This paper presents the sizing of a treatment plant for domestic wastewater generated in the urban planning zone of the Cooperativa de la Pequeña Empresa de Pastaza LTDA, located in Puyo, Ecuador. The population served will be 291 inhabitants and the average flow is 1.48 L/s. The most suitable option for the plant’s secondary treatment was selected using the prioritisation matrix methodology. The complete treatment proposal is composed of input, sieving and desanding, two parallel septic tanks, a trickling filter, a secondary decanter and finally a disinfection process of two 12GPM ultraviolet light lamps at the outlet of the secondary decanter. There is also an area for drying the sludge after it has been digested in the septic tank. The calculated cost of the plant was $57,032.88USD and it would occupy an area of 86m².