City Of Ann Arbor Wastewater Treatment Plant

FACILITY BASIC INFORMATION

Plant Name: City of Ann Arbor Wastewater Treatment Plant (AAWWTP)

Location: 49 South Dixboro Road, Ann Arbor, Washtenaw County, Michigan

Operating Authority: City of Ann Arbor Public Services Area (Systems Planning Unit)

Design Capacity: 29.5 MGD (Average Daily Flow)

Current Average Flow: ~18.5 MGD

Population Served: ~130,000 residents (plus commuter/student influx)

Service Area: City of Ann Arbor, Ann Arbor Township, Pittsfield Township, Scio Township

Receiving Water Body: Huron River

NPDES Permit Number: MI0023256

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Year Commissioned: 1937 (Major expansions in 1978, 1990s, 2000s)

The City of Ann Arbor Wastewater Treatment Plant (AAWWTP) serves as the critical sanitation hub for one of Michigan’s most vital economic and educational centers. Located on the eastern edge of the city, adjacent to the Huron River, this Class A facility treats an average of 18.5 million gallons daily (MGD) with a hydraulic design capacity of 29.5 MGD. Operated by the City of Ann Arbor, the plant provides wastewater services to a permanent population of approximately 130,000, along with the significant transient population associated with the University of Michigan.

Originally commissioned in 1937 as a primary treatment facility, the AAWWTP has evolved through multiple capital improvement cycles into a sophisticated tertiary treatment complex. It is distinguished by its use of advanced tertiary filtration, ultraviolet (UV) disinfection, and on-site fluidized bed incineration for solids handling. As the guardian of water quality for the Huron River—a designated “Country Scenic River” downstream—the facility operates under strict NPDES parameters, particularly concerning nutrient loading and emerging contaminants such as PFAS.

Facility Overview

A. Service Area & Coverage

The AAWWTP operates as a regional facility. While owned by the City of Ann Arbor, it processes wastewater from a broader sewershed that includes Ann Arbor Township, Pittsfield Township, and Scio Township via contractual agreements. The collection system feeding the plant spans over 475 miles of sanitary sewer mains. The service area is unique due to the seasonal fluctuation in load driven by the university academic calendar, requiring operational flexibility to handle varying biological loading rates.

B. Operational Capacity

The facility is designed for an average daily flow of 29.5 MGD, with a peak hydraulic capacity exceeding 50 MGD during wet weather events. Historical data indicates a stable average flow between 18 and 20 MGD over the last decade, suggesting adequate reserve capacity for planned regional growth. However, the plant is subject to significant inflow and infiltration (I/I) challenges during heavy precipitation, necessitating robust hydraulic management strategies at the headworks and flow equalization stages.

C. Discharge & Compliance

Treated effluent is discharged directly into the Huron River. The plant operates under a National Pollutant Discharge Elimination System (NPDES) permit issued by the Michigan Department of Environment, Great Lakes, and Energy (EGLE). The discharge is located upstream of Geddes Pond and Ford Lake, making nutrient removal (specifically phosphorus) a critical compliance parameter to prevent eutrophication in downstream impoundments. The facility consistently maintains a strong compliance record, frequently receiving Platinum and Gold Awards from the National Association of Clean Water Agencies (NACWA).

Treatment Process

The AAWWTP utilizes a conventional activated sludge process augmented by tertiary filtration and UV disinfection. The treatment train is designed to achieve high-level removal of BOD, TSS, and Phosphorus.

A. Preliminary Treatment

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Raw influent enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit chambers. These chambers reduce the velocity of the wastewater, allowing heavier inorganic materials (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit is dewatered and disposed of in a sanitary landfill. The headworks also includes provisions for septage receiving from hauled waste haulers.

B. Primary Treatment

Wastewater flows into rectangular primary settling tanks. Here, the hydraulic retention time allows settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. Approximately 50-60% of suspended solids and 30-35% of BOD are removed in this stage. Ferric chloride is frequently added at this stage or prior to it to precipitate phosphorus and enhance sedimentation.

C. Secondary Treatment (Activated Sludge)

The biological treatment core consists of aeration basins utilizing a conventional activated sludge process. The plant utilizes fine-bubble diffusion to maximize oxygen transfer efficiency. Microorganisms consume the dissolved organic matter and convert non-settleable solids into settleable biological floc. The mixed liquor then flows to secondary clarifiers, where the biomass settles out. A portion is returned to the aeration tanks (RAS), while excess biomass is wasted (WAS) to the solids handling train. The secondary system is configured to manage ammonia nitrogen through nitrification during warmer months.

D. Tertiary Treatment

To meet stringent effluent limits for suspended solids and phosphorus, the AAWWTP employs tertiary treatment via gravity filtration. The secondary effluent is passed through dual-media filters (typically anthracite coal and sand). This polishing step is critical for removing fine particulate matter that may contain bound phosphorus, ensuring the effluent meets the low-level limits required for the Huron River watershed.

E. Disinfection

Historically a chlorine gas facility, the AAWWTP converted to Ultraviolet (UV) disinfection to improve safety and environmental performance. The effluent passes through channels equipped with low-pressure, high-intensity UV lamp banks. This physical process neutralizes pathogens (bacteria and viruses) by disrupting their DNA without adding chemicals to the water, thereby eliminating the need for dechlorination and reducing the formation of disinfection byproducts.

F. Solids Handling & Incineration

Solids handling is a notable feature of the Ann Arbor facility. Primary sludge and Waste Activated Sludge (WAS) are thickened using gravity thickeners and centrifuges, respectively. The thickened sludge is dewatered using high-performance centrifuges to achieve a cake solid concentration suitable for thermal reduction.

The dewatered cake is processed in an on-site Fluidized Bed Incinerator. This thermal oxidation process reduces the sludge volume by approximately 90%, leaving only inert ash. The ash is collected via scrubbers and electrostatic precipitators to meet air quality standards before being disposed of at a sanitary landfill. This method significantly reduces the volume of material requiring trucking and landfill space.

Infrastructure & Facilities

A. Physical Plant

The site encompasses a significant footprint along the Huron River corridor. It houses the East and West Plant structures (reflecting different expansion eras), administration buildings, a fully accredited environmental laboratory, and maintenance workshops. The architecture reflects a functional industrial aesthetic typical of mid-20th-century public works, with modern additions housing the UV and solids handling systems.

B. Energy Systems

The plant is a major energy consumer, primarily driven by aeration blowers and the incineration process. The Fluidized Bed Incinerator requires natural gas for startup and temperature maintenance, though autogenous combustion is maximized when sludge characteristics allow. The facility has implemented Variable Frequency Drives (VFDs) on major pumps and blowers to optimize electrical consumption.

C. Odor Control

Given the plant’s location near recreational trails and residential zones, odor control is a priority. The headworks and solids handling buildings are maintained under negative pressure, with foul air treated through chemical scrubbers and activated carbon adsorption systems before release.

Recent Upgrades & Major Projects

The City of Ann Arbor maintains an aggressive Capital Improvement Plan (CIP) to address aging infrastructure and evolving regulatory requirements.

Solids Handling & Incinerator Upgrade (2015-2018)

• Project Scope: Comprehensive rehabilitation of the sewage sludge incineration system to meet new EPA SSI (Sewage Sludge Incineration) MACT emission standards. Included the installation of a new mercury removal system and upgrades to the fluidized bed incinerator.
• Budget: ~$16 Million
• Drivers: Regulatory compliance (Clean Air Act) and aging equipment reliability.
• Technical Highlights: Installation of a sorbent polymer composite mercury scrubber system, one of the first of its kind for this application, ensuring air emissions remain well below federal limits.

UV Disinfection System Replacement (2018-2020)

• Project Scope: Replacement of the aging UV disinfection system originally installed in the early 2000s. The project involved hydraulic modifications and the installation of new, more energy-efficient UV banks.
• Budget: ~$4.5 Million
• Drivers: Equipment obsolescence and energy efficiency.
• Results: Improved pathogen inactivation reliability and reduced electrical consumption for the disinfection stage.

Upcoming: Headworks & Grit Removal Improvements (2024-2026 Planning)

The city is currently in the planning and design phases for significant upgrades to the preliminary treatment facilities. This project aims to replace aging bar screens and improve grit removal efficiency to protect downstream pumps and the digesters/incinerator from abrasion.

Regulatory Compliance & Environmental Performance

A. Permit Requirements

Under NPDES Permit MI0023256, the AAWWTP adheres to strict limits protecting the Huron River. Key parameters include:

• Phosphorus: Limits are seasonally variable but generally require effluent concentrations below 1.0 mg/L, with goals often set lower (0.5 – 0.8 mg/L) to prevent algae blooms.
• E. Coli: Strict limits during the recreation season (May-October) requiring effective UV disinfection.
• PFAS/PFOS: As a Michigan facility, the plant is subject to some of the nation’s strictest monitoring requirements for Per- and Polyfluoroalkyl Substances. The plant actively monitors industrial users to trace and reduce source loading.

B. Environmental Stewardship

The City of Ann Arbor is a leader in watershed management. The WWTP staff works closely with the Huron River Watershed Council to monitor river health. The shift to incineration with advanced scrubbing demonstrates a commitment to minimizing land application risks associated with emerging contaminants.

Operational Excellence

A. Staffing

The facility functions 24/7/365, staffed by a team of approximately 30-40 personnel, including operations supervisors, maintenance mechanics, electricians, and laboratory technicians. Operations staff are required to hold Michigan EGLE Municipal Wastewater Treatment Certification, with senior staff holding Class A licenses.

B. Laboratory Capabilities

The on-site laboratory is state-certified for wastewater analysis. It performs daily compliance testing for BOD, TSS, pH, Ammonia, and Phosphorus, as well as process control testing to optimize the activated sludge biology. This internal capability allows for real-time process adjustments.

Challenges & Future Planning

PFAS Management: Like many Michigan utilities, Ann Arbor faces the challenge of “forever chemicals.” While the WWTP does not generate PFAS, it receives them from upstream users. The plant is focused on source control and monitoring, as standard wastewater treatment does not remove these compounds.

Aging Infrastructure: Portions of the plant date back to the 1930s and 1970s. Concrete deterioration and mechanical obsolescence require continuous investment. The city utilizes a detailed Asset Management Plan to prioritize replacements.

Climate Resilience: Increasing rainfall intensity in the Great Lakes region poses risks of hydraulic overloading. Future planning focuses on wet weather flow management and hardening critical electrical infrastructure against potential flooding events.

Technical Specifications Summary

Related Facilities

The AAWWTP relies on a network of remote lift stations to convey wastewater over the hilly topography of Ann Arbor. Additionally, the plant accepts limited hauled septage from unsewered areas in Washtenaw County. The facility works in concert with the City’s Water Treatment Plant (located upstream on the Huron River) to manage the integrated water cycle of the region.

Frequently Asked Questions

Technical Questions

Q: What is the specific incineration technology used?
A: The plant utilizes a Fluidized Bed Incinerator, which suspends the sludge in a hot sand bed using air pressure to ensure complete and efficient combustion.

Q: Does the plant perform biological nutrient removal (BNR)?
A: The plant is configured for nitrification (ammonia removal) and uses chemical precipitation (ferric chloride) combined with tertiary filtration for phosphorus removal. It does not currently utilize a full biological phosphorus removal implementation.

Q: How is the ash from incineration handled?
A: The inert ash resulting from incineration is captured, wetted to prevent dust, and transported to a licensed sanitary landfill for disposal.

Public Interest Questions

Q: Does the plant smell?
A: While wastewater treatment naturally produces odors, the AAWWTP uses advanced chemical scrubbers and carbon filters, particularly at the headworks and solids handling buildings, to minimize odors reaching the surrounding community.

Q: Is the water released into the Huron River clean?
A: Yes. The effluent is treated to tertiary standards, disinfected with UV light, and must meet strict state and federal water quality standards before discharge. It is generally cleaner than the river water it enters.

Q: Can I tour the facility?
A: The City of Ann Arbor frequently organizes tours for educational groups, particularly from the University of Michigan and local schools. Interested parties should contact the City Public Services Area for current availability.