Innovative, Sustainable and Cost-Effective Nitrogen Removal Strategies to Meet New Colorado Regulations (TN < 2 mgN/L): Insights and Case Studies

ABSTRACT: This study identifies cost-effective strategies for N removal to meet Colorado's regulation 31 of 2 mg N/L TN. Three levels of approaches are recommended. Case studies from operational plants validate the findings, guiding WRRFs in CO towards efficient regulatory compliance.

INTRODUCTION: Conventional BNR with advanced process control methods (Advanced BNR or ABNR) with AvN and ABAC have further enhanced efficiency and reduced costs. Building on these advancements, PdNA has emerged as a promising strategy, reducing aeration energy and carbon requirements. For meeting the most stringent TN limit of 2 mg/L, using PdNA as a polishing step provides opportunities for almost any activated sludge plant, instead of being constrained to a very limited set of process layouts. Additionally, sidestream PNA will aid the overall N removal performance. Cost remains a critical aspect for upgrades and meeting a stringent TN limit, necessitating thorough evaluation to ensure compliance and efficiency.

OBJECTIVES: This study evaluates advanced N removal strategies to meet Colorado's Regulation 31 (annual median TN of 2 mg N/L). By developing standard flow sheets and conceptual designs, the study compares air demands, chemical addition, sludge yield, and capital costs for both sidestream and mainstream.

METHODS: Three N removal strategies are evaluated. Table 1 shows the advantages and disadvantages of each improvement level.
Level 1: Conventional BNR: Achieves ammonia levels below 1 mg N/L using existing conventional BNR processes, serving as the baseline.
Level 2: ABNR with Densified Activated Sludge (DAS): Integrates advanced aeration control techniques and selective wasting to promote to enhance nutrient removal efficiency, increase capacity, and reduce energy consumption.
Level 3: ABNR/DAS with PdNA Polishing: Combines advanced BNR and DAS with PdNA polishing and sidestream PNA to improve N removal efficiency to drive sustainability and reduce operational costs.

While P removal is not discussed in this paper, there are synergies between N and P removal with the proposed improvements. Densification in Level 2 is expected to improve phosphorus removal efficiency by retaining PAOs. Implementing PdNA and sidestream deammonification will allow for reductions in carbon requirement and N load to the mainstream process, which can benefit EBPR process. For Level 3, reductions in operational SRT resulted in reductions in aeration demand and increased sludge production. For this study, PdNA efficiency of 65% was used.

CASE EXAMPLES
City of Boulder WRRF (BWRRF): BWRRF operates a 25 MGD facility with a 4-stage Bardenpho process with PAD for sidestream N removal. Future upgrades include a DAS system with a low DO Anaerobic/Oxic (A/O) configuration to promote SND using hydrocyclones. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2). Flow sheets for each scenario are presented in Figure 1. Figure 2 compares monthly electricity usage for the scenarios under existing flow, 2040 Maximum Month flow, and build-out conditions. Implementing advanced aeration control results in approximately 30% energy savings. Comparing Scenario 1 with Scenario 3, nearly 40% energy savings are achieved by implementing PdNA in the Solids Contact Tanks. The majority of energy saving for Scenario 3 was attributed to sidestream deammonification. A comparison of sludge production as a result of SRT changes will be provided in the full paper.

Confidential Client: Client's secondary treatment system uses a two-pass, five-stage Bardenpho processes with step-feed, at current flow of 4.0 MGD. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2 and Figure 1). In Scenario 3, existing tertiary filters are used for PdNA-based N removal. Table 3 compares the cost per unit of N for the three scenarios, including aeration and carbon costs for denitrification. Similar to Case Study 1, the major aeration saving was achieved from Scenario 1 to Scenario 2. Scenario 3 resulted in additional cost savings for aeration and carbon source. Net Present Worth comparisons for Scenarios 2 and 3 are presented on Figure 3. Scenario 3 enables the WRRF to meet TN of 2 mg/L at a lower cost compared with Scenario 2 at TN of 3 mg/L.

South Platte Renew: SPR is the third largest WRRF in CO, with a design capacity of 50 MGD. The treatment process includes trickling filter/solids contact, clarifiers, nitrifying trickling filters, and denitrification filters. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2 and Figure 1). In Scenario 3, a portion of the solids contact tanks effluent is directed to denitrification filters to promote PdNA. Figure 4 shows that methanol usage decreases by about 50% in Scenario 3 (with PdNA and sidestream PNA) compared to Scenario 1, while achieving an effluent TN of 2 mg/L. With PdNA in denitrification filters, the N loading to the filters increases, as presented in Table 4. The changes in sludge production for this case is related to changes in methanol consuption only.

CONCLUSIONS: This study highlights the strategies for WRRFs to comply with stringent TN discharge limit of 2 mg N/L while maintaining operational and cost efficiency. Implementation of aeration control, densification, polishing PdNA, and sidestream PNA are proposed in three improvement levels.