The particle size distribution (PSD) of the effluent from a typical activated sludge process was measured. The relation between the particle size distribution (PSD), residual biomass and conventional suspended solids (SS) and turbidity of a typical activated sludge process effluent was established with the help of laser scattering, oxygen uptake rate (OUR) measurement and the IWA model analysis. The laser scattering technique is capable of measuring particles between 20 nm and 2000 pm. The final effluent also consists of residual biomass which is unsettled in the secondary settling tank (SST) and the amount of this residual biomass was estimated using OUR measurement and IWA model. The final effluent was found to be dominated by the particles of size 1-10 mu m, the size range for most of the bacterial cells (Ekama et al. 1997). The results showed that the total number of particles was correlated to the amount of residual biomass. Correlation determining the relation between laser particle size monitoring results and SS/turbidity measurements was also found. The results showed that the particle monitoring was more sensitive than the SS/turbidity measurement and with further development and improved control, it can used for more complex and sensitive data. (C) 2009 Elsevier B.V. All rights reserved.