Thursday, June 18, 2020

An insight into municipal wastewater treatment processes

Wastewater treatment processes are becoming a prerequisite procedure nowadays, as the world is suffering from the problem of available water resources. Today we are facing the problem of shortage of water because of water pollution. 

Trash from household activities and industrial waste are often released into the streams. Such waste is not only harmful for aquatic life but is also harmful for human settlement.
Wastewater treatment processes include both industrial wastewater process and municipal wastewater treatment process.Particularly, Municipal wastewater treatment process is also known as sewage treatment.  

municipal wastewater treatment processes

Wastewater treatment methods

Municipal wastewater treatment process is classified into a decentralized system, centralized system and combined sewer systems. Decentralized system involves treating the sewage on site from where it is collected. Centralized system involves treating the sewage off site and combined sewer systems are capable of handling the storm run off.

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Wastewater treatment plant

Decentralized system

Decentralized system is also known as an on-site system. It includes septic tanks, biofilters and aerobic treatment systems. Of them, septic tanks are widely in use.   

Septic tanks

The sewage collected is initially brought into the settling tank. Settling tank allows the solid and dense materials to settle down. Bacteria start playing their role here and they decompose the organic solid matter

municipal wastewater treatment processes

The effluent or remaining liquid now contains dissolved organic matter and some pathogens. Such pathogens are responsible for causing diseases and so they are harmful for human health. The dissolved organic matter serves as a source of nourishment for these bacterias.

 The effluent is then allowed to pass through the distributor box into various porous pipes.
The pipes are porous in nature so that the effluent gets released in the soil of the leaching or adsorption field. In the soil, the process of aerobic respiration takes place. If the soil is fine grained it is capable of filtering out large pathogens and remaining fine suspended particles.

However, in septic tanks the main issue is Nitrate. As nitrate is not absorbed by the soil and plants. Also there are many  criterias that need to be fulfilled.
  • The soil depth in the leaching field should allow the seepage of wastewater or sewage for the filtration purpose.
  • The soil should have medium penetration to allow passage of wastewater. However, if the penetration is too high, the wastewater may contaminate groundwater readily
  • The leaching field should be away from nearby surface water sources.
  • Wells should be away from such leaching fields.
  • Wells should penetrate an aquifer that is surrounded by low permeable  material such as clay to prevent pollution.

Municipal wastewater treatment process

It is also known as a centralized system or sewage treatment plant. It incorporates physical, chemical and biological processes for the treatment of wastewater.

Municipal wastewater treatment processes

Primary treatment stage

Primary treatment is simply the physical processing of the sewage. It is a composite of bar screen, grit chamber and settling tank. 

Initially, the sewage is allowed to pass through bar screens. Bar screens filter out the macro particles such as inorganic trash and also some fine sediments. The solid waste disposal can be done either in landfill or through burning the waste. The size of the screen can vary depending on the size of material to be collected. 

Then the wastewater approaches the grit chamber. In the grit chamber, the flow is hampered so that sand and gravels get settled down.

 There are three types of grit chamber.
  • Horizontal grit chamber
  • Aerated grit chamber
  • Vortex grit chamber
Again the vortex grit chamber can be mechanically induced, moved by liquid and multi tray vortex separators.

Abolition of grit is vital to prevent the abrasion of equipment and to minimize the heavy deposits in the aeration tank.

Then the effluent is allowed to pass through the sedimentation tanks. The suspended solids and organic and inorganic trash is allowed to settle down. Fats and grease needs to be skimmed in the primary treatment stage only. If industrial waste like certain acids or alkalis are present, it needs to be neutralized.

Such tanks may sometimes contain mechanically working scrapers for the sludge treatment. The sludge is collected at the base of the tank and is allowed to pump for sludge treatment facilities.  

Secondary treatment stage

Secondary treatment stage involves biological action. The primarily treated wastewater is passed through an aeration tank. The aeration tank acts like oxidation ponds. 

The effluent is aerated for the bacteria and fungi to work on it. Bacteria and fungi are capable of breaking down the dissolved and suspended solids. Such organisms form a flock.

Municipal wastewater treatment processes

The effluent is now allowed to enter the final settling tank. Here the solids along with bacterias and fungi settle down. Such material is semi solid known as sludge. 

The sludge is collected and used again in the aeration process. It is known as an activated sludge process. The remaining sludge undergoes anaerobic respiration which gives out gases like carbon dioxide, methane, hydrogen sulphide and biogas. 

Sometimes, the water treatment is done by the process of chlorination. Chlorination can have harmful effects. It can form chlorinated hydrocarbons like chloroform or can have some carcinogens.

Tertiary treatment stage

The stage is also referred to as an effluent polishing stage. The stage can vary depending upon the contaminant present and the motive. The tertiary wastewater treatment process steps may include filtration, passage through activated charcoal, chlorination and various chemical treatment of water.

The purpose of the tertiary stage is to pull out the remaining inorganic compounds. Also the harmful bacteria and viruses are also removed.

Some tertiary wastewater treatment process steps are mentioned below: 

Municipal wastewater treatment processes

  • The remaining suspended solids can be abolished by the process of sand filtration. Also the process of carbon adsorption is utilized to abolish the residue.
  • Oxidation ponds or lagoons are utilized to carry out aerobic respiration. Such ponds can be the habitats for certain phytes. 
  • Small invertebrates such as water fleas and some species of wheel animals also provide great help in removing the fine particles. 
  • Also some species can help in the removal of the most significant issue which is nitrogen. Initially, ammonia is transformed into nitrite through the nitroso species. Later on the nitrite is oxidised to nitrate in the presence of nitro species. Both the processes require oxic conditions.
Removal of nitrate from the water and fixing the nitrogen in the atmosphere is carried out through various processes. The most usable mechanism is the activated sludge process.

Wastewater to drinking water process

Wastewater can be treated for its usability as drinking water. The technique for the conversion of wastewater into drinking water varies with the type and the concentration of contaminant present.

The process can be as simple as above mentioned. The wastewater may transform into drinkable water by the primary treatment, secondary treatment and sometimes tertiary or advanced treatment.

But sometimes the process can be more complex. The advanced treatments need to be done. Also advanced equipment with higher efficiency is required. The advanced treatments include micro filtration, reverse osmosis, filtration through U.V rays and storage in groundwater or surface water reservoirs. 

Micro filtration

Micro filtration helps in removing the remaining solids. 

Reverse osmosis

Reverse osmosis is the process of purifying water by passing the water through a membrane. The wastewater is pressurized and is passed through the membrane. It is helpful in abolishing the bacteria, viruses and other pathogens. 

However reverse osmosis also filters out the needed micro nutrients. Such micro nutrients are essential for the human body. Also wastage of water is more in the R.O system. Nowadays, R.O with zero wastage are used.But again such a process has an effect on the life of the R.O system.

Passing through U.V rays

The water is also filtered by passing it through the U.V chamber. The water is also disinfected using ozone and hydrogen peroxide.

Storage in reservoirs

The final step is storing the water in the groundwater or surface water reservoirs. The process may take upto six months.

Finally, after getting recycled the water is treated according to the standard norms and is ready to use as potable water.


Thus, wastewater treatment is now inevitable. However, there can be some adverse effects which are unknown today. But for now to solve the water crisis, this is to be implemented.
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