Products In Action: Chemicals
The Johnstown, Penn., Wastewater Treatment Plant has found a simple and relatively inexpensive chemical solution to a chronic hydrogen sulfide odor problem.
About a year ago the staff began to add to its sludge GenClear® N ? a blend of sodium nitrite and sodium nitrate from General Chemical Corporation of Parsippany, N.J. The result: hydrogen sulfide levels have been reduced to less then 1 ppm from peaks of over 10 ppm.
"The air here is much more tolerable since we began using the nitrate/nitrate mix," said Jeff Mulligan, chief plant operator at this pure-oxygen activated sludge facility. Because it acts as a chemical source of oxygen, the blend also has eliminated the need for a mechanical-aeration tank.
The 40-year-old plant serves 65,000 people in Johnstown and nearby municipalities. It routinely processes 12 mgd, although throughput has gone as high as 20 mgd.
Mulligan, who has worked at the plant for eight years and has been chief operator for the last three, said the hydrogen sulfide odor has always been powerful.
"I visited the plant in the mid-80s, and the presence of hydrogen sulfide was very evident," he said. "This noxious gas had many effects. It impacted the physical and psychological well being of everyone at the site and led to corrosion of steel and other metals, as well as of concrete. It also posed such a serious problem for the electronics on site that we installed ventilation equipment for the control center."
Mulligan estimates that hydrogen sulfide levels at the Johnstown plant had peaked for short times between 10 ppm and 15 ppm, based on data from a hydrogen sulfide gas detector.
"This is within U.S. Occupational Safety and Health Administration guidelines for hydrogen sulfide exposure," he said. "Even so, since the levels we were seeing at the plant were quite noticeable and made work more difficult, we took steps to reduce them."
The hydrogen sulfide problem begins in the holding tanks for thickened sludge. Just upstream of these tanks is a 110,000-gallon gravity thickener where settling occurs over several hours. The supernatant is returned to headworks, while waste activated sludge (WAS) is pumped from the bottom of the thickener into two 28,000-gallon concrete holding tanks.
The WAS stays in these tanks for a day or two. During this time, bacteria use up the dissolved oxygen and the WAS becomes anaerobic. Anaerobic and facultative bacteria then turn to oxygen-containing compounds for the oxygen they need. Since sulfates are the most abundant such compounds in the WAS, bacterial action leads to the formation of hydrogen sulfide gas.
Hydrogen sulfide is freed when WAS reaches the open air as it leaves the holding tanks to be dewatered in belt presses. After this, they are treated with lime to raise pH, reduce pathogens and control disease vectors.
Mulligan now neutralizes the odor by adding sodium nitrite/nitrate liquid solution between the thickener and the holding tanks. In treating the sludge, Mulligan injects the solution into the waste sludge line with a metering pump at a rate of 30 to 40 gallons per day. The nitrate level is monitored in the supernatant from the gravity thickener and in the filtrate from the belt press.
Workers use these readings to adjust the product feed rate so the filtrate?s nitrate residual stays at 1 ppm. This optimizes the process by both reducing odors and ensuring the chemical is not overfed.
The nitrite/nitrate mix stops hydrogen sulfide generation in two ways. First, it replaces sulfates as the oxygen source of choice; as long as nitrite and/or nitrate are present, sulfate will not be broken down by SRB and no hydrogen sulfide will form. Second, the alternate oxygen source allows other common bacteria to consume dissolved sulfide ions by oxidizing them back to sulfates. The nitrate/nitrite mix can thus provide an effective barrier to problems associated with the generation of hydrogen sulfide in WAS, as well as municipal wastewater collection systems.
"Bottom line, we've cut our cost and improved our air," Mulligan said. "Our hydrogen sulfide gas detector now continually registers it at less than one part-per-million. But you don?t need a detector to tell the difference. All you need to do is breathe.
"It's been a real relief to everyone who works here."