09 PAR 1304 2018 Facility Plan_Chapter 07_RWHTF Existing Conditions.pdf

2018 Facility Plan 7-1 PAR 1304

CHAPTER 7- ROBERT W. HITE TREATMENT FACILITY

This chapter includes a description of RWHTF existing Major Systems, Subsystems, and Major Components and their key design criteria. This section also describes hydraulic and process capacity evaluations performed under the 2018 Facility Plan and summarizes results. Hydraulic and process capacities were combined to develop the RWHTF Major System treatment capacities presented in Section 7.2. This section builds off existing facility descriptions and capacity evaluations provided under the 2013 Facility Plan and incorporates changes implemented under the treatment improvement projects listed below. Updates to system capacities also consider LOS criteria developed under the 2018 Facility Plan.

• North Secondary Aeration Supply Optimization Study (PAR 1172)

• 2013 Facility Plan (PAR 1180)

• Plant Water Chlorinated (PWC) System Improvements project (PAR 1212)

• South Headworks and Grease Processing Improvements project (PAR 1225)

• Sidestream Nutrient Removal project (PAR 1237)

• Solids Processing Building Improvements project (PAR 1244)

• Digester Complex Rehabilitation project (PAR 1259)

• Nuisance Struvite and Dewaterability Improvements (PAR 1280)

• South Secondary Energy Analysis (PAR 1295)

• RWHTF Peracetic Acid Disinfection project (PAR 1314)

• TU 08 Load Center and Transformer Replacements project (PAR 1333)

The RWHTF is located eight miles northeast of Denver on a 134-acre site within the limits of Commerce City. The treatment facility is bordered by Sand Creek on the North East, Suncor on the South East, and the South Platte River on the West. An aerial view of the RWHTF and boundary lines of property owned by the Metro District is presented in Figure 7-1. Based on Site Approval 4726, the capacity of the RWHTF is 220 MGD for hydraulic flow (maximum monthly average daily flow) and 212 tons per day (tpd) of five-day biological oxygen demand (BOD5) for organic loading (maximum monthly average daily loading).

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Figure 7-1 RWHTF Property and Boundary Lines

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RWHTF EXISTING PROCESS FACILITIES

The RWHTF consists of seven Major Systems, as summarized in Table 7-1 and illustrated in Figure 7-2. In general, RWHTF systems comprise two independent liquid stream treatment processes (North Treatment Complex and South Treatment Complex) and one common solids treatment process (Solids Processing & Handling Complex). The North and South Complex each feature mechanical screening and grit removal, primary clarification, BNR, sidestream treatment, and disinfection processes. The Solids Processing & Handling Complex includes gravity thickening of primary sludge; grease processing; dissolved air flotation (DAF) thickening of waste activated sludge (WAS), surface waste activated sludge (SWAS), and secondary scum; sludge equalization and fermentation; acid phase and methane phase anaerobic digestion; centrifuge dewatering; phosphorous recovery; and biosolids conveyance, storage, and loading. A process flow diagram (PFD) of the entire treatment facility is presented in Figure 7-3 and schematics of the overall liquid and solid stream processes are shown in Figure 7-4 through Figure 7-8.

Table 7-1 RWHTF Major Systems, Subsystems, and Major Components

Major System Subsystem Major Component 100 Influent Interceptors and Flumes

Influent Interceptors and Flow Metering Facilities

North: Denver-Central

North: Clear Creek

North: Platte River

South: Denver-Sand Creek

South: Republic-Paper

South: Denver-Globeville

Primary Influent Flow Split (PIFS)

Sand Creek North Diversion

300 Preliminary Treatment

North Bar Screen Facility (301)

South Bar Screen Facility (302)

Headworks/Septage Receiving (310)

North Overflow Channel

South Overflow Channel

North Grit Handling Facility (321)

South Grit Handling Facility (322)

North Grit Pump Station (331)

South Grit Pump Station (332)

North Grit Basins (341)

South Grit Basins (342)

Preliminary Effluent (399) 400 Primary Treatment North Primary Clarifiers (401) North Primary Clarifiers 1-3 (NPRI1)

North Primary Clarifiers 4-6 (NPRI2)

North Primary Clarifiers 7-10 (NPRI3)

South Primary Clarifiers (402) South Primary Clarifiers 1-4

Primary Sludge Pumping North Primary Pump Station - 1 (411)

North Primary Pump Station - 2 (413)

North Primary Pump Station - 3 (415)

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Major System Subsystem Major Component South Primary Pump Station (412)

NPRI1 and NPRI2 Primary Scum Pumps

NPRI3 Primary Scum Pumps

SPC 1-4 Primary Scum Pumps

Primary Effluent (499) Primary Effluent Flow Split

500 Secondary Treatment North Secondary Aeration Basins (551)

North Secondary Aeration System North Secondary West Blower Facility (523)

North Secondary East Blower Facility (525)

North Secondary Pumping North MLR (1) Pumps

North RAS (2) Pumps

North WAS Pumps

North SWAS Pumps

North Scum Pumps

North Aeration Basin 2 North Basin 2 Pump Station (579)

North Primary Effluent Pump Stations (521)

North Secondary Clarifiers (541) -

South Secondary Aeration Basins (552)

Stage 5 Basins South Aeration Stage 5 West (534)

South Aeration Stage 5 East (536)

South Secondary Blower Facility (562)

South Secondary Pumping South MLR Pumps

South West RAS Pump Station (554)

South East RAS Pump Station (556)

South WAS Pumps

South SWAS Pumps

South Scum Pumps

South Primary Effluent Pump Station (522)

South Secondary Clarifiers (542) -

North Secondary Sidestream Reactor (531)

North CaRRBs (3)

North SAR

South Secondary Sidestream Reactor (532)

South CaRRBs

Deammonification

Secondary Effluent (599) North Secondary Effluent Channel

South Secondary Effluent Channel

North Carbon Addition Facility (597)

Carbon Stock Storage

Carbon Feed Pumps

700 Disinfection Plant Water Chlorinated PWC Pump Station (720) PWC Booster Pumps

Ammonia Ammonia Feed Facility/Tanks (701)

North Ammonia Sampling Facility (703)

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South Ammonia Sampling Facility (702)

Sodium Bisulfite SBS Storage

SBS Feed Pumps

Sodium Hypochlorite SHC Storage Facility

North Disinfection Dosing Facility (711)

South Disinfection Dosing Facility (712)

North Upstream Cl Sampling

South Upstream Cl Sampling

North and South Downstream Cl Sampling

Disinfection Contact Basins (717) North Parshall Flume

North Static Mixers

South Parshall Flume

South Static Mixers

Peracetic Acid North PAA Storage

South PAA Storage

North PAA Dosing

South PAA Dosing

Disinfection/Effluent Outfall (799) Reaeration Structures

Outfall Sample Facility (730)

800 Solids Processing and Handling

Dissolved Air Flotation Facility (809)

DAF Thickeners

ConWAS (4) Pumps

Gravity Thickeners Facility (810) Gravity Thickeners

Thickened Sludge Pumps

Primary Sludge Screening

Sludge Holding Tanks (827) SHT-1 (Fermenter)

SHT-2

SHT-3

Digester Feed Pumps (SHT-1)

Digester Feed Pumps (SHT-3)

Anaerobic Digestion Digester Complex - Tanks (1-10) (830)

Digester Complex - Tanks (11-12) (831)

Digested Sludge Pumps

Acid Sludge Pumps

Acid Recirculation Pumps

Phosphorus Recovery (867) Phosphorus Recovery Reactor

Phosphorus Recovery Aeration System

Struvite Pumps

Struvite Classifier

Biosolids Dewatering and Storage Facility (840)

Centrifuge Supply Pumps

Centrifuge Feed Pumps

Centrifuges

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Centrate System Centrate Foam Tank

Centrate Transfer Pumps

Centrate Holding Tank (807)

Centrate Feed Pumps

Grease Processing Facility (820) Scum Concentrators

Screened Grease Pumps

Cake Conveyance and Storage Conveyor System

Cake Storage Facility (850)

Acid Storage Facility (857) Ferric Chloride Storage

Ferric Chloride Feed Pumps to Centrifuge Feed Lines Ferric Chloride Feed Pumps to Digesters, DAF, and Centrifuge Discharge

Magnesium Chloride Storage

Magnesium Chloride Feed Pumps to Phosphorus Recovery

Polymer SP Emulsion Polymer Bulk Storage

SP Dry Polymer Solution Transfer Pumps

BDSF (5) Polymer Feed Pumps

DAF Liquid Polymer Bulk Storage

DAF Polymer Feed Pumps

900 Energy Recovery Cogeneration Cogeneration Facility (901)

Digester Complex Flares (920)

Channels & Conduits – Energy (999) 1Mixed Liquor Recycle 2Return Activated Sludge 3Centrate and RAS Reaeration Basin 4Concentrated WAS 5Biosolids Dewatering and Storage Facility 6 Major Systems, Subsystems, and Major Components were defined by the 2018 Facility Plan to support LOS evaluations and consist of process systems only. RWHTF systems and components in the table above and described in the following sections were organized in coordination with the RWHTF Asset Management Facility/Building Codes MAP-C-SP-069, excluding electrical, I&C, and support facilities.

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Figure 7-2 RWHTF Major Systems

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Figure 7-3 RWHTF Process Flow Diagram

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Figure 7-4 RWHTF Flow Schematic: Influent Interceptors, Preliminary Treatment, and Primary Treatment

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Figure 7-5 RWHTF Flow Schematic: Preliminary Treatment and Primary Treatment Solids Conveyance

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Figure 7-6 RWHTF Flow Schematic: Secondary Treatment Liquid Streams

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Figure 7-7 RWHTF Flow Schematic: Secondary Treatment Solids Conveyance

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Figure 7-8 RWHTF Flow Schematic: Solids Processing and Handling

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RWHTF PROCESS CAPACITY EVALUATION

The process capacity for each Major System at the RWHTF was determined by evaluating the hydraulic and treatment capacities of associated Subsystems and/or Major Components. The term hydraulic capacity refers to the maximum amount flow that can be passed through a component while treatment capacity refers to the amount of flow that can be treated in a system to a required standard. A system’s process capacity considers both hydraulic and treatment constraints. Hydraulic and/or treatment capacities are documented in this section based on CDPHE design criteria for domestic wastewater treatment works (WPC-DR-1 dated 09/15/2012) and are summarized in terms of their key design parameters. The WPC-DR-1 defines unit process capacities as follows:

• Fixed/Total Capacity = Capacity of a unit process with all facilities or units in operation

• Firm Capacity = Unit process capacity with the largest unit out of service

Hydraulic capacities for pumping systems supporting Major Systems are summarized in terms of the duty pumping capacity. Standby pumps are not included in the aggregate capacities.

Hydraulic and organic loading conditions of the fixed/total or firm capacity for Subsystems are listed where applicable under each Major System.

In Chapter 3, flow and load projections were developed through 2040. A summary of the RWHTF AAF projections are provided in Table 7-2. The AAF can be converted to MMF by multiplying the flow by the MMF peaking factor of 1.13. Similarly, the AAF can be converted to PHF by multiplying by the PHF peaking factor of 1.6.

Table 7-2 RWHTF Annual Average Flow and Loading Projections

Parameter 2020 2025 2030 2035 2040 Flow (MGD) 135 141 148 154 160 BOD (ppd) 322,000 337,000 354,000 369,000 383,000 TSS (ppd) 343,000 359,000 378,000 394,000 408,000 TKN (ppd) 51,300 53,700 56,500 58,900 61,000 TP (ppd) 7,470 7,790 8,160 8,480 8,760

The projected 2040 AAF of 160 MGD and MMF of 181 MGD were used as the basis for determining if a system has adequate process capacity through the planning period. Because process capacities are not always expressed in terms of AAF or MMF, they cannot be readily compared to the projected 2040 AAF/MMF. A direct comparison was facilitated by converting process capacities to their equivalent annual average flow (EAAF) and equivalent maximum month flow (EMMF). The resulting equivalent capacities were then directly compared to the 2040 AAF and MMF to determine if sufficient capacity is available through the planning period. The following methods were used to calculate equivalent capacities.

1. Establish fixed and firm flow or load capacity based on the unit(s) design criteria (e.g. peak hour flow, surface overflow rate, solids loading rate, etc.)

2. For Major Systems with hydraulically-governed capacity, flow peaking factors and flow projections are used to calculate EAAF and EMMF. For example:

a. Prior RWHTF designs utilized a PHF factor of 1.6 for design and rating of hydraulically governed processes. Therefore, EAAF is calculated first by dividing

2018 Facility Plan 7-15 PAR 1304 the peak hour flow (PHF) by 1.6. EMMF is then calculated by multiplying the EAAF by 1.13

b. The North Bar Screens firm capacity at PHF is 225 MGD. Dividing by the PHF pealing factor of 1.6 yields an EAAF of 141 MGD. The EMMF of 159 MGD is calculated by multiplying the EAAF by the 1.13 MMF peaking factor.

c. Similarly, the South Bar Screen firm capacity at PHF is 110 MGD. The corresponding EAAF and EMMF are 69 MGD and 78 MGD, respectively.

d. The combined North and South Bar Screens firm capacity is therefore 210 MGD EAAF and 237 MGD EMMF.

3. For unit processes with load-dependent capacity, load peaking factors and load projections are used to calculate EAAF and EMMF. For example:

a. BioWin™ modeling indicated that NSEC and SSEC have the combined capacity to treat 480,000 ppd BOD at maximum month load (based on flow splits, primary clarifier removal rates, CDPHE Regulation 85 effluent nutrient limits and other assumptions documented in following subsections).

b. Average annual and maximum month constituent concentrations were used to convert the load to an EAAF or EMMF. For example, the maximum month influent BOD concentration for the planning horizon is calculated to be 289 mg/L. The EMMF for the combined NSEC and SSEC is calculated as follows:

𝟒𝟒𝟒𝟒𝟒𝟒,𝟒𝟒𝟒𝟒𝟒𝟒 𝒍𝒍𝒍𝒍𝒅𝒅 𝑩𝑩𝑩𝑩𝑩𝑩

(𝟐𝟐𝟒𝟒𝟐𝟐𝒎𝒎𝒎𝒎𝑳𝑳 𝑩𝑩𝑩𝑩𝑩𝑩 × 𝟒𝟒.𝟑𝟑𝟒𝟒)

= 𝟏𝟏𝟐𝟐𝟐𝟐 𝑴𝑴𝑴𝑴𝑩𝑩

In this example, the 480,000 ppd BOD load rating at a BOD concentration of 289 mg/L corresponds to an EMMF of 199 MGD EMMF.

As noted previously, process capacities combine both hydraulic and treatment capacities and are a function of the performance of the individual unit processes, the interrelationships between these processes, and the redundancy required by CDPHE and/or stipulated by the Metro District requirements (i.e. redundancy LOS and ELOS). The results of the treatment capacity analyses for the liquid and solids treatment trains are presented in Figure 7-9. Between 2025 and 2040, the capacity of the RWHTF will be limited by the north aeration basins and secondary clarifiers, primary sludge gravity thickeners, and anaerobic digesters.

Because the RWHTF North and South raw influent concentrations are different, the combined plant flow, load, and concentration projections assume the total raw influent flow contribution from the North and South is constantly 70 and 30 percent, respectively. Subsystem and Major Component EMMFs can be compared to MMF projections through 2040 to determine when capacity-based process and equipment needs are triggered through the planning period. The projections and corresponding EMMFs are dependent on the per capita water usage. Changes to water usage impacts both flow and concentrations of constituents (e.g., BOD, TSS, TKN, TP, etc.) in plant influent. If constituent concentrations increase over time, the EMMF would decrease and vice-versa. However, the load-based rating of a particular unit process would remain the same. Therefore, as the Metro District continues to track unit process performance, it is critical that both flows and loads be monitored and benchmarked against the criteria limiting the capacity of each unit process.

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Figure 7-9 RWHTF Liquid and Solids Capacity - EMMF Condition

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RWHTF INFLUENT INTERCEPTORS AND PRELIMINARY

TREATMENT

Wastewater collected from the Metro District service area is conveyed to the RWHTF through a number of influent interceptors. The influent undergoes preliminary treatment at the North and South Headworks to remove screenings and grit. This section summarizes key design parameters and capacities for the associated Subsystems presented in Figure 7-10.

Wastewater is conveyed to the RWHTF through the Clear Creek, Central Denver, Platte River, Globeville/Denver Water Recycle, Republic Paperboard, and Sand Creek interceptors. The North Headworks receives flows from the Clear Creek, Central Denver, and Platte River Interceptors;

the South Headworks receives flows from the Sand Creek Parallel, Globeville, and Republic Paperboard Interceptors. Table 7-3 summarizes the raw influent interceptors and diversion flow metering facilities and their flow metering (hydraulic) capacities. Flume sizes are listed in terms of throat width as measured in feet.

Table 7-3 RWHTF Influent Interceptors and Flow Metering Design Information Major Component Equipment Description Capacity (MGD) North Interceptors Central Denver 12-ft Parshall Flume 164 Clear Creek 10-ft Parshall Flume 86 Platte River 6-ft Parshall Flume 75 South Interceptors Sand Creek Parallel 8-ft Parshall Flume 75 Republic Paperboard 4-in Magnetic Flow Meter 2 Globeville 16-in/18-in Magnetic Flow Meters 7 Flow Diversions Primary Influent Flow Split (PIFS) 3-ft Parshall Flume 96 Sand Creek North Diversion 48-in Magnetic Flow Meter 45

Total Influent Flow Measurement Capacity (excluding PIFS) (1) 454 1 PIFS flow measurement is not included because any influent flows transferred via PIFS are measured by upstream influent flumes;

the PIFS flume is not required to measure all influent flows.

The North Headworks also receives variable contributions from septage providers and currently receives in-plant waste (IPW) pump station return flows. The septage and IPW returns have minimal impact on the plant hydraulics but may affect organic loadings. The South Headworks can receive flow through the Denver Water Recycling Plant force mains and the Stormwater Pump Station, however the Metro District has not received flow from Denver Water in many years. The District is currently evaluating utilizing the Denver Water Recycling Plant force mains for alternate purposes. The South Headworks may also receive IPW depending on operating strategies.

The original Sand Creek “triple-barrel” Interceptor siphon is comprised of three parallel 48, 24, and 18-inch pipes. This interceptor was historically used to convey influent flows to the South Headworks prior to construction of the Sand Creek Parallel interceptor siphon, which consists of a single 60-inch pipe. The “triple-barrel” siphon has not been used in several years and is only used during high-flow events. There are concerns with the amount of debris that has accumulated in the pipelines and the potential impact on the South Headworks if this debris were to be flushed out. In an effort to address these concerns and update the infrastructure, PAR 1232 is currently designing a realignment and replacement of the Sand Creek triple-barrel interceptor siphon. The tentative plan is to install a 60-inch pipeline adjacent to the Sand Creek Parallel.

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Figure 7-10 RWHTF Major System: Influent Interceptors and Preliminary Treatment

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Diversion structures located throughout the facility are used to transfer influent and primary effluent flow between the North and South Treatment Complexes based on available capacity, discharge permit requirements, and/or construction sequencing constraints. Two hydraulic diversion structures provide the capability to transfer influent flows between the North and South Headworks: the Primary Influent Flow Split (North-to-South flow diversion) and the Sand Creek North Diversion (South-to-North flow diversion). PAR 1225 included the installation of a third diversion structure on the Platte River and Globeville interceptor lines. This new diversion structure provides the ability to divert Platte River influent to the South Complex via a new 54” pipeline. A schematic of the influent interceptors and diversion structures is provided in Figure 7-11.

Figure 7-11 RWHTF Influent Interceptors Plan

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The North Headworks consists of a 120-ft long by 20-ft wide influent channel with 7.5-ft side walls, which leads to the inlet structure of the North Bar Screen Facility. The normal sidewater depth in the influent channel is approximately 2.7 ft. The shallow channel depth allows for mixing of the various influent flows and prevents grit deposition. There is a passive overflow channel around the North Bar Screen Facility to the grit basins that is designed to prevent an overflow condition if the bar screens are blinded or incoming peak flow surpasses their hydraulic capacity. The overflow channel has an approximately 80-ft long weir wall along the west side of the inlet channel at elevation 5127.50 ft.

The South Headworks consists of a 38-ft long by 17-ft wide influent channel with 9.5-ft side walls, which leads to the inlet structure of the South Bar Screen Facility. The normal sidewater depth in this channel is approximately 3.25 ft, and the turbulence of the channel mixes the influent flows and prevents grit deposition. The South Headworks also has provisions to handle screen blinding events and flows in excess of its capacity via a 37-ft long overflow weir at elevation 5122.00 ft.

This overflow structure transfers flow around the bar screens to the South Grit Basins.

The North Bar Screen Facility houses six mechanically cleaned climber screens with 3/8-inch openings. The South Bar Screen Facility houses five mechanically cleaned climber screens with 3/8-inch openings. The bar screens remove large debris and rags to protect downstream equipment. Screenings are collected and conveyed to one of two auger-type washer/compactor units via a single sluiceway in each bar screen facility. These washer/compactors wash out organics and compact the screenings. Dewatered screenings are conveyed to a tipping trough in each facility where the screenings are evenly distributed into a roll-off dumpster prior to hauling for landfill disposal. Organics that are washed from the screenings are conveyed to the IPW pump station and are ultimately conveyed back to the North influent channel.

Table 7-4 summarizes the North and South bar screen capacities. WPC-DR-1 requires screening facilities to be designed with a firm capacity meeting PHF conditions. The North bar screen capacity is based on a 6-inch maximum headloss at 30 percent screen blinding. The South bar screen capacity is based on 8.5-inch maximum headloss at 40 percent screen blinding.

Table 7-4 RWHTF North and South Bar Screens Design Information Major Component Equipment Description Capacity (MGD) (3)

North Bar Screens Quantity: 6 Type: 3/8-inch mechanical climber screens PHF Capacity: 45 MGD (ea.) (1)

South Bar Screens Quantity: 5 Type: 3/8-inch mechanical climber screens PHF Capacity: 27.5 MGD (ea.) (2)

Combined North and South Firm Screening Capacity 335 1 PAR 1116 Site Location Amendment Approval Letter 2 PAR 1225 PDR Approval and Streamlined-Design Request Letter Acceptance 3 Firm capacity designed for PHF conditions

The bar screen inlet overflow channel capacities are summarized in Table 7-5. The overflow weirs are sized to accommodate design peak hour flow conditions. If the screens become blinded, the influent channel water surface elevation will increase until the influent overflows a weir into the overflow channel.

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Table 7-5 RWHTF North and South Overflow Channel Design Information

Major Component Capacity (MGD) (3)

North Overflow Channel 230 (1) South Overflow Channel 110 (2)

Combined North and South Overflow Channel Capacity 340 1 PAR 1116 Site Location Amendment Approval Letter 2 PAR 1225 PDR Approval and Streamlined-Design Request Letter Acceptance 3 Designed for PHF conditions

The PHF-based capacities above correspond to a North Bar Screens firm equivalent capacity of 141 MGD EAAF / 159 MGD EMMF and a South Bar Screen firm equivalent capacity of 69 MGD EAAF / 78 MGD EMMF. The combined North and South facilities provide a total capacity of 210 MGD EAAF / 237 MGD EMMF, which is projected to be sufficient through the planning period.

The North Headworks has six, 20-ft diameter vortex-type grit basins and the South Headworks will have four, 20-ft diameter vortex-type grit basins following completion of PAR 1225. Grit removed from the screened raw influent is pumped to the grit washers in the North and South Grit Facilities. The grit is washed to remove organics and then dewatered in vortex washer/classifiers before it is distributed in roll-off dumpsters and hauled to a landfill for final disposal. The organics flow from the grit washers is conveyed to the IPW pump station where it is ultimately conveyed back to the North Influent Channel. Table 7-6 summarizes the North and South grit removal capacities. WPC-DR-1 requires the firm capacity of grit removal facilities to meet PHF conditions.

The North and South grit basin capacities are based on a grit loading rate of 2 cubic feet per million gallons (CF/MG).

Table 7-6 RWHTF North and South Grit Basins Design Information Major Component Equipment Description Capacity (MGD) (3) North Grit Basins Quantity: 6

Size: 20-ft diameter Type: Vortex grit basins Capacity: 50 MGD (ea.) (1)

South Grit Basins Quantity: 4 Size: 20-ft diameter Type: Vortex grit basins Capacity: 37 MGD (ea.) (2)

Combined North and South Firm Grit Removal Capacity 361 1 PAR 1116 Site Location Amendment Approval Letter 2 PAR 1225 PDR Approval and Streamlined-Design Request Letter Acceptance 3 Firm capacity designed for PHF conditions

The PHF-based capacities above correspond to a North Grit Basins firm equivalent capacity of 156 MGD EAAF / 177 MGD EMMF and a South Grit Basin firm equivalent capacity of 69 MGD EAAF / 78 MGD EMMF. The combined North and South grit basins provide a total capacity of 225 MGD EAAF / 255 MGD EMMF, which is projected to be sufficient through the planning period.

RWHTF PRIMARY TREATMENT

Primary treatment at the RWHTF consists of primary clarification, primary sludge pumping, combined primary sludge thickening, and primary effluent flow transfers as shown in Figure 7-12.

Primary influent is conveyed from the grit basins to the primary clarifiers, which provide physical separation of settleable solids and contribute to the plant’s organics removal capacity. The current rated design capacity of the primary treatment system is 225 MGD and 215 tpd of TSS on an

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MMF basis. This capacity corresponds to the largest unit out of service in both the North and the South Treatment Complexes.

There are fourteen primary clarifiers at the RWHTF, ten in the North Primary Complex (NPC) and four in the South Primary Complex (SPC). All of the primary clarifiers in both complexes are center feed, horizontal flow units with single peripheral effluent weirs. The NPC is divided into three areas: three 158-ft diameter clarifiers in area 1 (NPRI1), three 158-ft diameter clarifiers in area 2 (NPRI2), and four 106-ft diameter clarifiers in area 3 (NPRI3). The SPC has four 150-ft diameter primary clarifiers.

The process capacities for the primary clarifiers are summarized in Table 7-7. These capacities use the surface overflow rate (SOR) as the critical hydraulic condition for treatment. CDPHE sets maximum surface overflow rates for primary clarifiers from 800 to 1,200 gpd/sf under MMF conditions and from 2,000 to 3,000 gpd/sf under PHF conditions. The SOR values listed below are in exceedance of this standard but were approved by the WQCD under the Primary Treatment Improvements project (PAR 877).

Table 7-7 RWHTF North and South Primary Clarifier Design Information Major Component Equipment Description Capacity (MGD) (2)

North Primary Clarifiers 1-6 (NPRI1 and NPRI2)

Quantity: 6 Diameter: 158-ft Sidewater Depth: 10-ft SOR: 1,070 gpd/sf (1)

Capacity: 21 MGD (ea.)

North Primary Clarifiers 7-10

(NPRI3)

Quantity: 4 Diameter: 106-ft Sidewater Depth: 9.5-ft SOR: 1,350 gpd/sf (1)

Capacity: 12 MGD (ea.)

South Primary Clarifiers 1-4 Quantity: 4 Diameter: 150-ft Sidewater Depth: 11-ft SOR: 1,350 gpd/sf (1)

Capacity: 24 MGD (ea.)

Combined North and South Fixed Primary Clarifier Capacity 270 1 PAR 877 Site Location Amendment Approval Letter 2 Fixed capacity based on SOR at MMF conditions

For the process capacity evaluation, it was assumed that the design MMF SORs listed in Table 7-7 are the limiting conditions and the firm capacity is calculated with the largest unit out of service in both the North and the South Treatment Complexes. Given these assumptions, the NPC firm equivalent capacity is 135 MGD EAAF / 153 MGD EMMF and the SPC firm equivalent capacity is 63 MGD EAAF / 72 MGD EMMF. The combined North and South primary clarifiers provide a firm equivalent capacity 198 MGD EAAF / 225 MGD EMMF. This capacity is projected to be sufficient through the planning period.

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Figure 7-12 RWHTF Major System Primary Treatment

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Primary clarifier BOD removal efficiency is typically assumed to be 25 percent for the current rated overall treatment capacity. However, the Primary Clarifier Performance Evaluation observed average combined BOD removals of 32 percent and average TSS removal of 70 percent. As a result of the 2014 Primary Clarifier Performance Evaluation, the Metro District established a maximum SOR target of 1,000 gpd/sf in the summer season and 700 gpd/sf in the winter season to achieve 70 percent TSS removal as a performance goal for the North Primary Clarifiers. This study assumed the same performance criteria for the South. An updated evaluation is recommended to confirm these assumptions. Historical primary clarifier performance data, from 2015 to 2017, indicates that the average North Treatment Complex primary clarifier TSS removal efficiency is 55 percent while the average South Treatment Complex primary clarifier TSS removal efficiency is 70 percent, which is considered high relative to typical primary clarifier performance.

To prevent fermentation of settled solids and to increase clarifier efficiency, primary sludge pumping rates are set to obtain a thin underflow concentration and short sludge blanket retention time (i.e., thin-sludge pumping). The Metro District targets 1.5 percent total solids (TS) concentrations in the settled sludge before it is pumped to the gravity thickeners for primary sludge thickening. Recessed impeller primary sludge pumps in the North (one pump station per clarifier area) and South (one pump station) are operated to pump sludge between 0.5 to 2.0 percent solids. Primary sludge pumping capacities in gallons per minute (gpm) are provided in Table 7-8.

The primary sludge pumps were sized to accommodate the peak daily solids loading (PDSL).

Table 7-8 RWHTF North and South Primary Sludge Pumping Design Information

Major Component Equipment Description Duty Capacity (gpm) NPRI1 and NPRI2 Primary Sludge Pumps

Quantity: 12 (6 duty) Type: Centrifugal recessed impeller Capacity: 400 gpm (ea.)

2,400

NPRI3 Primary Sludge Pumps Quantity: 6 (4 duty) Type: Centrifugal recessed impeller Capacity: 240 gpm (ea.)

South Primary Clarifiers 1-4 Primary Sludge Pumps

Quantity: 5 (4 duty) Type: Centrifugal recessed impeller Capacity: 500 gpm (ea.)

2,000

Primary scum from the primary clarifiers is transferred to scum pits where primary scum pumps recirculate and transfer the contents of the scum pit to the Grease Processing Facility. Scum is recirculated and transferred by centrifugal chopper pumps. Primary scum pumping capacities are provided in Table 7-9.

Table 7-9 RWHTF North and South Primary Scum Pumping Design Information

Major Component Equipment Description Duty Capacity (gpm) NPRI1 and NPRI2 Primary Scum Pumps

Quantity: 8 (6 duty) Type: Centrifugal chopper Capacity: 220 gpm

1,320

NPRI3 Primary Scum Pumps

Quantity: 4 (2 duty) Type: Centrifugal chopper Capacity: 220 gpm

South Primary Clarifiers 1-4 Primary Scum Pumps

Quantity: 4 (2 duty) Type: Centrifugal chopper Capacity: 220 gpm

2018 Facility Plan 7-25 PAR 1304

North primary effluent is conveyed to the North Secondary Influent (NSINF) Channel where it can either flow to the NSEC or be diverted to the SSEC via the Primary Effluent Flow Split (PEFS) Control Structure. This flow transfer provides flexibility for the Metro District to manage varying flow and loading conditions and allows the District to take advantage of the greater capacity in the North Headworks and the NPC. The PEFS utilized continuously to balance the primary effluent flows between the NSEC and SSEC.

The PEFS Control Structure is located on the west end of the NSINF Channel. The PEFS inlet gate allows flow to enter the control structure via a 48-inch pipeline. The transfer flow is controlled by a modulating flow control valve and an ultrasonic flow meter within the structure. The transfer flow then enters the 72-inch North Primary Effluent (NPEFF) Transfer pipeline.

The hydraulic capacity of the PEFS Control Structure and the downstream NPEFF Transfer pipeline is approximately 75 MGD, depending on the hydraulic grade line between the NSINF Channel and the downstream wet well at the Primary Effluent Pump Station (PEPS). The transfer flow discharges into the South Primary Effluent (SPEFF) Conduit, where it combines with effluent flows from the South Primary Clarifiers. The combined flow is conveyed to the PEPS and pumped to the South Secondary Aeration Basins.

RWHTF NORTH SECONDARY TREATMENT

The NSEC consists of twelve activated sludge trains (i.e., aeration basins and secondary clarifiers), each fed primary effluent from the NSINF Channel. Space is available for an additional two aeration basins and two clarifiers to be installed on the east side of the NSEC. Within each activated sludge treatment train, mixed liquor flows from an aeration basin to a dedicated secondary (i.e., final) clarifier. The NSINF Channel also conveys return activated sludge (RAS), sidestream centrate treatment return flows, and sidestream anaerobic reactor (SAR) effluent flows to each treatment train. Secondary influent is diverted to each train using a modulating weir gate.

The locations of associated NSEC Subsystems are shown in Figure 7-13 and an overview of the NSEC biological nutrient removal process flow is presented in Figure 7-14.

A Secondary Treatment process capacity evaluation summary is provided in Section 7.7 including BioWin™ model updates. Detailed descriptions for both the 2018 Facility Plan Process Modeling and 2018 Facility Plan Aeration Systems Evaluation are provided in Appendix G and Appendix H, respectively.

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Figure 7-13 RWHTF Major System: North Secondary Treatment

2018 Facility Plan 7-27 PAR 1304

Figure 7-14 RWHTF North Secondary Biological Nutrient Removal Process Flow Diagram

2018 Facility Plan 7-28 PAR 1304

The NSEC aeration basins are configured in a Modified Ludzack-Ettinger (MLE) process for nitrogen removal with a three-pass plug flow layout. The aeration basins can operate in conjunction with the SARs for Enhanced Biological Phosphorus Removal (EBPR). The A-Pass consists of three anoxic zones for denitrification (reduction of nitrate to nitrogen gas), followed by a swing zone and two oxic zones. The B-Pass and C-Pass are operated aerobically for BOD oxidation and nitrification (oxidation of ammonia to nitrite and further to nitrate). The nitrate-rich mixed liquor recycle (MLR) is pumped from the end of C-Pass to the anoxic zones at the front of the A-Pass for denitrification. Primary effluent, CaRRB effluent, SAR effluent, and RAS are mixed in the NSINF channel and distributed to each aeration basin.

Mechanical mixers are located in the NSINF channel and anoxic zones to help maintain solids suspension when needed; however, based on testing, the Metro District found that the mixers are generally not required to operate continuously for adequate solids suspension.

Fine bubble diffuser grids deliver oxygen and provide mixing in the oxic zones. The NSEC process operates at an SRT in the range of three to six days, with longer SRTs required at lower mixed liquor temperatures. Table 7-10 summarizes the NSEC aeration basin design hydraulic and organic loading capacities. The design capacities are based on MMF conditions with all basins in service.

Table 7-10 RWHTF North Secondary Aeration Basin Treatment Design Information Subsystem Equipment Description Capacity (2) Aeration Basins 1-12 Quantity: 12

Type: Three-pass activated sludge Configuration: MLE SRT: 3 to 6 days Volume: 2.05 MG (ea.)

Capacity: 8.8 MGD (ea.)

106 MGD (1)

79 tpd BOD (1)

Fixed NSEC Aeration Basin Capacity 106 MGD 1 North Secondary Quads Project (PAR 942) PDR Approval Letter 2 Fixed capacity based on MMF conditions

The NSEC Aeration System consists of the fine bubble diffusers (membrane in swing zones and ceramic in aerobic zones), piping, and single stage blowers. Blowers are located in the East and West Blower Facilities. The blowers supply air to the aeration basins through the interconnected aeration supply piping network. The NSEC blower design capacities are summarized in Table 7- 11 in terms of standard cubic feet per minute (scfm).

Table 7-11 RWHTF North Secondary Aeration System Design Information Major Component Equipment Description Duty Capacity (scfm) West Blower System Quantity: 4 (3 duty)

Type: Single stage blowers Capacity: 32,500 scfm (ea.)

Horsepower: 1,500 horsepower (hp)

97,500

East Blower System Quantity: 2 (1 duty) Type: Single stage blowers Capacity: 45,450 scfm (ea.)

Horsepower: 2,500 hp

45,450

Combined East and West Duty Blower Capacity 175,450

2018 Facility Plan 7-29 PAR 1304

The Metro District has the ability to control process aeration through either a dissolved oxygen (DO) or an ammonia-based aeration control (ABAC) strategy. Operating with the ABAC control strategy allows the District to reduce over-aeration in the activated sludge process, which can reduce energy costs and improve denitrification performance. Due to turn-down limitations of the existing blowers and the diurnal low-flow periods, the aeration system control process cannot be fully optimized. Over-aeration still occurs during periods of low flow and loading. Typically, the District operates one large blower (i.e., 2,500 hp) and one small blower (i.e., 1,500 hp). During peak conditions, a second small blower is brought into service.

The blowers in the West Blower Facility have been operational since 1968, and the blowers in the East Blower Facility have been operational since 1988. Throughout the years, the blowers have been reliable, and a number of upgrades have been performed; however, the major equipment (i.e., the blowers and motors) and the infrastructure (i.e., piping, valves, controls, etc.) are aging and require increased maintenance and resources.

Sidestream treatment in the NSEC supports the management of recycle nitrogen loads in the centrate stream, which can contribute up to 25 percent of the total ammonia load treated at the facility. Additional sidestream volume is utilized to support enhanced biological phosphorus removal. Managing the phosphorus loads and centrate recycle loads is critical to meeting current and future effluent permit limits for nitrogen and phosphorus.

Treating centrate in the sidestream reduces the total ammonia load requiring treatment in the mainstream secondary treatment process. Sidestream centrate (or deammonification effluent) treatment in the NSEC is provided through CaRRBs. The sidestream phosphorus load will be managed prior to dewatering via a new phosphorus recovery process. The sidestream anaerobic reactors provide the anaerobic volume necessary to support mainstream EBPR. The sidestream phosphorus load is managed prior to the centrate via the phosphorus recovery process.

The CaRRB process provides partial nitrification of recycle ammonia loads by commingling centrate with RAS and supplying diffused air. The extent of nitrification is a function of the total ammonia load applied, the available hydraulic retention time (HRT), and the alkalinity available in the reactors. There are two CaRRBs in the NSEC. The Metro District has demonstrated that the NSEC CaRRBs can achieve 50 percent nitrification of the entire recycle centrate ammonia load without supplemental alkalinity addition (PAR 942 PDR). Because this level of sidestream nitrification has provided sufficient nitrogen removal in the NSEC, supplemental alkalinity delivery systems were decommissioned in 2017.

The NSEC CaRRBs can operate in denitrification mode in the last two zones of the reactor’s plug-flow configuration by turning off the air supply, mixing the mixed liquor suspended solids (MLSS) with vertical shaft mixers in lieu of diffused air mixing, and providing a carbon source in the form of acetic acid, for example. These zones can be supplied with external carbon stock in the form of acetic acid from dedicated storage and feed facilities. Historically, the Metro District has not operated these anoxic zones with the supplemental acetic acid feed. The NSEC CaRRB design information is summarized in Table 7-12. The overall capacity of the North and South CaRRBs is discussed in Section 7.6.4.2.

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Table 7-12 RWHTF NSEC CaRRB Design Information Subsystem Equipment Details Capacity (MGD) (1) North CaRRBs (CaRRB 1 and 3)

Quantity: 2 Volume: 0.68 MG (ea.)

Centrate Flow: 2.0 MGD (ea.)

RAS Flow: 30.6 MGD (ea.)

HRT: 3.5 hr (ea.)

2.0

1 Capacity shown is specific to centrate treatment capacity; total hydraulic capacity includes RAS flows

The SARs are repurposed CaRRBs used to anaerobically condition RAS for EBPR in the NSEC.

The SARs are provided with soluble carbon, primarily in the form of volatile fatty acids (VFAs), to drive the release of orthophosphate from polyphosphate accumulating organisms (PAOs) present in the RAS. The soluble carbon source is Gravity Thickener Effluent (GTE), which is the clarified supernatant of the gravity thickeners.

The SARs are located on the southwest side of the NSEC aeration basins. The system consists of two reactors: SAR No. 2 and SAR No. 4, which were previously CaRRB No. 2 and No. 4. Each SAR is divided into an anaerobic zone with three 2-hp mixers, followed by two swing zones each with a 5-hp mixer and fine bubble diffusers. During normal operation, the SARs will operate in a completely anaerobic condition, without aerating the swing zones. Effluent from SARs flows into the NSINF Channel where it combines with RAS bypass flow, CaRRB effluent, and primary effluent flows. The flows are mixed in the NSINF Channel prior to distribution to the NSEC Aeration Basins.

SAR design information is summarized in Table 7-13. This information is based on design flows with both reactors in service. The total capacity of the SARs is a function of the NSEC RAS pumping system and the GTE conveyance system. The SARs are capable of receiving all pumped flow from RAS Pump Stations No. 2 and No. 4 in the NSEC South Gallery. The amount of GTE conveyed to the SARs is a function of gravity thickener (GVT) operations. The GTE flow control system is capable of conveying up to 3.5 MGD to each SAR. If supplemental internal carbon is required in the SSEC, a portion of the GTE flow can be diverted via the PEFS Control Structure and the GTE Diversion Structure.

Table 7-13 RWHTF Sidestream Anaerobic Reactors Design Information Subsystem Equipment Description Capacity (MGD) Sidestream Anaerobic Reactors 2 and 4

Quantity: 2 Volume: 0.72 MG (ea.)

RAS Flow Range: 10 – 26 MGD (ea.)

GTE Flow Range: 2.5 – 3.5 MGD (ea.)

Hydraulic Retention Time (HRT): 0.6 – 1.3 hours

59 (1)

1Hydrualic capacity assuming an HRT of 6.0 hours

Assuming a fixed total SAR volume of 1.44 MG and the minimum retention time of 0.6 hours, the maximum total flow to the SARs is 59 MGD. In 2040, the maximum RAS flow to the SARs is 45.5 MGD, while GTE flow is 7.2 MGD, resulting in a total flow to SARs of 52.7 MGD. This suggests the SARs have sufficient capacity through the planning period.

2018 Facility Plan 7-31 PAR 1304

North secondary pumping systems include MLR pumping, RAS pumping, and WAS and SWAS pumping as described in the following sections.

Each NSEC aeration basin has two MLR pumps (one constant speed and one variable speed).

The variable speed pump has a capacity of 9.5 MGD, and the constant speed pump has a capacity of 9.8 MGD. MLR pumping capacity is summarized in Table 7-14. The pumps include independent suction piping from the aeration basin effluent box and independent discharge piping to the MLR channel. Both MLR pumps are intended to operate at all times. The 9.5 MGD MLR pump has a magnetic flow meter on the discharge to record the MLR flowrate. The MLR pumps can return a maximum of 218 percent of the maximum month flow back to the anoxic zones. The VFD is used to vary the total MLR rate to optimize nitrate reduction without returning too much oxygen to the anoxic zones.

Table 7-14 RWHTF North Secondary MLR Pumping Design Information Major Component Equipment Description Duty Capacity (MGD) (1) MLR Pumps

Quantity: 24 (2 per basin) Type: Horizontal non-clog centrifugal Capacity, Constant Speed Pumps (1-12A): 9.8 MGD (ea.)

Capacity, Variable Speed Pumps (1-12B): 9.5 MGD (ea.)

1 PAR 942 PDR Approval Letter; rated capacity assumes all pumps in service

The NSEC RAS pumping system consists of four RAS pumping stations, or quads, where one quad is a grouping of three clarifiers. Each quad includes four variable speed pumps, with an individual RAS pump directly connected to a clarifier, and a fourth standby pump for redundancy.

Each pump station conveys RAS flows either to the CaRRBs (Quad 1 and 3), the SARs (Quad 2 and 4), or directly to the NSINF Channel and are controlled using flow meters, control valves, and VFDs. RAS pumping system design information is summarized in Table 7-15. The RAS pumping capacity allows for returning a maximum of 98 percent of the design maximum month flow.

Table 7-15 RWHTF North Secondary RAS Pumping Design Information Major Component Equipment Description Duty Capacity (MGD) (1) RAS Pumps Total Quantity: 16 (4 per quad)

Duty Quantity: 12 (3 per quad) Type: Vertical non-clog centrifugal Capacity: 8.7 MGD (ea.)

104.4

1 PAR 942 PDR Approval Letter; rated capacity assumes all duty pumps in service

The NSEC includes two methods for sludge wasting. One method wastes directly from the RAS header pipelines through two pumping stations that convey WAS to the DAF thickening process.

Each station utilizes two WAS pumps (one duty, one standby). The second wasting method uses surface wasting weirs from the sidestream reactors (i.e., CaRRB and SAR). The SWAS pumping system consists of two pumping stations (one duty and one standby pump per station) and one wet well per pump station that receives surface wasted flows from adjacent reactors via scum collection troughs. The SWAS pumps convey flows to the DAF thickening process.

2018 Facility Plan 7-32 PAR 1304

While scum collection and SWAS pumping capabilities are available for the SARs, no wasting from these reactors will occur when they are operating in EBPR mode with VFA present in the mixed liquor (i.e., GTE conveyance in service). NSEC surface wasting capacity is effectively reduced to the SWAS pump station associated with the CaRRBs. The North WAS/SWAS Pumping System is summarized in Table 7-16.

Table 7-16 RWHTF North Secondary WAS/SWAS Pumping Design Information Major Component Equipment Description Duty Capacity (gpm) (1) WAS Pumps Quantity: 4 (2 duty)

Type: Horizontal non-clog centrifugal Capacity: 750 gpm (ea.)

1,500

SWAS Pumps Quantity: 4 (2 duty) Type: Horizontal non-clog centrifugal Capacity: 250 gpm (ea.)

Total NSEC Wasting Capacity 2,000 gpm (2.9 MGD) 1 PAR 942 PDR Approval Letter; rated capacity assumes all duty pumps in service

The North Carbon Addition Facility was installed under North Secondary Improvements Supplemental Carbon Addition project (PAR 1073). This facility was primarily used as an acetic acid feed system for supporting denitrification in the NSEC CaRRBs. The system was modified under PAR 1237 to provide backup soluble carbon to the SARs when the GTE system is out of service. The feed facility includes two, 5,400-gallon double-walled high-density polyethylene (HDPE) storage tanks and two peristaltic metering pumps. The supplemental carbon has not yet been required for either CaRRB or SAR operation, but the feed system is available, if needed.

Carbon addition capacity information is provided in Table 7-17.

Table 7-17 RWHTF Carbon Stock Addition Design Information Major Components Equipment Details Capacity Carbon Stock Storage Tanks Quantity: 2

Type: Double-wall HDPE Capacity: 5,400 gallons (ea.)

10,800 gallons

Carbon Stock Feed Pumps Quantity: 3 Type: Peristaltic Capacity: 200 gpm (ea.)

600 gpm

The system was temporarily retrofitted in 2016 to conduct the Carbon Augmentation Investigation.

The intent of the evaluation was to determine, and quantify, if a liquid waste stream generated by Waste Management (WM) in the recycling of consumer-packaged beverage containers could be beneficially used at a local wastewater treatment facility. The liquid waste stream (referred to as “beer”) from WM was predominantly composed of expired beer and small amounts of soda, which are byproducts of the container recycling process.

Major objectives of the Carbon Augmentation Investigation included identifying logistical and operational considerations for importing and using beer, determination of chemical composition and consistency, and demonstration of the beneficial use of beer in the wastewater treatment process. Results of the six-month pilot test consistently demonstrated that beer was an effective alternative carbon source for EBPR and denitrification in an activated sludge wastewater treatment process.

2018 Facility Plan 7-33 PAR 1304

The secondary clarifiers provide final liquid-solids separation prior to disinfection. The North clarifiers consist of six main elements: the center column, the energy dissipating inlet (EDI), the flocculation zone, the effluent launders, the sludge collection mechanism, and the secondary scum handling system.