Buffer tanks are often designed and implemented for control purposes, yet control theory is rarely used when sizing and designing buffer tanks and their control system. Instead, rules of thumb such as using '10 min residence time' are used. The objective of this paper is to provide a systematic approach. We consider mainly the case where the objective of the buffer tank is to dampen ('average out') the fast (i.e. high frequency) disturbances, e.g. in flow and concentration, which cannot be handled by the feedback control system.