Knocking the Foul Odors Out of the Ballpark: Solving a Stinky Collection System Problem Through Field Investigation and Ventilation Modeling

Introduction The San Antonio Water System (SAWS) is a public water and wastewater utility serving over 2 million people in the San Antonio area. SAWS owns and operates a collection system consisting of nearly 6,000 miles of sewer mains and over 150 lift stations. Like many wastewater utilities, SAWS experiences odor and corrosion issues in the collection system. One particularly troublesome area is at Stinson Park and consists of parallel 72-inch trunk mains and a 60-inch sewer that combine at a junction box and pass under Six Mile Creek via a siphon. Sulfide and other odorous compounds generated in the upstream collection system volatilize to hydrogen sulfide (H2S) gas and other odorous gases in the sewer headspace. The odorous air cannot continue downstream past the siphon and would vent from the Apollo Street junction box about 1700 feet north of Six Mile Creek, resulting in numerous odor complaints. SAWS has dosed ferrous sulfate in the collection system for many years to precipitate liquid sulfide and control volatilization of sulfide. In 2012, a vapor phase biotower odor control system was constructed at Apollo Street to capture and treat odorous air from the junction box and the surrounding collection system, but odor complaints persisted from an adjacent school and a nearby youth sports complex. In 2015, several improvements were made to the Apollo Street odor control system by cleaning accumulated grease and adding carbon polishing. Odor complaints from the school ceased, but odor complaints from the sports complex continued. In 2018, SAWS began a thorough, stepwise investigation to determine the source of the odor complaints and how to control them. Objectives The objectives of the investigation were as follows: -Assess the cause and source of odor complaints near Six Mile Creek. -Develop odor control options to mitigate odor complaints near Six Mile Creek. -Evaluate options to improve the performance of the Apollo Street odor control system to better mitigate odor complaints. -Evaluate the option of constructing an air jumper at the Six Mile Creek siphon to convey odorous air past the creek to be treated at the downstream Rilling Road odor control system. Methods/Findings Figure 1 shows prominent features of the evaluation area including the Apollo Street odor control system, sports complex, and Six Mile Creek. The media in the carbon vessel was replaced before the 2018 baseball season, but odor complaints from the sports complex continued. No major sources were identified around the baseball fields at the complex, but siphons were cleaned. In 2019, SAWS staff attended a baseball game upon receiving an odor complaint at the start of the baseball season and there were indications the odors came from the Apollo Street odor control system (see Figure 2). Investigative efforts revealed partially treated air discharged from the Apollo Street odor control system was the source of odors at the sports complex. Flow pacing of ferrous sulfate addition was implemented to control the diurnal H2S concentration allowing the biotower media to effectively remove H2S and not overload the carbon vessel. Spent carbon media was also replaced. In addition, better and more consistent maintenance of the Apollo Street and Rilling Road odor control system began in 2020. Following these improvements, only one odor complaint was received between 2019 and the beginning of 2022. Odor complaints returned in the spring of 2022. Investigation indicated the biotower was not operating properly and repairs were made. An odor process and ventilation model of the area was developed that validated the results of the field investigation. The sewer ventilation assessment determined the airflows needed to better depressurize the collection system and to determine the airflow required to potentially convey odorous air downstream via a siphon air jumper to the Rilling Road odor control system for treatment. The ventilation model provided a properly sized system rather than an oversized system using overly conservative estimates or an expensive fan test saving hundreds of thousands of dollars in construction costs. The model was also used to identify improvements to optimize the odor control system, making the system more resilient and reducing operational costs. The recommended improvements included: -Install high performance structured plastic media in the biotower -Increase airflow to 6,000 cfm -Expand the activated carbon bed -Replace the virgin carbon with both high capacity H2S carbon and virgin carbon -Improve flow pacing of ferrous sulfate Conclusions The following are conclusions as a result of the field investigation and ventilation modelling: Capital and operational improvements made to the Apollo Street odor control system since 2015 and upstream flow paced ferrous sulfate addition have mitigated odor complaints. -Optimizing performance of the odor control system through further relatively low-cost improvements will make the system more resilient and reduce operational costs by reducing the frequency of carbon replacement and decreasing ferrous sulfate addition. -Constructing an air jumper and treating odorous air at the Rilling Road odor control system is extremely expensive and not recommended as the Apollo Street odor control system can adequately treat odorous air at a lower cost. Lessons learned for utilities: -Be aware of the area odor history -Make note of all potential sources of odor -Do not dismiss complaints -Note the time, wind direction/speed, odor type for odor complaints -Assess all aspects of available mitigation options including reliability -Assess performance and condition of equipment -Maximize equipment efficiency -Revisit and continuously improve -Utilize third party experts to validate results This presentation will demonstrate how combining a stepwise approach encompassing field investigations with modeling tools is used to identify and develop solutions to mitigate odor complaints and control corrosion in a collection system.