ppm Dissolved oxygen measurement

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Courtesy of IC Controls Ltd.

Dissolved oxygen (D.O.) levels are used as a general indicator of water quality. Oxygen is essential to life and vital for countless aquatic forms. D.O. level control is critical in numerous applications ranging from the monitoring of industrial waste water to environmental surveillance.

What is Dissolved Oxygen?
Dissolved oxygen (D.O.) is the amount of oxygen gas dissolved in a given quantity of solvent (usually water) at a given temperature and atmospheric pressure. As such, it should not be confused with combined oxygen as found in the water molecule, H2O. It is usually expressed as a concenn'ation in parts per million or mg/1. It can also be expressed as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in Mater at a given altitude and temperature.

Aerobic bacteria plays a key role in the measurement of dissolved oxygen levels both from an environmental and industrial perspective. Microorganisms such as bacteria are responsible for the decomposition of organic waste. When organic matter such as dead plants, manure, sewage and food waste are present in a water supply, the bacteria will begin to break down this w;aste. Much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they require to live. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. Water supplies containmg organic waste will also contain bacteria to break down this w:aste. When BOD levels are high, dissolved oxygen levels decrease because the available oxygen in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive. Hence, measurement of D.O. and BOD levels are vital in order to maintain sufficient oxygen levels to sustain the natural ecosystem. The use of aerobic bacteria for the decomposition of organic waste has become a fundamental aspect of wastewater treatment. Bacterial attack in the presence of oxygen results in the biochemical breakdown of sewage into inorganic byproducts, specifically, carbon dioxide, wrater and sludge. This oxidation process, known as the activated sludge process, is the most widely used method of secondary waste treatment today. The process typically occurs in an aeration basm and is achieved by aeratmg or bubbling pure oxygen through the wastewater. The oxygen, which is depleted by the bacteria, is replenished to allow the process to continue. When the measured D.O. falls below a specified concentration, air is added to the basin which provides oxygen to sustain the microorganisms and mixes the waste. Without enough dissolved oxygen, beneficial bacteria will die; conversely, too much oxygen is wasteful and inefficient resulting in unnecessary energy expenses. To maintain optimal dissolved oxygen levels. on-line analytical instruments are employed to measure and control the concentration of D.O.

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