An acoustic backscatter system for in situ concentration profiling of settling flocculated dispersions

An in situ concentration profiler is demonstrated, using a novel application of a commercial acoustic backscatter system, which is suited to a wide range of dispersion and sludge settling systems. Specifically, the system utilises 2, 4 and 5 MHz transducer–receivers that both emit high frequency ultrasonic pulses and receive the echo backscatter signals, with the strength of the return being directly proportional to the particulate concentration. It was found using well mixed flocculated Spheriglass dispersions from 2 to 10 wt% that the backscatter signal was attenuated in an approximated linear mode with column depth (on a dB scale). In turn, the gradient of the attenuation was also found to increase linearly with respect to particle concentration, which allowed the formation of a direct correlation relationship. The backscatter response of a 5 wt% settling dispersion was also observed. It was found that a peak formed in the measured signal at a depth relating to the cloud-front, and the bulk change in particulate concentration could be measured by comparing the difference in the linear slope of the acoustic attenuation. Further, by taking the interpolated differentiation of the backscatter signal at particular times, concentration changes throughout the column depth could be profiled. High levels of segregation within the sludge at intermediate time intervals were highlighted (due to the heterogeneous distribution of the particulates) as well as a build up in hindered settling effects.