Water aeration is often required in water bodies that suffer from hypoxic or anoxic conditions, often caused by upstream human activities such as sewage discharges, agricultural run-off, or over-baiting a fishing lake. Aeration can be achieved through the infusion of air into the bottom of the lake, lagoon or pond or by surface agitation from a fountain or spray-like device to allow for oxygen exchange at the surface and the release of gasses such as carbon dioxide, methane or hydrogen sulfide.
Dissolved oxygen (DO) is a major contributor to water quality. Not only do fish and most other aquatic animals need it,span> but aerobic bacteria help decompose organic matter. When oxygen concentrations become low, anoxic conditions may develop which can decrease the ability of the water body to support life.
Any procedure by which oxygen is added to water can be considered a type of water aeration. This being the only criterion, there are a variety of ways to aerate water. These fall into two broad areas – surface aeration and subsurface aeration. There are a number of techniques and technologies available for both approaches.
In diffused air aeration method compressed air is blown through the sewage and air is diffused in sewage by diffuser Diffusers are of two types: Plate diffuser and Tube diffuser
They are rectangular/square plates made of crystalline alumina or high silica sand. In this method the compressed air is blown through a perforated plate diffuser. The air comes out through the holes of the diffuser plate and rises upward in the form of bubbles. Thus the sewage absorbs oxygen from the air.
It consists of a perforated tube suspended in the waste water near the bottom and can be taken out while cleaning. The compressed air is dent through the tube. The air comes out through the holes with great force and agitates the sewage.
In this method the surface of sewage is agitated violently with the help of some mechanical equipment to encourage absorption of oxygen from atmosphere. There are two well known forms of mechanical aerator. Vertical surface aerator and Horizontal surface aerator.
They consist of electrically driven propellers (vanes) mounted in either a floating or fixed supports. They throw the bulk liquid (sewage) through air and oxygen transfer occurs both at the surface of the droplets and at the surface of the bulk liquid and is then mixed by the currents produced by agitation. In this method the performance is seriously affected by ice formation in winter.
In this system, diffused air aeration and mechanical aeration are combine in a single unit. The well known type of such combination is Dorroco aerator. The aeration of sewage is done by air diffusers as well as mechanical aerators. Air diffuser plates are located at the bottom of tank and the submerged paddles rotate in the direction opposite to that in which the compressed air rises up from the air diffusers. Paddles are rotated by a motor on a horizontal shaft with a speed of 10-12 rpm.