City Of Lincoln Theresa Street Wastewater Treatment Facility
1. Introduction
The Theresa Street Water Resource Recovery Facility (WRRF) serves as the primary wastewater treatment hub for the City of Lincoln, Nebraska. As one of the most advanced facilities in the Midwest, it processes approximately 60% of the city’s total wastewater flow, serving a population of over 290,000 in conjunction with the Northeast WRRF. Originally commissioned in the late 19th century and substantially modernized over subsequent decades, Theresa Street has evolved from a basic treatment plant into a model of energy recovery and sustainability.
Operated by Lincoln Transportation and Utilities (LTU), the 27.6-MGD facility is distinguished not only by its critical role in protecting the Salt Creek watershed but also by its pioneering adoption of biogas-to-renewable natural gas (RNG) technology. In recent years, the facility has transitioned its identity from a “treatment plant” to a “resource recovery facility,” reflecting a strategic shift toward circular economy principles, capturing energy, nutrients, and purified effluent to support the regional ecosystem.
2. Facility Overview
A. Service Area & Coverage
The Theresa Street WRRF services the central, southern, and western portions of Lincoln, covering the densest urban sectors including the University of Nebraska-Lincoln, the State Capitol district, and established residential neighborhoods. The collection system feeding the plant relies heavily on gravity flow due to the facility’s location at a low elevation point near Salt Creek, supplemented by major lift stations in the southern expansion zones.
B. Operational Capacity
The facility is designed for an average daily flow (ADF) of 27.6 Million Gallons per Day (MGD) with a peak instantaneous hydraulic capacity exceeding 60 MGD. Historically, the plant operates near 22-24 MGD on average. Due to the combined nature of older parts of the collection system and infiltration/inflow (I/I) challenges common in the region, wet weather events can cause rapid spikes in influent volume, necessitating robust flow equalization and peak flow management strategies.
C. Discharge & Compliance
Treated effluent is discharged into Salt Creek. Because Salt Creek is an effluent-dominated stream during dry periods (where treated wastewater can comprise the majority of stream flow), the facility operates under strict NPDES permit limits regarding ammonia toxicity, E. coli, and dissolved oxygen. The plant has consistently maintained compliance with the Nebraska Department of Environment and Energy (NDEE) standards, acting as a critical barrier preventing nutrient loading into the downstream Platte River basin.
3. Treatment Process
The Theresa Street WRRF utilizes a conventional activated sludge process train augmented with advanced solids handling and disinfection capabilities.
A. Preliminary Treatment
Influent wastewater 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 where velocity is reduced to allow inorganic solids (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit and screenings are washed, compacted, and disposed of at the local landfill. This stage is critical for protecting downstream pumps and digesters from abrasion and accumulation.
B. Primary Treatment
Wastewater flows into rectangular primary clarifiers. Here, flow velocity is further reduced, allowing settleable organic solids to form a raw sludge blanket on the tank floor, which is scraped to hoppers for removal to the solids handling train. Grease and oils (scum) floating to the surface are skimmed off. The primary treatment stage typically removes 30-40% of BOD and 50-60% of Total Suspended Solids (TSS).
C. Secondary Treatment (Activated Sludge)
The primary effluent enters the biological treatment phase, consisting of aeration basins and secondary clarifiers.
- Aeration Basins: The facility utilizes a plug-flow activated sludge configuration. Microorganisms (mixed liquor) consume the remaining dissolved organic matter and convert ammonia to nitrate (nitrification). Air is supplied via fine-bubble diffusers to maintain dissolved oxygen levels required for biological activity.
- Secondary Clarifiers: The mixed liquor flows into circular secondary clarifiers where the biological solids settle out. A portion of these solids is returned to the aeration basins as Return Activated Sludge (RAS) to maintain the biological population, while the excess is removed as Waste Activated Sludge (WAS).
D. Disinfection
To ensure pathogen destruction before discharge, the clarified effluent passes through a closed-vessel Ultraviolet (UV) disinfection system. The facility transitioned from chlorine gas to UV technology to eliminate the risks associated with hazardous chemical storage in an urban environment and to reduce disinfection byproducts in the receiving water. The UV system is sized to handle peak wet-weather flows while ensuring consistent dosage for E. coli inactivation.
E. Solids Handling & Biogas Recovery
Solids handling is a standout feature of the Theresa Street facility:
- Thickening: Primary sludge and WAS are thickened (using gravity thickeners and/or rotary drum thickeners) to reduce hydraulic volume.
- Anaerobic Digestion: Thickened sludge is pumped to anaerobic digesters heated to mesophilic temperatures (approx. 98°F). Here, bacteria break down volatile solids, reducing sludge volume and pathogen content while generating biogas (methane and CO2).
- Dewatering: Digested sludge is dewatered using high-solids centrifuges, producing a cake that is suitable for land application on local agricultural fields as a soil amendment (biosolids).
4. Infrastructure & Facilities
A. Physical Plant
The Theresa Street site is physically constrained, bordered by Salt Creek, the University of Nebraska-Lincoln’s Innovation Campus, and the Devaney Center. This “landlocked” status requires engineering ingenuity for any expansion, often necessitating vertical construction or retrofitting existing footprints rather than expanding outward.
B. Energy Systems & RNG
The facility is a net energy producer. Historically, biogas was flared or used in boilers. However, LTU implemented a state-of-the-art Biogas Conditioning System. This system purifies the digester gas to pipeline-quality Renewable Natural Gas (RNG). This RNG is injected into the commercial natural gas grid and sold as renewable vehicle fuel, generating significant revenue through Renewable Identification Numbers (RINs) under the EPA’s Renewable Fuel Standard.
C. Odor Control
Given its proximity to university facilities and event venues, odor control is paramount. The facility employs covers on primary clarifiers and utilizes chemical scrubbers and biotrickling filters to treat foul air from the headworks and solids processing buildings, maintaining good relations with the neighboring community.
5. Recent Upgrades & Major Projects
Biogas-to-RNG Resource Recovery Project (Completed ~2018/2019)
Cost: Approx. $8 Million
Scope: Installation of a gas conditioning system to remove siloxanes, moisture, and CO2 from digester gas. Implementation of an injection station to the Black Hills Energy pipeline.
Impact: This project transformed the facility into a revenue-generating asset, reducing greenhouse gas emissions and providing a renewable fuel source for the transportation sector. It is projected to generate millions in revenue over its lifespan, offsetting operational costs.
Solids Dewatering Improvements (2020-2022)
Scope: Replacement of aging belt filter presses with high-performance centrifuges.
Technical Highlight: The switch to centrifuges improved the dryness of the biosolids cake (increasing % solids), which significantly reduced hauling costs and improved the efficiency of the land application program. This upgrade also included polymer feed system enhancements and conveyor modifications.
Headworks & Grit Removal Upgrade
Drivers: Aging equipment and the need for better grit capture to protect downstream digesters.
Details: Installation of improved screening and vortex grit removal technologies to handle peak flows and reduce the inorganic load on the anaerobic digesters, thereby maximizing digestion volume for volatile solids reduction.
6. Regulatory Compliance & Environmental Performance
The Theresa Street WRRF operates under NPDES Permit NE0020290 administered by the NDEE. The facility faces stringent requirements due to the low dilution ratio of Salt Creek.
- Ammonia-Nitrogen: Seasonal limits are strictly enforced to prevent toxicity to aquatic life.
- E. Coli: Disinfection must meet recreational water quality standards during the recreation season (May-September).
- Nutrients: While Nebraska is currently focusing on nutrient monitoring, the facility is planning for future Total Nitrogen (TN) and Total Phosphorus (TP) limits as part of the broader Mississippi River/Gulf of Mexico Hypoxia Task Force initiatives.
The facility consistently receives Gold or Silver awards from the National Association of Clean Water Agencies (NACWA) for peak performance and permit compliance.
7. Challenges & Future Planning
Aging Infrastructure & Site Constraints
Portions of the plant infrastructure date back decades. A primary challenge for LTU engineers is maintaining continuous operations while rehabilitating concrete tanks and underground piping within a highly congested site footprint. Future master plans emphasize asset management and rehabilitation over expansion.
Flood Resiliency
Located in the Salt Creek floodplain, the facility must maintain resilience against rising river levels. Recent improvements have focused on protecting critical electrical gear and pump stations from potential 100-year flood events.
Future Nutrient Regulations
The City is proactively evaluating technologies for Biological Nutrient Removal (BNR). Future capital improvement plans (CIP) likely include retrofitting aeration basins with selector zones to promote biological phosphorus removal and denitrification.
8. Technical Specifications Summary
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Secondary Treatment (Activated Sludge) |
| Design Capacity (ADF) | 27.6 MGD |
| Peak Hydraulic Capacity | > 60 MGD |
| Primary Treatment | Rectangular Clarifiers |
| Secondary Treatment | Plug Flow Activated Sludge |
| Disinfection | Ultraviolet (UV) Irradiation |
| Solids Stabilization | Mesophilic Anaerobic Digestion |
| Dewatering | High-Solids Centrifuges |
| Biogas Utilization | Conditioning to RNG (Pipeline Injection) |
| Biosolids Disposal | Land Application (Class B) |
| Receiving Water | Salt Creek |
| Year Commissioned | Original: Late 1800s / Major Expansions: 1960s, 1980s, 2010s |
9. FAQ
1. What is the difference between the Theresa Street and Northeast facilities?
Theresa Street is the older, central plant handling roughly 60% of the city’s flow. The Northeast WRRF was built to handle expansion and growth in the northern and eastern basins. They function as a unified system under LTU.
2. Does the plant produce its own power?
While it doesn’t generate electricity on-site (co-generation), it produces renewable energy in the form of Renewable Natural Gas (RNG). This gas is injected into the grid and offsets the use of fossil fuels in vehicles, which is equivalent to taking thousands of cars off the road annually.
3. How does the facility handle heavy rain?
The plant utilizes large retention basins and flow equalization strategies. If flows exceed the biological treatment capacity, the plant is designed to provide primary treatment and disinfection to the excess flow to protect the biological process from washing out.
4. Is the facility regulated for Phosphorus removal?
Currently, the facility monitors for phosphorus. While strict numeric limits are not yet in full force in the permit, the City is planning for future compliance requirements involving nutrient reduction.
5. What happens to the “sludge”?
The solids are digested (stabilized), dewatered, and transported to local agricultural land. Farmers use this nutrient-rich “biosolid” material as a fertilizer, closing the loop on the nutrient cycle.