Abstract: A tank for holding a liquid for a predetermined and relatively long time in a continuous flow process. The tank includes an inner compartment with an inlet and an outer compartment with an outlet. The outer compartment encloses the inner compartment except its inlet. The liquid enters through the inlet and is forced upwardly in a non-preferential flow, it overflows to the outer compartment and comes out through the outlet. A pump is connected to the outlet and a variable throttle valve is in turn connected to the output of the pump. A level sensor for the liquid is connected to a computerized control unit to monitor the liquid level and to activate the throttle valve to either restrict or increase the flow to adjust the liquid level. Since the flow rate then is brought back to what it was when throttle valve is restricted to its original state, then changing the liquid level in the second compartment is equivalent to changing the holding time.