2017 Excellence in Environmental Engineering and Science® Awards Competition Winner

Grand Prize - Design

Robert W. Hite Treatment Facility South Secondary Improvements Project

Entrant: Carollo Engineers, Inc.
Engineer in Charge: John Fraser, P.E.
Location: Denver, Colorado
Media Contact: John Fraser, Carollo Engineers, Inc.

Entrant Profile

Metro Wastewater Reclamation District's service area in Denver, Colorado.

Metro Wastewater Reclamation District (District) is comprised of 60 local governments providing wastewater services to about 1.8 million people in a 715-square-mile service area throughout greater Denver. The District currently handles 130 million gallons of wastewater each day (mgd) at the Robert W. Hite Treatment Facility (RWHTF). The RWHTF is the largest wastewater plant between the Mississippi River and the west coast, and is rated for a 220 mgd treatment capacity. Originally constructed in the mid 1960s, the RWHTF has under-gone several upgrades including the recent $151 million South Secondary Improvements Project that allows the entire plant to remove ammonia and nitrogen to below permitted discharge levels, and sets the District up for future phosphorus removal. More than 95 percent of the raw wastewater pollutants are removed at the RWHTF prior to discharge to the South Platte River.

In 2016 the District commissioned the Northern Treatment Plant (NTP) located in Brighton, CO to provide 24 mgd of capacity for connectors northeast of Denver. The NTP uses advanced treatment prior to dis- charge to the South Platte River.

The District's treated water makes up more than 85 percent of the South Platte River nine months of the year and is suitable for agriculture, fish and aquatic life, industrial use, water supply, and recreation.

The District land applies stabilized biosolids at private farms and the District's METROGRO Farm. The award winning Resource Recovery and Reuse programs at the District serve as a national example of stewardship of our nation's agricultural and prairie lands.

Owner: Metropolitan Wastewater Reclamation District
Engineer: Carollo Engineers, Inc.
Construction Contractor: Wester Summit/McCarthy (WSMC)

Project Description

The Largest Operating Conversion of Pure Oxygen Activated Sludge to BNR in the US

Metro Wastewater Reclamation District's (MWRD) 90-mgd High Purity Oxygen System (HiPOS), that provided nearly half of the 220-mgd capacity at the Robert W. Hite Treatment Facility (RWHTF), could not meet future nutrient limits. As a result, MWRD constructed a new South Aeration Complex (SAC) to treat 114-mgd to an effluent below 1.75 mg/L ammonia, 8.68 mg/L nitrogen (NO₂+NO₃), and prepare for future phosphorus (TP) limits.

This $134 million construction project included:

Initiating a Compost Demolition and Site Preparation project one year prior to SAC construction.
Constructing the SAC with six BNR aeration basins, three sidestream Centrate and RAS Reaeration Basins (CaRRB), a Primary Effluent Pump Station (PEPS), a blower building, two electrical buildings, and piping and equipment galleries.
Repurposing eight obsolete HiPOS basins into two RAS/WAS pump stations, stage-5 aeration, and mixed liquor splitter boxes.
Renovating 10 secondary clarifiers.

Throughout the five-year construction duration, the existing plant remained in operation and compliant with discharge criteria.

3D CADD image of the intergrated simple structure SAC, providing 114-mgd of BNR capacity including spacious piping and equipment galleries for safe O&M.

"One Plant" Integrated Approach

For decades, MWRD operated two different types of processes. This provided flexibility for effluent compliance but was difficult and costly to operate and maintain. This project created "one plant" with similar operations and maintenance (O&M) characteristics for simplified O&M.

Construction costs were well under $1.50 per gallon. Savings were achieved by designing the SAC as one common, integrated structure larger than three football fields. PEPS, blower building, and electrical rooms were integrated into one multi-level West Mechanical Building.

HiPOS basins were repurposed with new RAS/WAS pump stations. The existing clarifier capacity increased by 25 percent by integrating unique maze-type Energy Dissipating Inlets (EDIs).

50-mgd RAS/WAS pump station repurposed inside the existing HiPOS aeration basins.

Integration Accomplishments

"One Plant" operations with new SAC.
SAC structure integrating aeration basins, sidestream treatment, pumping, piping, galleries, and West Mechanical Building.
Repurposing HiPOS basins to integrate RAS/WAS pump stations, stage-5 aeration, and mixed liquor splitter boxes.

Quality, User Satisfaction, and Performance

The project provides low-cost BNR capacity, exceptional plant performance, operational flexibility, and logical layout for ease of O&M.

MWRD Satisfaction and System Performance Accomplishments

114-mgd of BNR treatment for under $1.50/ gallon.
Effluent quality that exceeds NPDES criteria.
Significant reduction of pollutants to the South Platte River.
Flexibility to operate in a variety of modes.
Logical layout and accessible equipment, piping, and electrical gear.

Treated effluent quality is currently below the required nutrient discharge standards as indicated in the table below.

Figure 1 - Six months of data operating under A2O configuation resulted in effluent nitrate (Sec Eff NO3) ranging from about 3 to 8 mg/L and averaging less than 5 mg/L. Secondary effluent ortho phosphate (SE OP) and total phosphorus (SE TP) was consistently less than 1 mg/L.

Figure 2 - Effluent ammonia prior to new plant startup ranged from 10 to 13 mg/L with significant variability. After full operation in 2015, the facility consistently achieved values of less than 1 mg/L.

Figure 3 - Effluent total inorganic nitrogen (TIN) prior to new plant startup ranged from 13 to 17 mg/L with significant variability. After full operation in 2015, average TIN values were consistently under 5 mg/L.

Figure 4 - Flexible aeration basin configuration allows MWRD staff to operation under a variety of modes optimizing performance and reducing treatment costs. The CaRRB sidestream treatment process provides reduction of ammonia and mainstream bioaugmentation for supercharged performance.

Rendering of interconnected SAC and modified existing HiPOS, clarifiers, and underground piping illustrates the project complexity.

Effluent nitrogen and phosphorus are presented in Figure 1. Figures 2 and 3 illustrate before and after improvement for ammonia and nitrogen, respectively. The project reduced pollutant loadings to the South Platte River by over 4.0 million pounds per year.

The BNR process provides a wide range of operating modes to meet changing operational goals and can be modified to 5-stage Bardenpho. Flexibility provides the operations staff with the ability to run in MLE, A2O, or other modes. The process illustrated in Figure 4 consists of four anaerobic/anoxic zones to manage nitrogen and phosphorus reduction, three larger aerobic zones for nitrification, and a "swing zone" that can operate anoxic or aerobic.

CaRRB sidestream treatment reduces ammonia from dewatering and provides bioaugmentation to supercharge mainstream performance.

Originality and Innovation

The project includes examples of new and innovative processes, and improvement of established engineering procedures to create one of a kind originality.

Key Accomplishments in Innovation and Originality

Treatment Process

Sidestream treatment using Carollo-developed CaRRB system.
Surface wasting to reduce foaming and filament growth.
Zone walls to reduce short circuiting and prevent scum trapping.
Secondary clarifiers with high rate maze-type EDIs.

Structural Design

Limiting the use of drilled piers to only pump stations, mechanical buildings, and galleries as required to control differential pipe movement.
Aeration basins and CaRRB placed on compacted soils using Rammed Aggregate Piers (RAP).
A single interconnected continuous SAC and West Mechanical Building structure.
Vehicle accessible elevated operating decks and below grade galleries.

Mechanical Equipment

Open floor dry pit PEPS for equipment removal at grade level.
Low speed (350 rpm) PEPS pumps reduce power and limit wear and tear.
Blower building with unique air inlet plenum and "dual filtration walls."
High flow/low pressure axial propeller pumps adapted from the power industry used for mixed liquor return.

Electrical Gear

Main electrical supply located directly adjacent to PEPS and blower building reduces costs of wiring and power.
Full access cable room below switchgear and MCCs.
3D CADD to route extensive array of exposed cable tray through galleries.

Complexity

The size of the new South Secondary Complex alone required complex engineering solutions.

Notable Project Complexities:

Multiple interconnected process areas tie-in to existing facilities.
Repurposing HiPOS within existing footprint.
Large equipment and piping.
Continuous compliant operations using extensive bypass pumping.

Large, complex basins and equipment incorporated into the project:

Six parallel aeration basins 50-feet wide and 526-feet long.
74,000 CY of concrete to construct SAC.
Rehabilitation of ten 140-foot diameter secondary clarifiers.
Five 56-mgd, 450 HP VFD driven primary effluent pumps.
Five 29,000 scfm, 2,000 HP single stage centrifugal blowers.
Twelve 25-mgd, 75 HP, axial flow MLR pumps.
Fourteen 11.4-mgd, 125 Hp RAS pumps.

Final tie-ins and startup used a complex temporary bypass pumping system. The 128-mgd pumps were staffed 24/7 and operated continuously for six months.

Social and Economic Advancement

At a unit cost of under $1.50 per gallon, the project was completed significantly below the national average cost for secondary treatment, which ranges from $2 to $4 per gallon. Innovations such as CaRRB sidestream treatment, anti-foam trapping baffle walls, axial flow dry pit MLR pumps adapted from the power industry, and single structure design serve as a model to help others reduce construction costs and improve O&M. Managing treatment costs reduces user fees for ratepayers contributing to economic sustainability.

The elimination of more than 4.0 million pounds of nitrogen discharge to the South Platte River and provision for future phosphorus removal protects receiving waters, improves aquatic habitat, and reduces impacts on lakes and reservoirs used for downstream water supply.

Click images to enlarge in separate window.


Aerial view of RWHTF during South Secondary Improvements Project construction. The operational North Secondary Complex treating 106-mgd can be seen to the right and the 114-mgd South Secondary Complex can be seen under construction to the left.	Complete and operational South Secondary Improvements Project including SAC consisting of six parallel aeration basins, three CaRRB sidestream treatment basins, West Mechanical Building, repurposed HiPOS into RAS/WAS pump stations, and ten retrofit secondary clarifiers.

The sun rises over the new South Aeration Complex aeration basins capable of treating 114-mgd to less than 1.75 mg/L ammonia, 7.6 mg/L nitrogen, and is set-up to provide 1 mg/L effluent total phosphorus in the future.	Aeration basin transition between anoxic and aerobic zones allows flexible operations.

Six parallel BNR aeration basins with multiple zones for anaerobic, anoxic and aerobic operations. Unaerated zone mixer motors (green) sit on concrete access bridges. Bridges and aeration decks are fully vehicle accessible for ease of O&M.	Hydraulically optimized zone baffle walls separate aerated from unaerated zones to prevent short circuiting and eliminate foam trapping that can cause detrimental filament growth.

One of two large upflow passive mixed liquor splitter boxes capable of distributing 150 mgd between five secondary clarifiers. The dual splitter boxes were repurposed into former HiPOS aeration basins matching existing plant hydraulic profile.	One of ten 140-foot diameter secondary clarifiers retrofit with maze type EDIs and suction header type sludge withdrawal mechanisms to increase throughput capacity by 25 percent.

West Mechanical Building housing 256-mgd PEPS pump station and dual wet wells in the foreground, 150,000 scfm blower building housing five 2,000 HP single stage centrifugal blowers, and electrical power distribution center between the high load areas on either end of the building.	PEPS housing five 56-mgd pumps with 450-HP, VFD driven pumps. Open dry pit design allows all mechanical equipment to be removed using a 15-ton at-grade bridge crane.

Blower building housing five 29,000 scfm 2,000 HP single stage centrifugal blowers (blue). Inlet air is filtered through a unique dual filtration wall. Air is collected and distributed to aeration basins through a 84-inch diameter air header on top of air filtration plenum room.	Spacious piping and equipment galleries include twelve 25-mgd axial flow MLR pumps, large diameter RAS, CaRRB feed, and air piping. Cable trays are used throughout the galleries to carry power and instrumentation wiring to equipment and devices.