Comprehensive Rehabilitation: Measuring the Effectiveness of Public vs. Private I/I Removal
The City of Revere, Massachusetts entered into a consent decree (CD) with the U.S. Department of Justice that required the removal of infiltration and inflow (I/I) to eliminate sanitary sewer overflows (SSOs). In response to the CD, the City immediately began a series of sewer system evaluations to determine the condition of the collection system.
In addition, a System Capacity Assessment was conducted by installing 15 temporary flow meters, one rain gauge, and three groundwater gauges, as well as utilizing a permanent MWRA flow meter immediately downstream of Revere’s service area, to monitor sewer flows in the collection system. This phase of the project characterized the sewer system operations during dry weather, wet weather and tidal conditions as a baseline.
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The data collected during the flow metering program was used to develop a SWMM model of the sewer system. A range of design storms were input into the SWMM model to simulate the effects that these various storm events would have on the wastewater collection system; one-year, five-year, 10-year, 25-year, 50-year, and 100-year design storms were simulated with varying results based on the size of the storm simulated. This step was crucial in relating specific areas of flooding (SSOs) with sections of the collection system identified as having capacity issues. This also formulated a baseline for the evaluation of alternatives which would ultimately reduce overflows in the system.
The SWMM model indicated that a very large portion of the sewer collection system would need to be rehabilitated to remove sufficient extraneous flow to mitigate SSOs and to meet requirements outlined in the CD. In fact, the model showed that in order to reach a 10-year level of protection against SSOs, approximately 40 to 50 percent of the extraneous flow would need to be removed citywide.
Initially, the City focused on rehabilitation and repair of public sewers to address the I/I issue. Pre-construction flow meters were installed in the spring 2014 to quantify sewer flows prior to rehabilitation efforts. The $6.25 million Phase I Rehabilitation Program, initiated in Flow Meter areas FM3 and FM12, consisted of the following:
- Lining of approximately 57,400 ft of mainline sewer pipe;
- Spot replacement of collapsed sewer pipe in 75 locations;
- Service lateral connection lining of approximately 1,445 service laterals; and
- Lining and rehabilitation of approximately 340 manholes.
Figure 1. Infiltration Removal in FM3-North from Comprehensive Rehabilitation of the Public System
Following the completion of construction work, a post-construction flow metering program was conducted in spring 2016 utilizing the same temporary flow meters in FM3 and FM12 that had been used prior to construction. Using this flow meter data, together with pre-construction metering results and data from the permanent flow meters, allowed the project team to determine the extraneous flow removed because of rehabilitation efforts on the public sewer system. As can be seen in Figure 1, the average dry weather flow in FM3- North during the nighttime hours, when approximately 90 percent of the flow is from infiltration, was reduced from 0.36 mgd in spring 2014 prior to rehabilitation, to 0.28 mgd in spring 2016 after rehabilitation. This equates to a 22 percent reduction of base infiltration because of rehabilitation of the public system only. As such, the goal of 50 percent I/I removal could not be achieved by simply rehabilitating the public sewer system. As result, the City decided to embark on a comprehensive rehabilitation approach that would address both public and private I/I sources.
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Based on local knowledge of the system, it was stipulated that sump pumps were likely the most common private inflow source in the City along with roof leaders and driveway drains. The first step in the identification and removal of private inflow sources was the initiation of a Sump Pump Amnesty Program (SPAP) to encourage participation. In addition, the City amended local ordinances that provided for enforcement of illegal discharges of unpolluted water into the City sewer system. The amendments required building owners to redirect any illicit sources from the sewer system. These sources included, but were not limited to, sump pumps, roof drains (gutter downspouts), yard drains, driveway drains and patio drains.
Following a multi-year private inflow removal program, an analysis was conducted by comparing similar rain events occurring before and after private inflow sources were removed in FM3. A storm event that occurred on Sept. 30, 2015, lasting 11.5 hours (from 3 a.m. to 2:30 p.m.), and producing 1.74 in. of rain was used for estimating the I/I prior to removal of private sources. A storm event that occurred on Oct. 9, 2016, lasting 14.5 hours (from 6 a.m. to 8:30 p.m.) and producing 1.70 in. of rain was used for estimating the I/I after removal of private sources. The peak flow observed as a result of the rainfall in 2015 was 3.73 mgd, while the peak flow due to rainfall in 2016, following the removal of 258 private inflow sources, was 2.13 mgd. This equates to a 43 percent reduction in inflow as a result of the private inflow removal program.
The results of this project indicated that the implementation of a comprehensive rehabilitation program of the public sewer collection can remove as much as 22 percent of infiltration. Furthermore, an aggressive and diligent private inflow identification and removal program, if successful, can remove as much as 43 percent of the private inflow within a particular flow meter area. The combination of these two programs focusing on public and private inflow sources could remove up to 65 percent of the extraneous flow in a sewer collection system.
In conclusion, the City was able to achieve the goal of 50 percent I/I reduction by addressing both the public and private sources of I/I. The public sources were primarily removed by lining all mainline sewers; lining all service lateral connections from the main to the first joint; lining all manholes rehabilitation of sewer lines by CIPP lining, lining of manholes and grouting of service lateral connections. The private sources were primarily removed through the Sump Pump Amnesty Program.
Nicholas J. Rystrom, P.E., is with the City of Revere, Massachusetts. Jonathan E. Kunay, P.E., is with CDM Smith, Boston. Ahmad Habibian, Ph.D, P.E., is with CDM Smith, Fairfax, Virginia.
