Trends in Improved Assessment and Implementation of Odor and Corrosion Control Strategies in Wastewater Collection Systems

INTRODUCTION
Urban wastewater has historically been managed by separately optimizing the collection system (CS) and the treatment facilities. CS have been optimized through platforms that primarily focus on conveyance capacity defined by wet weather flow conditions. However, during dry weather conditions, the system becomes a bioreactor changing the characteristics of the wastewater and the environment within the pipe, often leading to sulfide production causing odors and costly corrosion problems. Considering that these are common and costly problems for utilities, there is a need for the development of cost-effective strategies to address them, and as an alternative to current trial-and-error practice and expensive sampling and piloting projects. The paper shares insights gained through the development and implementation of innovative tools that simulate biological, chemical and physical processes in CS including sewer ventilation. This understanding is key to effectively solving sewer odor and corrosion and determining wastewater transformations. Large and complex CS have been evaluated using sewer process models with the goal of solving odor and corrosion problems and related management strategies including: - Establishing risk profiles along wastewater conveyance - Developing management strategies for multiple zones of the systems while determining upstream and downstream impacts - Prioritizing areas for mitigation based on estimated pipe life and likelihood of odor complaints - Quantifying wastewater characteristics for wastewater treatment plant influent to determine potential impacts on performance. METHODS
Three case studies where sewer process modeling was used were selected for this paper. Further details of each project and a summary of the outcomes are presented in the Results and Discussion section.
Sewer Process Modeling
The projects utilized the Mega-WATS simulator to evaluate the CS, which is based on the WATS model (Hvitved-Jacobsen et al, 2013) targeting sewer biological, chemical, and physical processes within CS. This includes transformation of organic matter and sulfurous compounds under aerobic, anoxic, and anaerobic conditions. Mega-WATS represents a unique tool to estimate sulfide production for the potential for odors and corrosion. Changes in key plant influent characteristics can be estimated given changes in CS operation such as changes in flow rates, storage and chemical addition. With appropriate physical inputs and field data (e.g. COD, pH, hydrogen sulfide -H2S) the Mega-WATS models presented in this abstract were calibrated and used to simulate odor and corrosion problems and related management strategies.
RESULTS AND DISCUSSION
Albuquerque-Bernalillo County Utility Water Authority (Water Authority) The Water Authority has implemented a range of methods and technologies for controlling odors and corrosion in the CS that conveys 50MGD. Nevertheless, the sewer network still faces corrosion challenges with numerous pipes that are at risk of collapse. This study aimed to optimize odor and corrosion control efforts across the entire CS, in conjunction with and considering the pipe rehabilitation program and the identified operational constrains of the downstream plant (SWRP). The Mega-WATS model included approximately 250 miles of pipes. Model calibration used 16 sampling locations; an example of calibration results for hydrogen sulfide using stochastic simulations is presented in Figure1. An existing plant process model provided the considerations for the influent at the SWRP: 1)a minimum pH of 7 to meet the permitted effluent pH; 2)a maximum magnesium concentration to prevent formation of nuisance struvite, thus limiting the use of magnesium hydroxide for pH adjustment; 3)limit inert suspended solids concentration to avoid impacting the biological process. Iron-rich solids produced in the potable water treatment plant were considered for free sulfide control. Figure2 shows the distribution and type of chemical dosing and Table1 shows the proposed optimized chemical dosing rates and the expected savings, which are substantial. Milwaukee Metropolitan Sewerage District (District) The District has had a history of odor and corrosion issues, caused mostly by H2S in its collection system. The District's CS serves 28 communities with 1.1 million people and covers 423 square miles. Wastewater is transported to the Jones Island and South Shore WRFs. Physical sewer data were extracted from the District's hydraulic model to build the entire sewer network in Mega-WATS (295 miles of pipes between 8 and 150 inches). The project used a stepwise approach to determine the areas in the system that required attention and developed recommendations on the best solutions: 1. Field sampling was conducted to facilitate model calibration. 2. Using the calibrated model and historical odor complaints, 15 zones were identified for further evaluation of odor and corrosion issues. 3. A second sampling was conducted in ten zones where Mega-WATS modeling indicated the potential for odors and corrosion. The results led to the identification of eight priority zones to evaluate. Figure3 shows these areas. All zones were located at drops or near siphons. 4. Conceptual designs using liquid- and vapor-phase mitigation were completed using results from the model. 5. Cost and non-monetary factors were used to select the recommended alternatives (Table2). Oakland and Macomb County (OMID) The OMID wastewater CS has experienced corrosion and degradation in some sewer reaches due to microbial induced corrosion. Combined, the population of the two counties is more than 2 million people with a land area of more than 1,400 square miles. The counties CS includes sewers with depths up to more than 100 feet and diameters up to 11 feet. The H2S generated in the interceptor system is significant because of long detention times and large turbulent drop structures. In addition, the CS is used to store wastewater which results in anaerobic conditions and high sulfide generation. To provide a robust evaluation and selection of the best approach for mitigating odor and corrosion, the project included field sampling, Mega-WATS modeling and fan testing. Outcomes from this project included: - The identification of potential corrosion impacts (concrete loss) due to different flow regimes (Figure4) showing where a reduction in flow would increase corrosion rates. - Determination of chemical dosing (Figure5) and air flow rates for comparing mitigation alternatives and provide recommendations. - Air dispersion modeling was conducted to determine the levels of odor control required to mitigate odors and reduce complaints. - Recommendations led to an integrated approach with multiple zones for vapor-phase treatment and chemical dosing (Figure6). - Modeling of potential corrosion impacts due to different operating schemes including wastewater storage and release (Figure7) to estimate the increase in concrete loss per event (Figure8) due to increased sulfide production.
CONCLUSIONS New modeling approaches and simulation platforms such as Mega-WATS allows for the integration of wastewater collection and treatment systems. While process modelling of collection systems allows for proper asset management (corrosion prediction) and odor mitigation strategies of a wide range of system characteristics, as shown in the diversity of the case studies, it also indicates transformations in the wastewater characteristics that may impact treatment facilities.