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Wastewater Treatment Process

 

Wastewater Treatment

The water that has been used in various activities is termed as wastewater. It must be treated before its discharge into the environment. There are several stages of wastewater treatment. Wastewater treatment plants use primary, secondary and tertiary stages.

1.Primary treatment

In this step waste water treatment primary treatment involves a number of physical process these include

  • Screening

In this process bar screens with 10 millimeters mesh size are used to block the flow of large floating debris like plastic cans, wrappers, wood pieces etc. so that they don’t enter into the next unit as these large debris may hinder the function of machinery to a great extent.

  • Comminution

In this process a comminutor is used to grind the debris that pass through the bar screen later on these grinded residue is removed by sedimentation.

  • Grit removal

In this physical process a large narrow tank is used to slow down the flow of incoming wastewater so that the small solid particles such as sand, silt and gravel settle down at the bottom of the tank.

  • Sedimentation

In this process sedimentation tanks are used to remove the suspended solids that pass through the above mentioned processes, here the incoming wastewater is given a considerable resting time so that the suspended solids may settle down due to gravity, the solid which is settled at the bottom of sedimentation tank is known as sludge which is removed from the tank through its bottom by mechanical scrappers from where it collected in hoppers

Primary treatment remove

  • 50% of BOD
  • 90% suspended solids
  • 55% fecal coliform

2.Secondary treatment

Secondary treatment of waste water is a biological process in which microbes act on organic impurities and suspended solids that escapes primary treatment. Microbes consume organic matter as their food and convert it into carbondoixide, water and energy for their growth and reproduction in this process following three methods are used

  • Trickling filters

Trickling filters are specially designed tanks in which a layer of stones is provided at the bottom of considerable height, air is also circulated upward through the pores present in stone bed, where microbes multiply and flourish easily when the waste water is passed through this tank its flow is reduces over the coarse surface of stones and it starts trickling down where the microbes absorb dissolved organic impurities thus lowering the BOD of wastewater

  • Activated sludge

In this process the settled sewage mixed with fresh sludge is subjected to an aeration tank in which compressed air is injected from the bottom of the tank to provide enough oxygen for the metabolism of microbes, this oxygen rich condition forms a healthy suspension of biological activated sludge

  • Oxidation pond

In oxidation microbes especially bacteria and algae are allowed to grow in waste water. Algae prepare their food by the process of photosynthesis and release oxygen which is used by bacteria to digest the dissolved organic pollutants from the wastewater this process is accelerated by supplying additional oxygen with the help of mechanical aerators

Secondary treatment removes

  • 90-95% of free pollutants
  • 85-90 % of BOD and suspended solids
  • 90-99 % coliform bacteria

3.Tartary treatment

In this process the wastewater is allowed to pass through biological reactor where alum is used to remove additional phosphorous in the next step wastewater is passed through gravity sand filters in order to trap the flock which are formed on addition of alum, in order to remove odor carbon reactors, regulated amount of chlorine hydrogen peroxide etc. are used, similarly in the next step the clear water is allowed to enter chlorine contact tank this tank contain disinfectants such as chlorine, ozone and ultraviolet light  is used to remove microbes from treated water including bacteria, viruses and parasites likes giardia

Finally, the chlorine removed by adding sodium bisulphate just before the discharge into the environment for further reuse

Figure: Wastewater Treatment Plant

 

 

 

 

 

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