Recent changes in the regulations concerning the treatment of waste water imply a number of technical requirements that purification plants must satisfy. These requirements are not met by most Italian plants. Therefore, a large amount of investment will be necessary in the near future. The aim of this paper is to investigate scale and characteristics of economic inefficiency in the purification process, particularly in the activity of nitrogen reduction in waste water. The analysis has been accomplished by using recently developed techniques in the analysis of stochastic production frontiers. It is argued in the paper that, when technical efficiency is achieved by a lant, there is evidence of non-decreasing returns to scale; on the contrary, enlarging a plant is likely to be ineffective if allocative efficiency is not achieved.