Seneca Wastewater Treatment Plant Montgomery County
1. INTRODUCTION
The Seneca Water Resource Recovery Facility (WRRF) serves as a cornerstone of wastewater infrastructure for the rapidly expanding I-270 corridor in Montgomery County, Maryland. Operated by WSSC Water, one of the largest water and wastewater utilities in the United States, this 26-MGD facility is designated as an Enhanced Nutrient Removal (ENR) plant, playing a pivotal role in the multi-state effort to restore the Chesapeake Bay.
Located in Germantown, the facility treats wastewater from approximately 180,000 residents and businesses in the Gaithersburg, Germantown, and Clarksburg areas. Unlike many plants of its era that focus solely on liquid treatment, Seneca has established itself as a model of resource recovery and energy efficiency. Following extensive upgrades over the last decade, including a substantial energy performance contract, the plant now generates a significant portion of its own power through Combined Heat and Power (CHP) systems utilizing digester gas. As WSSC Water continues its strategic shift from “wastewater treatment” to “water resource recovery,” the Seneca facility stands as a testament to engineering innovation, balancing strict effluent limits with operational sustainability.
2. FACILITY OVERVIEW
A. Service Area & Coverage
The Seneca WRRF services the Seneca Creek Basin, covering roughly 73 square miles of northern Montgomery County. This service area includes the dense residential and commercial sectors of Germantown and Gaithersburg, as well as the rapidly developing Clarksburg region. The collection system is gravity-dominated, following the natural topography of the Seneca Creek watershed, supplemented by several key pumping stations, including the Great Seneca Pump Station. The demographic mix is primarily residential (approx. 85%), with a growing sector of light industrial and biotechnology facilities associated with the I-270 technology corridor.
B. Operational Capacity
Constructed with a hydraulic design capacity of 26 MGD, the plant currently processes an average daily flow (ADF) ranging between 16 and 19 MGD. The facility is designed to handle significant wet weather events, with a peak hydraulic capacity exceeding 50 MGD. Historical flow trends indicate a steady increase consistent with the urbanization of Upcounty Montgomery. While current capacity utilization sits near 70%, WSSC Water’s long-term master planning accounts for the continued build-out of the Clarksburg area, ensuring the facility remains within permit limits through 2040.
C. Discharge & Compliance
Treated effluent is discharged into Great Seneca Creek, which flows approximately 6 miles before converging with the Potomac River. Because the Potomac is the primary drinking water source for the Washington, D.C. metropolitan area and a major tributary to the Chesapeake Bay, discharge standards are exceptionally stringent. The facility operates under NPDES Permit MD0021717, requiring Enhanced Nutrient Removal (ENR) levels—specifically targeting Total Nitrogen (TN) levels at or below 3.0 mg/L and Total Phosphorus (TP) at or below 0.3 mg/L. The facility consistently earns Platinum and Gold Peak Performance Awards from the National Association of Clean Water Agencies (NACWA) for compliance excellence.
3. TREATMENT PROCESS
The Seneca WRRF utilizes a complex treatment train designed to meet ENR standards. The process flow moves from preliminary physical removal to advanced biological treatment, tertiary filtration, and UV disinfection.
A. Preliminary Treatment
Raw influent enters the headworks where it passes through mechanically cleaned bar screens (typically 6mm to 10mm spacing) to remove large debris, rags, and plastics. Screenings are washed, compacted, and sent to landfill. Following screening, the flow enters aerated grit chambers. These chambers introduce air to create a spiral flow pattern, allowing heavier inorganic grit (sand, gravel, coffee grounds) to settle while keeping lighter organic material in suspension. The removed grit is classified and dewatered prior to disposal. Odor control at the headworks is managed via chemical scrubbers to mitigate impacts on the surrounding suburban community.
B. Primary Treatment
Wastewater flows to rectangular primary clarifiers equipped with chain-and-flight sludge collectors. These tanks reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge and floatables (grease/oil) to be skimmed from the surface. This stage typically removes 50-60% of Total Suspended Solids (TSS) and 30-35% of Biochemical Oxygen Demand (BOD). The primary sludge is pumped directly to the solids handling train for thickening and digestion.
C. Secondary Treatment (Biological Nutrient Removal)
The core of the Seneca facility is its activated sludge system, configured for Biological Nutrient Removal (BNR). The aeration basins utilize a staged anaerobic/anoxic/aerobic configuration to achieve nitrification and denitrification:
- Anaerobic Zone: Promotes the growth of Phosphorus Accumulating Organisms (PAOs) for biological phosphorus removal.
- Anoxic Zone: Mixers keep solids in suspension without oxygen addition, allowing bacteria to convert nitrates into nitrogen gas (denitrification).
- Aerobic Zone: Fine bubble diffusers provide oxygen for the conversion of ammonia to nitrate (nitrification) and BOD reduction.
Following aeration, the mixed liquor flows to secondary clarifiers, where the biological biomass settles. Return Activated Sludge (RAS) is pumped back to the aeration basins, while Waste Activated Sludge (WAS) is removed for processing.
D. Tertiary / Advanced Treatment
To meet the strict limit of 0.3 mg/L Total Phosphorus, the facility utilizes deep bed gravity sand filters. These filters provide physical polishing of the secondary effluent, capturing remaining suspended solids and particulate phosphorus. Chemical addition facilities (utilizing Aluminum Sulfate or Ferric Chloride) are available upstream of the filters to precipitate soluble phosphorus if biological removal requires supplementation during process upsets or cold weather.
E. Disinfection
Historically a chlorination facility, Seneca converted to Ultraviolet (UV) Disinfection to improve safety and eliminate the formation of disinfection byproducts (DBPs). The effluent passes through banks of low-pressure, high-intensity UV lamps which scramble the DNA of pathogens, preventing reproduction. Following disinfection, the water is re-aerated (cascade aeration) to ensure adequate Dissolved Oxygen (DO) levels before entering Great Seneca Creek.
F. Solids Handling & Energy Recovery
Seneca is a leader in solids processing within the WSSC network:
- Thickening: Primary sludge utilizes gravity thickening, while WAS is thickened via Dissolved Air Flotation (DAF) or rotary drum thickeners.
- Digestion: The blended sludge undergoes Anaerobic Digestion in mesophilic digesters. This process stabilizes the sludge, reduces pathogen counts (Class B biosolids), and produces methane-rich biogas.
- Dewatering: Digested sludge is dewatered using high-speed centrifuges to achieve a cake solid concentration of roughly 20-25%.
- Disposal: The resulting biosolids are hauled off-site for land application on agricultural fields in Maryland and Virginia.
G. Process Control
The entire plant is monitored via a centralized SCADA (Supervisory Control and Data Acquisition) system. Online instrumentation continuously monitors DO, pH, ORP, Ammonia, and Nitrate levels, allowing operators to adjust aeration rates and recycle flows in real-time to optimize ENR performance and energy consumption.
4. INFRASTRUCTURE & FACILITIES
A. Physical Plant
The facility occupies a footprint designed to blend with the surrounding topography. Critical structures include the Administration Building (housing the control room and laboratory), Maintenance Building, and the extensive tankage of the process trains. The laboratory is certified for process control testing, ensuring immediate feedback for operators.
B. Energy Systems
Energy management is a standout feature at Seneca. The plant utilizes a Combined Heat and Power (CHP) system. Two cogeneration engines (approx. 660 kW capacity) run on the biogas produced by the anaerobic digesters. This system generates electricity to power plant equipment while capturing waste heat to maintain digester temperatures, creating a highly efficient closed loop. Additionally, a ground-mounted 2 MW solar photovoltaic array contributes to the facility’s power needs, making it one of the greenest plants in the region.
C. Odor Control
Given the proximity to residential zones, robust odor control is mandatory. The plant utilizes covered processes at the headworks and primary clarifiers, ducting foul air to chemical wet scrubbers (typically caustic/bleach) and biological filters. WSSC maintains a 24-hour odor complaint hotline to address community concerns immediately.
5. RECENT UPGRADES & MAJOR PROJECTS
Energy Performance Contract (2018-2020)
- Project Scope: Comprehensive energy overhaul including new CHP engines, boiler replacements, HVAC upgrades, and building envelope improvements.
- Budget: ~$23.8 Million (Part of a larger $58M contract across WSSC facilities).
- Partner: Schneider Electric (Energy Service Company).
- Technical Highlights: Installation of new biogas pretreatment systems to protect engines from siloxanes and H2S; implementation of a 2 MW solar array on facility grounds.
- Results: Reduced greenhouse gas emissions by approximately 18% and secured guaranteed annual energy savings.
Seneca WRRF Evaluation and Assessment (2022-2024)
- Project Scope: A holistic condition assessment of mechanical, electrical, and structural assets to develop a 20-year prioritization plan.
- Drivers: Aging infrastructure (original 1978 assets reaching end of life) and resiliency planning against climate change.
- Key Focus: Headworks screening reliability, secondary clarifier mechanism rehabilitation, and electrical distribution redundancy.
Current/Upcoming Projects (2024-2027)
Solids Handling Upgrades: Design and construction are underway to modernize the dewatering centrifuges and polymer feed systems. Estimated budget: $15 Million. This project aims to increase cake solids percentage, reducing hauling costs and truck traffic.
Electrical Distribution Upgrade: Replacement of main switchgear and substations to ensure reliability and arc flash safety compliance.
6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE
A. Permit Requirements
The NPDES permit (MD0021717) sets strict effluent limits to protect the Chesapeake Bay:
- Total Nitrogen (TN): Monthly average ~3.0 mg/L (Annual Load Cap).
- Total Phosphorus (TP): Monthly average ~0.3 mg/L (Annual Load Cap).
- TSS & BOD: Generally < 10 mg/L (Monthly Average).
- E. Coli: < 126 MPN/100mL (Geometric Mean).
- Dissolved Oxygen: Minimum 5.0 mg/L.
B. Compliance History
Seneca WRRF maintains an exemplary compliance record. The facility has received multiple NACWA Platinum Awards, recognizing consecutive years of 100% permit compliance. Occasional challenges have arisen regarding wet weather peak flows, but no significant consent decree violations are currently active specific to the plant’s treatment performance.
C. Environmental Stewardship
Beyond effluent quality, the facility contributes to the “Green Germantown” initiative through its solar production. The biosolids program supports regional agriculture, returning carbon and nutrients to the soil rather than landfilling.
7. OPERATIONAL EXCELLENCE
A. Staffing
The plant is staffed 24/7/365. The team comprises approximately 30-35 full-time employees, including Maryland Class 5A (Wastewater) licensed operators, industrial mechanics, instrument technicians, and laboratory analysts. WSSC Water invests heavily in operator training, offering apprenticeship programs to combat industry-wide workforce shortages.
B. Performance Metrics
The facility typically achieves removal efficiencies exceeding 98% for both BOD and TSS. The unit cost of treatment has been stabilized through the energy performance upgrades, decoupling a portion of operating costs from grid electricity price fluctuations.
8. CHALLENGES & FUTURE PLANNING
A. Current Challenges
Aging Infrastructure: Much of the concrete tankage dates to the late 1970s. Concrete degradation and mechanism fatigue in clarifiers are ongoing maintenance priorities.
PFAS Regulations: Like all utilities, Seneca is monitoring emerging EPA regulations regarding Per- and Polyfluoroalkyl Substances (PFAS). Future regulations may require capital-intensive technologies like Granular Activated Carbon (GAC) or Ion Exchange.
B. Future Planning
The Bi-County Water and Sewer Plan outlines capacity needs. While current capacity is sufficient, the build-out of the “Upcounty” region is monitored closely. Future expansions will likely focus on intensification of treatment (e.g., MBR retrofits or densified activated sludge) rather than physical footprint expansion, due to land constraints.
9. COMMUNITY & REGIONAL IMPACT
The Seneca WRRF is a silent economic engine for Montgomery County. By providing reliable sewer capacity, it enables the continued growth of the I-270 biotechnology corridor. The facility engages proactively with the community, offering tours to engineering students from the University of Maryland and local high schools to foster the next generation of water professionals. WSSC Water also maintains a transparent “Near Neighbors” communication strategy to alert residents of any unusual maintenance activities that might generate noise or odors.
10. TECHNICAL SPECIFICATIONS SUMMARY
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Tertiary Treatment with BNR/ENR |
| Design Capacity | 26.0 MGD |
| Current Average Flow | 16 – 19 MGD |
| Peak Hydraulic Capacity | > 50 MGD |
| Primary Treatment | Rectangular Clarifiers |
| Secondary Treatment | Activated Sludge (Anaerobic/Anoxic/Aerobic Zones) |
| Tertiary Treatment | Deep Bed Gravity Sand Filtration |
| Disinfection | Ultraviolet (UV) Irradiation |
| Solids Stabilization | Mesophilic Anaerobic Digestion |
| Dewatering | High-Speed Centrifuges |
| Power Generation | CHP (Cogeneration) + 2MW Solar Array |
| Service Area | ~73 Square Miles (Germantown/Gaithersburg) |
| Receiving Water | Great Seneca Creek |
| Operating Authority | WSSC Water |
| Commissioned | 1978 (Original) |
11. RELATED FACILITIES
Piscataway WRRF: WSSC’s Bio-Energy hub located in Prince George’s County, which is pioneering thermal hydrolysis processes.
Damascus WRRF: A smaller satellite facility located upstream in Montgomery County.
Great Seneca Pump Station: The primary conveyance asset lifting flow into the Seneca headworks.
12. FAQ SECTION
Q1: What is the treatment capacity of the Seneca WRRF?
The facility has a design capacity of 26 Million Gallons per Day (MGD) and currently treats an average of 16-19 MGD.
Q2: Does Seneca WRRF perform nutrient removal?
Yes. It is an Enhanced Nutrient Removal (ENR) facility, utilizing biological and chemical processes to reduce Nitrogen and Phosphorus to levels safe for the Chesapeake Bay (TN < 3.0 mg/L, TP < 0.3 mg/L).
Q3: How is power generated at the plant?
The plant uses a Combined Heat and Power (CHP) system that burns methane biogas produced during sludge digestion to generate electricity and heat. This is supplemented by a 2 MW on-site solar array.
Q4: What method of disinfection is used?
The facility uses Ultraviolet (UV) light disinfection, which avoids the safety hazards and environmental toxicity associated with chlorine gas.
Q5: Who operates the facility?
The facility is owned and operated by WSSC Water (Washington Suburban Sanitary Commission), a bi-county agency serving Montgomery and Prince George’s Counties.
Q6: Where does the treated water go?
Treated effluent is discharged into Great Seneca Creek, which eventually flows into the Potomac River.
Q7: How are biosolids handled?
Solids are thickened, anaerobically digested, dewatered via centrifuge, and hauled off-site for agricultural land application as a fertilizer/soil conditioner.
Q8: Are there tours available?
WSSC Water occasionally offers tours for educational groups and community organizations. Requests should be routed through the WSSC Office of Communications and Community Relations.
Disclaimer: This article is for informational purposes for engineering and industry professionals. Data presented is based on public records, WSSC Water annual reports, and NPDES permit documentation available as of late 2023. Specific operational parameters may vary based on real-time conditions.