Deep Bed Denitrification Meets Strict, Varying
Nitrogen Limits
(Featured at WEFTEC.09)

The cities of Littleton and Englewood, Colo., just south of Denver, share a wastewater plant – the Littleton/Englewood advanced wastewater treatment (AWT) plant located in Englewood. The 50-mgd facility receives sewage from the two cities as well as from 21 districts within a 75 square mile service area. The facility serves more than 300,000 residents in the southern Denver metropolitan area. Plant effluent is discharged to the Denver metro area’s major watershed, the South Platte River.

With the Denver metro area’s population growth consistently outpacing the U.S. national average for the past 50 years, the cities of Littleton and Englewood in 2001 began planning to expand the 36-mgd wastewater treatment plant to its current 50-mgd capacity. At the onset of planning and design for the expansion project, the plant’s inflow had reached 80 percent of its design capacity. The goals of the expansion were three-fold:

1. Improve the effluent quality to South Platte River to meet several new USEPA regulations.
2. Increase capacity to meet demand of the growing population.
3. Modernize the plant’s infrastructure, as much of the existing treatment plant was 25 years old.

The plant process upgrades, including enhancing nitrate removal with a new in-plant recycling system and new denitrification filters, were designed and managed by Brown and Caldwell’s Denver office. The final plant design included eight deep bed denitrification gravity filters, each 11.8 ft by 96.1 ft, containing 7.9 ft of 2-3 mm rounded sand. The filters were designed with individual carbon source feeds for denitrification and variable influent flow splitting for maximum flexibility. Advanced control strategies and instrumentation were included to improve reliability in meeting the new daily permit requirements. The previous permit was based on monthly averages.

Design work was completed in 2004. Western Summit Constructors, Inc. of Denver handled construction and the new plant was dedicated in December 2008. The filter design and process equipment – the TETRA® DeepBed™ Denite® system – were supplied by Severn Trent Services.

Flexibility needed to meet USEPA regulations
The Littleton/Englewood AWT plant needed a reliable and flexible treatment process design that could comply with new daily effluent permit requirements for total inorganic nitrogen (TIN) that are more stringent at different times of the year and only require partial denitrification during other times. This presented a challenge, since denitrification filters work best when fully denitrifying. Otherwise, partially converted nitrogen, in the nitrite form, discharges from the filter and causes a very high chlorine demand downstream.

To solve the challenge of varying denitrification requirements, the plant was designed with individual methanol feeds to each deep bed denitrification filter. As a result, each filter could be individually controlled to either produce full, efficient denitrification or simply provide solids filtration. This produced a blended effluent quality that met the daily requirements for TIN while using variable influent flow to maximize performance and system flexibility.

Since phosphorus removal might be required in the future, the filters were equipped with two inlet gate valves that feed evenly dividing, cut-throat flumes. Filters not being used for denitrification can be run with both flumes open to allow for doubled filtration rates.

System operation
To optimize the operation and reliability of the deep bed denitrification system, advanced instrumentation technologies and control strategies were used. Flow entering the filter main influent channel is measured by a flow meter. An automatic sampler draws water samples from the filter influent channel every 30 to 60 minutes, and a second sampler operates in the filter clearwell where the effluent from the eight filters combines. The composites are collected once daily and analyzed in the plant laboratory.

There are also two ultraviolet-based online analyzers for nitrate-nitrogen and phosphate. The first analyzer tests water from five locations, including the influent channel feeding all filters and individual effluent samples from filters one through four. The second analyzer also tests five samples, including effluent from filters five through eight and the combined effluent of all filters before it enters the clearwell. The analyzers test each location one at a time and repeat the testing continuously every few minutes.

A computer algorithm uses the flow and analyzer outputs to individually control and dose methanol carbon source to each filter influent. Methanol dosing is varied proportionately with influent flow and influent nitrate values for each filter. Individual methanol pumps dose each filter, assisted by a constant flow of dilution water injected into the pumped methanol. This transports the methanol to the filter inlet quickly to ensure responsive dosing and lowered flammability.

The computed dosage is corrected periodically based on the resulting effluent nitrate nitrogen from individual filters. Methanol pumps can also be set to adjust based on either individual or combined effluent nitrate-nitrogen values.

Rigorous system testing
During the initial filter biological startup in April 2008, methanol feed was started at 25 percent of theoretical dose and increased an additional 25 percent per day until a full dosage was fed. Despite the 15º C process water at that time, significant denitrification was observed within a week and consistent NO₃-N removal was achieved within 10 days.

A five-day filter process performance test was conducted in denitrification mode during the week of July 7, 2008. The filters demonstrated the capability to remove more than 20 µg/L of NO₃-N by themselves even at max-day flows. Max-day flows were applied to the filters almost continuously throughout the five day test. The inlet NO₃-N peaked near 30 µg/L in the morning hours and was treated without problem. Gregory Ellard, senior process engineer for Severn Trent Services, noted that the Littleton/Englewood test was possibly the most rigorous full-scale operation of deep-bed denitrification filters ever performed.

During the performance test, each filter needed a daily backwash to remove solids and excess biomass. Nitrogen gas was also purged or bumped from the filters by reversing flow through each filter with the backwash pumps. Rapid gas accumulation caused by the high nitrate and flow loading was found to be best handled by short but frequent bump cycles of 60 seconds per filter every 30 minutes.

Backwash water usage was about 3.4 percent of forward flow, meeting the process guarantee. And not only was there no net pickup of TOC across the filters, there often was a significant reduction instead.

Continuing operations, estimated savings
Plant operators are now using the flexibility of the innovative individual filter controls to meet daily permits, and the plant’s design has allowed cost-effective control of plant effluent quality to a degree not previously possible. Presently, two of the eight filters are in filtration-only mode, and the other six filters are in denitrification mode. Five of the denitrifying filters have effluent set points of 2.5 µg/L NO₃-N. The remaining denitrifying filter is operated off of the sampler from the combined effluent of the eight filters to trim the combined concentration to 8 to 9 µg/L NO₃-N, ensuring that methanol is not overfed and that nitrite residual is minimal.

According to Ellard, calculations reveal that if the Littleton-Englewood AWT filter plant had been processing an average daily flow of 14 mgd and was reducing nitrate-nitrogen from 20 µg/L to 1 µg/L, the yearly methanol cost at USD $1.15 per gallon would be $412,000. Achieving denitrification down to 9 µg/L using the innovative individual filter controls would cost $251,000 per year under the same scenario, representing a 39 percent reduction in potential methanol usage at a savings of $161,000 per year.

Representatives from Severn Trent Services will present findings on the use of deep bed denitrification at the Littleton/Englewood Advanced Wastewater Treatment Plant at the WEFTEC.09 Municipal Wastewater Treatment Process session, "Nitrogen Removal," on Monday, October 12. (Poster Presentation entitled "Cool Water High-Rate Full-Scale Denitrification in Deep Bed Filters")

For more information, e-mail info@severntrentservices.com.