September 2010, Vol. 22, No.9

Problem Solvers

Fog lifts wastewater treatment plant odors

Problem: Hydrogen sulfide odors plague staff and nearby citizens.
Solution: Installation of hydroxyl-ion fogging units to pretreat odors ahead of carbon scrubbers.

 

Even though the Leucadia Wastewater District (LWD; Carlsbad, Calif.) takes pride in its environmental integrity and near elimination of sewer overflows and spills, odors troubled the plant.

Created as a special district in 1959, LWD collects and treats 17,000 m3/d (4.5 mgd) of wastewater and maintains nine lift stations, two pumping stations, 24 km (15 mi) of force mains, and more than 321 km (200 mi) of pipeline. But maintaning the system did not stop odor complaints about two of the district’s pumping stations.

“One pump station is located next to a shopping center and is very near our headquarters office,” explained Leo Schempp, field services manager for the district. “The wet well wasn’t sealed properly, and we were getting serious odor issues there.” Some of the loudest complaints were coming from the district’s employees, many of whom refused to park their cars in the office parking lot because of the odors, Schempp said.

The other pump station is situated on a point along Batiquitos Lagoon, a coastal wetland and nature reserve operated by the California Department of Fish and Game. The pump station is next to South Carlsbad State Beach, a popular site for swimming, diving, fishing, and picnicking.

About 5 years ago, with odors emanating from both locations — especially those fugitive odors that occur during first flush periods and upstream pump starts — the district sought a solution.

“We knew of a casino nearby that had to close temporarily because of hydrogen sulfide odors,” Schempp said. “They had solved the problem with O-MEGA hydroxyl fogging units from Vapex Environmental Technologies [Oveido, Fla.].”

The units use air, water, and electricity to generate a hydroxyl-ion fog that is injected into odorous spaces through an atomizing nozzle. The hydroxyl ions in the fog oxidize the odorous compounds, then the fog condenses back into the water stream. No chemicals or hazardous materials are used.

The casino flooded its odorous area with fog, which eliminated the smell, but when the fogging unit was turned off, the smell returned, Schempp explained. Based on this reference, LWD agreed to install two units — one at each pumping station – on a 6-week trial basis.

“The units were very effective within minutes of turning them on,” Schempp said. Following the trial run, the district purchased the units.

 

Shopping center solution

The pump station near the shopping center is designed to handle up to 13,000 m3/d (3.5 mgd). Initially, LWD installed an O-MEGA V800 unit with oxidant output of up to 0.36 kg/d (0.80 lb/d) to fog the wet well and control odors. After about 8 months, the pump station underwent a major rehabilitation to reseal the wet well and install a 2.4-m-diameter (8-ft-diameter) carbon canister. The carbon filter holds approximately 1089 kg (2400 lb) of activated carbon media. The fogging unit was repositioned ahead of the carbon scrubber as a form of pretreatment.

“We now use the O-MEGA unit to feed the hydroxyl-ion fog into the water flow ahead of the wet well,” Schempp said. “Then we use the carbon scrubbers as a polishing step. It’s a belt-and-suspenders approach.” This redundant approach is intended to prevent odor complaints if one system goes down and during scheduled maintenance, he said.

The results have been good. Hydrogen sulfide levels generally measure about 15 ppm without the system and less than 5 ppm with the system operating. While extracting foul air from the well reduces the reaction time and effectiveness of the hydroxyl-ion fog, it still provides significant reduction in the loading to the carbon scrubber. The scrubber outlet normally measures zero ppm of hydrogen sulfide. The district uses OdaLog odor-measurement devices from App-Tek International Pty Ltd. (Brendale, Queensland, Australia), positioning one in the wet well and one at the scrubber stack outlet.

 

Coastal cure

The district implemented a similar two-part upgrade and reconfiguration at the coastal pumping station, which is designed to handle up to 19,000 m3/d (5 mgd). Hydrogen sulfide levels, which are as high as 120 ppm when the station’s fogging unit is off, drop to less than 20 ppm when the unit is operating. This unit has an oxidant output of up to 0.7 kg/d (1.5 lb/d).

The carbon canister at this station holds about 1450 kg (3200 lb) of media, and the normal hydrogen sulfide measurement at the scrubber outlet is essentially zero, Schempp said.

In addition to reducing odors and odor complaints, the fogging units have yielded a significant cost savings, because the carbon scrubber units do not have to be recharged with fresh media as often as they would without the fogging units providing pretreatment. Schempp estimates a savings in fresh carbon costs of about $6000 per year at the shopping center pump station and about $20,000 per year at the coastal pump station.

In addition, the fogging units have required little if any maintenance. “In the 5 years of operation, we’ve needed to replace a few air filters, and the air compressors require the expected amount of parts and upkeep,” Schempp said.

The biggest benefit of the odor-control upgrade may be improved staff morale, Schempp said. LWD asked the employees who had registered the most complaints to be on an odor panel to address the problem; they are pleased with the results. As for Schempp, he said, “It’s like a dream come true.” n

 

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