Quantifying Sub-Surface Discharges from Individual Sewer Defects

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ABSTRACT
The California Regional Water Quality Control Board, Santa Ana Region (RWQCB) required wastewater collection agencies in Orange County to report discharges from “pipe breaks, leaking sewer pipes and joints, and other subsurface discharges of wastewater.” The Orange County Sanitation District (OCSD), in conjunction with a research team from University of California, Irvine (UCI), developed a device to quantify the subsurface discharge of individual sewer defects. The instrument developed to measure discharge rates of individual sewer defects has been named the “Exfiltrometer.” Soil adjacent to each sewer defect tested for exfiltration was tested for the presence of wastewater. Indicators of wastewater used in this study include: E. coli and total coliform bacteria, caffeine, and major anions. A variety of sewer defect types were tested. Some of the same defect types exhibited similar exfiltration rates. Others did not. Ultimately, a greater number of exfiltration tests need to be performed to draw more statistically significant conclusions about a wastewater collection system’s sub-surface discharge of wastewater.

INTRODUCTION
On April 26, 2002 the California Regional Water Quality Control Board, Santa Ana Region (RWQCB) established a Monitoring and Reporting Program as part of its General Waste Discharge Requirements for Wastewater Collection Agencies in Orange County. It included a requirement to develop a methodology to report discharges from “pipe breaks, leaking sewer pipes and joints, and other subsurface discharges of wastewater.” Orange County Sanitation District (OCSD), as the lead agency in the compliance effort, receives wastewater from satellite systems serving 24 cities and 7 special districts. Approximately 400 miles of interceptors and trunk lines and 175 miles of local small diameter sewers are OCSD owned, and about 5,500 miles of local sewers owned and operated by their satellites. The OCSD service area covers about 470 square miles and overlies a deep underground aquifer along the Santa Ana River basin. This aquifer serves as a major source of drinking water. The sanitary sewers in the region are up to 100 years old and are mostly made of clay and vitrified clay pipe materials.

OCSD and its satellite partners retained Brown and Caldwell to assist in reviewing the Orders requirements, and the Water Quality and Treatment Laboratory at University of California, Irvine, to develop a methodology to quantify exfiltration from individual locations in sewers. OCSD has submitted a report to the Regional Board titled: Status Report on the Development of a Reporting Methodology for Subsurface Discharges of Sewage covering this effort.

A survey of the literature found that the available methods to measure exfiltration fall into three broad categories. The first is based on placing a sewer with a defect in a box above ground and determining the rate of leakage. It provides a result that does not take into account the retarding effect attributed to the soil outside the defect, such as soil clogging, etc. The second method requires isolation of a sewer reach, surcharging the reach, and recording the rate of water level drop in the manhole. This method significantly overestimates the rate of exfiltration because it includes flows in the upper part of the sewer, while most small diameter sewers flow less than one-half full. The third method relies on area-wide mass balances or tracers, but the reported results have been difficult to quantify.

METHODS
In order to quantify leakage rates through sewer defects for this study, an Exfiltrometer has been developed. When a sewer defect was selected for study, the Exfiltrometer was taken out into the field, placed into the sewer, and exfiltration measurements were made. There are a number of test procedures that were utilized to ensure that the exfiltration measurements are reliable.

An Exfiltrometer prototype (Gifford, 2006) was produced prior to this study.

The main components of the Exfiltrometer (Figures 1, 2, 3, & 4) are the inflatable plugs, waterdepth sensors, reservoir, controller, atmospheric float, and umbilical cord.

The Exfiltrometer quantifies the amount of water that leaks through a sewer defect utilizing a mass balance. First, the two plugs are inflated to isolate the sewer section containing a known defect (Figure 1). The controller is then programmed to pump water from the reservoir to maintain a constant water depth in the test section of the sewer. If water leaks through the sewer defect in question, the water level in the test section drops. The water level is monitored by a water-depth sensor in the test section and its output is received by the controller. When/if the water level in the test section drops below the programmed level, typically 2 mm, the controller switches on a pump that delivers water from the reservoir. When the water level in the test section returns to the programmed level, the controller turns the pump drawing water from the reservoir off. The volume of water used from the reservoir versus the time period of the test is then plotted to show the leakage rate of the sewer defect and, thus, the defect’s exfiltration rate.

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