A method includes forming an aqueous fibre suspension including cellulosic fibres from one or more raw material flows, and applying at least one chemical and/or physical control measure to the aqueous fibre suspension or at least one of its raw material flows for control of microbial activity in the aqueous fibre suspension or the raw material flow before an inlet of an intermediate residence entity. In this manner a starting ORP value for the aqueous fibre suspension is obtained. The aqueous fibre suspension is in the intermediate residence entity at least a minimum delay time. A final ORP value is measured for the aqueous fibre suspension after an outlet of the intermediate residence entity before the formation of the fibrous web. An ORP difference value between the starting ORP and final ORP values is calculated. Finally, the aqueous fibre suspension is formed into a fibrous web and dried.