Denitrification Filters Meet Strict, Varying Nitrogen Limits
The 50-million-gal-per-day (mgd) Littleton/Englewood advanced wastewater treatment (AWT) plant serves more than 300,000 residents in the Denver metropolitan area. The facility also receives sewage from 21 districts within a 75-sq-mile service area. Plant effluent is discharged to the Denver metro area’s major watershed, the South Platte River.
In 2001, the cities of Littleton and Englewood began planning to expand the then-36-mgd wastewater treatment plant to its current 50-mgd capacity to accommodate the area’s fast-growing population. At the onset of planning and design, the plant’s inflow had reached 80% of its design capacity.
The goals of the expansion were three-fold:
- Improve the effluent quality to meet new U.S. Environmental Protection Agency (EPA) regulations;
- Increase capacity to meet demand of the growing population; and
- Modernize the plant’s infrastructure.
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 .
EPA Regulations Require Flexible System
The Littleton/Englewood AWT plant needed a reliable and flexible treatment process design to 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.
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.
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 flowmeter. 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, which test water from 10 locations: the influent channel; from each of the eight filters; and the combined effluent of all filters before it enters the clearwell. Each location is tested one at a time, and testing is repeated 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% of theoretical dose and increased an additional 25% per day until a full dosage was fed. Despite the 15?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 mg/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 mg/L in the morning hours and was treated without problem. 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% of forward flow, meeting the process guarantee. And not only was there no net pickup of total organic carbon across the filters, there often was a significant reduction instead.
Continuing Operations, Estimated Savings
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 mg/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 mg/L NO3-N, ensuring that methanol is not overfed and that nitrite residual is minimal.
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 mg/L to 1 mg/L, the yearly methanol cost at $1.15 per gallon would be $412,000. Achieving denitrification down to 9 mg/L using the innovative individual filter controls would cost $251,000 per year under the same scenario, representing a 39% reduction in potential methanol usage at a savings of $161,000 per year.
Featured at WEFTEC.09
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, Oct. 12. The poster presentation is titled "Cool Water High-Rate Full-Scale Denitrification in Deep Bed Filters."