Back River Wastewater Treatment Plant Baltimore

1. INTRODUCTION

The Back River Wastewater Treatment Plant (BRWWTP) serves as the cornerstone of wastewater infrastructure for the Baltimore metropolitan region and stands as the largest wastewater treatment facility in the State of Maryland. Owned and operated by the Baltimore City Department of Public Works (DPW), the facility treats wastewater for approximately 1.6 million residents across Baltimore City and Baltimore County. With a design capacity of 180 million gallons per day (MGD) and a peak hydraulic capacity exceeding 400 MGD following recent hydraulic upgrades, the plant is critical to the environmental health of the Chesapeake Bay.

Originally commissioned in 1911, Back River has evolved from a basic trickling filter plant into a sophisticated Enhanced Nutrient Removal (ENR) facility. The site occupies a 466-acre campus in Dundalk, Maryland. In recent years, the facility has been the subject of massive capital investment, including the $430 million Headworks Project aimed at eliminating sanitary sewer overflows (SSOs). As a focal point for regional environmental compliance, the plant utilizes advanced denitrification technology to meet stringent total nitrogen (TN) and total phosphorus (TP) limits mandated to protect the sensitive Back River and Chesapeake Bay ecosystems.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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The BRWWTP operates under a unique inter-jurisdictional arrangement. While the facility is owned and operated by the City of Baltimore, it is physically located within Baltimore County. The plant manages wastewater for a dense urban and suburban service area encompassing Baltimore City and the majority of Baltimore County. The collection system feeding the plant is extensive, characterized by aging infrastructure that is currently subject to a federal Consent Decree aimed at reducing infiltration/inflow (I/I) and eliminating overflows.

B. Operational Capacity

The plant is designed for an average daily flow of 180 MGD. Historically, the plant treats an average range of 130–160 MGD during dry weather. However, the facility is subject to extreme peaking factors during storm events due to the age of the collection system. The recently completed Headworks Project was specifically designed to handle wet weather flows of up to 400+ MGD, mitigating hydraulic bottlenecks that previously caused backups in the collection system up to 10 miles upstream.

C. Discharge & Compliance

Treated effluent is discharged via Outfall 001 into the Back River, an estuarine tributary of the Chesapeake Bay. Due to the bay’s nutrient impairment, the facility operates under strict NPDES parameters for nutrient loading. The plant is classified as an Enhanced Nutrient Removal (ENR) facility, with performance goals of achieving annual average effluent concentrations of 3.0 mg/L Total Nitrogen (TN) and 0.3 mg/L Total Phosphorus (TP).

3. TREATMENT PROCESS

The Back River WWTP utilizes a multi-stage treatment train designed to handle high organic loads while achieving tertiary nutrient removal standards.

A. PRELIMINARY TREATMENT (Headworks)

The new Headworks facility, commissioned in 2021, represents a hydraulic modernization of the plant.

• Fine Screening: The flow enters through mechanized fine screens to remove rags, plastics, and large debris, protecting downstream pumps.
• Grit Removal: Vortex grit chambers remove sand, gravel, and heavy particulates to prevent abrasion in mechanical equipment.
• Pumping: The heart of the preliminary stage consists of massive Archimedes screw pumps and centrifugal pumps capable of lifting over 400 MGD of influent against the plant’s hydraulic grade line.

B. PRIMARY TREATMENT

Wastewater flows by gravity to Primary Settling Tanks. These large circular clarifiers reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. This stage typically removes 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).

C. SECONDARY TREATMENT (Activated Sludge)

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The biological treatment utilizes a conventional Activated Sludge process.

• Aeration Basins: The plant utilizes multiple reactor trains where settled sewage is mixed with Return Activated Sludge (RAS). Large blowers provide oxygen via fine-bubble diffusers to support microbial growth, which consumes organic matter and converts ammonia to nitrate (nitrification).
• Secondary Clarifiers: Mixed liquor flows to secondary clarifiers where the biological floc settles. The clear supernatant flows to the tertiary stage, while settled biomass is returned to the aeration basins or wasted (WAS) to solids handling.

D. TERTIARY/ADVANCED TREATMENT (ENR)

To meet Chesapeake Bay restoration goals, Back River employs advanced denitrification technology.

• Deep Bed Denitrification Filters: The facility utilizes massive Denite® filters (deep bed sand filters). Methanol is added as a carbon source to facilitate the biological conversion of nitrate to nitrogen gas (denitrification).
• Phosphorus Removal: Chemical precipitation (typically using ferric chloride or alum) is utilized alongside biological uptake to precipitate phosphorus, which is then captured in the filtration stage.

E. DISINFECTION

Effluent is disinfected using Chlorination (liquid sodium hypochlorite) to eliminate pathogens. Prior to discharge, the water undergoes Dechlorination using sodium bisulfite to neutralize residual chlorine, preventing toxicity to aquatic life in the Back River. A post-aeration step ensures the effluent meets Dissolved Oxygen (DO) requirements.

F. SOLIDS HANDLING

Solids processing is a critical and complex operation at Back River:

• Thickening: Primary sludge and WAS are thickened (gravity thickening and dissolved air flotation/centrifugation) to reduce water content.
• Anaerobic Digestion: Thickened sludge is stabilized in large egg-shaped anaerobic digesters. This process reduces pathogen counts and volatile solids while producing biogas.
• Dewatering & Pelletization: Digested sludge is dewatered using high-solids centrifuges. The facility includes a dedicated Pelletizer Facility (historically operated via contract, e.g., Synagro) that heat-dries the biosolids into Class A fertilizer pellets for beneficial reuse.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The 466-acre site is a mix of historic architecture and modern industrial design. The campus includes extensive maintenance shops, a central laboratory responsible for compliance testing, and the distinct egg-shaped digesters which are visible landmarks in the area.

B. Energy Systems

Back River is a significant energy consumer but employs cogeneration technology to offset demand. The facility captures methane gas produced during anaerobic digestion to fuel on-site power generation systems. These cogeneration units provide heat for the digesters and electricity for plant operations, enhancing sustainability and reducing operating costs.

C. Odor Control

Given its proximity to residential areas in Essex and Dundalk, odor control is a priority. The plant utilizes chemical scrubbers and biofilters, particularly at the Headworks, primary settling areas, and sludge processing facilities, to strip odorous compounds (H2S) from the air stream.

5. RECENT UPGRADES & MAJOR PROJECTS

Headworks Improvement Project – $430 Million (2017-2021)

• Project Scope: Construction of a new influent pumping station, grit removal facility, and fine screen facility.
• Technical Highlights: Installation of eight massive Archimedes screw pumps (12 feet in diameter) and large centrifugal pumps.
• Drivers: Specifically designed to eliminate a 10-mile hydraulic restriction causing sewer backups into homes and waterways during wet weather.
• Results: Increased peak wet weather capacity to over 400 MGD, significantly reducing sanitary sewer overflows (SSOs) in the collection system.

Enhanced Nutrient Removal (ENR) Upgrades – Various Phases

• Project Scope: Upgrades to denitrification filters and chemical feed systems.
• Funding: largely supported by the Bay Restoration Fund (“Flush Tax”).
• Drivers: Compliance with Chesapeake Bay Total Maximum Daily Load (TMDL) requirements.
• Results: Enabled the plant to target effluent limits of 3 mg/L TN and 0.3 mg/L TP.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Operating under NPDES Permit MD0021555, the facility faces some of the strictest nutrient limits in the region due to the Chesapeake Bay TMDL. Strict monitoring is required for Nitrogen, Phosphorus, TSS, BOD, and Fecal Coliform.

B. Compliance History & Challenges

While the facility is capable of high-level treatment, it has faced significant operational challenges recently. In 2022, the Maryland Department of the Environment (MDE) temporarily directed the Maryland Environmental Service (MES) to oversee operations at Back River following a period of compliance failures. These issues were primarily related to solids inventory management, equipment maintenance backlogs (particularly in the primary clarifiers and denitrification filters), and staffing shortages.

Following this intervention, substantial progress was made in repairing equipment and reducing solids inventory. Control has since transitioned back to Baltimore City DPW, with ongoing oversight and requirements to adhere to modified consent orders regarding maintenance and staffing levels.

7. OPERATIONAL EXCELLENCE & CHALLENGES

A. Staffing and Expertise

The facility requires a large staff of licensed Maryland wastewater operators, maintenance mechanics, and instrumentation technicians. Like many utilities, Back River faces challenges in workforce development and retention. DPW has implemented apprenticeship programs and revised salary structures to attract Class 5A licensed operators.

B. Current Challenges

• Solids Management: The reliability of the pelletizer facility and centrifuge performance is the critical path for the plant. If solids cannot be processed and removed from the site, the liquid treatment train becomes overloaded.
• Aging Infrastructure: While the Headworks is new, much of the secondary treatment and solids handling infrastructure requires continuous rehabilitation.
• Inflow & Infiltration (I/I): The collection system is old, leading to massive spikes in flow during rain events, testing the hydraulic limits of the secondary clarifiers.

8. COMMUNITY & REGIONAL IMPACT

The Back River WWTP is vital to the ecological recovery of the Chesapeake Bay. By removing millions of pounds of nitrogen and phosphorus annually, the plant directly contributes to the reduction of “dead zones” in the Bay, supporting the regional crab and oyster industries. Economically, the plant enables continued residential and commercial growth in the Baltimore metropolitan area by providing essential sanitation capacity.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. RELATED FACILITIES

• Patapsco Wastewater Treatment Plant: Baltimore’s second major facility (63 MGD), serving the southern portions of the City and County.
• Back River Pelletizer Facility: Located on-site, this facility processes the digested sludge into fertilizer pellets. It has historically been operated via third-party contracts (e.g., Synagro).
• Collection System Pump Stations: Numerous major pumping stations throughout Baltimore County convey flow to the Back River headworks.

11. FAQ SECTION

Technical Questions

Q: What is the primary purpose of the Back River Headworks Project?
A: The $430 million project was designed to eliminate hydraulic restrictions at the plant entrance. By installing massive screw pumps, the plant can now accept peak wet weather flows without causing sewage to back up into the collection system, significantly reducing Sanitary Sewer Overflows (SSOs).

Q: Does Back River WWTP perform Biological Nutrient Removal (BNR)?
A: Yes, and specifically Enhanced Nutrient Removal (ENR). The plant uses biological nitrification in the activated sludge basins followed by denitrification in deep bed sand filters (using methanol as a food source) to convert nitrate to nitrogen gas.

Q: How are biosolids managed at the facility?
A: Solids are thickened, anaerobically digested (producing biogas), dewatered via centrifuges, and then heat-dried into Class A pellets for use as fertilizer. The plant also has contingency plans for hauling dewatered cake if the dryers are offline.

Public Interest Questions

Q: Why is a Baltimore City plant located in Baltimore County?
A: The plant was sited in 1911 based on gravity/topography to serve the region. Through inter-municipal agreements, the City owns and operates the land and facility, but it serves residents of both jurisdictions. The County pays the City for treatment services based on flow volume.

Q: Is the plant currently complying with environmental regulations?
A: Following significant challenges in 2021-2022 that led to state oversight, the plant has seen substantial improvements in equipment availability and solids inventory reduction. It is currently operating under strict regulatory consent orders to ensure continued compliance and maintenance.