Praxair, Inc. has announced results of a test of a technology for emission reduction in wastewater treatment. The company claims that the In-Situ OxygenatorTM system reduces atmospheric emissions of volatile organic compounds (VOCs) by 98%.
The test demonstrates the capability of the company's system to lower VOC emissions at municipal and industrial wastewater treatment facilities, enabling them to meet regulatory demands. The system also enables wastewater treatment facilities to increase capacity easily and inexpensively while, at the same time, reducing emissions.
The $540,000 study was financed jointly by Praxair and the New York State Energy Research and Development Authority (NYSERDA). Tests were conducted by scientists and engineers from Praxair and the State University of New York at Buffalo.
During the tests, a small amount of methyl ethyl ketone (MEK), a representative VOC, was added to a wastewater side stream which was then fed to a portable pilot plant. There, half of the stream was treated with a conventional air-based, fine-bubble diffuser system and half with the oxygen-based Praxair In-Situ Oxygenator system. Air and oxygen flows were adjusted so that the level of dissolved oxygen in both treatment tanks was the same. The tanks were covered so that the amount of MEK in the headspace under the treatment tank covers could be measured. Data showed that the Oxygenator system reduced the amount of MEK emissions to the atmosphere by 98%. Oxygen utilization efficiency was found to be 92%, substantially higher than the 15-30% utilization rate of air based fine-bubble diffuser systems.
The fine bubble diffuser technology pumps air into the wastewater. Because of low oxygen utilization and the fact that air is about 79% nitrogen and only 21% oxygen, it is necessary to pump a large volume of air through the water to achieve the proper level of dissolved oxygen. As the nitrogen and unused oxygen in the air bubble up through the wastewater, they pick up VOCs, such as MEK, and carry them to the atmosphere.
Praxair's system mixes high-purity oxygen into the waste stream. It operates with very high oxygen transfer efficiency and utilization rates, maximizing the biodegradation of VOCs in the liquid while minimizing emissions. In many cases, acceptable VOC emission levels can be achieved by using the system without a treatment tank cover and vent control system.
An additional advantage of the system is lower power requirements. The total power needed to maintain the same level of dissolved oxygen, including the energy required to run an oxygen-generating plant, is significantly lower than a comparable air-based system.
The Buffalo Sewer Authority's Bird Island Treatment Plant was chosen for the evaluation because of its reliable and consistent operation. Any differences noted could be attributed to differences in the two technologies and not to variations in the plant's basic operation.