SURFACE AERATORS

Description

A surface aerator employs blades or vanes rotating about a vertical or horizontal shaft to mix and aerate wastewater. Air is entrained behind the blades, turbulence and spray are produced, and oxygen is transferred into the wastewater. As a result, aerobic microorganisms consume the BOD in the effluent. Efficient operation can also lead to oxidation of ammoniacal nitrogen. Surface aerators can be designed to minimise foaming and may be covered for soundproofing.

Vertical shaft aerators consist of a conical spray head covered with regularly spaced blades or vanes. The depth of tank is dictated by the ability of the surface aerators to prevent stratification of oxygen concentration. To combat this, surface aerators can be arranged to encourage rotational flow of all the wastewater in the tank. Alternatively a draft tube or baffles can be used to encourage deep mixing.

Slow speed surface aerators with rotational speeds of about 50 rpm operate in deep tanks. Medium speed aerators, with smaller gearboxes, are slightly less efficient but less expensive. High speed surface aerators without gearboxes have the lowest capital cost but also have the lowest oxygen transfer and mixing efficiencies. They are mainly used for liquid depths of about 2 m.

Surface aerators may have a horizontal shaft equipped with square or triangular blades that lies across the surface of a tank and rotates at about 75 rpm. They are used to aerate oxidation ditches and to provide the momentum to establish a circulatory motion throughout the entire treatment unit. The location of these aerators within the wastewater vessel has to be carefully chosen to eliminate dead areas.

Application

Surface aerators are widely used in sewage treatment, and in the treatment of effluents from the food, paper, chemical, pharmaceutical and steel industries and for the treatment of agricultural waste. They may also be used to aerate lakes and lagoons.

Surface aerators transfer a smaller percentage of the oxygen in the air into solution, but this is balanced by the large volumes of air that are introduced. For the latter reason surface aerators are used to remove excess carbon dioxide in pure oxygen systems.

Sizing

Surface aerator power requirements vary from about 1 kW to over 100 kW. The oxygen transfer efficiency is about 1-2 kg O2/kWh transferred to the liquid. Horizontal shaft aerators have slightly higher oxygen transfer efficiencies than vertical shaft aerators. The efficiency of surface aerators is dependent on their depth of immersion. Some surface aerators are windpowered and have no additional power requirements.