Lessons Learned from Fan Testing in Multiple Large Diameter Sewer Systems in Austin, Texas

The City of Austin has regularly tracked odor complaints within its major interceptor systems and has sought ways to reduce complaints. In order to assess the feasibility and effectiveness of head space air extraction for odor control within four of its large diameter interceptors, the City initiated a comprehensive fan test in early 2021. Large-scale tests of the impact of air extraction were performed in areas served by the City's Walnut Creek Interceptor (WCI), Little Walnut Creek Interceptor (LWCI), and Williamson Creek Interceptors (WICI) and the Crosstown Tunnel / (CTI), and Onion Creek Interceptor (OCI) systems. All are large diameter pipeline systems (diameters up to 96-inch) with headspace restrictions at downstream ends. Fan testing and monitoring was performed on five different combinations of extraction points and associated monitoring locations. For each extraction point, The City of Austin and its consultants selected a combination of upstream and downstream monitoring manholes to provide an indication of the zone of influence of the air extraction. The testing area included 34 separate monitoring locations along approximately 50 miles of interceptor. The purpose of the fan test monitoring was to provide data on headspace pressurization and hydrogen sulfide concentrations, and the variability of each over time, both under existing conditions and with air extracted from the sewer headspace. Perkins Engineering Consultants, Inc. (now Mead & Hunt) served as the prime contractor for the testing events, placing and maintaining equipment within the manholes for monitoring hydrogen sulfide and differential pressure, monitoring results during testing, and transmitting results to the project team on a daily basis. V&A Consulting Engineers, Inc. providing equipment, ducting, and personnel for fan operation during the tests. Differential pressure and hydrogen sulfide loggers were installed two days prior to starting the fan test at each location to establish baseline sewer pressure and hydrogen sulfide conditions. The loggers continued to monitor differential pressure and hydrogen sulfide during the two-day fan test, with most loggers transmitting data continuously during the test. Loggers were left in place at least six hours after each test was completed to ensure the system had reverted to normal conditions. During the two-day fan test, the fans were run over a range of airflows to provide a more accurate picture of the zone of influence potentially available within each interceptor. Two fan test locations were tested on the WCI: one at Balcones Park, and the other at Loyola Lane, located upstream of Balcones Park. The Loyola Lane test showed a zone of influence of at least 0.05 in-w.c. 16,000 ft upstream and 8,500 ft downstream of the extraction point. The Balcones Park test had a zone of influence 24,000 ft upstream. The LWCI test showed a zone of influence of 15,000 ft upstream. Testing on the CTI siphon Produced a zone of influence 75,000 ft upstream in certain areas. Testing on the WICI showed a zone of influence of 20,000 ft downstream on the OCI and only 10,000 ft upstream. This paper presentation the test methodology and results, and presents lessons learned and considerations that should be incorporated when choosing fan test extraction and monitoring points for testing. Extreme weather conditions were presented during part of the testing by Winter Storm Uri. The experience of running the test and having instruments in place through the extreme weather event will also be discussed.