Establishing Basin Size and Uniformity

Avoid the three major pitfalls of rainfall-dependent inflow and infiltration studies for more accurate results.

Establishing Basin Size and Uniformity

FIGURE 2: Capture coefficients in two model basins in King County.

Understanding the impact of basin size and uniformity will improve the odds of a successful flow monitoring project and help you spend the public’s money wisely.

For the purposes of this article, a basin or sewer shed is defined as the portion of a sewer system or sewer shed upstream of a flowmeter or between meters. Basin sizes can be defined by the length of public sewer, inch/diameter/miles of public sewer or acres of developed area in the sewer shed. The concept of limiting basin size and having roughly equal basins to study is born out of the idea that rainfall-dependent inflow and infiltration is not uniformly distributed within a collections system.

The 80-20 rule, or Pareto Principle, states generally that most things in nature are not uniformly distributed and the majority of occurrences appear in a minority of events. For example, 80 percent of the revenue from a grocery store comes from 20 percent of the items. Originally, the Pareto Principle referred to Vilfredo Pareto’s observation that 80 percent of Italy’s wealth belonged to only 20 percent of the population.

Our historical approach to removing RDII has tended to ignore this principle, and most of the procedures we created generally assumed that the intrusion of RDII into sewers was uniformly distributed. After all, the prime tools for locating sources of RDII were based on CCTV and smoke testing, and those tools turned up defects most everywhere.

Pinpointing problems

The most important benefit of controlling basin size is the ability to pinpoint the offending sewer segments. Figure 1 shows how the location of apparent areas of excessive RDII changes as a function of basin size. At a basin size of 31,000 linear feet, 60 percent of this 385,000-linear-foot study area appeared to have excessive RDII. At a basin size of 8,100 linear feet, only 42 percent of the study area appeared to have excessive RDII, and the location of excessive RDII (red areas) shifted with the change in basin size.

There are two levels of offenses that can be committed by the manager responsible for spending the public’s money. The first is to accomplish the task but spend more money than was necessary. This could arise from a change order that was avoidable. The second and more severe offense is to spend the money and not accomplish the task. A manager starting with the results (Figure 1) of the flow study on the left (basin size 31,000 linear feet) would ignore problem areas that are evident in a flow study on the right (basin size 8,100 linear feet). 

Much of the flowmetering data collected for RDII reduction projects is simultaneously used to set up and calibrate a hydraulic model. Hydraulic modelers will select calibration points at logical nodes and typically are not looking to create basins with uniform sizes. The result is a mix of very large and small basins.

A corollary to the basin size rule is: The larger the basin, the closer the RDII severity will be to the system average. If basins are not uniform in size, the analyst may be tricked into believing that RDII is less severe in the larger basins. The larger the metered basin, the narrower the measured performance will be.

You can demonstrate this by calculating the capture coefficient for your entire collections system. During the wet season, calculate the total volume of rain falling on the sewered area and calculate the percentage of extra flow arriving at the treatment plant. There is an 80 percent chance that your calculated value will fall between 3 and 7 percent, whereas a smaller basin size study will produce results in a range of 0.5 to 20 percent.  

Reducing cost

Utilizing smaller study basins can also help reduce cost. Most modern RDII studies start with some form of flowmetering followed by CCTV and perhaps smoke testing in the basin with the most severe RDII. Smaller meter basins will result in isolating sources of RDII in smaller portions of the system and will require less CCTV and smoke testing to develop rehabilitation plans. 

An example of the cost reduction comes from the widely published King County Regional I&I Control Program. A long-term metering project was conducted on modeling basins that averaged around 300,000 feet in size. A short-term metering program divided the modeling basins into smaller mini basins of around 21,000 feet.

The county had established the threshold for excessive RDII as a capture coefficient of 5 percent. Figure 2 displays the service area with modeling basins outlined in blue. We will look at the two modeling basins shaded in pink and blue. The upper basin (pink) had a capture coefficient of 8.9 percent (excessive), and the lower basin (blue) had a capture coefficient of 3.5 percent (nonexcessive). Had they conducted the RDII study with these basins, the upper basin would have undergone a sanitary sewer evaluation survey and the lower basin would have been ignored.

When the upper modeling basin was divided into smaller basins, a more detailed picture emerged. Approximately half of the modeling basin suffered from excessive RDII. Had the SSES activity (estimated at $2 per foot) been conducted for the model basin, the SSES cost would have been approximately $600,000, but at the smaller basin level, SSES cost would have been just over $300,000. The metering cost for the smaller basin size was $65,000 for a savings of nearly $300,000. There would certainly be as much or more savings in rehabilitation costs, but it is difficult to develop two comparative rehabilitation scenarios. 

In the lower modeling basin, division into smaller basins discovered two mini basins with excessive RDII. In this case, the manager would have walked away from sources of RDII because the model basin was nonexcessive. This phenomenon is the second problem with working with large meter basins: It is easy to miss poorly performing sections of sewer because they are lost in the close-to-average performance of a large basin. 

The third problem with large basins is the difficulty in measuring relatively small RDII reduction in basins with high flows. If the manager found these defects through conventional CCTV and smoke testing techniques and made repairs, the improvement would be statistically hard to find if the pre- and post-rehabilitation analysis was conducted on the model basin level. This phenomenon contributes to the common belief that RDII reduction doesn’t work well. 

Two-tier approach

In Saco, Maine, wastewater operators took an innovative two-tier approach to locating the sources of RDII in the Bear Brook sewer shed. 

The Bear Brook pump station has experienced severe RDII flows, and sewer overflows have occurred during heavy rains. Part of the sewer shed parallels Bear Brook, and it was assumed that most of the RDII was originating in that section of sewer. Instead of using the traditional smoke testing and CCTV in the 109,000-linear-foot sewer shed, the city elected to deploy a two-tier mini basin metering strategy. 

Tier 2 metering involved relocating and installing additional meters to create eight smaller mini basins in the three worst Tier 1 basins. Of the eight mini basins, two showed severe RDII. Further investigation revealed a source of RDII from Thornton Academy and Bear Brook itself.

By following the two-tier metering process, the city was able to better delineate areas of low and high RDII without requiring extensive follow-up SSES activity. It was found that during one particular storm of 2.13 inches, 88 percent of the RDII was generated in two of the mini basins. The two basins contained 24,106 linear feet or 20 percent of the total system size. The results showed an 88/20 ratio — 88 percent of the RDII volume originated in 20 percent of the sewer shed. 

For projects conducted in basins over 30,000 linear feet, studies show that 80 percent of RDII volume originates in 50 to 60 percent of the system. This would require further metering or SSES to more specifically locate the RDII source. In contrast, for small basins, 80 percent of the RDII volume can be isolated to less than 20 percent of the study area.

Diagnostic savings

Adherence to the Pareto Principle is more than just an interesting phenomenon. There are significant time and financial savings implications. This project identified three possible options for locating sources of RDII:

  1. The old way of doing SSES over the entire Bear Brook sewer shed
  2. Doing SSES on the worst of the five large basins
  3. Doing SSES on the worst of the mini basins

The metering cost for both phases of the work using option 3 was $25,000, and the payback over straight SSES was approximately 13:1. Savings from reduced construction will be even greater.

Shortly after the conclusion of the metering project, the stormwater connection at Thornton Academy was removed and the sewer along Bear Brook was relined (as was originally planned). 

The preliminary result is that the peak flows arriving at the Bear Brook pump station have been reduced from a 6:1 peaking factor to a 2:1 peaking factor. This improvement also helps reduce the volume of flow bypassing secondary treatment that occurs downstream at the treatment plant.

Summary

Basin size is the most important variable that a project manager can control. If basin size is sufficiently small, the distribution of RDII volume will approximately conform to the 80-20 rule. Several practitioners have attempted to develop an optimum basin and, based on their specific projects, the values have varied from 3,000 to 9,000 linear feet. It is not apparent that there is a specific basin size that should be deployed for best results, but it is clear that it is less than 10,000 linear feet.

The cost reduction in SSES work alone is usually greater than the cost to reduce basin size. Construction cost is also reduced because a smaller portion of the system is repaired based on the mini basin metering data.

The second benefit to conducting a project with small basins is that it is much easier to demonstrate that RDII has been reduced. The RDII reduction is easier to spot in a smaller basin with lower flows than trying to spot the reduction in a larger flow. All other things being equal, starting with small meter basins provides a better chance of answering the question, “What have you accomplished with the money?”



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