Odor Control

The formation of objectionable odors is very likely when millions of gallons per day of wastewater are collected and sent to one location for treatment. One of the most common odors associated with wastewater treatment is the "rotten egg" smell of hydrogen sulfide. It is unpleasant to smell but can also be hazardous to sewer workers and corrode expensive treatment systems. That's why the Division of Wastewater Treatment has taken steps to combat odors and the problems they cause.

• Hydrogen Peroxide Addition: The presence of the "rotten egg" smell of hydrogen sulfide is nature's way of saying the water has run out of oxygen. One of the most effective ways of restoring oxygen back to the water is to add a liquid chemical called hydrogen peroxide. Most people are familiar with 3% hydrogen peroxide available at drug stores, grocery stores, etc. that is used as a mild disinfectant. The Division of Wastewater Treatment uses a much stronger 50% solution to add to the wastewater being pumped to the Wastewater Treatment Plant. It does not disinfect the wastewater - it adds oxygen to the wastewater. This helps prevent the "rotten egg" smell of hydrogen sulfide from forming at the pumping stations and at the treatment plant. About 19 BILLION gallons of wastewater is treated each year with hydrogen peroxide, at a cost of about $350,000.
• Air Scrubbing: After the wastewater gets to the Wastewater Treatment Plant, it enters treatment tanks. During the initial stages of treatment, biological action uses up available oxygen, and again hydrogen sulfide begins to form. This time, however, the rotten egg smell gets trapped under fiberglass covers installed on the treatment tanks in areas where odors are released into the air. A large blower draws odorous air from the covered areas to an Air Scrubber where chemicals are applied to remove the hydrogen sulfide odor from that air. The scrubber then releases the treated air, free of odor causing chemicals, into the atmosphere. About 24,000 cubic feet of air each minute of the day is treated in the air scrubber. This air treatment process cost about $2 million to build, and the chemicals used for treatment cost another $30,000 each year.

Air Permit Compliance

Methane gas (the major component of natural gas) is produced as a by-product gas at the Wastewater Treatment Plant during a biological treatment process called anaerobic digestion. This gas is burned in various other processes to produce hot water and electricity. This saves a lot of money because the gas does not have to be purchased from the local utility.

Anaerobic digestion can also produce Hydrogen sulfide, which becomes a component of the by-product gas. When the combined gas is burned, the sulfur in the hydrogen sulfide is oxidized and becomes an air pollutant. Therefore, the Environmental Protection Agency (EPA) has issued permits for processes that burn this gas. The permits limit the amount of oxidized sulfur that can be discharged to the air from these processes.

To make sure the by-product gas contains only a small about of hydrogen sulfide, the Division of Wastewater Treatment adds a chemical to the anaerobic digestion process that "ties up" the hydrogen sulfide. That chemical is ferrous chloride. The iron in ferrous chloride combines with the hydrogen sulfide to make iron sulfide. This dramatically reduces the amount of hydrogen sulfide in the gas and the amount of oxidized sulfides discharged to the air.

It costs about $40,000 a year for the ferrous chloride chemical to treat the 100 million gallons of material annually that creates the by-product gas.