10 PAR 1304 2018 Facility Plan_Chapter 08_RWHTF Process Improvements.pdf

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CHAPTER 8- RWHTF PROCESS IMPROVEMENTS

This section explores opportunities to improve LOS and achieve near and long-term treatment goals established for the RWHTF. LOS goals and planning drivers impact the RWHTF’s Major Systems in two ways.

1. LOS goals inform near and long-term strategies for the RWHTF’s core processes. For example, long-term performance LOS goals based on CDPHE Regulation 31 may require fundamental core process upgrades and treatment intensification within the planning horizon.

2. LOS goals trigger capital improvements and opportunities for Subsystems or Major Components that need LOS-related capital projects or capital projects as a result of equipment reaching the end of its service life (e.g. projects that are listed for replacement under the FARS).

Under the 2013 Facility Plan, treatment improvement recommendations were made based on regulatory, capacity, fixed asset replacement, and operational efficiency drivers through 2035.

The focus of the 2013 Facility Plan approach was maximizing performance within existing facilities through operational optimization before initiating capital improvements projects. Many of these improvement recommendations have been or are being implemented at the RWHTF including south headworks and grease processing improvements, EBPR and deammonification, phosphorous sequestration and struvite recovery, and a new solids processing building. Addition of tertiary treatment including flocculation, sedimentation, filtration, an ultraviolet (UV) disinfection were recommended before 2025 to meet anticipated TN and TP regulations of 2.01 mg-N/L and

0.1 mg-P/L, respectively.

The 2018 Facility Plan builds upon 2013 recommendations and approaches by incorporating the LOS framework into alternatives evaluations and developing Integrated Solutions that string together near and long-term improvements to meet various LOS drivers occurring at different times through the planning period. The 2018 Facility Plan also incorporates new insights to future nutrient regulations and considers phased timing of improvements to best facilitate compliance with current (Regulation 85), VIP, USEPA ammonia, and Regulation 31 limits when they are expected to be enforced.

APPROACH TO RWHTF EXPLORING AND CONVERGING

The 2018 Facility Plan developed four liquid stream and three solids stream process themes under which near and long-term treatment alternatives and improvements were identified. These process themes included carbon management, BNR, tertiary/advanced treatment, disinfection, solids stabilization, biosolids reduction, and sidestream treatment. A technology pre-screening was conducted in coordination with the Metro District which led to the converging to four liquid stream integrated solutions and three proposed solids subsystem improvements. Specifically, this section addresses the objectives listed below.

• Define core liquid and solids stream core processes and summarize critical LOS and planning criteria that impact those core processes.

• Explore the universe of technologies available to address near and long-term treatment opportunities and converge on best-fit solutions through comprehensive screening.

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• Identify piloting and innovation opportunities and considerations for triggering primary treatment carbon diversion in contrast with secondary treatment expansion.

• Propose integrated solutions that achieve long-term liquid stream LOS capacity, performance, and redundancy goals.

• Consider options for balancing BNR between secondary and tertiary treatment systems, while identifying opportunities for secondary BNR optimization.

• Provide a comprehensive business case evaluation for liquid stream integrated solutions including capital and O&M net present costs and a qualitative comparison of benefits.

• Conceptually develop space planning and capital costs for solids stream improvements, including GVT expansion, WAS thickening improvements, and digester mixing upgrades.

• Refine liquid streams and solids stream road maps while identifying opportunities for off-ramp triggers, including research and/or piloting opportunities.

SUMMARY OF RWHTF TREATMENT RECOMMENDATIONS

Through the Exploring and Converging evaluations, the 2018 Facility Plan selected one Integrated Solution for liquid stream improvements and three solids stream improvements to meet LOS goals through the planning period. Integrated Solution No. 2b (IS2b), which includes BNR intensification, SSEC expansion, denitrification moving bed bioreactors, floc/sed, filtration, and UV disinfection, was selected as the planned liquid stream path as indicated on the RWHTF 20- Year Treatment Road Map (Figure 8-22). The road map also shows available off-ramps to the selected approach including minor modifications or enhancements to IS2b depending on outcomes of the various recommended studies and pilot projects.

For the solids stream, the 2018 Facility Plan evaluated both conventional and intensified solids stabilization opportunities. Technologies were screened and narrowed to three recommended improvements to support LOS goals through the planning period: expansion of existing GVT facilities, conversion of WAS thickening from DAF to centrifuges, and digester cover replacement and mixer installation. As indicated on the RWHTF 20-Year Treatment Road Map, a Solids Improvements Evaluation is recommended in the near-term to further evaluate the proposed alternatives and identify other intensification opportunities. Solids handling and carbon management strategies may be re-evaluated in the long term for opportunities to further optimize and intensify treatment.

Other recommended improvements and evaluations are listed below.

• Continuation of AB-2 pilot (intensification piloting)

• High speed blower pilot

• North Secondary Blower and Basin Rehabilitation and Intensification

• Dissolved organic nitrogen and phosphorous evaluation and piloting

• Disinfection evaluation in coordination with liquid stream piloting

• Solids Handling and Carbon Management Re-evaluations A RWHTF site layout incorporating the proposed liquid and solids stream improvements through the planning period is provided in Figure 8-23.

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BASIS OF TREATMENT RECOMMENDATIONS

Core processes are defined as processes that form the backbone of the RWHTF process flow sheet. A core process can be comprised of one or more Major Systems and related supporting Subsystems and Major Components. More specifically, core processes can be defined for the RWHTF liquid and solids streams. The current core process for the liquid stream consists of primary treatment followed by three-stage BNR and disinfection. The current core process for the solids stream can be defined as sludge thickening followed by acid phase mesophilic anaerobic digestion and dewatering. These two core processes provide the baseline for exploring alternatives and establish the backbone of the liquid and solids road maps.

At the heart of the exploring effort is the need to chart a path forward for the RWHTF that aligns with the LOS and planning criteria. The 2018 Facility Plan identified key process themes to help categorize potential technologies and opportunities. These themes were intended to address treatment principles within the core processes at the RWHTF and account for industry trends related to treatment goals and resource recovery. This thematic approach allowed for identifying not only technologies, but overall treatment strategies that ultimately drove development of the integrated solutions. Four process themes were identified for liquid core processes and three process themes were identified for solids core processes.

• Liquid Stream Theme 1 - Carbon Management: Exploring carbon management technologies at the RWHTF is a question of how to best position the Metro District to effectively manage carbon through the liquid and solids treatment processes. The use of simple and cost-effective technologies can provide balanced carbon diversion while improving capacity of the secondary biological process. Optimizing the diversion of carbon to the solids stabilization process can position the District to increase biogas and energy production. Once carbon is diverted, it is also critical to manage the conveyance and processing of the material through solids processing. Therefore, an exploration of systems downstream of carbon diversion processes was also performed.

• Liquid Stream Theme 2 - Secondary Biological Nutrient Removal: Exploring secondary treatment technologies at the RWHTF requires careful consideration of performance LOS goals and a keen understanding of current operations and process capacity. Overarching drivers like minimizing footprint, chemicals, and energy; leveraging existing infrastructure; and reducing complexity of operation and maintenance are also important considerations as they have the potential to impact how intensification and resource recovery influences technology evaluation and selection. For the exploration of secondary process treatment technologies, the following structure was employed:

o Conventional BNR Technologies – These technologies represent the conventional biological treatments or physical separation steps that can be implemented to achieve secondary treatment LOS goals.

o Intensification Technologies – These technologies represent enhancements to the conventional technologies. These enhancements can result in capacity increase with the same or reduced footprint or reduction of chemicals or energy demands.

• Liquid Stream Theme 3 - Tertiary and Advanced Treatment: Because of future stringent compliance targets, the Metro District must consider a wide range of integrated alternatives that will ensure long-term, reliable compliance at a reasonable cost.

Therefore, the focus of tertiary and advanced treatment process exploring is on technologies that can complement secondary BNR technologies and achieve high efficiency removal of TP and TN.

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• Liquid Stream Theme 4 - Disinfection: Exploring tertiary and advanced treatment impacts the current approach to disinfection since these improvements assume the North and South secondary effluents will be combined. There may be hydraulic limitations with using the existing North contact basins to achieve CTs under the combined effluent flow conditions. However, a new chemical contact basin or UV disinfection facility following tertiary treatment are also viable alternatives. The exploration of disinfection options builds upon current piloting and testing efforts.

• Solids Stream Theme 1 – Solids Stabilization: Exploring solids core process technologies at the RWHTF focuses on unlocking treatment capacity by leveraging existing infrastructure. In addition, it focuses on positioning the Metro District to continue meeting treatment goals and reducing operation costs, while achieving effective resource recovery. For the exploration of solids stabilization technologies, the following structure was employed:

o Conventional Solids Stabilization Technologies – These technologies represent the conventional anaerobic digestion treatment schemes that can be implemented to achieve desired LOS.

o Intensification Technologies – These technologies represent enhancements to the conventional technologies. These enhancements can result in capacity increase with existing or reduced footprint.

• Solids Stream Theme 2 - Biosolids Reduction Technologies: Biosolids reduction technologies can provide flexibility to further reduce the mass of biosolids produced at the RWHTF that would need to be transported to the Farm. These technologies are beyond conventional and intensified solids stabilization and may not be implemented in the near-term. However, they are included to provide perspective into the long-term horizon of biosolids treatment trends in the industry.

• Solids Stream Theme 3 - Sidestream Treatment: Treatment of sidestream process flows, such as centrate from dewatering operations, helps manage nutrient recycle loads to the mainstream BNR process. The Metro District has invested and implemented a robust sidestream management program. For this reason, the exploration of technology options for sidestream treatment is more limited and focused on the impact of potential new solids stabilization processes on sidestream, mainstream, and resource recovery processes.

Technologies were then screened using the following two-step approach:

• Step 1 - Fatal flaw screening: This step included consideration of technology feasibility, historical performance, and alignment with Metro District goals and industry trends.

• Step 2 – Multi-criteria qualitative screening: This step involved a multi-criteria approach to screen technologies by theme using qualitative measures derived from engineering experience and judgement. The top-ranked processes were then considered for inclusion within different integrated solutions. For this step, a four-dimensional analysis framework was developed and used to screen and rank technologies as illustrated in Table 8-1.

Integrated solutions were developed to meet the needs of the RWHTF through 2040. Due to the scale and complexity of the facility, and the need to provide the Metro District with the infrastructure at the right time, consideration was given to how phased implementation of the integrated solutions might occur. Therefore, for the purpose of a comparative evaluation, the proposed sequencing for each solution is also provided.

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Table 8-1 Technology Screening Criteria Criteria Sub-criteria 1 Sub-criteria 2 Sub-criteria 3 Sub-criteria 4

Process Compatibility

Is the process compatible with existing upstream/ downstream processes?

Will the process reduce O&M relative to current processes?

Will the process reduce the need for specialized maintenance/ startup?

Will the process minimize the handling of new chemicals/ streams?

Ease of Physical Implementation

Will the process reduce footprint?

Can the process or technology be retrofitted within the existing footprint?

Will this technology minimize significant modifications to conduits, structural, electrical?

Will the process improve health and safety of MWRD employees?

Technology Maturity/ Adaptability

Are there full-scale installations of this technology in municipal applications?

Will this technology improve ability to meet regulatory challenges?

Are spare parts and technical know-how to troubleshoot this technology available in the USA?

n/a

Improvement to Level of Service

Will the process provide benefit impacts to capacity?

Will the process improve redundancy/reliabilit y of performance?

Will the process improve overall performance?

Will the process improve O&M/FARS level of effort?

Improvement to Enhanced LOS

Will the technology/ process position

MWRD to meet 3R goals?

Will the technology/ process position

MWRD to meet EMI goals?

Will the technology/ process position

MWRD to achieve Innovation 2.0?

n/a

Cost What is the relative CAPEX cost?

What is the relative OPEX cost?

Will the process reduce the need for specialized maintenance/ startup?

Will the process minimize the handling of new chemicals/ streams?

CRITICAL LEVEL OF SERVICE AND PLANNING CRITERIA

This section summarizes the LOS goals and requirements established during the Planning Phase.

These drivers are LOS goals that can fundamentally change the RWHTF process flow sheet and include capacity, redundancy, performance, O&M, and Enhanced LOS considerations.

The projected 2040 flow and load projections were used for alternatives evaluations and are summarized in Table 8-2. Projected capacities presented herein are based on the MMF and maximum month load (MML) conditions at winter temperatures (i.e., 15°C). This criteria is considered conservative as it is unlikely that both conditions would occur simultaneously.

Table 8-2 2040 Projected Flows and Loads

Parameter Unit Annual Average Maximum Month Flow MGD 160 181 Biochemical Oxygen Demand (BOD) tpd (lb/day) 383,000 (192) 446,000 (223) Total Suspended Solids (TSS) lb/day 408,000 481,000 Total Kjeldahl Nitrogen (TKN) lb/day 61,000 68,000 Total Phosphorus (TP) lb/day 8,760 9,800

The capacity analyses provided in the existing facility descriptions includes a comprehensive evaluation of the RWHTF capacity from headworks to outfall. Critical capacity LOS that impact core processes include the secondary treatment capacity and the anaerobic digestion capacity.

It is important to consider the capacity LOS from two perspectives:

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1. Capacity of existing core processes to meet critical LOS goals and requirements with no additional process units through the end of the planning period, and

2. Capacity of the existing core process to meet critical LOS goals and requirements at site build out through estimated service area build out conditions.

Nine scenarios (six for liquid and three for solids) were considered to evaluate critical capacity LOS with respect to the core processes. These scenarios are summarized in Table 8-3.

Figure 8-1 and Figure 8-2 illustrate relevant site layout alternatives for secondary treatment. The current liquid stream capacity with all units in service is approximately 199 MGD EMMF. At buildout, with an enhanced redundancy and performance LOS, the capacity is estimated to be approximately 157 MGD EMMF. This possible capacity range makes a strong case for development of a technology-based secondary treatment process intensification program, since there is a potential for capacity shortage in the future, depending on the LOS criteria. The substantial drop in capacity results from a more robust minimum aerobic SRT for nitrification targeting non-detect ammonia in the effluent in order to meet the performance LOS criteria of 2.01 mg/L TN on an annual median basis.

Figure 8-3 illustrates site layout alternatives for anaerobic digestion. The current EMMF capacity with all units in service is approximately 217 MGD. At site buildout and with an enhanced redundancy LOS, the EMMF capacity is estimated to be in the order of 188 MGD with one additional digester.

Table 8-3 Capacity Scenarios for Liquid and Solids Core Processes

Scenario Major System

Redundancy Units OOS

Performance

LOS

Operational Criteria(1)

EMMF Capacity

(MGD)

(Baseline) Liquid

North and South

Secondary Treatment

Aeration Basins and Clarifiers

N – 0 S – 0

TIN – 7.0 mg/L TP – 0.7 mg/L

MLSS – 3,000 mg/L Temperature 15ºC Min. Aerobic SRT –

6 days

(Baseline)

N – 1 S – 1 173

(Buildout)

N – 1 S – 1 221

(Full Nitrif.)

N – 0 S – 0

TN – 2.01 mg/L TP – 0.1 mg/L

MLSS – 3,000 mg/L Temperature 15ºC Min. Aerobic SRT –

9 days

(Full Nitrif.)

N – 1 S – 1 128

(Buildout/Full Nitrif.)

N – 1 S – 1 157

(Baseline) Solids

Anaerobic Digesters

Class B Biosolids

Total SRT – 15 Days at Feed 5.1% TS

(Baseline) 2 181

(Buildout) 2 188

1 MLSS criteria variable depending on flow and SVI.

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Figure 8-1 North Secondary Treatment Site Buildout Alternatives

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Figure 8-2 South Secondary Treatment Site Buildout Alternatives

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Figure 8-3 Anaerobic Digestion Site Buildout Alternatives

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Redundancy LOS criteria for major systems and subsystems were developed in coordination with the Metro District and are documented in the LOS Matrix (Appendix D). Table 8-4 lists key redundancy LOS criteria that were used as part of the Converging Phase evaluation. Redundancy criteria established by the District may be more conservative than those required by CDPHE in design approvals.

Table 8-4 RWHTF Required and ELOS Redundancy Subsystem/ Major Component Required Redundancy (CDPHE) ELOS Redundancy

North and South Primary Clarifiers No units out of service One unit out of service North Aeration Basins No units out of service One unit out of service South Aeration Basins No units out of service One unit out of service

North Secondary Clarifiers No units out of service One unit out of service South Secondary Clarifiers No units out of service One unit out of service

Gravity Thickeners No units out of service One unit out of service Anaerobic Digesters One unit out of service Two units out of service

Critical performance LOS criteria that impact core processes are based on key BNR regulatory drivers as illustrated in Figure 8-4. The regulatory drivers with the greatest potential impact to core processes are the nutrient regulations laid out by the CDPHE road map as well as temperature limits. The 2018 Facility Plan categorized BNR performance targets based on these regulations as described below.

• Conventional BNR – This corresponds to the current CDPHE Regulation 85 effluent nutrient limits at the RWHTF (i.e., TIN < 15 mg/L and TP < 1.0 mg/L on a running annual median basis).

• Level 1 BNR – This corresponds to the VIP for early nutrient reductions. Under Level 1 BNR, the RWHTF would need to achieve an annual median TIN < 7 mg/L and TP < 0.7 mg/L to maximize incentive benefits.

• Level 2 BNR – This corresponds to the VIP limits plus anticipated limits under U.S. EPA aquatic life ambient water quality criteria for ammonia. Under Level 2 BNR, the RWHTF would need to achieve Level 1 BNR as well as a maximum daily total ammonia nitrogen (TAN) limit of 2.8 mg/L.

• Level 3 BNR – This corresponds to the anticipated limits under CDPHE Regulation 31 and the associated Barr-Milton TMDL. Under Level 3 BNR, the RWHTF would need to achieve an annual median of TN < 2.0 mg/L, TP < 0.1 mg/L, and a daily maximum TAN limit of 2.8 mg/L.

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Figure 8-4 Critical BNR Performance for ELOS

The influence of dissolved organic nitrogen (DON) is an important consideration towards achieving Level 3 BNR performance. Preliminary effluent samples showed background DON concentrations range between 1.0 and 1.8 mg/L, with an average of 1.4 mg/L. While this data represents a limited number of samples (i.e. 32 samples), it suggests background DON levels may be near the TN concentration limit of 2.0 mg/L anticipated under Regulation 31, thus affecting secondary and tertiary BNR processes’ ability to achieve Level 3 BNR performance. More rigorous and comprehensive sampling should be conducted to better understand the fraction and characterization of DON species in the wastewater influent, which will help to inform future decision-making regarding advanced treatment.

The Exploring Phase considered and discussed several advanced treatment options for DON removal, including post-filtration granular activated carbon (GAC), ozonation followed by biologically active filtration, and enhanced coagulation, flocculation, and sedimentation. It is recommended that these technologies be evaluated through future pilot studies to validate their efficacy and support the decision-making process. Opportunities to incorporate these technologies into the proposed integrated solutions are discussed herein.

ELOS significantly impacts the exploring of opportunities for core processes. ELOS are defined in the LOS Matrix as enhanced capacity, reliability, performance, or O&M criteria. For example, near-term regulatory incentives like the CDPHE Incentive Program are considered an ELOS since the actual discharge permit is the major regulatory requirement and constitutes the base LOS.

However, ELOS also includes principles previously discussed including:

• Planning drivers that align with the District’s Strategic Plan directives,

• Resource recovery and sustainability stewardship considerations,

• EMI goals, and

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• Regional Integration of Urban Infrastructure.

PRIMARY TREATMENT AND CARBON MANAGEMENT

The traditional driver for primary treatment at WRFs like the RWHTF is diversion of suspended solids and organic carbon. Diverting solids and carbon in this manner serves two purposes. First, it reduces the load to the secondary process by conveying mostly soluble and colloidal organic carbon to the secondary biological process. Secondly, it diverts a large fraction of particulate readily biodegradable material to the solids stabilization process.

Stabilization of this material results in the generation of biogas, which is then utilized for energy recovery at the RWHTF. Balancing carbon removal through the whole treatment process is critical since carbon in the raw wastewater is also needed for enhanced biological phosphorous removal and biological nitrogen removal. As future treatment targets change, carbon management will become increasingly important.

The RWHTF currently employs fourteen primary clarifiers throughout the facility, ten in the NPC and four in the SPC. Historical primary clarifier performance data from 2015 to 2017 indicates that the average North and South primary clarifier TSS efficiency are 55 and 70 percent, respectively.

Increasing primary treatment carbon diversion capacity at RWHTF using conventional technology would require construction of new primary settling tanks. Up to two new conventional primary clarifiers can potentially be constructed in the space adjacent to the existing South primary clarifiers to provide additional capacity if needed. The capacity analysis indicates that the existing facilities have adequate capacity for service through the planning period and beyond. Therefore, the Metro District elected to reserve this space for future primaries or use it for another purpose.

Carbon management technologies were screened according to the fatal flaw and screening criteria. Additional details on the evaluation is provided in Appendix I and a summary of findings is provided in Table 8-5. Key insights from the screening effort are listed below.

• CEPT (excluding use of non-iron based coagulants) were lowly ranked based on the Metro District’s decision to phase out CEPT, the high O&M costs incurred by a potential substitute chemical such as polymers and potential for District to have an effluent iron discharge limit in the future. In addition, given the high performance of the conventional primary treatment at the RWHTF, the adoption of a CEPT approach may not be cost effective for a marginal enhancement to solids/carbon diversion.

• High Rate Activated Sludge (HRAS), A/B, and Anaerobic MBR (AnaMBR) were identified as infeasible options at present due to significant infrastructure modifications that would be needed for these technologies. These processes would require considerably high capital expenditures (CAPEX).

• Conventional primary treatment, BFPT, and primary effluent filtration technologies will be considered as core primary carbon diversion technologies during integrated alternatives development and analyses.

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Table 8-5 Summary of Primary Treatment Carbon Diversion Technologies

Criteria Convent. PT CEPT Ballasted Floc PT HRAS PE Filter AnaMBR

Process Compatibility High High Low Low Medium Low

Ease of Physical Implementation High High Low Low Low Low

Maturity and Adaptability High High High Medium High Low

Positive Impact on LOS Medium Medium Medium High High Medium

Positive Impact on ELOS Low Medium High High High High

CAPEX $$ $ $$ $$$ $$ $$$

OPEX $ $$ $$$ $ $$ $

Fatal Flaw X X X Consider for Integrated Alternative

X X X

The RWHTF currently attains average BOD and TSS removal efficiencies in the upper range of what can be achieved with conventional primary clarification. Conventional primary clarification is therefore an attractive long-term solution and fits well with all integrated solutions because it allows the Metro District to continue using existing infrastructure and rely on proven technology for managing the diversion of carbon.

Further enhancement of carbon diversion at the RWHTF could be achieved through chemical addition or two-sludge configuration (e.g. A-B process) or bio-flocculation processes. The Metro District recently submitted a request to CDPHE to decommission chemically enhanced primary clarification capabilities. In addition, CEPT may not provide a significant improvement given that the baseline process is already performing well.

The A-B process and its derivatives, while infeasible given the current infrastructure and process requirements, could evolve to even more advanced technologies in the future. If this occurs, some version of an A-stage could be converted to increase TSS and BOD removals to more than 80 and 60 percent, respectively. The existing secondary aeration basins and clarifiers would become the B-stage. The enhanced carbon removal in the A-stage would likely result in additional capacity in the B-stage but may require carbon supplementation to meet future nitrogen and phosphorus limits. As the technology further develops, achieving two-sludge process performance within the existing infrastructure could become more feasible. Therefore, it is recommended that the Metro District re-evaluate the technology in the future. If the overall cost to build and operate the A-B configuration becomes less than conventional primary clarification and BNR, the Metro District may then consider conversion of the facility.

For the 2018 Facility Plan, conventional primary clarification was selected and included in all four integrated solutions. It is important to note that investing in the primary clarifiers and associated

2018 Facility Plan 8-14 PAR 1304 equipment/infrastructure will continue to serve the Metro District and achieve the LOS goals even if a decision is made in the future to pursue a two-sludge configuration.

SECONDARY BIOLOGICAL NUTRIENT REMOVAL AND

TERTIARY TREATMENT

As noted previously, exploring potential secondary BNR technologies at the RWHTF was conducted to examine opportunities to leverage existing infrastructure to increase capacity, provide flexibility to meet future treatment goals (e.g., nutrients, organics, etc.), improve reliability while minimizing footprint, reduce chemicals and energy, and reduce complexity of operation.

Further consideration was given to compatibility with carbon management and solids treatment goals.

Secondary treatment at the RWHTF currently utilizes a combination of biological, chemical, and physical processes to remove solids, carbon, nitrogen and phosphorus from primary effluent. The NSEC employs an MLE process for nitrogen removal with a three-pass plug flow layout. The aeration basins can operate in conjunction with the SARs for EBPR. The SSEC uses a two-stage (anoxic, oxic) MLE process to achieve nitrification and denitrification. These facilities are also capable of operating in an EBPR configuration using a three-stage A2O (anaerobic, anoxic, oxic) configuration. This system is designed with flexibility for conversion to a five-stage process with the Stage 5 Basins already in service.

As estimated from the capacity evaluation, the existing NSEC capacity is limited by aeration basin and secondary clarifier capacities. SSEC capacity is limited by aeration basin capacity. Both systems are currently designed with aggressive minimum aerobic SRTs (4.0 to 4.6 day), which could cause challenges for meeting future ammonia and TIN standards. Additionally, settleability issues limit the ability to reliably operate at MLSS values greater than 3,000 mg/L, particularly in the NSEC where the clarifiers are shallow.

Although limited, both NSEC and SSEC have available space for expansion. For NSEC, there is space to build two additional aeration basins (using the existing MLE configuration) as well as two additional 130-ft clarifiers. For SSEC, there is space to build two additional aeration basins (using the existing A2O configuration) as well as two additional 140-ft clarifiers.

Moving into the future, technologies that allow the Metro District to cost effectively increase aerobic and total SRT while improving sludge settling characteristics would significantly benefit BNR capacity at the RWHTF without the need for additional aeration and clarification infrastructure

Solids separation at the RWHTF is currently achieved using secondary clarification. The NSEC and SSEC clarifiers are described in Chapter 7 and a secondary clarifier capacity evaluation is presented in Section 7.7.3. The NSEC clarifiers have a 10-foot sidewater depth while the SSEC clarifiers have a 14-foot sidewater depth. Historical performance at the RWHTF indicates that secondary effluent TSS can be variable (Figure 8-5 and Figure 8-6), particularly for the NSEC.

This is likely due to periods of filamentous growth in the activated sludge basins, which, when combined with the shallow NSEC clarifier depth, can lead to suboptimal TSS removal performance. Variable TSS removal is a concern as the RWHTF discharge permit limits effluent TSS to 30 mg/L on a 30-day average.

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Figure 8-5 Performance of NSEC Secondary Clarifiers

Figure 8-6 Performance of SSEC Secondary Clarifiers

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Optimization opportunities for clarification include physical and mechanical enhancements as well as process enhancements. The Metro District has already implemented several physical and mechanical enhancements to their secondary clarifiers including suction header-type sludge withdrawal mechanisms and optimized energy dissipation inlet with MEDIC technology (Multilayer Energy Dissipation Inlet Column). Although these practices helped to improve performance, they do not achieve the level of performance determined necessary to support LOS drivers discussed in Chapters 3 through 5. Therefore, additional technologies and approaches based on physical and mechanical enhancements are considered in the following sections.

Several options are available to potentially improve performance of secondary solids removal systems, including:

• retrofit and expansion of existing secondary clarifiers. This option includes replacing the existing North Clarifiers with deeper clarifiers and building additional units as needed;

• HydroGrav Adapt;

• plate settlers; and

• membrane separation.

These optimization alternatives were screened according to fatal flaw and technology screening criteria. Key insights from the screening are listed below.

• Replacing existing NSEC clarifiers was determined to be infeasible due to the significant impact to plant operation that would be incurred if this project were pursued, as well as the capital costs associated with this concept.

• Expansion of clarification to include deeper and more modern secondary clarifier designs was considered to be the baseline from which intensification technologies could be applied.

• Installation of plate settlers to enhance secondary clarification was determined to have potential for improving performance of the existing clarifiers. Further piloting would be needed to determine how to best implement this technology within the existing footprint.

• Solids removal enhancements to clarifiers (e.g., Hydrograv Adapt) were identified to have potential for improving performance of the existing clarifiers. However, this type of technology is not expected to increase overall capacity. Correspondingly, this type of technology is considered to be modular to integrated solutions in the sense that they could be applied in the future if/when the Metro District needs to improve removal of solids from the secondary clarifiers.

• MBR technologies would have significant CAPEX and OPEX at the RWHTF. However, the improvements to site layout and simplicity of operation are significant, especially when considering future potential limits and increases to flows and loads.

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Table 8-6 Summary of Secondary Separation Technologies

Criteria Replace existing SCs

Build Additional SCs Plate Settlers HydroGrav

Adapt MBR Process Compatibility High High Medium Low Low

Ease of Physical Implementation Low High Low Low Medium

Maturity and Adaptability High High High Medium High

Positive Impact on LOS Medium High Medium High High

Positive Impact on ELOS Low High High Medium Medium

CAPEX $$$ $$ $$ $ $$$

OPEX $ $ $ $$ $$$

Fatal Flaw X Consider for Integrated Alternative

X X X X

Given the outcomes of the screening process, it was determined that the baseline option for the RWHTF integrated solutions would include expansion of the existing clarifiers and inclusion of more modern secondary clarification mechanisms where feasible. Hydrograv Adapt was identified as promising and is considered a modular enhancement to any of the integrated solutions that maintain secondary clarification. Similarly, examining the benefits of implementing MBR also warrants consideration, particularly as it relates to both solids removal efficiency and overall intensification potential.

Optimization through process enhancements include technologies that improve sludge settleability. The Metro District is currently exploring this type of enhancements through pilot testing. A full-scale pilot basin in the NSEC, AB-2, is currently being operated as an isolated treatment train to evaluate TSS removal optimization and process intensification technologies.

Results of AB-2 evaluations will be used to identify the optimal alternative for achieving better secondary clarifier performance. Outcomes will not only support capacity LOS needs, but will also render more robust nitrogen removal performance, a more reliable and stable process, and lower risk operation. The technology currently under evaluation, inDenseTM, has shown favorable results. Several other technologies should also be considered for full-scale evaluation and are discussed in subsequent sections.

Conventional BNR technologies represent industry standard biological or physical separation steps that can be implemented to achieve secondary treatment goals. Intensification technologies represent enhancements to the conventional technologies to reduce footprint, chemicals or energy demand while improving capacity and reliability of meeting treatment goals. Five conventional and twelve intensified BNR options were considered. Details on these technologies, as well as considerations for implementation at RWHTF, are provided in Appendix I.

Appendix I Conventional Technologies

• 3-Stage BNR

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• 5-Stage BNR

• Step Feed BNR

• Two Stage BNR

• Conventional Secondary Clarification

Intensification Technologies

• Integrated Fixed -Film Activated Sludge (IFAS)

• Membrane Aerated Bioreactors (MABR)

• inDENSE™

• Biomag®

• Aerobic Granular Sludge (AGS)

• Biocos®

• Hydrograv

• Membrane Bioreactors (MBR)

• Simultaneous Nitrification and Denitrification (SND)

• Mainstream Deammonification (DMX)

• PANDA

• RAS - Fermentation

The secondary BNR technologies were screened according to fatal flaw and technology screening criteria. Details on the evaluation are provided in Appendix I while a summary of findings is provided in Figure 8-4. Key insights from the screening are as follows:

• Two-stage BNR, Biomag and Biocos were identified as infeasible due to the significant modifications to infrastructure that would be needed at the RWHTF as well as high operational cost (Biomag) that would be required.

• Existing conventional BNR technologies can still allow MWRD to meet future goals;

however, footprint, CAPEX and OPEX would become limiting factors. Therefore, intensified BNR becomes attractive to help the District achieve secondary treatment goals in a cost-effective manner.

• A flexible BNR concept combining elements of 3-Stage, 5-Stage and Step-Feed BNR may be considered as the baseline. This flexible BNR concept can then be enhanced using intensification technologies.

2018 Facility Plan 8-19 PAR 1304

• Fixed film processes like IFAS and MABR are promising options to increase basin capacity; however, these technologies will not address settleability issues and will require modification to the hydraulic profile of the facility.

• Hybrid Aerobic Granular Sludge (AGS) concepts, utilizing a combination of BNR enhancements (RAS fermentation, inDENSE, AGS) and secondary clarifier modifications has significant potential for the RWHTF.

• MBR technologies have significant CAPEX and OPEX requirements for RWHTF.

However, the improvements to site layout and simplicity of operation are significant especially when considering future potential limits and increases to flows and loads.

• BNR intensification focused on energy and chemical reduction like SND, Mainstream DMX, PANDA and RAS Fermentation can be considered as standard enhancements to integrated solutions that might be triggered by emerging regulatory needs, changes to unit energy, chemical prices and/or specific desire to pursue resource recovery and EMI goals.

• Solids removal enhancements to clarifiers (e.g., Hydrograv Adapt) were identified to have potential in improving performance of the clarifier; however, it is not expected that this type of technology would increase overall capacity. Correspondingly, this type of technology may be modular to integrated solutions in the sense that they could be applied in the future if/when the Metro District needs to improve removal of solids from the secondary clarifiers.

2018 Facility Plan 8-20 PAR 1304

Table 8-7 Summary of Secondary BNR Treatment Technologies

Core Process Approach

Process Compatibility

Ease of Physical

Implementation

Maturity

Adaptability

Positive Impact on

LOS

Positive Impact on

ELOS

CAPEX OPEX Fatal Flaw

Consider for Integrated Alternative

Conventional

3-Stage BNR High Medium High Low Medium $ $ X

5 Stage BNR High Medium High Medium High $$$ $ X

Step Feed BNR Medium Medium High Medium Medium $$ $$ X

Two Stage BNR Low Low High Medium Medium $$$ $$$ X Conventional SC Medium Low High High Low $$ $ X

Intensification

IFAS Medium Medium High High Medium $$ $ X

MABR Medium Medium Medium High High $$ $ X inDENSE Medium Medium Low Medium High $ $ X

Biomag Low Medium High High Low $$ $$ X

AGS Medium Medium High High High $$ $ X

Biocos Low Low High High Medium $$$ $$ X Hydrograv Adapt Low Low Medium High Medium $ $$ X

MBR Low Medium High High Medium $$$ $$$ X

NGN- SND High Medium High High High $ $ X

NGN-

Mainstream

DMX

Medium Medium Medium Low High $$ $ X

NGN - PANDA Medium Medium Low Low High $ $$ X

RAS

Fermentation Low High High High High $ $ X

2018 Facility Plan 8-21 PAR 1304

A significant focus of planning future treatment process facilities for the RWHTF revolves around performance LOS. Because of the stringent longer-term compliance targets, the Metro District is faced with considering a wide range of integrated solutions that ensure the RWHTF will be capable of reliable permit compliance at a reasonable cost. The need for tertiary treatment processes focuses on high efficiency removals of TP and TN.

The RWHTF plant operates in an Enhanced Biological Phosphorus Removal mode. Therefore, TSS removal is crucial to meet the performance LOS for effluent TP concentrations through removal of particulates containing high levels of phosphorus. This LOS requires a consistent TP effluent concentration below 0.10 mg/L on an annual median basis per CDPHE Regulation 31 and the potential Barr-Milton TMDL. The 2013 Facility Plan and Phosphorus Initiative evaluated alternatives for Tertiary Phosphorus Removal including ballasted sedimentation, contact clarification, conventional flocculation/clarification, dual-media filtration, mono-media filtration, and membrane filtration. The Metro District selected conventional flocculation/sedimentation and dual media filtration for planning. This approach is still applicable to the 2018 Facility Plan and will be considered as the baseline condition. The District may consider piloting flocculation and membrane filtration as an alternative to media filtration given the future stringent performance LOS imposed by Regulation 31. Direct filtration with chemical addition for phosphate trimming may also be an option for consideration and piloting.

The RWHTF also operates in a TN removal mode, and residual dissolved nitrogen species in the treated effluent must be removed to meet the ultimate performance LOS. This LOS requires a consistent TN effluent concentration below 2.01 mg/L on an annual median basis per CDPHE Regulation 31. The traditional approach for achieving low TN has been to add a high-level denitrification step generally in the form of some type of biofilm process. However, for very low TN concertation limits, advanced treatment to remove DON fractions may be required. Though effective, these tertiary processes are capital and O&M intensive. The 2013 Facility Plan considered alternatives for tertiary denitrification including Moving Bed Biofilm Reactor (MBBR), Biological Activated Filter (BAF), Denitrification Filter, Membrane Bioreactor (MBR), and Fluidized Bed Biofilm Reactor. The District selected MBBR for planning. This approach is still applicable to the 2018 Facility Plan and will be considered as the baseline condition.

In recent years, much knowledge has been gained through research and demonstrations at pilot and full scale to illustrate the effectiveness of intensifying the degree of treatment at the secondary stage to achieve much greater nutrient removal. This work has decreased the focus on tertiary processes to meet the more stringent TN limits. The design and construction of the Aeration Basin No. 2 (AB2) full-scale technology test train will allow the District to experiment and develop innovative approaches to intensify its secondary treatment in lieu of certain tertiary treatment processes. The results of this technology-based approach can reveal technical solutions that improve the treatment performance such that more treatment can be achieved within the existing secondary treatment process and in smaller footprints. The effectiveness of these intensive technology approaches reduce, and may even eliminate, the need for some of the secondary and tertiary treatment infrastructure considered in the 2013 Facility Plan.

Integration of intensification technologies into the RWHTF core process does not guarantee that tertiary facilities will never be needed. However, regulatory measures carry a degree of

2018 Facility Plan 8-22 PAR 1304 uncertainty, particularly those that are slated for promulgation well into the future. As such, it is prudent for the Metro District to invest in intensification technologies now, which in and of themselves are continually improving in performance, scale, and readiness levels, through ongoing research, development, and demonstration. This approach will provide the District flexibility in maximizing utilization of existing infrastructure to optimize performance benefits (i.e., a “no regrets” approach) without locking in to a tertiary solution that could prove unnecessary in the future. Furthermore, a focus on secondary process intensification full-scale demonstration using AB2 provides a potential for significant cost saving opportunities that can leverage public buy-in for those capital and O&M funding increases that will inevitably be needed to meet near term LOS goals.

Coupled with the secondary treatment piloting studies at AB2, the Metro District may consider tertiary nitrogen removal piloting. Technologies other than MBBR that could be considered for tertiary nitrogen removal piloting include:

• Denitrification Filters,

• Fluidized Bed Reactors (FBR),

• Microalgae Reactors, and

• Microbial Encapsulated Media Reactors (Biocatalysts).

Previous studies conducted by the Metro District have recommended the use of microfiltration and reverse osmosis to meet the performance LOS required by Regulation 31. As pointed out by the 2013 Facility Plan this option would lead to a significant financial burden to the District and its member agencies. In parallel to proposed secondary treatment technology testing in AB2, the District may consider the following: implementation of a DON characterization program;

establishment of a data base that could be used to plan advanced treatment facilities; and piloting of advanced nitrogen removal technologies. Technologies other than reverse osmosis that could be considered for advanced nitrogen removal piloting include:

• Advanced flocculation sedimentation and filtration,

• Biocatalysts,

• Microalgae,

• Ozone/BAC, and

• Post filtration GAC.

Emerging effluent quality concerns were discussed under Regulatory Drivers. The following considerations are given as it relates to exploring of tertiary treatment options.

• E. coli limits may become more stringent around the 2025 permit renewal period. Further in the future, EPA water quality criteria for viruses (coliphages) may enter into permit requirements. A more robust disinfection approach may be considered to address these concerns favoring technologies that can be augmented or easily retrofitted to address potential future ambient water quality criteria for bacteriophage. One potential impact of water quality criteria for viruses is a substantial increase of the UV dose required to achieve inactivation.

• Based on coordination with Metro District staff, this Facility plan assumes the 10-year compliance schedule for effluent temperature with the goal of reducing effluent temperature by one degree Celsius by 2020, two and a half degrees Celsius by 2030, and

2018 Facility Plan 8-23 PAR 1304 four degrees Celsius thereafter. The District has already started investigating ways to meet the temperature standards such as through studies like the Evaluation of Physical Changes to Segment 15 to Dissipate In-stream Temperature project (PAR 1194) and the Segment 15 Temperature Mitigation Alternatives Analysis and Feasibility Study (PAR 1279). Caution should be taken on exploring opportunities for heat recovery upstream of secondary treatment and in the collection system. Historically the District has adopted an aggressive minimum SRT for nitrification at 15 degrees Celsius for secondary treatment planning and design. If this minimum 15-degree temperature drops upstream in the process, there could be implications on treatment capacity, since the minimum SRT for nitrification criteria would need to be increased.

• Trace inorganic and organic constituents of concern were discussed in the regulatory watch list.