Čističky odpadových vôd

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.
Secondary sedimentation
The final step in the secondary treatment stage is to settle out the biological floc or filter material and produce sewage water containing very low levels of organic material and suspended matter.

Tertiary treatment provides a final stage to raise the effluent quality before it is discharged to the receiving environment. More than one tertiary treatment process may be used at any treatment plant.

Filtration
After the solids are removed, the liquid sewage is filtered through a substance, usually sand, by the action of gravity. This method gets rid of almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron. Filtration over activated carbon removes residual toxins.

Disinfection
The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.) Common methods of disinfection include ozone, chlorine, or ultraviolet light. Chloramine is usually not used in wastewater treatment because of its persistence.

Chloramine is used for drinking water, the wastewater flows into a 'chlorine contact' tank, where the chemical chlorine is added to kill bacteria, which could pose a health risk, just as is done in swimming pools. The chlorine is mostly eliminated as the bacteria are destroyed, but sometimes it must be neutralized by adding other chemicals. This protects fish and other marine organisms, which can be harmed by the smallest amounts of chlorine.


CONCLUSION: However, recent evidence has demonstrated that very low levels of certain contaminants in wastewater and synthetic materials can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water. In the US and EU, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to be met. A significant threat in the coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries.