I/I Reduction for System Management & Compliance
Sewer Capacity & Capacity Recovery
Cost of Alternatives
|Sewer Capacity Management||
Sewer Capacity & Capacity Recovery
Cost of Alternatives
The main reasons for conducting sewer rehabilitation for I/I reduction are to recover capacity in the sewer system, and reduce or eliminate SSO (Sanitary Sewer Overflows) events. When a system has SSOs then regulatory agencies begin to act progressively in the form of NOVs (Notices of Violation) and Administrative or Consent Orders. Sometimes these Orders may specifically require sewer rehabilitation for addressing SSOs. However, from an enforcement standpoint, regulatory agencies do not care how problems are corrected or how much it costs. For these agencies, it is strictly a matter of compliance. In many cases, the Orders require development and submission of a CAP-ER (Correction Action Plan/Engineering Report – or some equivalent) which lays out a plan to stop overflows. Such plans may propose new facilities, upsizing, replacement, rehabilitation, or some combination of these actions.
When the customer load on a system (or a particular basin) has grown to the point where most of the capacity is being used (perhaps 75% of the design capacity), or where a facility has reached the end of its useful life, then replacement or upsizing cannot be avoided. Otherwise, a program of well-planned sewer rehabilitation is likely the least cost, long-term solution. This statement is contingent on the level of effectiveness of I/I reduction that can be achieved by sewer rehabilitation. The early, disappointing history of sewer rehabilitation projects was discussed in Section 4 c iii of Chapter 1. However, projects in the past 20 years using a strategic system approach (with good before-after flow monitoring verification) have achieved aggregate results in the range of 49%. Table 1 summarizes the overall results from 29 projects in Nashville and Brentwood, Tennessee for three significant I/I parameters. These projects comprised a total of 97 miles of lining and rehabilitation of contiguous manholes and service laterals (to the property line or easement line). (Kurz, 2012a; Kurz et al, 2012b; Kurz, 2015)
Table 1. Summary Results of Effectiveness for 97 miles of Sewer Rehabilitation.
I/I Parameter Reduction %
24-hour RDI/I (mgd) 79.5 51
Peak-hour RDI/I (mgd) 139.5 48
Annual I/I (MG/year) 3,835 47
The RDI/I parameters (24-hour RDI/I and Peak-hour RDI/I) were standardized based on the 5-year, 24-hour rainfall event recurrence for middle Tennessee. That event has a depth of 4.5 inches. Additionally, each analysis compiles the results as a straight-line linear regression in the form of y=ax+b. Using this standardization technique allows comparison with programs that may use a different 24-hour design storm. The Peak-hour RDI/I is the best parameter for evaluating progress with SSO reduction. Annual I/I is a good parameter for evaluating O&M (Operation and Maintenance) cost savings. Practical applications will be illustrated in following sections.
Capacity and Capacity Recovery
Recovering sewer capacity by removing excess flow is intuitive. However, there are some interesting ways to analyze and depict this process. (Kurz, 1997) Figure 1 shows comparable graphs of projected peak-hour RDI/I before and after sewer rehabilitation in basin E-11 in Brentwood. There was an overall reduction of 62% for the peak-hour RDI/I flows. These projections are shown in comparison to the available remaining capacity for RDI/I (the ADDWF – Average 7-day Dry Weather Flow hydrograph has already been subtracted from the storm event results). The peak-hour RDI/I before rehabilitation was about 5.6 mgd and the intercept with the 2007 capacity line shows that the pipe was likely to surcharge for storms greater than 2 inches in 24 hours. After rehabilitation, the peak-hour RDI/I was reduced to about 2.1 mgd (a 62% reduction). Additionally, when this was compared to the remaining capacity threshold (recalculated from the 2011 ADDWF), then it is clear that the capacity has been increased by about 0.7 mgd, and that this pipe is not likely to surcharge from RDI/I from the 5-year, 4.5 inch storm. This picture is not perfect, however. In this case, the E-11 meter is close to a trunk line which is influenced by I/I and ocassionally backs up and causes the E-11 meter to record a surcharge.
ABOVE - left side bar
Figure 1. Peak-hour RDI/I Reduction Following Sewer Rehabilitation in Basin E-11.
Theoretically, federal regulations do not allow any SSOs from sanitary sewer systems under any conditions. From a practical standpoint, there is a possibility that any manhole low in a sewer system could overflow under the right rainfall or flooding conditions. It is not economical to design a gravity system that is leak-proof under any rainfall conditions. Therefore, it is useful to use the hydrologic approach of rainfall recurrence interval depths to define a “design storm”. As an example, the five-year, 24-hour rainfall event was used in Nashville for design purposes and to define overflows for corrective action. Early in the program, a list was compiled of overflow complaints that resulted in defining 137 locations for elimination. After the system was modeled, additional locations were identified for potential oveflows under the design storm conditions. Usually, these were in remote locations where direct observation was unlikely. Field inspections were conducted, and a final list of 167 SSO locations was accepted. By 2005, this list had been reduced to 27 (84% reduction) as a result of 324 miles of sewer rehabilitation (about 15% of the whole system). (Kurz, et al 2000a & 2000b; Kurz, 2012b) Many of the overflowing manhole type SSOs could not be properly monitored. However, the duration of overflows was tracked and the yearly results are shown in Figure2. The annual rainfall is also shown on the graph. While there is not a perfect relationship between annual rainfall depth and annual I/I, some of the annual fluctuation shows a rough correspondence. A good example for comparison is the maximum duration measured in 1994 when there was 60 inches of rainfall compared to two similar later years a decade later with significantly shorter durations.
ABOVE - LEFT SIDEBAR
Figure 2. Nashville Reduction of Duration of Rainfall Induced Duration of SSOs Resulting from Rehabilitation.
Cost of Alternatives
There is controversy (or at least uncertainty) today about the efficacy and economy of different approaches to providing adequate capacity and eliminating SSOs in sanitary sewer systems. Not much is publicly written, but the effects of this uncertainty are apparent in the widely varying approaches to developing corrective action plans to eliminate SSOs. These approaches may be characterized generally as:
The engineer’s decisions to select any particular approach (or mix of approaches) on this list are likely to be highly influenced by their perception of different levels of effectiveness. If the perception is that rehabilitation is only effective for confidently removing 25% to 30% of the I/I, then decisions will likely shift more towards new construction. However, if the designer is confident about removing 50%-55% of the excess flow, then rehabilitation looks more desirable. The shift in these factors may not make much difference in the economic analysis of construction costs. However, when the long-term costs are considered of allowing I/I to remain in the system rather than eliminating it, then this difference of perception takes on a greater significance. A dramatic example of that difference was shown in the Brentwood rehabilitation program where the cost of the whole program will be recouped in about 12 years from savings on the metered flow to Nashville for treatment. Additionally, a long-term economic analysis should include the O&M costs on any new facilities. In summary, system managers should consider long-term costs associated with various decisions in addition to expeditiously correcting the immediate problems of SSOs.
Kurz, G., Woodard, S. (2000a) Impacts of Overflow Reduction Resulting From Sewer Renewal, ASCE Convergence 2000, July, 2000, Kansas City, MO.
Kurz, G., Woodard, S., Ballard, G. (2000b) Nashville Cuts Wet Weather SSOs Using Sewer Rehabilitation, WEF Urban Wet Weather Conference, May 2000, Rochester, NY.
Kurz, G. (1997), Predicting I/I Reduction for Planning Sewer Rehabilitation, ASCE International Pipeline Congress, June 1997 Boston, MA
Kurz, G. (2012a) An I/I Program Case History: Nashville Removes 3.6 Billion Gallons of I/I and 137 SSOs, UCT 2012 Sewer Strategies Rehab Workshop, January 25, 2012, San Antonio, Texas.
Kurz, G., Milton, C., Colvett, K., Muirhead, D., (2012b) Sewer Rehabilitation Pays in Brentwood Tennessee, No-Dig 2012, March 11-15, 2012, Nashville, Tennessee.
Kurz, G, (2015) Unpublished Updates for Ongoing Projects.