E-Zine March 2014
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Click here to read "Wastewater Applications"
There are several ways that have commonly been used in wastewater treatment to measure density. Nuclear densitometers were tried early, and found to be difficult to use, and relatively inaccurate, especially on RAS and WAS, due to the high hydrogen content of the fluid and the narrow density range from which to compute “percent solids.” Typically, a RAS or WAS density range is from 1.04 to 1.4 SGU. Not a very large range. Nuclear density gauges worked better on primary sludge or thickened WAS, but the difficulties associated with licensing have driven most wastewater authorities to abandon nuclear densitometers entirely.
Coriolis mass flowmeters have been used successfully in wastewater treatment, but they are prone to difficulties with line size and with plugging. Few manufacturers make a Coriolis mass flowmeter larger than 6” (150 mm) diameter, and most medium sized and larger wastewater plants have sludge lines larger than that. Besides that, even a 6” Coriolis mass flowmeter is expensive.
Ultrasonic density meters have also been tried, with mixed success including an insertion sensor that can be inserted in large pipes.
“Percent solids” measurement is much easier in applications like mining where the specific gravity of the dry solids is constant, and the dry solids value can simply be entered into the density computer. In heavy media separation, for example, the dry solids gravity is always going to be approximately 4.0 SGU. This means that it is quite easy to determine the “percent solids” value using 4.0 as a constant.
Measurement of mass flow can be accomplished by measuring volumetric flow, and measuring density, and multiplying the two terms. In dredging, for example, typically either a magnetic flowmeter or a Doppler ultrasonic flowmeter is used to measure flow, and a nuclear densitometer is used to measure slurry density. Coriolis meters are too small, and too abrasion and vibration sensitive to be used for this application.
This is another application where research may yield new ways to measure a very important variable. There are many applications outside of the range of Coriolis mass flowmeters where the measurement of density is problematic at best, and difficult.
From Flow Control (September 2002)