Basics of Biological Wastewater Treatment

BASICS OF BIOLOGICAL WASTEWATER TREATMENT

The Sequence Batch Reactor is just one of the available technologies providing biological treatment of sewage water. As with other biological water treatment plants, (e.g. the activated sludge process in a continuous throughput plant or plants using trickling filters or submerged contact aerators), the sewage water is treated with the help of micro-organisms, which work to remove the polluted matter. These micro-organisms can reproduce quickly and mass together to form sludge flakes (activated sludge). This sludge is also called "secondary sludge", because it only forms within the plant and exists due to the high amount of micro-organisms present, whereas the normal sludge, which is carried along with the sewage water, is mostly polluted matter.
This "normal" sludge, otherwise known as "primary sludge" settles in the preliminary clarification tank. In plants without a preliminary clarification tank the sludge is a mixture of primary and secondary sludge.
These micro-organisms require oxygen to dismantle the pollutants. In a Sequence Batch Reactor, the oxygen-supply is mostly added through the turbine action. The turbine mixes air and water at the tank surface, so that oxygen form the surrounding air is mixed into the water. This "oxygenated water" can be then used by the micor-organisms and if performed properly boosts and accelerates their activity. At the same time the turbine's action creates a deep vortex current within the tank, this keeps the activated sludge in a balanced state and intensively mixes it with the wastewater.
The micro-organisms use the wastewater pollutants as a source of food and change them through the use of oxygen into a cellular substance. The amount of activated sludge - also called biomass - constantly grows. From time to time it has to be removed from the system to guarantee a steady proportion between the volume of wastewater and the biomass.
The sludge, which is removed from the tank, is called "excess sludge". Depending on the plant design, the excess sludge may already be stabilised or it could be necessary to take a further stabilisation step.
Before the excess sludge - mostly called "sewage sludge" - can be used as a fertiliser it is usual to store it in a sludge tank. Through natural evaporation the sludge is able to reach a higher dry matter content.
The biological wastewater treatement stage is usually considered to be the second cleaning step, following the first step, which is mechanical cleaning. During the mechanical cleaning phase, a screen filters the coarse matter and any sand, which may be present, is removed via a "grit chamber"
In larger plants a designated amount of sludge, existing in the in-flowing sewage water is retained in a primary settling tank.
Some plants use a third and final cleaning step after the biological-cleaning phase. during this step the remaining final pollutants are removed by extra filters, either mechanically with micro-filtration, chemical precipitation or with ultra-violet light.