This paper deals with adaptively tuning the discrete-time PID controllers to be able to regulate some infinite-dimensional plants in the presence of unmeasured load disturbances having an arbitrary form. The key idea is that the PID control law is derived on the basis of a second order model of AR structure. Such an approach leads to the appearance of unmodelled dynamics. To cope with the unmodelled uncertainty, the projection together with an adjustable dead zone is utilized in the adaptation algorithm. The boundedness of all the signals is established. The results are evaluated by simulation studies.