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Puerto Rico Aqueduct and Sewer Authority (PRASA) Determine Excessive I/I Performance Metrics for Consent Decree Compliance Using Creative Benchmarking Application

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In April 2006 PRASA was issued a Consent Decree (CD) by EPA which required that sanitary sewer system evaluation surveys (SSES) and repairs be performed in the wastewater service areas experiencing excessive infiltration/inflow (I/I). The CD specified that the determination of excessive I/I be done according to service basin size and divided the approach for PRASA's island-wide system into two groupings. The first grouping was a list of seven high-priority areas, such as Bayamón (the largest of the seven). A comprehensive I/I analysis would be conducted in each of these areas. The second grouping was the 52 remaining island service basins. For these the I/I analysis would initially be conducted based on the treatment plant flows and then progress into those service basins found to exhibit excessive I/I.

The CD also provided a requirement that PRASA evaluate and select an appropriate performance indicator and target for delineating excessive I/I. Selection of a performance measure and target was a key decision for PRASA because the subsequent SSES and rehabilitation activities could create significant funding strain for PRASA. Equally of concern was PRASA's recognition that excessive I/I was a problematic issue that needed to be systematically and expeditiously addressed.

The Bayamón service area was the first service area that PRASA evaluated. The Bayamón service area is located west of San Juan; covers over 50 square miles; serves a population of about 315,000; and includes approximately 500 miles of public sewer that generate an average daily flow of about 40 mgd.

One of the initial preparatory activities conducted for the Bayamón work was to do a literature review of how the wastewater industry and regulatory agencies defined excessive I/I. PRASA found varying definitions and several suggested benchmarking values. They also found that the literature stated excessiveness should be determined based on a cost effective analysis when possible. This presented a challenge to PRASA since a hydraulic model would be the appropriate tool to support a locally based analysis and they did not have a model. PRASA decided to build on a technique used by CH2M HILL and approved by EPA for an excessive I/I study in 1999 in PRASA's Carolina service area. Excessive I/I in the Carolina service area was determined by benchmarking against other wastewater service areas across the United States that were previously found to be excessive using hydraulic modeling and comparing the cost and savings of transporting and treating I/I. Rainfall derived infiltration and inflow (RDII) was normalized to service area. The paper will describe PRASA's evaluation of multiple performance indicators and decision to use RDII per inch of rain per linear foot of sewer as their performance indicator. The paper will also show how PRASA developed an excessive I/I benchmarking trend line based on a regression analysis of the benchmarking data.

PRASA installed 65 flow monitors and 10 rain gauges throughout the Bayamón service area in the summer of 2006 and for 60 days the flow and rainfall data where collected. Using the excessive I/I benchmarking trend line, 18 Bayamón sub-basin service areas were determined to be excessive. A phased SSES and rehabilitation program is currently in progress for the Bayamón basin. The SSES and rehabilitation work in Bayamón is estimated to cost 21 million over the next 10 to 15 years. PRASA has also started their excessive I/I analysis in the other six Group 1 service areas.

A second requirement of the CD was for PRASA to develop and implement a Preliminary Sewer System Evaluation Plan (PSSSEP) for the second group of 52 basin service areas and treatment facilities operated by PRASA. The island of Puerto Rico contains a population of approximately 4 million, most of which is concentrated in the metropolitan service areas. The majority (71%) of the population live in urban areas versus 29% for rural areas.

The concept for evaluating excessive I/I for the Group 2 basins was to use available information. Then the basins were separated into two categories on the basis of multi-criteria using both wet and dry-weather flow metrics. The criteria basically revealed:

Those that leaked excessively during wet weather and needed to be included in the next phases of work, and

Those that were relatively rainproof, free of chronic operational problems, and could be excluded from the PSSSEP.

Daily treatment plant flow data and daily rainfall data over a 44-month period was evaluated. The rain gauges were from 129 NOAA and 59 USGS rain gauge sites. Rainfall quantity and duration criteria were applied to an average 130 days of rain in excess of 0.5-inches per basin. This resulted in over 6,700 storms which were evaluated for RDII. Dry weather periods were also delineated for the non-wet periods. The paper will describe the data analysis techniques that were used to process the massive amount of data and maintain quality control of the data. It also discusses how PRASA used the benchmark trend-line to determine the performance target foreach of the 52 basins and whether the basin was deemed excessive.

The Group 2 basin analysis findings showed that 19 of the 52 basins exhibited excessive RDII. SSES and Rehabilitation costs are estimated to cost $1.6 billion over the next 10 to 15 years. PRASA submitted their findings to EPA.

This paper will be beneficial to utilities and their engineers who perform I/I analyses because it presents the results of the excessive I/I literature review; an illustration and discussion of the importance of performance indicator selection; and several data management and analysis tools for processing large amounts of data from multiple sources.

Keywords: Performance metrics; RDII; SSES; benchmarking; cost-effectiveness; flow monitoring

Document Type: Research Article


Publication date: January 1, 2009

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