The importance of aeration does not restrain itself to a way of oxygen transfer to the wastewater in order to allow the occurrence of biological degradation of organic matter. The addition of oxygen is also important as a way to remove some compounds, such as iron, magnesium and carbon dioxide, as well as a way to promote chemical oxidation, eliminating organic compounds that resist biological treatments and oxygenating the effluent before its rejection.
The aeration equipment commonly used consists of air diffusion units, turbine aeration systems in which air is released below the rotating blades of an impeller, and surface aeration units in which oxygen transfer is accomplished by high surface turbulence and liquid sprays.
Most of the organic compounds present in an wastewater is removed by biological processes.
When organic matter is removed from the solution and degraded by micro-organisms, there are an oxygen consumption and a microbial growth associated. But, besides the presence of organic matter and oxygen, there are other limiting factors to the performance of biological treatments.
Several minerals are essential to the metabolism of degradation of organic compounds. All, except phosphorous and nitrogen, are usually present in sufficient amounts in the wastewater.
Domestic wastewater provide a well balanced microbial diet, but some industrial activities, for example, pulp and paper production, do not have effluents with enough quantities of phosphorus and nitrogen, making it necessary to add them to the effluent.
Other factors to have in account when dealing with biological treatments are the temperature, the pH and the toxicity of some pollutants.
There are several systems of application of biological degradation to wastewater treatment. The most common is the activated sludge system. The activated sludge system has as its functioning principle the degradation of organic matter and its transformation in a floc suspension easy to settle, allowing the use of gravitational liquid-solid separation technics.
This system consists of two different steps. On the first step, in a well mixed and aerated reactor, occurs the microbial reaction, consuming the organic matter. On the second, the settling of sludges, formed by flocs and micro-organisms, occurs, being a portion of these reintroduced in the process. This recycle has as objective the maintenance of the high microbial levels.