E-Zine February 2014
Click here to read "Introduction"
As an example, look at the typical wastewater treatment plant. There are normally four locations where the measurement of density is important to the plant operation process.
First of these is primary sludge density, which is measured after the primary clarifier. This is a biologically active slurry with water and large solids with varying density.
Second, there is return activated sludge (RAS), which is measured in the return line from the secondary clarifier to the aeration basin. This is an extremely biologically active slurry, with some high-density solids, but mostly low-density biomass.
Third, there is waste activated sludge (WAS), which is return activated sludge being wasted due to sludge age (the older the sludge, the lower the concentration of beneficial life forms). It is quite similar to RAS in makeup.
Fourth is thickened waste activated sludge. This is WAS after it has been thickened, on its way to being dewatered for disposal. All of these sludges are abrasive, and RAS and WAS have densities that are very close to water.
If that weren’t enough, the desired measurement is NOT density. It is “percent solids.” In order to achieve this measurement, you must measure the relative density of the liquid, and compute “percent solids.” To compute “percent solids,” you must know what the density of the solid component of the slurry is. In a wastewater treatment plant, this is calculated using laboratory methods that take a considerable amount of time. (See Standard Methods for the Examination of Water and Wastewater a WEF/AWWA publication.) So, sludge density is computed in the lab only once or twice daily. However the dry solids density of the solids in primary sludge, RAS and WAS is changing moment-to-moment due to the composition change of the solids.
Click here to read "Measuring Tools"
From Flow Control (September 2002)