An adaptive controller for activated sludge processes is introduced. The control objective is to keep, in the presence of input constraints, the concentration of the biomass proportional to the influent flow rate, where a prespecified small tracking error of size lambda is tolerated. This is achieved by the so called lambda -tracker which is simple in its design, relies only on structural properties of the process and weak feasibility properties, and does not invoke any estimation or identification mechanism or probing signals. lambda -Tracking is proved for a model of an activated sludge process with unknown reaction kinetics and including unknown time-varying process parameters. It is illustrated by simulations that the lambda -tracker works successfully, and even under practical circumstances which go beyond what we can prove mathematically, it can cope with 'white noise' corrupting the measurement and periodically acting disturbances.