© SGEM2014. The alternatives for treating urban waste water are many and very varied. The choice depends not only on the characteristics of the water to be treated, but also on other factors such as the size of the town it serves, the economics, environmental factors, etc. While many works have dealt with the subject of the treatment of waste water from small towns, various industrial processes or inflow from towns with a large industrial load, not many have carried out comparative studies of various types of Waste Water Treatment Plants (WWTPs) in towns with inflows which are predominantly domestic using statistical processes. This work aims to carry out a study, using Fuzzy Logic, of the various analytical parameters analysed in the water at the inlet (inflow) and at the outlet (outflow) of two different WWTPs which receive domestic contributions, once the water has undergone purification treatment. Two waste water treatment plants serving different towns in the province of Valencia (Spain) and which treat the inflowing waters using different purification systems were selected for this. The Utiel WWTP treats the waters using activated sludge and a prolonged aeration process with a “two-stage” purifier, while the Cheste plant uses a treatment involving bacteria beds with trickling filters. An analytical series is available from these WWTPs covering some 10 years with a frequency of two weeks. The parameters available, at the inlet and the outlet, are: rain (Lluv), daily input volume (VolD), pH, conductivity (Cond), Solids in Suspension (SS), DBO5, DQO, total Nitrogen (Nt), total Phosphorus (Pt) and a sampling period expressed in months. The PreFuRGe (Predictive Fuzzy Rules Generator) tool was used to process the data, allowing us to obtain an immediate qualitative analysis of the information contained in the resulting volume of data. Thus, dependency relationships were established between the parameters, not numerically, but logically, by also analysing the var...