Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | ocTober 2015 | 39 In the know Technically speaking: dissolved oxygen measurement Improving dissolved oxygen control Developments in dissolved oxygen sensing technology are opening up new possibilities for both accurate measurement and significant energy and maintenance costs savings in aeration processes Jonathan Penn Global ProDucT ManaGer conTinuous WaTer analysers abb MeasureMenT anD analyTics to break down organic carbons into carbon dioxide, water and energy, clearing the water of harmful substances. The aeration process efficiency relies on close control of dissolved oxygen levels. Both excessively low and excessively high levels of dissolved oxygen can be equally as harmful to aquatic life, making it essential for water treatment plants to ensure that levels are as close to ideal as possible before water is discharged. Under ideal conditions, dissolved oxygen levels should be maintained at between 1.5ppm to 2ppm. Various methods have historically been used to measure dissolved oxygen, including the Winkler Titration method and portable handheld meters. Although both methods can provide a reliable measurement, the results are only an indication of dissolved oxygen levels for a particular period and set of conditions, making them unsuitable for applications such as dissolved oxygen blower control where the supply of oxygen needs to be constantly adjusted. online measurement Continuously measuring dissolved oxygen levels offers the best way of ensuring the right conditions for maximum aeration efficiency. When used with modern sensing technology, an online dissolved oxygen measurement system can offer much tighter control of dissolved oxygen levels, matching them to actual oxygen demand. When coupled with automatic blower control, significant energy cost savings can also be realised through reduced air consumption. Sensor technology There are two main types of sensors available for dissolved oxygen monitoring – electrochemical and optical. Electrochemical sensors work on either the polarographic or galvanic cell principles. Both work in a similar way, featuring a polarised anode and cathode with an electrolyte solution surrounded by an oxygen permeable membrane. The measurement is derived based on the difference in oxygen pressure outside and inside of the membrane. Variations in the oxygen pressure outside of the membrane affect the rate of diffusion of oxygen through the membrane itself. The cathode reduces the oxygen molecules, producing an electrical signal that is relayed first to the anode and then to a transmitter, which converts the signal into a reading. The consumption of the oxygen at the cathode requires a constant sample flow in order for a reading to Problems associated with traditional dissolved oxygen sensing systems can now be eliminated using optical dissolved oxygen sensors D issolved oxygen is a key ingredient in the efficient treatment of waste in water processes. A typical wastewater treatment plant uses four main stages of treatment – Primary, Secondary, Tertiary and Sludge. The secondary treatment stage is the point at which organic waste is oxidised to form carbon dioxide, water and nitrogen compounds. Most modern plants use an activated sludge system, which uses a culture of bacteria and other organisms to feed on the organic materials in the sewage. Under the right temperature, these bacteria and organisms use dissolved oxygen

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