A method for processing pulp in an apparatus for washing and dewatering pulp, and a system for controlling this apparatus, the apparatus comprising two rotatable press rolls (102, 104) having a permeable outer surface (106, 108), and a vat (114, 116, 118), the press rolls (102, 104) defining a press nip (112) between them. The processing is determined by a set of variable operating parameters, which are adjusted and/or maintained during operation in response to deviations of measured values of control parameters in relation to their respective predetermined threshold values. The variable operating parameters include rotation speed of the press rolls. The control parameters include vat pressure and/or linear load.