I Smell a New Approach - A Utility's Adoption and Use of a Sewer Process Model

Summary--Using a sewer process model to predict extensive collection system parameters related to odor and corrosion, e.g., sulfide and pH, the Albuquerque Bernalillo County Water Utility Authority (WUA) better understands its own system. The WUA has adopted and used the model in-house to address and resolve, quickly, confidently, and with documented, analytical justification, a wide range of operational, design, and planning issues. This model allows chemical feed optimization and assurance that appropriate wastewater is delivered to the Water Resource Recovery Facility (WRRF) to meet performance requirements. The model is complex and a significant undertaking, yet the WUA has been more effective and efficient since its adoption. This paper describes how to integrate these tools into the Utility's operations, including practical issues of structuring staff, growing the team with consultant support, and developing depth of staff. This paper also describes specific studies performed by WUA staff utilizing the model. Introduction The WUA owns and maintains over 2,400 miles of pipes in the collection system to convey approximately 50 MGD wastewater from the service area to the WRRF. The WUA has implemented various methods and technologies for controlling odors and corrosion in the collection system, resulting in significant institutional knowledge and experience in this engineering science. As part of the 2019 Collection System Odor and Corrosion Control Master Plan, Jacobs Engineering (Jacobs) and the WUA implemented a systematic approach to address odor and corrosion issues by developing a sewer process model of the WUA system. The model was created using WUA ArcGIS piping assets, hydraulic data, and wastewater characteristic data. The model used is the Wastewater Aerobic/Anaerobic Transformations in Sewers (WATS). The WATS model simulates sewers' complex chemical, biological, and physical processes through extensive mass balances and differential equations. The WATS software can predict hydrogen sulfide odor generation, concrete asset corrosion, chemical management, analyze wastewater characteristics at inflows to wastewater treatment plants, etc. The WUA's objective in using the model is to respond in real-time to unexpected-immediate, near-term predictable, conceptual design-phase and routine chemical feed considerations. The Collection Section operates and utilizes the model successfully for a wide range of tasks. Methods The framework for developing and running the WATS model in-house is shown in Figure 1. We identified an in-house engineer (Engineer) with strong technical skills but minimal previous experience in sewer odor control and assigned the responsibility of learning and utilizing the model. We provided a learning environment with adequate resources, strong support, and reviews. Technical support was provided by the modeler's direct supervisor and, under a contract established to provide training and related support, the lead process engineer (Consultant) for developing the WATS model of the WUA system. As noted in Figure 1, per typical WUA practice, the Engineer created a Standard Operating Procedure (SOP) that provides specific documentation and procedures for using the WATS model. The Engineer's knowledge is solidified through the process of writing the SOP. It serves as the Engineer's reference document and a training guide for staff assigned to assist or even to learn to operate the WATS model. After the Consultant provided basic training, it was time for the assigned Engineer to learn by performing or 'doing' repeated model use cases. This WUA Learn-by-doing approach is shown in Figure 2. Results Our first use of the model was to develop specific proposed locations, on three interceptors, for ferric chloride and magnesium hydroxide stations proposed by the Master Plan. Specific locations were found based on land availability, truck access, and other criteria. Chemical feeds were simulated with a target of 0.2 mg/L free iron downstream. Reports were developed in-house and provided to WUA Engineering for consultant design of the new facilities. The next use was to develop flow times from each permitted industrial user to the WUA's only significant WRRF. The WUA's WATS network is based on export data from the WUA's InfoSWWM model, including lengths and average velocities. In the typical use of the WATS model, upstream and downstream pipes are connected, establishing the line being evaluated. The model provides the flow time and multiple parameters pertinent to odor and corrosion control. A report was delivered to Pretreatment, providing a total flow time for each user. Pretreatment has used the report data once for a high BOD spill, but there were no obvious spill impacts to the WRRF influent. An ongoing use is managing the WUA's chemical feed program to prevent odors, manage corrosion, and deliver appropriate wastewater to the WRRF. The WUA utilizes six liquid-phase chemicals: ferric chloride, calcium nitrate, calcium hydroxide, magnesium hydroxide, hydrogen peroxide, and residual iron from the surface water treatment plant (SWTP). The WATS model is the tool used to optimize these feeds. In two studies, the model has been used to protect the WRRF. First, a calcium nitrate tank was to be replaced, and the tank had to be quickly emptied. The model indicated the nitrates would be consumed and would not reach the WRRF. Second, a low-pH Fluorosilicic Acid needed to be disposed of at the SWTP. The model indicated the pH impact at the WRRF would be minimal. In both cases, the model matched the actual results and provided confidence for the action taken. Recently, the model network was extended in two locations to study lines in design or under construction. One was a new gravity line subject to very high BOD. The network extended downstream included an existing gravity, a lift station, and a force main. Results indicated H_2 S levels in the gravity lines would be acceptable, but the unexpectedly high force main discharge would be investigated. The other is a proposed lift station and force main discharging to a gravity line unavailable for service connections. Figures 3 and 4 show results indicating the sulfide levels reached minimal levels well before a location where service connections could result in odor issues. We plan to use this 'free treatment' in the gravity line and not include liquid phase odor control but to provide managed airflow in the gravity line and construct corrosion resistance pipes and manholes. The WUA is currently designing a future satellite WRRF. The service area includes a calcium nitrate station that is to be replaced by a ferric chloride station at an upstream location. We are currently utilizing the model to estimate the increase in BOD when calcium nitrate is no longer fed, to optimize the ferric chloride feed and to estimate the resulting decrease in pH and alkalinity. Lessons Learned Modeling proficiency requires regular use. Once the designated Engineer has learned the model basics, the time allocated to the model may average 15% full-time employment. Naturally, some weeks may require nearly full-time efforts. We observed increased task requests as other groups within the WUA became aware of this new in-house capability. The WATS model has proven effective, but the utility investment and commitment must be recognized. The designated Engineer will ideally have a background in process engineering, but many of these subjects can be taught and learned over time. Sustaining depth of staff requires additional employees to be competent in the model. Writing a basic training SOP helps create and maintain the depth of staff. SOPs should be written with new advancements in the model and should be updated regularly. Benefits and Significance A utility benefits from operating a tool such as the WATS model in several ways. First, should anyone understand your system better than the utility itself? We believe a utility should best understand its system due to ownership and continuity. A utility is best served if it is prepared for all eventualities and can respond quickly. The WATS model allows the utility to do so. A utility continuously adds facilities and systems under a consultant-designed contract describing typical responsibilities. This contract structure often cannot direct timely decisions that optimize the utility-operated sewer systems. Finally, the utility benefits in responding quickly and adequately to a wide range of occurrences that all utilities endure.