Comparison of single-stage and two-stage activated sludge processes for the expansion of the Innsbruck WWTP

A modification of the legal framework conditions shortly prior to the commencement of the construction works in the course of the expansion of the Innsbruck WWTP, necessitated a design adaptation for the new effluent standards to be met. The time pressure and doubts whether the initially chosen AB process did in fact constitute the optimum solution for the WWTP expansion under the new boundary conditions, led to the installation of a pilot plant with the aim of simultaneously testing the single-stage and AB processes. During the elaboration of the pilot plant tests, the AB process was optimised step by step so as to allow the achievement of the required effluent concentrations with a similar degree of reliability as accomplished with the single-stage plant. The comparison of investment and operating costs as well as plant operation and plant layout ultimately led to the choice of the single-stage alternative.