OXYGEN INJECTION USING PSA TECHNOLOGY

The emergence of pressure swing adsorption (PSA) technology has made injection of oxygen into wastewater force mains for hydrogen sulfide (H2S) control a cost effective odor control alternative to other chemical treatments, such as calcium nitrate.The direct injection of oxygen into wastewater force mains has been practiced successfully in Australia and Great Britain for over 25 years, and on a limited basis in the U.S. for over 20 years. Oxygen can either be delivered in liquid form and stored on-site in large tanks, or generated onsite. Until recently, oxygen had to be delivered in liquid form and stored on-site in large tanks, which made it expensive and inappropriate for many sites. Recently, the patent on the PSA technology lapsed, and two manufacturers of PSA equipment have now made it economically feasible to generate the relatively small quantities of oxygen required by wastewater systems onsite. PSA systems are available with oxygen outputs ranging from two (2) standard cubic feet per hour (scfh) to 5,000 scfh.The PSA technology separates the oxygen from air using a process centered around a molecular sieve known as Zeolite. At high pressures the sieve adsorbs nitrogen, and at low pressures it desorbs nitrogen. The PSA oxygen generators consist of two tanks filled with molecular sieves. High pressure air is routed into the first tank, where it passes through the sieve and nitrogen is adsorbed. The remaining 90 – 95% pure oxygen is routed to a storage tank. Just before the first tank becomes completely saturated with nitrogen, the air is redirected to the second tank, and the process is continued. The first tank is then vented to atmosphere, which allows the nitrogen to be desorbed from the sieve while depressurizing. To complete the regeneration of the first tank, a small amount of the pure oxygen is used to purge it. This cycle is continuously repeated as long as there is a demand for oxygen, thus the name “pressure swing adsorption”.The typical outlet pressure of PSA systems is approximately 45 psi. Therefore, in force main applications operating above 45 psi, additional equipment is necessary for the oxygen pressure to overcome the main pressure. While oxygen booster compressors may be used, a preferred method is injection through a venturi aspirator installed in a side stream taken from the main. The venturi aspirator injection method has the added benefit of improved oxygen dissolution (> 90%) in the wastewater due to the turbulence created in the venturi. Force main applications should have a minimum pressure of 15 psi for oxygen injection systems to work efficiently.The following is a summary of a cost comparison recently completed for a Louisville/Jefferson County Metropolitan Sewer District 4.7 MGD pump station. The comparison is between a PSA oxygen injection system and a calcium nitrate addition system.In addition to lower total annual costs, oxygen injection has several advantages over calcium nitrate addition, including:No chemical handling or storageEasier to maintain proper dosage of chemicalIncreased dissolved oxygen concentration of the wastewaterPotential for reduced BOD of the wastewaterThe information in this paper will assist municipalities in determining required oxygen dosages and evaluating oxygen injection with PSA technology as an odor control alternative to calcium nitrate and other chemical treatments in wastewater force mains.