BALANCING TANKS

Description

A balancing tank has sufficient volume to permit a nonuniform flow of wastewater to be collected, mixed and pumped forward to a treatment system at a more uniform rate. Pumping is controlled by level sensors and its rate varies according to the depth of liquid in the balancing tank.

A balancing tank should be mixed to prevent the settlement of solids and to ensure that the wastewater quality is as uniform as possible. To prevent anaerobic conditions and odours developing prior to treatment, the contents of balancing tanks may need to be aerated. Venturi aerators will mix and aerate, while high speed propellers are used to keep solids in suspension.

Application

Balancing tanks are used where the flowrate of wastewater varies greatly or where its composition, strength or temperature varies sufficiently. Balancing tanks are intended to supply a secondary biological treatment system with a uniform flow of wastewater to allow more stable operating conditions. In this way, equipment may be sized to the average flow rather than to a peak flow. As a result, the capital expenditure on secondary treatment systems is reduced, the system is more energy efficient and the running costs are less. A balancing tank can store effluent produced during the day in order to treat a higher proportion of it at night. In this way, energy costs may be reduced. A balancing tank may also improve dosage of treatment chemicals or nutrients.

Sizing

To size a balancing tank, the unbalanced flow must be monitored and recorded. The average flow must be calculated to determine the rate at which the pump must feed liquid forward. The profile of the flow must be understood, so that the tank can accept surges without overflow.

A typical 500 m3 balancing tank might require a constant mixing load of about 7 kW and an intermittent pumping load of 5 kW. Mixers are available within the range 1-20 kW. Venturi aeration installed at 20-25 W/m3 of balancing tank capacity will supply adequate mixing and pre-aeration.