Pulling Double Duty

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Operations Forum July 1999

Brown and Caldwell
Mike Lutz and Greg Farmer

The 32-mgd (120-M3/D Littleton- Englewood Wastewater Treatment Plant serves a population of approximately 220 000 in southern metropolitan Denver, Colo. The 45-ac (18-ha) site adjoins residential and commercial zones, light industrial areas, parks, and a greenbelt along the South Platte River. Because of the plant’s urban location, a high priority is placed on odor control. In 1988 a systematic control program was initiated to reduce odor emissions and potentially adverse effects on the surrounding community.

Meanwhile, new water quality standards that were established to protect aquatic life in the South Platte River required the plant to meet new monthly variable ammonia limits, which range from 5.1 to 13.5 mg/L. After several nitrification options were evaluated, plant management decided that adding nitrifying trickling filters to secondary treatment facilities was most cost-effective.

Odor treatment units similar to the filters had been pilot-tested successfully as early as 1971 at the Hyperion Wastewater Treatment Plant in Los Angeles, Calif. Since then, several biological air-treatment systems using secondary process effluent have been constructed throughout the United States. Operating experience at these facilities suggested that nitrifying trickling filters could provide a favorable environment for sulfur-metabolizing bacteria, and in 1992, the filters were installed at the Littleton- Englewood plant with the dual purpose of achieving nitrification and odor control.

How It Works

Each filter is 105 feet (32-meters) in diameter and contains 24 feet (7 meters) of medium-density, cross-flow plastic media. The filters, covered with geodesic domes, are equipped with forced-air ventilation systems that deliver exhaust air from other treatment processes (click here for process schematic). More than 19,800 ft3/min (538 m3/min) of odorous air from two 45-ft-diameter 14 m diameter) dissolved-air flotation thickeners and a 320,000-gal (1211 m3) capacity centrate equalization tank discharges into the top of the enclosures and moves downward through the media with wastewater flow. Negative pressure within the filters ensures the air is contained and fully treated before discharge, which is accomplished with a centrifugal exhaust fan. A network of supply ducts arranged and fastened inside the domes and exhaust ducts uniformly distributes and collects air from within the filters.

Each filter provides approximately 20 minutes of air-residence time, treating roughly 9900 ft3/min (280 m3/min). Because the filters treat secondary effluent, the organic loading rate is low (typically 4.8 pounds [2 kg] of biochemical oxygen demand per 1000 cubic feet [28 cubic meters] per day). The oxygen content of the supply air sustains the biological process without requiring fresh air during normal operation. During maintenance or air-supply system shut down, bypass louvers in the dome enclosures can be opened.

The Bug Community

Aerobic sulfur-oxidizing bacteria coexist with nitrifying bacteria (Nitrosomonas and Nitrobacter) in the filter biofilm along with other microorganisms. The most well-known sulfur-oxidizing bacteria (Thiobacillus, Thiosphaera, Thiomicrospira, Thermothrix, Beggiatoa, and Thiothrix) use sulfide, hydrogen sulfide, thiosulfates, elemental sulfur, and partially reduced sulfur oxides as energy sources.

Sulfur-oxidizing bacteria use dissolved oxygen to convert sulfur compounds to sulfate, extracting energy in the process. Most of these bacteria are autotrophs that use carbonate as a carbon source. Beggiatoa is not autotrophic, relying on organic carbon for cell growth rather than carbonate. Certain sulfur bacteria, including Beggiatoa and Thiothrix, can oxidize hydrogen sulfide into sulfur intermediates that can be stored in the cell as a reserve energy supply. Filamentous Beggiatoa often contains granules of stored elemental sulfur visible under a microscope.

Bacteria produce sulfuric acid as metabolic waste when oxidizing reduced sulfur compounds. When these bacteria are present in collection systems, acid accumulates at the pipe crown, leading to the corrosion of concrete and metal structures above the water line. However, wastewater flowing over concrete media supports in nitrifying trickling filters continually washes out sulfuric acid, precluding corrosion. Sulfur oxidation also decreases wastewater pH, but the effect is negligible when compared with the drop in alkalinity caused by nitrification.

Filter Operation

Daily operation of the filters for odor control could not be easier at the Littleton- Englewood plant. Simply put, the operator ensures that supply and exhaust fans are running and that pumps are moving water at the correct rate for nitrification. The biofilm does the rest. In contrast, a chemical treatment system used at the plant requires constant attention to chemical inventories, pumping rates, pH, and oxidation-reduction potential.

The strength of odor sources and the filters’ performance are evaluated by measuring atmospheric hydrogen sulfide and organic sulfur compounds upstream and downstream of the process units. The laboratory performs a low-resolution scan using gas chromatography to establish the presence of various odor compounds in samples taken from the dome enclosures and ductwork. This procedure is followed by a flame photometric analysis of each odor compound to determine concentration accurately.

Effectiveness

The nitrifying trickling filters have proven to be very effective as biological scrubbers, removing between 96% and 99% of gas-phase hydrogen sulfide. Atmospheric hydrogen sulfide measurements of filter supply air averaged 248 parts per billion (ppb), but concentrations in exhaust air was below the analytical detection limit of 4 ppb in all samples. Methyl mercaptan in supply air averaged 32.9 ppb, while exhaust air measured below 4 ppb.

Test

Hydrogen Sulfide

Methyl Mercaptan

Concentration (ppb)

Percent Removal

Concentration (ppb)

Percent Removal

Supply

Exhaust

Supply

Exhaust

1

102

<4

96.1

13.0

<4

69.9

2

432

<4

99.1

18.7

<4

72.8

3

127

<4

96.7

13.3

<4

69.9

4

366

<4

98.9

24.5

<4

83.6

5

135

<4

99.0

10.8

<4

62.7

6

323

<4

98.8

20.0

<4

79.9

Average

248

<4

98.4

16.7

<4

76.0

A well-designed and maintained chemical scrubber can achieve discharge H2S concentrations as low as 100 ppb. This concentration is still above the human olfactory detection threshold of approximately 1 ppb for H2S. With a discharge of less than 4 ppb, the scrubber emits virtually no odor.

The filters’ 20-minute detention time sets the Littleton- Englewood units apart from most biological odor treatment units that use wastewater for biofilm maintenance. However, even with a 45-second detention time, these types of systems discharge hydrogen sulfide concentrations that typically range from 1 part per million to as low as 300 ppb.

Because the domes were recycled from primary clarifiers the only costs wre attributed to moving and re-installion. The odor control components of the nitrifying trickle filters cost $370,000 less than a chemical scrubber would cost. In addition, the filters save money because no chemicals are needed and they are easier to operate, providing a total cost savings of $615,000.

System Components

NTF Scrubber (US$)

Chemical Scrubber1 (US$)

Chemical scrubber system

0

500,0002

Centrate tank ductwork, fan

51,000

51,000

DAFTS aluminum domes

162,000

162,000

DAFT ductwork, fan

83,000

83,000

NTF ductwork, fan

128,600

0

Dome moving and installation

53,000

0

Annual chemical cost

0

25,000

subtotal

426,600

796,000

Present chemical cost

0

245,0003

Total present worth

426,000

1,041,000


1Chemical System consists of one multi-stage packed tower, chemical tanks, pumps, and piping.

2Based on $25 in capital cost per ft3/min.

3Calculated at 20 years plus 8% interest.

 

Mike Lutz is a partner at Integra Engineering http://www.integraeng.com/ (Denver, Colo.) and Greg Farmer is a process specialist at the Littleton- Englewood Wastewater Treatment Plant.