Using Acoustic Inspections to Prioritize Sewer Cleaning: Test Results from Baltimore County’s Pilot Study

Like most municipalities throughout the country, Baltimore County’s most significant future challenge is to maintain reliable service for their wastewater customers while managing smaller annual budgets and shrinking staff resources. To answer this challenge the County is investigating alternative maintenance methods that could potentially minimize fiscal and staff resources. One alternative that the County is studying is acoustic sewer inspections to prioritize gravity sewer cleaning.In November 2012 the County began an acoustic inspection pilot study that compares field results using closed circuit television inspection (CCTV) and the Sewer Line Rapid Assessment Tool (SL-RAT), winner of the 2012 WEF Innovative Technology award. The SL-RAT assesses potential sewer hydraulic restrictions based on acoustic transmission measurements. A sewer’s acoustic profile varies depending on common operation and maintenance (O&M) defects such as root intrusion, grease attachments or pipe collapses. The acoustic inspection condition assessment is based on the aggregate blockage within the pipe segment. Obstructions which can block the flow of water will also block acoustic energy. There is a direct physical correlation between the acoustic assessment process and the objective of assessing the cleaning requirements. This correlation is based on the degree of blockage within the pipe.Field inspection work was performed in three phases and completed in October 2013. The pilot study included over 370 gravity-fed sewer line segments (approximately 25,330 meters, or 83,100 feet). Many of these same sewers were inspected with conventional CCTV; however, no pre-cleaning was performed. The first phase consisted of acoustic inspection work performed by an inspection and cleaning contractor over a five week period from November 2012 to December 2012 (101 segments, 6,800 meters, or 22,300 feet). Two measurements were taken at each segment, alternating the transmitter and receiver between the upstream and downstream manholes so that results could be compared. So while 101 unique segments were part of this phase, there were 202 acoustic measurements recorded. The second phase of inspections was conducted by Baltimore County employees during a two-day period in April 2013 (80 segments, 4,200 meters, or 13,800 feet). This work was performed using the recommended procedure of leap-frogging the transmitter and receiver when going from manhole to manhole, and so only one acoustic measurement was recorded for each segment. The third phase of inspections was completed during a 2 week period in October 2013 (191 segments, 14,325 meters, or 47,000 feet). This third phase was also performed by Baltimore County employees, using the same procedure as the Phase 2 study.At face value, comparing the cleaning condition assessment using the SL-RAT acoustic inspection with the cleaning recommendation based on CCTV inspection should be straightforward. In reality, the condition assessment methodologies provided by acoustic inspection and CCTV inspection are inherently dissimilar. Care needs to be taken to establish a consistent protocol for the comparison. A suggested approach is to view the CCTV video in order to assess the aggregate amount of blockage in conjunction with the degree of blockage within the pipe segment. Based on this assessment a ranking is assigned. The ranking provides a measure of the blockage observed within the pipe which can then be used as a well-defined prioritization for the cleaning requirements. To facilitate comparing CCTV inspection with the SL-RAT, a scale consistent with the SL-RAT assessment was used. This approach was applied to fifteen pipe segments randomly selected from Phase 1 of the Baltimore pilot study in order to illustrate the approach. The SL-RAT assessment, in general, was shown to provide a conservative assessment compared to CCTV inspection.Another result of this program was that a substantial reallocation of cleaning resources could be realized through the use of acoustic inspection technology. During the second day of the Phase 2 study, 60 segments (2,900 meters, or 9,500 feet) were inspected in a residential neighborhood during a 5 hour period. Initial results indicate that only 5 out of the 60 segments warrant further attention at this time, which would result in a 92% reduction in cleaning for this particular area.Based on results from other municipalities and this particular pilot study, a two-man crew can perform over 3,000 meters (10,000 linear feet) of sewer inspections per day using the SL-RAT. If properly implemented, the County can expect significant cost savings and increased staff and equipment efficiencies using acoustic inspection technology.