In 2001, the Littleton/Englewood Wastewater Treatment Plant began design of its Phase 2 expansion from 36 mgd to 50 mgd. At the same time, the Colorado Department of Public Health and Environment was conducting a total maximum daily load (TMDL) study of Segment 14 of the South Platte River in Denver, potentially affecting future discharge permit limits.

To meet the compliance schedule for nitrate removal set in the plant’s existing discharge permit, it was necessary and most cost effective to include denitrification in the design of the expansion to meet anticipated (and not yet specified) total inorganic nitrogen (TIN) requirements.

The denitrification design incorporates innovative features to increase operational control, reduce capital construction expense, and reduce operation and maintenance costs for nitrate removal.

 

Water quality challenges

Used for recreation and drinking water, Segment 14 of the South Platte River is identified as an impaired segment for several parameters, the most important being nitrogen.

Facing imminent nitrogen discharge limits, the plant took an innovative approach. Facility staff and engineering consultants Brown and Caldwell recommended, developed and designed two system enhancements:

• An innovative denitrification filter design.

• A newly designed in-plant nitrate recycling system.

 

Filter innovations

After an alternatives analysis and pilot testing of denitrification processes, the plant team selected the Severn Trent Services TETRA Denite filter design with eight individual downflow filters. This design provides a substrate for biological removal of nitrate along with solids filtration and best addresses anticipated phosphorus removal requirements in the future.

Using methanol as a carbon source for denitrifying microorganisms, the typical Severn Trent system approach is to apply methanol to the entire process flow. After looking at many operating Severn Trent denitrification systems, the Littleton/Englewood team recommended several design modifications.

In the Littleton/Englewood system, each filter was designed with an individual chemical feed system for denitrification. Filter influent flow distribution, using cutthroat flumes, also provides maximum flexibility in optional operating modes.

Since only partial denitrification is needed at certain times of the year, denitrification filters can be operated independently (in denitrification or filtration-only mode) to meet variable discharge limitations, while optimizing use of methanol. To attain this level of flexibility, an advanced process control strategy and instrumentation were included for each filter.

During pre-design meetings, parties debated the wisdom of a complex process control strategy using individually controlled filters. However, further evaluation and design discussion focused on the benefits of individual-filter denitrification control, and Severn Trent ultimately patented those innovations.

 

Nitrate recycling

The recycling of a nitrate-rich process stream into the plant influent is truly innovative. It is theorized that denitrification may occur in carbonaceous trickling filters. The Littleton/Englewood plant, which operates trickling filters as part of secondary treatment, implemented a variation of the recycle stream concept as a way to test the theory and temporarily reduce TIN.

In operation since August 2001, the system used existing infrastructure to provide “nearly free” nitrate removal while yielding other environmental benefits. As one side benefit to this nitrified effluent return (NER), plant staff noticed that odors normally generated in the primary clarifiers were not as noticeable. Testing confirmed that hydrogen sulfide production was virtually eliminated while nitrate was recycled via NER.

In addition, Severn Trent filtration, whether in the nitrate removal or filtration-only mode, has shown a benefit in the facility’s chlorine-based disinfection system. E. coli levels dropped from an average of approximately 60 colonies/100 ml to typically 4 colonies/100 ml at present, with no changes in disinfection system design.

As a result of testing, the plant team designed a full-scale NER system to take advantage of existing nitrate-reducing capabilities within the treatment process. The NER system also provides a relatively constant hydraulic flow within the facility, allowing for more stable operation.

By using a combination of denitrification filters and nitrate-rich process flow recycling, daily TIN and ammonia discharge limitations are met while providing capital construction savings, operational cost optimization, and other environmental benefits.

 

Benefits quantified

The Littleton/Englewood plant’s denitrification system design has led to several accomplishments:

• By reducing the need for redundant treatment capacity in the denitrification filter process, the inclusion of the NER system in the Phase 2 design has reduced denitrification filter capital construction costs by about $3 million. As a result of this design, the NER system is capable of recycling up to 50 percent of plant design flow.

• In addition to construction savings, NER reduces the nitrate load to the denitrification system. Based on 1,500 pounds of nitrate removed per day, recent cost analysis (including related electrical/methanol expenses) indicates it costs $0.69 per pound of nitrate removed in the denitrification process, versus $0.24 per pound of nitrate removed via NER. Overall, NER operation reduces denitrification operation and maintenance costs by about $246,000 per year.

• Process optimization efforts have also enabled the plant to discontinue methanol application for four months of the year (May through August) and rely on NER alone to meet discharge requirements. That constitutes a savings of about 750 gallons of methanol per day — or $134,000 for the four-month period, assuming a methanol price of $1.49 per gallon.

Innovative approaches to nutrient removal have effectively positioned the Littleton/Englewood Wastewater Treatment Plant to meet current and future permit requirements while supporting continued cost-effective operations.

 

About the authors

Jim Tallent is Operations Division manager and a Class A wastewater treatment plant operator at the Littleton/Englewood (Colo.) Wastewater Treatment Plant and can be reached at jtallent@englewoodgov.org. Greg Farmer is process control administrator for the plant and a Class A operator. He can be reached at gfarmer@englewoodgov.org. The authors acknowledge Brown and Caldwell and Severn Trent Services for their contributions to this article.

Sidney Biesterfeld, Greg Farmer, Linda Figueroa, Denny Parker, and Phil Russell “Quantification of Denitrification Potential in Carbonaceous Trickling Filters” Wat. Res. 37 (2003): 4011-4017.