Rainfall-derived infiltration and inflow (RDII) into sanitary sewer systems has long been recognized as a major source of operating problems, causing poor ...
SSO CONTROL: GUIDANCE ON METHODOLOGIES AND TOOLS FOR PREDICTING RAINFALL DERIVED INFILTRATION AND INFLOW Srini Vallabhaneni1, PE, BCEE, CDM S. Wayne Miles2, PE, BCEE, CDM 1 CDM, 151 North Delaware Street, Suite 1530, Indianapolis, IN 46204 2 CDM, 5400 Glenwood Avenue, Suite 300, Raleigh, NC 27612
ABSTRACT Rainfall-derived infiltration and inflow (RDII) into sanitary sewer systems has long been recognized as a major source of operating problems, causing poor performance of many sewer systems. RDII is the main cause of sanitary sewer overflows (SSOs) to streets or nearby streams and can also cause serious operating problems at wastewater treatment facilities. There is a need to develop proven methodologies and computer tools to assist communities in developing SSO control plans that are in line with their projected annual capital budgets and provide flexibility in future improvements.
KEYWORDS Wastewater collection, infiltration and inflow (I/I), sanitary sewer overflows (SSOs)
INTRODUCTION Managing a sanitary sewer system requires considerable understanding of the conveyance capacity, the dry-weather flows experienced in the system, and the system's wet-weather response from infiltration and inflow so that decisions may be made regarding issues such as: •
Day-to-day operation and maintenance
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Capital improvements planning
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Wet-weather wastewater management
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Long-term rehabilitation needs
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Customer satisfaction
•
Regulatory compliance
Often, a good understanding of a sewer system is developed over time by experience gained from trial and error operational strategies, feedback from field operations personnel, and customer complaints. This process of gaining an understanding of the sewer system can be expedited and strengthened by implementing a complementary program of system-wide hydraulic modeling and strategic flow monitoring. This approach has been used successfully by a number of communities recently to gain a better understanding of the current and future wastewater collection needs of their system. This understanding, as well as information gained from system flow monitoring and hydraulic modeling is essential to making cost-effective decisions related to all of the key issues outlined above. An important aspect of developing a model of the sanitary sewer system is the simulation of rainfallderived infiltration and inflow (RDII), which is often a significant portion of the flows in a system during wet weather. This paper will review available RDII prediction methods and present the Synthetic Unit Hydrograph (SUH) method, a proven RDII prediction methodology used in many communities across the country. This method was selected by the United States Environmental Protection Agency (U.S. EPA)
SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
recently as the method of choice for developing a computer toolbox to support SSO analysis and planning activities.
METHODOLOGY Wastewater Flow Components In general, wastewater flows can be divided into three components: base wastewater flow (BWWF), groundwater infiltration (GWI), and RDII. The wet-weather component (i.e., RDII) is of particular importance because it is the increased portion of flow that occurs during a rainfall event. The three components of the hydrograph are described in the following sections.
Base Wastewater Flow BWWF is domestic wastewater from residential, commercial, and institutional (schools, churches, hospitals, etc.) sources, as well as industrial wastewater sources. It is affected by the population and land uses in an area and varies throughout the day in response to personal habits and business operations. BWWF from future population growth is further evaluated as part of the flow projections task.
Groundwater Infiltration GWI is defined as groundwater entering the collection system through defective pipes, pipe joints, and manhole walls. The magnitude of GWI depends on the depth of the groundwater table above the pipelines, the percentage of the system that is submerged, and the physical condition of the sewer system. The variation in groundwater levels in the study area, and hence the amount of GWI, is seasonal in nature. While GWI is also affected by rainfall, it responds gradually and is not directly related to any individual rainfall event. It is evidenced by a general increase in wastewater flow that persists for periods of many days or weeks. From a practical standpoint, it is often not possible to differentiate infiltration of groundwater (saturated zone) from infiltration due to long-term drainage of unsaturated soils, and the term GWI is used in this paper to describe both types of flow.
Rainfall-Derived Infiltration and Inflow RDII refers to stormwater that enters the sanitary sewer system in direct response to the intensity and duration of rainfall events. RDII can be further broken down into stormwater inflow (SWI) and rainfalldependent infiltration (RDI), based upon the pathways through which the flow enters the sewers or manholes. SWI reaches the collection system by direct connections rather than by first percolating through the soil. SWI sources may include roof downspouts illegally connected to the sanitary sewers, yard and area drains, holes in manhole covers, cross-connections with storm drains, or catch basins. RDI includes all other rainfall-dependent flow that enters the collection system, including stormwater that enters defective pipes, pipe joints, and manhole walls after percolating through the soil.
Data Analysis Decomposition of Flow Monitoring Data Hydrograph decomposition is a method of estimating the different components of flow and was used to analyze flow monitoring data to estimate the quantities of BWWF, GWI, and RDII flow. Analysis procedures, which CDM developed, are used to assist in separating measured wastewater flows into base flow (including GWI) and RDII components. Average base flow hydrographs for a typical weekday and weekend days were developed from the recorded data for dry-weather conditions. To determine the RDII component for each storm event where more than 0.5 inches of rainfall was recorded, the typical base flow hydrographs are then subtracted from a wet-weather hydrograph. This method of hydrograph decomposition is an important step in analyzing and simulating wet-weather flows in the sewer system. An example hydrograph decomposition for a typical flow monitor is shown in Figure 1. The average weekday dry-weather flow (BWWF + GWI) for this meter is 1.75 mgd. For the recorded storm event, the peak total flow rate during the event is 3.45 mgd. The difference between the drySSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
weather hydrograph and the total wet-weather hydrograph gives the volume of rainfall that entered the collection system during the event.
Figure 1. Hydrograph decomposition helps to identify individual wastewater flow components Once the hydrograph decomposition is completed for each monitor, the volume of RDII is compared to the volume of rainfall that fell on the area contributing flow to each monitor. The ratio of RDII volume to rainfall volume (which is the inches of rain over the subbasin area) is defined as the R value. In other words, the R value is the fraction of rainfall from a storm event that enters the sewer system as RDII. The higher the R value, the more I/I is conveyed by the sewer system. A schematic of the hydrograph decomposition process is provided in Figure 2 on the next page.
Comparison of R Values for Municipalities in U.S. EPA Region 4 The R values calculation results for a number of municipalities in U.S. EPA Region 4 are provided in Figure 3. This figure shows the minimum, maximum, and average R values for twelve municipalities. The average R value for the municipalities was 3.4 percent. The average maximum R value for other municipalities was 22 percent.
SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
Figure 2. Schematic of Wastewater Decomposition and RDII Analysis Approach
Figure 3. Range of R Values for Utilities in U.S. EPA Region 4 SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
Unit Hydrograph Methodology The unit hydrograph methodology is used to determine a characteristic relationship between rainfall and RDII. This approach was developed by CDM and has been used on sewer master planning projects throughout the country since the early 1980s (Giguere and Riek, 1983). It also described in the U.S. EPA document, Computer Tools for Sanitary Sewer System Capacity Analysis and Planning (Vallabhaneni et al. 2007). A unit hydrograph is defined as the flow response that results from one unit of rainfall during one unit of time. Typically, a time unit was defined as one 5-minute time increment. The unit hydrograph procedure includes development of up to three unit hydrographs for each rainfall time step to estimate the RDII flow response. The responses from each unit hydrograph are summed to compute the total response to the observed rainfall. The triangular unit hydrograph procedure is shown in Figures 4, 5, and 6. Each of the three unit hydrographs is described by three calibration parameters: R, T, and K. 1. The R parameter is the fraction of the total rainfall volume that enters the sewer system as RDII. The sum of the three unit hydrographs (R1, R2, and R3) represents the total R-value for the storm event. For example, if the R-value equals 0.05, then each inch of rainfall that falls over a 100-acre sewershed produces an RDII response of 5 inch-acres or 0.14 million gallons. 2. The T parameter represents the time to the peak unit hydrograph flow. 3. The K parameter represents the declining portion of the triangular unit hydrograph. K is the ratio of the time of recession (time required for flow to decline from the peak flow to zero) to the time of peak flow, T. For example, if T equals 2 hours and K equals 3, then the time to decline is 6 hours, and the total hydrograph duration is 8 hours. Three unit hydrographs are generally used because the shape of an RDII hydrograph is often too complex to be well represented by a single unit hydrograph. The first unit hydrograph represents the most rapidly responding flow component (including stormwater inflow) and will typically have small T and K values. The third unit hydrograph will have the largest T and K values and can be thought of as representing the delayed infiltration response. The second unit hydrograph represents the intermediate infiltration and inflow response. The U.S. EPA recently released the Sanitary Sewer Overflow Analysis and Planning (SSOAP) Toolbox that incorporated this unit hydrograph methodology for RDII prediction.
Figure 4. Definition of the Unit Hydrograph Parameters SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
Figure 5. Summation of Three Unit Hydrograph is Used to Represent the RDII Response
Figure 6. Individual Unit Hydrographs Responses are Summed for Each Unit of Rainfall to Produce a Simulated Total RDII Response.
SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
CONCLUSIONS 1. The hydrograph decomposition approach presented improves the understanding of individual wastewater flow components, thus leading to a better understanding of base flows, groundwater infiltration rates, and rainfall-derived infiltration and inflow (RDII) responses. 2. The calculation of R values is beneficial in quantifying RDII responses in sanitary sewer systems. The results of this calculation can support setting priorities for sewer system evaluation surveys (SSESs) for sewer system condition assessment and can support the development of sanitary sewer system models. 3. Application of the RDII response method presented herein can be used to produce simulated RDII responses for design storm conditions and thus can improve the use of flow monitoring data and calibrated models for simulating a range of conditions. 4. These analyses are an important component in the development of a sanitary sewer system master plan in order to better understand improvements needed to meet system performance objectives.
REFERENCES Camp Dresser & McKee Inc. (1989). East Bay I/I Correction Program Certification Procedures Manual. East Bay Municipal Utility District (EMNUD), Oakland, California. October, 1989. Giguere, P.R. and G.C. Riek, (1983). Infiltration/Inflow Modeling for the East Bay (Oakland-Berkeley Area) I/I Study. In: Proceedings of the 1983 International Symposium on Urban Hydrology, Hydraulics and Sediment Control. University of Kentucky, Lexington, KY. July 25-28, 1983. Miles, S.W, J.L Dorn, and R.E. Tarker Jr. (1996). An I/I Analysis and Prediction Method to Help Guide Separate Sanitary Sewer Improvement Programs. In: Proceedings of the Water Environment Federation (WEF) Conference on Urban Wet-Weather Pollution: Controlling Sewer Overflows and Stormwater Runoff. Quebec City, Quebec. June 1996. Samson, J, G.M. Ju, G.C. Riek, W.H. Sukenik (1998) Analyses of Sanitary Sewer Flow Data and the Demonstration of I/I Reduction. In: Proceedings of the Water Environment Federation 71st Annual Conference and Exposition, Orlando, FL. October 1998. Vallabhaneni S, Koran, J, Moisio S, Moore, C. (2002) SSO Evaluations: Infiltration and Inflow using SWMM RUNOFF and EXTRAN” Proceedings of the International Conference on Stormwater and Urban Water Systems Modeling, Toronto, Canada. Vallabhaneni Si, Chan C., and Burgess E. (2007). Computer Tools for Sanitary Sewer System Capacity Analysis and Planning. United State Environmental Protection Agency – Report Number EPA/600/R07/111.
SSO Control: Guidance on Methodologies and Tools for Predicting Rainfall Derived Infiltration and Inflow
