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Superoxygenation: Facts and Myths
R. E. Speece
Centennial Professor Emeritus of Civil and Environmental Engineering Vanderbilt University, Nashville, TN 37215
Until recently the difficulties encountered in dissolving pure O2 into wastewater at high concentrations have hindered attempts to maximize its amazing usefulness in preventing odor formation and in causing the rapid metabolism of dissolved sulfides by resident bacteria possible only under oxic conditions. However, a new process, Superoxygenation, as used in this discussion, utilizes pure oxygen under pressure in a unique gas transfer reactor to raise the dissolved oxygen (D.O.) in the discharged water to 10 to 300 mg/L, depending on the application. These high concentrations of D.O. enable many extra years of infrastructure usefulness by the resulting corrosion control. Furthermore, by the prevention of H2S formation in head works, primary clarifiers, gravity interceptors and force main sewers, odors can be eliminated without costly and troublesome capturing and scrubbing of foul gases. Avoidance of tertiary treatment often needed to compensate for low reaeration rates in receiving waters is often possible by superoxygenating effluents to D.O. levels = BODult; D.O. standards for rivers, canals, lakes and bays thereby may be attained consistently. Odor and corrosion prevention in sludge holding tanks prior to dewatering are also potentially achievable by superoxygenation technology.
It is well known that oxic conditions support the rapid chemical and microbial oxidation of H2S and other reduced sulfur compounds present in raw wastewaters. As long as oxic conditions are maintained, additional H2S formation is prevented. But aeration technology is limited economically in raising D.O. concentrations above 4 - 5 mg/L, due to the limited solubility of O2 in water (air:21% O2) of 7 – 14 mg/L at 35 and 0 oC respectively. Aeration techniques produce only D. O. <10 mg/L. Therefore the limitations of engineered aeration techniques in remediating wastewater problems are well known, as evidenced by the serious odor and infrastructure corrosion issues which trouble cities and industries worldwide.
All of the D.O. required for oxic microbial metabolism (i.e. 300 mg/L), can be added to the primary effluent and then the superoxygenated wastewater subsequently introduced to the activated sludge reactor when utilizing this pure oxygen technology. Depressurization under increased hydrostatic head at the bottom of a deep shaft and/or immediate dilution by low D.O. mixed liquor, may be incorporated to retain the very high D.O. concentrations in solution, facilitating use of open-topped microbial reactors for escape of CO2 and negating the adverse impact of reduced oxygen transfer rates (alpha factor) caused by high mixed liquor suspended solids (MLSS) in membrane reactors. .
Pure O2 costs approximately one tenth as much as alternative odor prevention chemicals (iron salts, nitrates, peroxide and MgO), but when used in this proprietary system, superoxygenation results in H2S concentrations even lower than alternative chemicals can achieve, without a high financial burden and environmentally negative impact. In addition D.O. concentrations of <60 mg/L can be retained in solution for hours at ambient pressure, while D.O. concentrations of >100 mg/l can be diluted in seconds to retain the D.O. in solution. These critically important benefits make possible treatment applications heretofore very difficult to consider.1
Past and planned municipal, industrial and lake superoxygenation installations point to successful results in the future for new applications. Corroborating data available from several ongoing installations will be included. Since some water quality professionals may be unfamiliar with the superoxygenation approach to wastewater treatment, the author will also discuss 15 frequently mentioned misconceptions about superoxygenation technology and offer the scientific basis which refutes each perceived ‘myth.’
Oxygen Transfer Requirements and Devices
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