In wastewater treatment plants, measuring the total amount and flow of influent and effluent from one process to another is very important and normally a non-intrusive, easy to install device is needed to provide an accurate reading. The total flow reading will provide valuable information, things like how much you are feeding to the aeration pond, overall what is the volume of incoming wastewater and discharge, and all this info will help the plant operators to control and fine tune in order to optimize the process. Most engineers would opt for different range of flow meters from vortex, mass flow but the most cost effective measured in terms of reliability and also accuracy is the Ultrasonic Flow Meters.
There are basically two different types, one which is the open channel and the other type is a clamp-on transducers based on either the transmission or reflection principle. Clamp-on transmission based flow meters basically rely on the transit time differences between the ultrasonic beams while the reflection based employs the Doppler Effect which is basically the frequency shift caused by moving particles in the liquid. Measuring flow of wastewater poses a major challenge mainly because the water that is being transferred from one location to another comprises a lot of suspended solids, sludge particles and in certain process stage the liquid can be concentrated enough to become slurries. Along the process line, sometimes addition of chemicals especially on the DAF system will greatly alter and shift the pH range and with inline units, corrosion will become a big issue that needs to be taken into consideration.
Thus the characteristics of wastewater means that the conventional flow meters will not be suitable but both types of ultrasonic flow devices located outside the pipeline will work just fine. They are also the most preferred and the main advantages with using this clamp-on unit is that it can be installed without having to cut through the pipes in order to install fittings or disrupting the operation just for installation, thus a major plus point to consider. Using these for treatment plant applications is also the right choice especially when it comes to moving multiphase liquid that contains mixture of oil plus water with air bubbles and they will also work well in low flow conditions. With inflow or wetted units, sometimes trapped solids can cause error in getting accurate readings but with the clamp-on type, all these problems can be avoided especially on wear and tear. Generally when it comes to the transducers, a plus minus 1% accuracy is the acceptable range and standard of measurement.
Finally for non contact applications, an open channel flow meter can be selected and the measuring principle used for determining flow is based on the corresponding height of the liquid passing through a weir. Usually for this type, it is used at the wastewater discharge side and the readings can be fed onto an online data collection system to check the trending and recording purposes. Nevertheless accurate readings are needed so that all this information can be tabulated and presented for submission to governmental agencies related to environmental control.
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