Leveraging the Odor Toolbox--A Comprehensive Approach to Evaluating Odor and Corrosion in a Large Diameter Interceptor System

BACKGROUND
The Oakland-Macomb Interceptor Drain (OMID) is the main wastewater interceptor that serves nearly 850,000 customers in 25 municipalities in Oakland and Macomb Counties north of Detroit, MI, USA. The Oakland-Macomb Interceptor Drain Drainage District (OMIDDD) operates the approximately 22-mile (35.4-km) unreinforced concrete interceptor that includes piping from 3 feet (0.3 meters) to 12.75 feet (3.9 meters) in diameter, 10 metering facilities receiving flow from contributing community sewers, and the 273-MGD (1,033-MLD) rated capacity Northeast Sewage Pumping Station (NESPS). Upstream of the NESPS, the OMID also receives flow from the Macomb Interceptor Drain (MID) that collects flow from suburbs to the east and is operated by the Macomb Interceptor Drain Drainage District (MIDDD). The MID portion includes about 26 miles (41.8 km) of large-diameter unreinforced concrete interceptor piping, 23 metering facilities, and 2 pump stations. There is corrosion in some locations of the OMID due to sulfuric acid (H2SO4) formed from hydrogen sulfide (H2S). The H2S released from the sewer also results in recurring odor complaints. Hydrogen sulfide release is greatest at turbulent vertical drop manholes where community sewers at higher elevations discharge into the deeper OMID and when control gates are operated during the OMID's unique 'storage and release' practice of flow control. Odor and corrosion issues extend back to the 1970s when the OMID was constructed. Since 2011, the OMIDDD has taken steps to study these issues and initiate mitigative improvements on behalf of ratepayers.

OBJECTIVES
Through both comprehensive studies and mitigation strategies, the OMIDDD seeks to accomplish the following key objectives for the interceptor: -Reduce the rates of corrosion -Cost-effectively extend its operating life -Minimize odor complaints throughout the system Though this paper focuses on the efforts of the OMIDDD, the MIDDD maintains similar objectives with their interceptor. Together, both Districts are working toward complete odor and corrosion control coverage of their combined systems. The Districts currently have active interceptor grouting and lining operations; however, the size of the interceptors makes Contractor mobilization, access, and liner installation a costly effort. With appropriate odor and corrosion control, these lining operations could potentially be limited to specific problematic segments and result in a significant cost savings.

METHODOLOGY
Characterizing the impact of sulfide generation, H2S release, and sewer corrosion required a comprehensive approach. This included reviewing past studies and completing field sampling, sewer process modeling, air dispersion modeling, and fan testing. The extensive data collection program at 17 locations throughout the OMID included liquid-phase dissolved sulfide grab samples and about four weeks of continuous monitoring of sewer differential pressure and vapor-phase H2S concentration using digital monitors. The same sampling effort was completed at 19 locations in the MID; thus, allowing for complete characterization of the large diameter tunnel network. Field sampling, wastewater quality, and hydraulic modeling data were used to develop and calibrate a complete system sewer process model. With this model, multiple conditions that might influence odor generation and sewer corrosion were evaluated: -Dry weather (low flow), -Wet weather from inflow and infiltration (high flow), -'Storage and release' operations -Future phased rehabilitation (e.g., adjusting gate operation frequency/duration for routine Contractor access) -OMID realignment Rates of concrete corrosion predicted by the sewer process model were correlated to prior condition assessments completed using the Pipeline Assessment Certification Program (PACP) ratings and used to approximate remaining pipe life based on the long-term operating strategy of the system up to a maximum allowable concrete loss of 1 inch (25 mm). Concrete loss plots (see example, Figure 1) for the various conditions were created to identify locations along the interceptor alignment most susceptible to deterioration. Process modeling also accounted for 'storage and release' operation, where wastewater is stored in the OMID using flow control (gate) structures. Storing wastewater either for flow management or downstream maintenance access increases detention time, thus generating 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). This pressurizes the H2S-containing odorous air in the interceptor headspace, often leading to off-gassing through ground-level manhole cover pick holes. Sensitivity of ground-level receptors to sewer off-gassing under average dry weather flow and at drop connection manholes was evaluated through air dispersion modeling. The OMID system has relatively few venting locations. Manholes along the interceptors that are spaced relatively far apart (about 1,000 feet, or 300 m) result in low natural ventilation rates. Assuming that the ambient odor impact criteria should be at or below a 7-D/T for 1-hour of community impact 99.5% of the time, air dispersion modeling results characterized the likelihood that an odor control strategy would affect ground-level impacts through management of sewer pressure, H2S, or both. Finally, once the most appropriate odor and corrosion mitigation locations and vapor-phase treatment technologies were identified, field fan testing with sewer pressure monitoring was completed to right-size the future odor control system (Figure 3).

RESULTS AND CONCLUSIONS
This comprehensive approach to evaluating OMID-specific operating conditions enabled the design team to identify the most effective locations and methods to address odor and corrosion. Both liquid-phase and vapor-phase technologies were evaluated through sewer process modeling. Three locations for activated carbon vapor-phase odor control systems were identified. Two systems one actively ventilated and the other passively will be located at flow control gate structures. The third system will be actively ventilated and located at the drop manhole downstream of a community sewer metering facility. Together these systems are expected to address odors and corrosion across most of the OMID. During storage conditions, the vapor-phase systems will not provide continuous control due to headspace constrictions. Likewise, the sizes of the recommended vapor-phase devices will not be large enough to depressurize the sewers for some storage and release events when the release rates are relatively fast and the stored wastewater volumes are relatively high. Process modeling showed that if storage and release events were to be conducted frequently-such as every day-the pipe life could be significantly reduced. However, under normal dry weather conditions, these systems will effectively depressurize the sewers and odors will be effectively mitigated. Corrosion will also be greatly reduced in the main interceptor but may persist at some locations lacking continuously wetted surfaces. These areas may be appropriate candidates for lining. Design has begun at each location, providing unique challenges related to aesthetics, visibility, and site constraints for which unique solutions are being pursued. For example, the community metering station is surrounded by residential homes, therefore requiring concealment that closely reflects the neighborhood (Figure 4). Facility designs will be completed in 2023.