The city of Newport News, Va., entered into a Special Order by Consent with the Virginia Department of Environmental Quality, Hampton Roads Sanitation District and 12 other Hampton Roads localities in order to reduce the occurrence of sanitary sewer overflows (SSOs) in its sanitary sewer system.
Newport News provides sanitary sewer services to 185,700 people within its municipal limits. The sanitary sewer system is comprised of 172 sewer pumping stations, approximately 658 miles of gravity sewer lines, approximately 14,500 manholes and 40 miles of sewer force mains.
Brown & Caldwell initially drafted a Sanitary Sewer Evaluation Survey (SSES) Plan Report in December 2008 based on condition assessments conducted on specific sanitary sewer system assets, and a prioritized rehabilitation program that addresses deficiencies was developed. The city then entered into an agreement with AH Environmental Consultants Inc. and RJN Group Inc. to provide professional services required in managing field and related activities associated with the SSES and associated phases of the city’s efforts to comply with the consent order requirements.
As part of the consent order, the sanitary sewer system program that the city implemented called for the structural integrity to be inspected as well as capacity issues and maintenance issues that could impact the collection system performance negatively. Assessment activities needed to include manhole inspections, lateral inspections, closed-circuit television (CCTV) inspections, smoke testing, dye testing and night-flow isolations.
Night-flow isolations were used to trace sources of infiltration and to quantify the amount of infiltration entering the sewer system. Night-flow isolations characteristically are performed to narrow down and identify reaches with excessive infiltration that can be pinpointed for further investigations.
The isolations usually are performed between the hours of midnight and 6 a.m. Performing the isolations during such low-flow periods allows field crews to understand the flow data they are collecting. The residents in Newport News, however, are early risers due to the large military and shipyard populations in the area; like clockwork, the flow would begin to rise with the residents at 4:30 a.m. That meant the night-flow isolation work came to an end at 4:30 a.m.
Flow measurements were conducted with a weir structure suitable for the size of pipe being isolated. The upstream reaches were plugged whenever flow conditions warranted, providing a quantification of infiltration in each reach of line. When flow conditions did not allow for plugging, differential measurements were used upstream and downstream for the section of pipe being investigated. Any known sewage flows that contributed flow repeatedly under nighttime conditions were noted.
RJN Group worked numerous nights to perform approximately 500 flow isolation setups and managed traffic control during nighttime setup with AH Environmental. The night-flow isolation began in the summer of 2009 and was completed at the conclusion of 2010.
The results of the night-flow isolation work indicated that significant groundwater infiltration was entering the sewer pipelines. By isolating line segments in approximately 1,000-ln-ft pipe segments, flow isolation pinpointed segments for further investigation that would utilize CCTV inspection. Using the gallons-per-day-per-inch-diameter-per-mile-of-sewer method, the consultants and owners were able to determine a cost-effective cutoff for further field investigations.
The main objective of all SSES projects is eliminating excessive inflow and infiltration (I&I) from the sanitary sewer system in a cost-effective way while minimizing residential impacts. This can be forgotten when performing day-to-day activities such as finding sewer system defects and writing the reports. This concept should
be on the minds of all staff members, from the field crew to management.
If the goal of any SSES project is to reduce I&I, field workers must know how to classify I&I severity and the potential sources. Criteria for judging the severity needs to be established by the client at the project outset based on client-accepted practices and sources (e.g., the National Association of Sewer Service Companies).
What It’s All About
Each client has different concerns that may evolve throughout the project. Night-flow isolation is more than placing weirs into the sewer lines. Safety and coordination of traffic control can be major causes of concern. Ensuring the proper safety equipment, crash trucks, signage vests, police assistance, etc., is an important factor that, if overlooked, can result in project delays and injury to personnel.