This article analyses the relation between wastewater quality parameters and particle size distribution (PSD) in three real wastewater treatment plants with different biofilm technologies: submerged biofilter system, trickling filter system, and rotating biological contactor system. The main quality parameters, (suspended solids, turbidity and COD), and PSD in the influent and effluent water of each different biofilm treatment were analyzed during 1 year. The PSD was fitted using the power law (n(d(p))= partial derivative N(d(p))/partial derivative d(p)= A.d(p)(-b.Log(dp))) obtaining the coefficients A and b to define the particle distribution. Mathematical correlations between this coefficients and the rest of parameters studied were found (SS = 0.0713.A(0.585), turbidity = 4.549 x 10(-4).A(1.096). COD = 0.0201.A(0.774)). The relation with the average particle size by mass was also found, (d(pma) = 60.3137.b(-2.242)). Moreover a relation between PSD and the particle elimination efficiency of the secondary treatment was study, (eta = 2.844-2.498.b(relative) + 0.0863.A(relative)). These expressions are very useful to understand the behavior of the biofilm treatment system using PSD analyses. (c) 2010 Elsevier Ltd. All rights reserved.