Process Advisory Twin for Operations Support at T.Z. Osborne WRF

Introduction: The City of Greensboro (COG) owns and operates the T.Z. Osborne WRF (TZO). TZO was recently upgraded to expand the plant capacity from 40 mgd to 56 mgd and to accommodate new nutrient limit requirements based upon the Jordan Lake rules. TZO's influent loadings are unique due to heavy industrial influence, resulting in significant spikes in organic and nutrient loadings at times. Influent chemical oxygen demand (COD) concentrations can spike to over 1,000 mg/L and influent total phosphorus (TP) ranges from 5 – 30 mg/L presenting significant challenges for consistent nutrient removal. TZO was previously configured as a 2-stage secondary treatment process for enhanced biological phosphorus removal (EBPR) to meet an effluent TP limit of 2 mg/L. The new permit limit effective in 2021, requires TZO to meet a new annual wasteload allocation for TN (891,272 lbs/year) and TP (112,044 lb/year) which equates to a TN limit of 5.23 mg/L and a TP limit of 0.66 mg/L at the design 56 mgd. Currently TZO treats approximately 35 mgd on average, which results in a TN and TP concentration limit of 8.4 mg/L and 1.05 mg/L, respectively. As a part of the expansion project, TZO was converted to a 5-stage BNR process to meet the new more stringent TN and TP limits. To ensure a smooth transition to the new process operation, COG proactively identified the need to train operations staff both on the new process equipment and configuration, as well as the shift in operational philosophy to balancing both TN and TP removal. The water and wastewater industry has been advancing the application of digital tools to provide operations support. For this project, a digital tool was desired to: 1. accurately represent the new secondary treatment process, 2. train staff on the shift to implementing both TN and TP removal, 3. empower staff to use the tool to continually optimize the process after start-up.
Methods: Digital twin is a broad term used to describe the virtual representation of physical assets and systems that enable users to simulate decisions and analyze results. Varying widely in complexity and scale, different kinds of twins can be leveraged to support the entire spectrum of utility needs, from long term-planning to fully automated real-time operation and can grow in complexity over time to encompass more assets, systems, and capabilities, expanding the benefits they provide. A Process Advisory Twin (Twin) was created for TZO to leverage the calibrated process model to reflect the new process configuration and provide a user-friendly interface for training and process optimization by plant staff. A site-specific, computer representation of TZO was developed using SUMO® and calibrated to accurately reflect current plant operation with respect to solids production, overall treatment performance, and nutrient removal. A custom, user-friendly interface and output visualizations were developed to 'sit on-top' of the process model, to encourage facility operators to develop and test preset options and their own ideas for optimization.
The Twin provides a clickable aerial image of the plant, with a simplified set of inputs and outputs customized based on feedback from plant staff. In addition to whole plant inputs, this Twin is focused on the new aeration basin configuration, providing the user with the information needed on the flexibility of operation (Figure 1). The outputs of the Twin include key process parameters like solids retention time, secondary clarifier solids loading rates, and dials of the effluent TN and TP concentration were developed to help the user visualize the predicted performance relative to the permit limits. (Figure 2). The tool is currently set up for steady state simulations to facilitate fast simulation times suitable for the training environment, but the infrastructure of this Twin is set up for future expansion, with the ability to modify to allow dynamic inputs and outputs, to accept real time data, or link up with other predictive tools to create a more comprehensive operations support tool. One key barrier of any digital tool implementation is user buy-in. These types of tools are only as beneficial as they are used by the staff. A few methods were implemented during this project to help encourage the tool's use:
1. Engagement During Development – Both small and large group demonstrations were used to gather input and staff feedback on the Twin's functionality during development.
2. Hands-On Training –Detailed training modules were developed using the Twin to facilitate knowledge transfers on the new biological treatment process configuration, including opportunities for staff to use the tool in conjunction with optimization of 5-stage basins as they came online.
3. Accessibility – the Twin was deployed as a web-based application that can be launched by multiple staff on any device (e.g. tablet, phone, laptop…)
Results and Status of Work: The new 5-stage basins were brought online in stages in 2021 and the Twin was used to assist with training and start-up. Grab sampling and online instrumentation were used to verify the process model predictions throughout start-up. Construction was completed and all of the 5-stage basins were online in September 2021. The plot of the cumulative TN load with respect to their annual limit, presented in Figure 3, demonstrates that TZO has been able to implement the new nitrogen removal strategy and is on track to meet their annual limit, despite the challenges of construction and start up throughout 2021. This project is an example of a targeted approach to leverage digital twins for operational benefits. This tool was scoped so the level of effort and complexity kept pace with COG's desired adoption of the tool. The customized interface was developed to reflect plant layout and tailored to the specific goals of project. Input from operational staff was gathered during development to ensure an easy to use and informative user interface that is deployed via web application. The Twin's infrastructure allows it to be adaptable to include connected data streams, including SCADA inputs, weather data or other predictive tools like soft sensors.
In 2022, the COG and Hazen have further identified several specific initiatives to optimize nutrient removal performance and to increase operations staff engagement in optimization through use of the Process Advisory Twin. Early 2022 initiatives include:
- Additional jar testing and refinement of the chemical trim predictions in the Twin to support optimization of EBPR and reduction in chemical use at TZO.
- Expansion of the tool to allow for dynamic inputs and outputs to support operational strategies to address spikes in influent loading. Consider implementation of influent load equalization and use of online instrumentation to assist with attenuation of influent carbon or nutrient spikes due to industries.