Environmental Water Treatment Knowledge: The Meaning and Function of Surface Aeration
Surface aeration is a mechanical aeration method.
In wastewater treatment, the impeller is generally installed at the liquid surface of the aeration tank. Its rotation lifts and conveys water, creating a circulating flow inside the tank that continuously renews the gas–liquid contact surface and absorbs oxygen; it forms a hydraulic jump, violently agitating the liquid and entraining air; and it creates a low-pressure zone that draws in air for oxygenation.
A surface aerator uses a motor to directly drive an axial-flow impeller, which sprays the wastewater from the guide tube outward through the guide plate to form a thin water curtain. In flight the water contacts the air and breaks into droplets; when the droplets fall they strike the liquid surface, producing turbulence and large numbers of bubbles that increase the dissolved oxygen content of the water.
Surface aeration equipment is mounted at or around the water surface of the aeration tank and rotates under motor power. Through the three actions of hydraulic jump, water lifting and negative-pressure air intake, oxygen from the air is transferred into the wastewater. The equipment features a simple structure, easy operation and management, and low cost, making it suitable for small and medium-sized wastewater treatment plants. Depending on the orientation of the rotating shaft, it can be divided into vertical-shaft and horizontal-shaft types.
Its core principle is to transfer oxygen from the air into the sewage through the three actions of "hydraulic jump", "lifting" and "negative-pressure suction".
Research shows that in unbaffled surface aeration systems, the circulating flow rate and the resulting vortex depth are interrelated, and both have a profound influence on system performance, especially the oxygen transfer process. Scaling criteria for the oxygen transfer rate can be established on the basis of the circulating flow rate.
Research and development in the field of surface aeration focus mainly on mechanism investigation, energy-saving control technology and equipment structure optimization. In mechanism research, studies have examined the deep influence of circulating flow and vortex action on the performance and oxygen transfer of unbaffled surface aeration systems, and have established scaling criteria for oxygen transfer based on circulating flow. In energy-saving control, a surface-aeration energy-saving control system has been designed for Carrousel oxidation ditches, which optimizes aerator operation through variable-frequency control, model-based frequency selection and neural-network models to reduce energy consumption. In equipment optimization, one patent arranges flow-dispersing protrusions on the float surface above the water line to break up the flow, refine the droplets and enlarge the gas–liquid contact area, thereby enhancing aeration while also increasing float strength and slip resistance; another patent adopts a cylindrical helical-gear reducer, a spiral-bevel-gear output end and an integrated automatic/manual lifter, aiming to improve processing capacity and reliability, extend impeller life and make the oxygenation rate easy to adjust.
Surface aeration equipment is characterized by simple construction, convenient operation and maintenance, low cost and relatively high oxygenation efficiency. It is suitable for small and medium-sized wastewater treatment plants. However, since the aeration process can generate large amounts of foam that hinder oxygen uptake at the liquid surface and reduce oxygen dissolution efficiency, it is not suitable for wastewater that tends to foam.
To improve performance and service life, several improved designs have been introduced: flow-dispersing protrusions on the float surface to refine the droplets, enlarge the gas–liquid contact area and improve aeration while adding float strength and anti-slip safety; an integrated automatic or manual lifter that adjusts the impeller submergence depth (stroke between 100 and 200 mm) to control the oxygenation rate; and connecting reinforcing rods between the impeller blades to extend service life. The impeller diameter typically ranges from 600 to 3250 mm.
According to the direction of the rotating shaft, surface aerators fall into two categories: vertical-shaft and horizontal-shaft.
Surface aeration is suitable for domestic sewage, aeration for agricultural and livestock farming, and aeration of various industrial wastewaters, and is commonly used in small and medium-sized wastewater treatment plants. It is also used in drinking water treatment to remove carbon dioxide, hydrogen sulfide and similar substances and to oxidize iron and manganese.
The advantages of surface aerators include simple, centralized facilities: no blower room, extensive air distribution piping or diffusers are required, which saves cost. They are simple in construction, easy to operate and manage, and can increase the dissolved oxygen content of the water; the equipment is simple, easy to maintain and low in cost.
Disadvantages include the fact that they are not suitable for wastewater that generates large amounts of foam during aeration, because the foam hinders oxygen uptake at the liquid surface and lowers oxygen dissolution efficiency. When treating large flows, using multiple units loses the cost advantage and increases capital, operating and maintenance costs, in which case diffused-air (blower) aeration may be preferable. In addition, some surface aeration devices (such as float-mounted aerator mixers) may suffer from low aeration efficiency and high energy consumption.
A surface aerator is a mechanical aeration device typically composed of a motor, a drive shaft, an axial-flow impeller and a float. The motor drives the impeller to rotate, lifting and conveying water to create a circulating flow in the aeration tank, renewing the gas–liquid contact surface and transferring oxygen from the air into the water through hydraulic jump and negative-pressure suction. The main components are: the motor, usually a waterproof totally enclosed fan-cooled type installed above the spray zone; the drive shaft, made of stainless steel; the axial-flow impeller, also of stainless steel and designed to achieve maximum hydraulic transfer without overloading the motor; and the float, whose shell is made of reinforced fiberglass filled with high-density polyethylene foam, providing buoyancy and support.
