Processing, December 2015
www waterwaste com Water Waste Processing December 2015 23 is clearly a better option than ultrasonic sensors Because radar uses microwaves the gas in the head space is irrelevant While radar is certainly not a silver bullet for use in foam it performs better than ultrasonic when foam accumulates on the surface Sludge digesters are common in wastewater treatment Sludge digestion is a bacterial process that can be conducted either in the presence of oxygen aerobic digestion or the absence of oxygen anaerobic digestion In both processes the sludge is converted from complex proteins and sugars into more simple compounds such as water and carbon dioxide or methane These digesters often generate foam and contain gases that would rule out an ultrasonic sensors use Radar works well in this application Many chemicals are required in wastewater treatment Because a level measurement using ultrasonic sensors is based on the speed of sound in air if the empty space in the tank has anything other than air large measurement errors can occur For example the speed of sound in chlorine is around 38 percent slower than in air As a result a level may be reported that is 38 percent lower than the actual level The speed of sound in nitrogen is approximately 11 percent faster than in air which could indicate a level 11 percent higher than the actual level In these tanks radar is a better option than ultrasonic Summary Radar level measurement is an excellent high performing choice for many applications but it might not be the best choice in wastewater applications Some recently developed ultrasonic level instruments have features included specifically for the wastewater industry Ultrasonic level instruments can be a cost effective and beneficial part of an efficient and safe wastewater treatment plant and should be considered when selecting instruments Tim Thomas is level product business manager for Endress Hauser Endress Hauser www us endress com Write In 000 Ultrasonic sensors are small and can be installed in tight places Figure 2 Some ultrasonic sensors monitor levels on both sides of a bar screen
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