INTRODUCTION:
The world needs water. It is important to say, that we need pure water. As humans, we drink it, wash with it, and use it for growing crops, just to name a few uses. The natural world is sustained with water as well, but balancing all of these uses is a challenge. For the production of drinking water it has to be further treated. Many industries have a need to treat water to obtain very high quality water for demanding purposes. Sewage treatment, is the process of removing contaminants from wastewater, both industrial and domestic effluents. It includes physical, chemical and biological processes to remove contaminants. Its objective is to produce water for reuse and a solid waste or sludge suitable for discharging. This solid material is often accidentally contaminated with many toxic organic and inorganic compounds.

MATERIAL AND METHOD:Typically, sewage treatment involves three stages, called primary, secondary and tertiary treatment. The solids are separated from the wastewater stream, dissolved matter is progressively converted into a solid biological mass by using microorganisms and the treated water may be disinfected chemically or physically.

Primary treatment removes the materials that can be easily collected from the raw wastewater and disposed of. The typical materials that are removed include fats, oils, sand, gravels and rocks (also referred to as grit), larger settleable solids including human waste and floating materials. This step is done entirely with machinery, hence the name mechanical treatment. Most of these materials such as sand and grit are sent to a landfill.

Primary Sedimentation
Many wastewater plants have a sedimentation stage where the sewage is allowed to pass slowly through large tanks called primary sedimentation tanks. The tanks are large enough that fecal solids can settle and floating material such as grease and oils can rise to the surface and be skimmed off. The main purpose of the primary stage is to produce a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.

Secondary treatment is designed to degrade the biological content of the sewage such as human waste, food waste, soaps... The municipal and industrial plants treat the settled sewage using aerobic biological processes. After primary treatment, dissolved matter is converted into a solid biological mass. The microorganisms degrade dissolved organic matter in the presence oxygen. Hence, mentioned degradation requires oxygen in addition to the substrate for the microbial growth. Therefore air, eventually enriched with oxygen, bubbling through treated water is much needed. There are many different methods of biological treatment. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic carbon molecules, etc.)

Except different organic and inorganic waste substances, high levels of nitrogen and phosphorus containing chemicals may be present in the treated water. Therefore, a number of very different treatment procedures are required to remove this nutritive contamination.

The removal of nitrogen proceeds through biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by reduction of nitrate to nitrogen gas (denitrification). Nitrogen gas is released to the atmosphere and thus removed from the water.
Nitrification itself is a two-step aerobic process. Each step needs a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2−) is facilitated by Nitrosomonas. Nitrite oxidation to nitrate (NO3−) is performed in the environment almost exclusively by Nitrospira.

Denitrification requires anoxic conditions. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process can do the job the most easily. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor like methanol.