Studying Odor And Corrosion For 168 Kilometers of Large-Diameter Interceptor

Background Oakland and Macomb Counties, located north of Detroit, Michigan, USA, are largely served by three connected yet separate interceptor systems. These include the Clinton Oakland Sewage Disposal System (COSDS), Oakland-Macomb Interceptor Drain (OMID), and the Macomb Interceptor Drain (MID). These systems serve the Oakland County Water Resources Commissioner (OCWRC), the Oakland-Macomb Interceptor Drain Drainage District (OMIDDD), and the Macomb Interceptor Drain Drainage District (MIDDD), respectively. Together, these three systems total about 168 km (104 miles) of deep interceptor sewer. Maps of the systems are provided in Figure 1; system design profiles are provided in Table 1. Persistent corrosion issues since the 1970s have led to the deterioration and at times catastrophic failure of portions of the interceptors. At certain venting locations, fugitive H2S emissions result in recurring odor complaints. Objectives Since about 2011, actions have been taken to study these issues in a system-wide approach and to initiate mitigative improvements with three principal objectives in mind:

*Identify options to reduce the rates of corrosion

*Cost-effectively extend interceptor operating life

*Minimize odor complaints Together, they are working toward complete odor and corrosion control coverage of their combined systems. The Districts currently have active condition assessment, grouting, and lining operations; however, the size of the interceptors makes Contractor mobilization, access, and liner installation complicated and costly. Odor and corrosion control could potentially limit lining to specific problematic segments and result in significant cost savings. Methodology Characterizing the impact of sulfide generation, H2S release, and sewer corrosion required a comprehensive approach, including reviewing past studies and completing field sampling, developing sewer process models using the commercially available Wastewater Aerobic/Anaerobic Transformations in Sewers (WATS) modeling platform (The WATS Guys), air dispersion modeling, and fan testing. The extensive data collection program included wastewater characterization and continuous sewer differential pressure and vapor-phase H2S concentration monitoring using Acrulog (Clontarf, Queensland, AUS) digital monitors. Sampling was completed at 57 locations in total across all three systems. This information was used to develop and calibrate system sewer process models. For the OMID and MID systems, multiple unique conditions were modeled. Critical scenarios included (1) dry weather (low flow), (2) wet weather from inflow and infiltration (high flow), and (3) 'storage and release' operations. For the COSDS, the impact of the Elizabeth Lake Pumping Station (ELPS) on the downstream sewer and areas that were concurrently undergoing condition assessment inspections were most critical. Results From sewer process modeling, the factors leading to the most severe odor/corrosion conditions included:

*'Storage and release' operation of flow control (gate) structures in the OMID and MID — This operation, which is used either for flow management or downstream maintenance access, increases detention time and generates sulfide. When released — either by raising the low-flow sluice gates or overtopping the divider walls in the flow control structures — turbulence results in H2S stripping (Figure 2), sewer pressurization, and fugitive odorous emissions at ground-level.

*H2S stripping at MID and OMID drop manholes — Hydrogen sulfide is released at turbulent vertical drop manholes where community sewers that flow through metering facilities at higher elevations discharge into the deeper MID and OMID.

*Force mains and pump station operations within the COSDS and MID — Anaerobic conditions occurring in the force mains exacerbate sulfide generation, particularly for long or low-velocity force mains. For example, the ELPS in the COSDS switches between two 1.3-km (0.8-mile) long force mains every 12 hours, increasing H2S and modeled rates of corrosion (Figure 3). Jacobs worked collaboratively with another consulting team that was hired by OCWRC to assess through field inspection and in situ testing specific areas of the COSDS that may require rehabilitation. The extent of corrosion was independently confirmed by corrosion rates calculated by the sewer process model. Conclusions This comprehensive approach to evaluating specific operating conditions enabled the design team to identify the most effective locations and methods (to address odor and corrosion. Activated carbon treatment systems were proposed for the MID, as well as process improvements to the existing Fraser Biofilter (designed/constructed by others). As of January 2024, the design of the OMID systems (by Jacobs) is complete, which presented unique challenges related to aesthetics, visibility, and site constraints requiring unique solutions — e.g., concealing one system to reflect the surrounding neighborhood (Figure 4). Additionally, the sewer process model of the COSDS system has been completed, and the basis of design is under development, which will include odor and corrosion management recommendations. One recommendation will be to pilot a force main flushing protocol at the ELPS in early 2024 as a potentially cost-effective alternative to treatment.