount of I/I dilution, about a third of the total I/I was attributable to dry weather infiltration, which was not detected by flow analysis alone. In a few cases, infiltration may be overestimated due to significant, dilute industrial flows or flows from combined sewer systems.
Based on the current results at the halfway point of the study, I/I represents an annual cost of $188 million in Tennessee. This cost is based on assuming a uniform O&M cost of $1.80 per thousand gallons (or $1.80 per 3.79 m3). (This figure is conservative compared to EPA’s recommendation in 2014 for using an O&M rate of $2 to $5 per thousand gallons.)
The good news is that this situation can be reversed and the savings can offset the costs for correction. 27 project areas in the first Nashville OAP (Overflow Abatement Program 1989-2005) and four project areas in Brentwood (representing about 120 miles or 193 km of pipe lining for the 31 project areas) were rehabilitated and extensively analyzed to determine the amount of I/I reduction and the associated program costs (Kurz et al, 1997, 2004, 2012). The aggregate reduction was about 50% for annual I/I, 24-hour RDI/I and peak-hour RDI/I. Service lateral and manhole rehabilitation was required in each project of lining the public sewers to minimize groundwater migration. Based on that experience, the cost to achieve 50% reduction across the state was estimated to be $1,150 million (US) for installation (pipe, lateral & manhole rehabilitation), long-term before-after flow monitoring, CCTV inspection, and engineering. When compared to the potential O&M savings, then this expense could be paid off in ~12 years (interest rates not considered).
This approach of using treatment plant flow, rainfall and influent characteristics from MORs is not intended to replace comprehensive monitoring and engineering studies to evaluate I/I and RDI/I. Especially in larger systems, it is critical to divide systems into tributary basins for monitoring and prioritization (Kurz, et al, 2005 and Stevens, 1993). Instead, the intention is to analyze all the collection systems in the state using a uniform procedure to provide information for program managers and decision makers. Additionally, the Excel analytical spreadsheets have been adapted for use as an “I/I calculator” by individual operators of public systems. Small communities are often reluctant to spend any money for additional equipment or engineering studies. This tool may provide an opportunity for operators to help themselves to determine the presence and extent of I/I in their systems.
REFERENCES
Bounds, T.R., (1997) Design and Performance of Septic Tanks. Site Characterization and Design of Onsite Septic Systems ASTM STP 901, M.S. Bedinger, A.I. Johnson, and J.S. Fleming, Eds., American Society for Testing Materials, Philadelphia.
Carcich, I., et al, (1972) Pressure Sewer Demonstration Project. Journal Water Pollution Control Federation, Washington, D.C.
Federation of Canadian Municipalities and National Research Council (2003) Infiltration/Inflow Control/Reduction for Wastewater Collection Systems. Ottawa, Ontario.
Kurz, G. (1997) Predicting I/I Reduction for Planning Sewer Rehabilitation. ASCE International Pipeline Congress: Trenchless Pipeline Projects Practical Applications, Boston, Massachusetts.
Kurz, G., Qualls, S. (2001) Estimating the Extent of I/I in EPA Region 4 Using NPDES Monitoring Reports, Proceedings of WEFTEC-2001, Atlanta, Georgia.
Kurz, G., Ballard, G., Burgett, M. and Smith, J. (2003) A Proposal for Industry-Wide Standardization of I/I Calculations. Proceedings of WEFTEC-2003, Los Angeles, California.
Kurz, G., Ballard, G., and Stonecipher, P. (2004) Nashville’s Program Removes 3.2 Billion Gallons of I/I. No-Dig-04, New Orleans.
Kurz, G., Stonecipher, P., Freeman, B., Ballard, G. (2005) Sewer Renewal – A Strategic Plan as Part of EPA’s CMOM Program. Underground Construction Technology Conference, Houston, Texas.
Kurz, G.,and Ward, B., Ballard, G. (2009a) Simple Method for Estimating I/I Using Treatment Plant Flow Monitoring Reports – A Self Help Tool for Operators. Proceedings of WEF Collection Systems Specialty Conference, Louisville, Kentucky.
Kurz, G.,and Ward, B., Ballard, G. (2009b) Rainy Day Calculations. Water Environment and Technology, September 2009, pp 97-103.
Kurz, G. and Hamilton, W. (2010) Confidence Levels for Estimating RDI/I. WEFTEC 2010, New Orleans, Louisiana.
Kurz, G., Milton, C., Colvett, K., Muirhead, D. (2012) Sewer Rehab Pays in Brentwood Tennessee. No-Dig 2012, Nashville, TN.
Kurz, G. (2014) The Scope of I/I in Tennessee – Quantifying the Problem Using Public Records. WEF-Collections Systems Specialty Conference, Baltimore, Maryland.
Stevens, P., (1993) Basin Size is the Most Effective Variable an I/I Engineer Can Control. Water Environment Federation 66th Conference.
US EPA (1977) Alternatives for Small Wastewater Treatment Systems. EPA 625/477-011B, Washington, D.C.
US EPA (2002) Wastewater Technology Fact Sheet – Sewers, Pressure. EPA 832-F-02-006, Washington, D.C.
US EPA (2014a) Guide for Estimating Infiltration and Inflow. US EPA Region 1 New England Water Infrastructure Outreach.
US EPA (2014b) Quick Guide for Estimating Infiltration and Inflow – For Region 1 NPDES Annual Reporting. US EPA Region 1 New England Water Infrastructure Outreach.
- New Study Measures 44% in Tennessee
Municipal and federal agencies in Canada and the USA have long recognized the problems associated with I/I (inflow and infiltration) in separate sewage collection systems. I/I results from clear water (from rainfall or groundwater) entering a sewer system. This problem has been studied in detail in some municipalities – mostly in systems with overflows (SSOs – Sanitary Sewer Overflows) where cities have received an Administrative Order or Consent Order. However, little information is available (only as indirect estimates) on the extent of I/I as a national problem in the USA or Canada. Such information should be useful to managers and regulators for formulating national control strategies, and to municipalities for benchmarking the performance of their systems.
A current study to determine the quantity, scope, and characteristics of I/I for the entire state of Tennessee (USA) may significantly add to the understanding of this problem. This study is evaluating daily influent flow, influent organic load, and rainfall for one year for each of the 227 municipal wastewater systems in Tennessee. Early results and a detailed description of the methodology were presented at the 2014 WEF Collection Systems Specialty Conference. 126 systems (more than half) in the state have been completed. The results show that those systems are treating ~44% of clear water on an annual basis. I/I represented more than half the annual flow in two-thirds of those systems. Projecting this leakage rate to the total sewage flow in Tennessee results in an annual I/I estimate of 104,720 million gallons (396 million m3) for the state.
This study used data routinely recorded by plant operators in MORs (Monthly Operations Reports) required by Tennessee and other states. Those reports form the basis of the DMRs (Discharge Monitoring Reports – also known as NPDES reports) required to be submitted to the US EPA under each treatment plant’s operating permit. An earlier study (Kurz & Qualls, 2001) used the data in DMRs to estimate I/I in EPA Region 4. However, that approach was limited because the flows and organic levels (usually measured as BOD – Biochemical Oxygen Demand) were only reported as monthly averages which masked shorter periods of low flow needed to estimate base flow from users. The advantage of using DMRs in the 2001 study was that the data was available electronically in EPA’s PCS (Permit Compliance System) database. The current study is much more labor intensive since it required transcription of ~74,000 data values from paper records into Excel spreadsheets.
In addition to annual I/I, the effects of RDI/I (rainfall derived I/I) and long-term dry weather infiltration are also estimated for each system. RDI/I is estimated by linear regression of I/I quantity compared to rainfall depth for discrete rainfall events. Linear regression is a highly reliable method for this purpose (Kurz & Hamilton, 2010 and Kurz et al, 2003). In this case, the regression process was modified slightly and incorporates a simple data transformation procedure (Kurz et al, 2009a & 2009b) that greatly improves the level of confidence (shown as a 95% confidence interval) for the projected values. The results showed that 89% of the plants may exceed the nominal capacity listed in their permit when the RDI/I flow increases for a 2-year storm (about 3.4 inches or 8.6 cm in 24 hours for middle Tennessee). While that figure is greater than expected, it is not necessarily cause for alarm since most plants have additional capacity to absorb peak flows. However, that additional hydraulic capacity threshold was not recorded in the permit summary (as shown on the state website). Therefore, it was not possible to determine which plants may be nearing a critical hydraulic overload condition.
Recent guidance documents from EPA (2014a & 2014b) and The Federation of Canadian Municipalities (2003) describe an approach for estimating I/I which relies on hourly flow monitoring. However, most treatment plants in Tennessee record flows from chart recorders or totalizers on a daily basis. The point of the present study was to demonstrate a simple approach that uses data already routinely collected daily by plant operators that did not require installation of additional equipment or costly engineering studies. Additionally, analysis of flow alone – without considering dilution of BOD (or CBOD – Carbonaceous BOD), may not be sufficient to detect dry weather infiltration.
Average BOD concentrations in the influent to plants in Tennessee varied from 32 to 893 mg/l (minimum concentrations were much lower). The instances of high concentrations of BOD were usually associated with known industrial discharges. Other investigators (Bounds, 1997 and Carcich et al, 1972) found that undiluted residential BOD concentrations averaged around 350 mg/l. This figure was also used by EPA (1977 & 2002) as an average concentration for undiluted residential BOD in its guidance for design of pressure systems and small alternative treatment systems. When the Tennessee influent BOD levels were analyzed to estimate the amount of I/I dilution, about a third of the total I/I was attributable to dry weather infiltration, which was not detected by flow analysis alone. In a few cases, infiltration may be overestimated due to significant, dilute industrial flows or flows from combined sewer systems.
Based on the current results at the halfway point of the study, I/I represents an annual cost of $188 million in Tennessee. This cost is based on assuming a uniform O&M cost of $1.80 per thousand gallons (or $1.80 per 3.79 m3). (This figure is conservative compared to EPA’s recommendation in 2014 for using an O&M rate of $2 to $5 per thousand gallons.)
The good news is that this situation can be reversed and the savings can offset the costs for correction. 27 project areas in the first Nashville OAP (Overflow Abatement Program 1989-2005) and four project areas in Brentwood (representing about 120 miles or 193 km of pipe lining for the 31 project areas) were rehabilitated and extensively analyzed to determine the amount of I/I reduction and the associated program costs (Kurz et al, 1997, 2004, 2012). The aggregate reduction was about 50% for annual I/I, 24-hour RDI/I and peak-hour RDI/I. Service lateral and manhole rehabilitation was required in each project of lining the public sewers to minimize groundwater migration. Based on that experience, the cost to achieve 50% reduction across the state was estimated to be $1,150 million (US) for installation (pipe, lateral & manhole rehabilitation), long-term before-after flow monitoring, CCTV inspection, and engineering. When compared to the potential O&M savings, then this expense could be paid off in ~12 years (interest rates not considered).
This approach of using treatment plant flow, rainfall and influent characteristics from MORs is not intended to replace comprehensive monitoring and engineering studies to evaluate I/I and RDI/I. Especially in larger systems, it is critical to divide systems into tributary basins for monitoring and prioritization (Kurz, et al, 2005 and Stevens, 1993). Instead, the intention is to analyze all the collection systems in the state using a uniform procedure to provide information for program managers and decision makers. Additionally, the Excel analytical spreadsheets have been adapted for use as an “I/I calculator” by individual operators of public systems. Small communities are often reluctant to spend any money for additional equipment or engineering studies. This tool may provide an opportunity for operators to help themselves to determine the presence and extent of I/I in their systems.
REFERENCES
Bounds, T.R., (1997) Design and Performance of Septic Tanks. Site Characterization and Design of Onsite Septic Systems ASTM STP 901, M.S. Bedinger, A.I. Johnson, and J.S. Fleming, Eds., American Society for Testing Materials, Philadelphia.
Carcich, I., et al, (1972) Pressure Sewer Demonstration Project. Journal Water Pollution Control Federation, Washington, D.C.
Federation of Canadian Municipalities and National Research Council (2003) Infiltration/Inflow Control/Reduction for Wastewater Collection Systems. Ottawa, Ontario.
Kurz, G. (1997) Predicting I/I Reduction for Planning Sewer Rehabilitation. ASCE International Pipeline Congress: Trenchless Pipeline Projects Practical Applications, Boston, Massachusetts.
Kurz, G., Qualls, S. (2001) Estimating the Extent of I/I in EPA Region 4 Using NPDES Monitoring Reports, Proceedings of WEFTEC-2001, Atlanta, Georgia.
Kurz, G., Ballard, G., Burgett, M. and Smith, J. (2003) A Proposal for Industry-Wide Standardization of I/I Calculations. Proceedings of WEFTEC-2003, Los Angeles, California.
Kurz, G., Ballard, G., and Stonecipher, P. (2004) Nashville’s Program Removes 3.2 Billion Gallons of I/I. No-Dig-04, New Orleans.
Kurz, G., Stonecipher, P., Freeman, B., Ballard, G. (2005) Sewer Renewal – A Strategic Plan as Part of EPA’s CMOM Program. Underground Construction Technology Conference, Houston, Texas.
Kurz, G.,and Ward, B., Ballard, G. (2009a) Simple Method for Estimating I/I Using Treatment Plant Flow Monitoring Reports – A Self Help Tool for Operators. Proceedings of WEF Collection Systems Specialty Conference, Louisville, Kentucky.
Kurz, G.,and Ward, B., Ballard, G. (2009b) Rainy Day Calculations. Water Environment and Technology, September 2009, pp 97-103.
Kurz, G. and Hamilton, W. (2010) Confidence Levels for Estimating RDI/I. WEFTEC 2010, New Orleans, Louisiana.
Kurz, G., Milton, C., Colvett, K., Muirhead, D. (2012) Sewer Rehab Pays in Brentwood Tennessee. No-Dig 2012, Nashville, TN.
Kurz, G. (2014) The Scope of I/I in Tennessee – Quantifying the Problem Using Public Records. WEF-Collections Systems Specialty Conference, Baltimore, Maryland.
Stevens, P., (1993) Basin Size is the Most Effective Variable an I/I Engineer Can Control. Water Environment Federation 66th Conference.
US EPA (1977) Alternatives for Small Wastewater Treatment Systems. EPA 625/477-011B, Washington, D.C.
US EPA (2002) Wastewater Technology Fact Sheet – Sewers, Pressure. EPA 832-F-02-006, Washington, D.C.
US EPA (2014a) Guide for Estimating Infiltration and Inflow. US EPA Region 1 New England Water Infrastructure Outreach.
US EPA (2014b) Quick Guide for Estimating Infiltration and Inflow – For Region 1 NPDES Annual Reporting. US EPA Region 1 New England Water Infrastructure Outreach.