We describe a novel application for a microwave on-line sensor to measure the total solids (TS) load entering a municipal wastewater treatment plant (WWTP) from slaughterhouse sewage and some sanitary wastewaters. Measuring this kind of wastewater stream is very challenging, because it contains a high, but varying organic load with nitrogen, phosphorus and microorganisms. The reliability of the measured signal was studied by comparison with laboratory analyses and a correlation is presented of TS-value with other parameters that are typically followed in a wastewater treatment process. The results suggest that on-line microwave sensoring could be used to monitor total solids in wastewater influent. Our results show that the on-line microwave sensor and laboratory reference analyses give similar results with a good correlation between the two techniques. Furthermore, we demonstrate that the total solids values correlate well with conductivity, total nitrogen and BOD7 values but not with phosphorus, pH and temperature.