A biological process with activated sludge is used for the treatment of urban and industrial waste water produced by the Chelghoum Mid city. This station plays an important role within the framework of environmental protection. Our study is on one hand intended to estimate the quality of the untreated water and the water at various stages of treatment (screening, grit and oil separation, biological treatment and settling), and on the other hand to verify the efficiency of the station by considering the volume of muds indicator. According to the obtained results, we conclude that the pH and the dissolved oxygen meet the standards. What is more, an important decrease of the organic matter represented by the COD and BOD5 is to be noted. The experiments conducted to verify the settling. by measuring the volume of muds indicator, showed that the settling is made in proper conditions, assured by controlling this indicator regularly. (C) 2009 Published by Elsevier B.V.