Comprehensive Collection System Odor and Corrosion Control through Integrated Planning

PURPOSE
The presentation will provide an overview of the City of Raleigh's Public Utilities Department, Raleigh Water (Raleigh Water) complex collection system odor and corrosion challenges, current approach to treating and handling odor and corrosion, and how modeling was used to evaluate hotspots and optimize treatment. The presentation will benefit the industry by providing integrated approaches to collection system odor and corrosion control assessment, design criteria, planning, and detailed treatment solutions through advanced sewer modeling.

PROBLEM STATEMENT
Raleigh Water is grappling with a complex and rapidly growing collection system with interceptors of varying age and materials, both concrete and inert. Due to the complexity and frequent changes, the system has areas at high risk of high hydrogen sulfide, potential odor nuisance, and corrosion. Raleigh Water maintains an active odor control program with the goal of addressing elevated and consistent odors within the wastewater collection system. The odor control program includes a liquid chemical feed program in which different odor control chemicals (ferrous sulfate, hydrogen peroxide, and calcium nitrate) are added to the wastewater at different strategic points within the collection system to reduce and/or suppress the formation of hydrogen sulfide. Routine monitoring of liquid and gaseous hydrogen sulfide levels is conducted to assess performance at the different dosage locations and to identify any needed adjustment to chemical feed rates. In addition to the chemical dosage program, Raleigh Water also has vapor phase odor treatment systems at several of its regional pump stations to treat the odorous air collected at the pump station. Raleigh Water's strives to build a programmatic system approach for odor and corrosion. Although the Raleigh Water has an expansive chemical dosing system in place that has had success, there are still gaps and areas in need of improvement. Raleigh Water recognizes these gaps and has a goal to optimize their system both for treatment and cost efficiency, while also minimizing elevated and consistent odors system wide.

BACKGROUND AND METHODOLOGY
Raleigh Water operates a collection system that spans over 2,500 miles of pipe, both gravity and forcemain, and conveys wastewater to one of three resource recovery facilities. Figure 1 shows Raleigh Water's collection system, locations of pump stations, dosing stations, and resource recovery facilities. The odor control treatment program is comprised of liquid treatment at more than 20 chemical dosing stations (varying chemical usage) and several vapor phase systems throughout the collection system. The chemical dosing stations throughout the collection system treat the system with different chemicals depending on the location. Calcium nitrate, iron, and peroxide are used throughout the system. In 2017, Raleigh Water completed an odor and corrosion control master plan that evaluated the ongoing treatment program for areas of improvement and potential areas in need of treatment. This was completed through extensive sampling, both liquid and gas phase, and collection system modeling. In 2022, HDR, The WATS Guys, and Raleigh Water worked together to build upon the findings from the previous master plan and assess Raleigh Water's collection system for odor and corrosion control using the Wastewater Aerobic/Anaerobic Transformation in Sewers (Mega-WATS) model. The Mega-WATS model was used for the project because it analyzes and solves complex in-sewer transformations including microbial-induced concrete corrosion, odor formation, and potential for nuisance risk caused by hydrogen sulfide air emissions from pressurized sections of the sewer. Additionally, the model can predict anticipated effects and results of utilizing hydrogen sulfide control strategies, both liquid phase and vapor phase throughout a collection system. The model was used as a key tool in an integrated master planning effort that began in January 2022 and will be completed in December 2022, providing integrated approaches to collection system odor and corrosion control assessment, design criteria, planning, and detailed treatment solutions through advanced sewer modeling. The analysis was done using two modeling scenarios:
1. 2022 average weather and flow conditions
2. 2025 projected average weather and flow conditions

The 2022 model was calibrated using 2015 sampling data as well as updated sampling data completed in June 2022. The model input parameters incorporated existing chemical dosing and vapor phase treatment information to represent field conditions and predict the level of hydrogen sulfide control within the system. The 2022 model findings were evaluated for corrosion risk and areas with high concentrations of hydrogen sulfide in comparison with known odor complaint locations.

RESULTS AND FINDINGS
The 2022 model results were used to determine odor and corrosion hotspots throughout the entire collection system. These hotspots were detected by assessing areas with model predicted hydrogen sulfide presence, known odor complaints, and/or model predicted corrosion. Plots similar to Figure 2 were used to define the hotspot areas and prioritize them based on parameters such as: hydrogen sulfide presence, odor complaints in the area, known high risk asset, high predicted corrosion rate, and known planned monitoring or replacement. The presentation will provide a more detailed assessment of the hotspots and how they were evaluated and prioritized from the modeling results. The 2022 odor and corrosion hotspots were used to develop over ten capital improvements solutions as well as potential operational changes. The developed solutions were primarily related to adding chemical dosing sites in areas with hydrogen sulfide hotspots that were not already being treated, as well as pilot testing different chemicals and combinations of chemicals along flow paths to assess for improved and optimized treatment. One specific flow path evaluated is made up of several oversized forcemains and has an overall detention time of more than 60 hours. The flow path currently has three chemical dosing sites that treat the line with calcium nitrate upstream and iron followed by peroxide downstream. Although his treatment combination has helped mitigate some of the hydrogen sulfide formation, there is still concern for corrosion and odor along the flow path. The project team completed a desktop evaluation on the flow path and found that the addition of a pH adjustment (to a pH between 8-9), such as magnesium hydroxide, upstream of the iron dosing improved the iron's precipitation capabilities with dissolved sulfide. The combination of pH adjustment followed by iron was predicted to be an effective chemical optimization strategy on other flow paths in the system as well. The model also predicted downstream pH with these chemical combinations matched would not adversely impact the resource recovery facility. The team has recommended pilot testing in various locations in the collection system to validate the treatment combination. This presentation will provide the detailed flow path findings, results, evaluation (economic and non-economic benefits), and recommendations that evolved throughout the project. The presentation will provide insight and potential mitigation solutions for other utilities experiencing problematic hydrogen sulfide levels, odorous air, ineffective chemical treatment or corrosion throughout their collection systems.