The determination of particle concentration in liquids has important applications in the industry. In the last years, environmental applications increased the importance of the determination of concentrations in real time. This implies the necessity of robust and low cost techniques. In this work we show the application of the Time Reversal technique to determine the particle concentration level in liquid media. The signal obtained in the Time Reversal process has an acoustic signature of the media by which the waves propagate. This signature is sensible to global changes of media properties, like temperature or the statistical distribution of diffusers. As specific application example make we show the determination of bacteria concentration in a biological reactor for the treatment of remainders of the milky industry and the determination of the evolution in time of the mud level.