Compliance Conundrums: What's Best? How A Systematic and Comprehensive Approach For Wet Weather Flow Management and Treatment Will Keep You From Drifting Off Course

INTRODUCTION
Mandated wet weather control programs coupled with the impacts of climate change, has resulted in an increase of wet weather flow frequency, peak and duration of flows to wastewater treatment facilities (WWTF) across the country. This has posed tremendous challenges for wastewater utilities to maintain and enhance their performance – especially the secondary treatment processes – to meet the current and the increasingly stringent permit limits anticipated of the near future. Addressing these challenges will require a systematic and comprehensive facilities planning approach to balance immediate permit compliance needs with the long-term operational reliability of the WWTF. The decisions between doing more with less (such as operation optimization and process intensification) and adding new infrastructure is complicated and not as easy as it might seem. This paper showcases how one of the country's largest wet weather flow systems has developed solutions to enhance treatment resilience and achieve long-term goals. BACKGROUND The Narragansett Bay Commission (NBC) in Rhode Island embarked on a three-phase combined sewer overflow (CSO) control program under the 1992 Consent Order aimed at achieving 98% reduction of annual CSO volumes and 80% reduction of annual shellfish bed closures. Phases I and II of this program were completed in 2008 and 2015. The ongoing Phase III program includes a 58.5-MG CSO storage tunnel and extensive infrastructure improvements which will capture additional CSO flows for treatment at NBC's Bucklin Point Wastewater Treatment Facility (BPWWTF). As a result, there will be an increase in prolonged high flow periods to the BPWWTF during tunnel dewatering. NBC has also projected flows and loads to the BPWWTF over a 20-year planning period. To address these future impacts at BPWWTF, NBC has developed a Facilities Plan Amendment (FPA) to evaluate the upgrades required. These improvements will serve multiple benefits, but ultimately: 1) improve the wet weather flow controls and secondary treatment performance ensuring compliance with permit requirements for nutrients and other pollutants and 2) allow for system upgrades to reduce the risk associated with potential future regulations and climate change.
WET WEATHER PERFORMANCE CHALLENGES AT BPWWTF The BPWWTF serves a 75-square-mile area, treating flow from all or parts of six nearby localities. The BPWWTF provides primary and secondary treatment and UV disinfection for up to 46 MGD. During a wet weather event, up to an additional 70 MGD wet weather flow will be diverted to the wet weather storage (approximately 2-MG), primary treatment and disinfection (with chlorination and dechlorination) facilities for treatment prior to being discharged to the receiving water. After the wet weather event, the influent flow to the plant will stay at or below 46 MGD while the stored volume is pumped back to the plant headworks from the wet weather treatment facilities and the CSO storage tunnel. An aerial photo of the BFWWTF with the liquid treatment processes highlighted is shown on Figure 1. The existing plant effluent permit limits are shown in Table 1. The primary challenges for the wet weather flow treatment at the BPWWTF include: - Reduced primary clarifier performance during high wet weather flows. This has resulted in higher solids and BOD loads to the secondary treatment process. - Uneven mixed liquor flow distribution to the existing six secondary clarifiers and lack of dedicated return activated sludge (RAS) pumping for each clarifier. During high flow events, it is challenging to control the settled sludge blanket depth, and two of the clarifiers can be overloaded causing solids washout and posing risk of permit violation. - System stress testing conducted by NBC indicated that the performance of the biological nutrient reduction (BNR) process could be under stress during future prolonged wet weather flow conditions once the CSO tunnel is online.
COMPREHENSIVE FACILITIES PLANNING APPROACH AND RECOMMENDATIONS
To address wet weather performance challenges, NBC utilized a systemwide InfoWorks ICMTM hydrologic and hydraulic model to simulate various wet weather facilities operation scenarios to optimize the controls of the wet weather flows to the BPWWTF. The evaluation recommended upgrading the existing BPWWTF main sewage pumps to provide more flexible and reliable control of the flow during wet weather events and tunnel dewatering operation. To ensure permit compliance at the BPWWTF under the future flow and load conditions, NBC evaluated six process improvement alternatives as follows: - Alternative 1 – Install Two New Final Clarifiers - Alternative 2 – Convert Existing Bioreactor to Solids Storage During High Flows - Alternative 3 – Convert Bioreactors to Contact Stabilization During High Flows - Alternative 4 – Install Polymer Feed System - Alternative 5 – Increase RAS Pumping - Alternative 6 – Increase Bioreactor Volume A calibrated BioWinTM model was used to evaluate the process performance of the alternatives. The model was further validated using the data from a special sampling program conducted in 2020. Constructability, O&M requirements, and life cycle costs were also evaluated. Alternative 1 – Install Two New Final Clarifiers – was determined to be the most reliable option to achieve compliance during prolonged wet weather flow conditions, although it had the highest initial cost. This alternative provides greater activated sludge solids inventory control and additional unit process redundancy. Alternative 4 – Install Polymer Feed System – is a low-cost solution that was selected as an optional performance enhancement when the sludge experiences poor settling characteristics. Other key treatment system improvements include optimizing secondary clarifier influent flow split, providing dedicated RAS pumping systems, relocating the RAS feed points for flexible process controls, as well as upgrading the effluent UV disinfection and effluent pump station to provide more redundancy and manage 100-year flood risks. To assess the plant compliance risk with future flows and loads, the final selected alternative was simulated with both steady-state (for average and maximum month conditions) and 30-day dynamic (for maximum month condition only) models in BioWinTM. The bioreactors are set up as a 4-stage Bardenpho process with an option of carbon supplement feeding to the beginning of the second anoxic zone. The model evaluation indicated that the proposed improvements would meet permit limits reliably, as shown in Figures 2 and 3.
SUMMARY
Following a systematic and comprehensive facility planning approach – from both a flow control and treatment aspect – will effectively address challenges from an increase of wet weather flows to wastewater treatment facilities. The secondary treatment processes, which are the most critical to achieving regulatory compliance, are particularly under the stress during the wet weather conditions and will greatly benefit from this approach. During a utility's decision-making process, taking an in-depth look at long-term system reliability and redundancy, as well as operation simplicity is key to making sure both regulatory compliance and asset renewal needs are met.