The demand for water and water scarcity are the two parallel lines that increase day by day. Freshwater demand rises with the upsurge of population, urbanization, and industrialization, while natural water sources are polluted. There are two primary sources of water contamination, viz—industrial waste and sewage. 33,212 MLD sewage is generated in India, out of which 13.5 % sewage is treated. Similarly, only 60% of industrial waste is treated, and the rest is discharged in natural water streams. As a result, nine out of ten deaths from diarrhea are due to a lack of access to safe drinking water in India. Therefore, it is imperative to treat wastewater before its release into natural water sources. Wastewater Treatment Plant comes as a redeemer because the wastewater must comply with the rules and regulations before discharge in rivers and lakes. It can treat water to fulfill the limits set by government bodies and reduce the chemical and microbiological load in the water. Several types of wastewater treatment systems can treat different kinds of wastewater.
Types of wastewater treatment plants and their applications
Wastewater treatment plants treat water in numerous stages to produce clean water for reuse. Below are the different types of wastewater treatment plants available for various applications.
- Sewage Treatment Plants (STPs)
The wastewater that enters the sewerage systems from kitchens, toilets, drains, laundry, bathrooms, commercial buildings, and industries is categorized as sewage. Sewage treatment plants treat water in 5 steps, viz.- pre-treatment, primary treatment, secondary treatment, and tertiary treatment.
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- In pre-treatment, sewage passes through filters that remove large solids and debris.
- In primary treatment, wastewater flows slowly inside the clarifiers tanks, where organic solids settle at the bottom and light materials float for removal. The organic matter that settles down is the primary sludge blanket that goes further in aeration basins for the activated sludge process.
- The sludge in the secondary treatment undergoes an aerobic aeration process in the aeration basin, where a system of pumps and pipes infuse air bubbles into sludge. The interaction of oxygen and bacteria results in the bacterial digestion of the organic matter and the removal of harmful chemicals. The wastewater then flows in secondary clarification basins where bacteria spend a couple of days to form a sludge blanket, which can be later pumped out. The settled sludge blanket is called return activated sludge (RAS). The RAS is cycled through primary and secondary basins numerous times to digest incoming sewage and becomes the waste-activated sludge (WAS) which moves to aerobic sludge digesters, and the bacteria digest one another. As a result, most of the sludge disappears. Lastly, the residual sludge moves to dewater facility, where the remaining water is removed from sludge through a belt press mechanism.
- The tertiary treatment follows the primary and secondary treatments and combines mechanical and photochemical processes. In addition, the wastewater undergoes sand filtration and UV light treatment in advanced tertiary treatment. UV lights attenuate viruses and bacteria and make them unable to infect.
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Applications- STPs are found in commercial buildings, residential areas, municipal wastewater treatment facilities.
- Effluent Treatment Plants (ETPs)
Effluent treatment plants work in various stages to treat wastewater like sewage treatment plants.
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- The preliminary wastewater treatment in effluent treatment plants focuses on physical screening through sedimentation, filtration, and clarification. The maximum amount of large solid particles are removed in this stage before the effluent is sent to the next step.
- The primary stage removes additional solid particles and organic matter with chemicals and breaks down the solid and organic waste through flocculation, chemical precipitation, and chemical coagulation methods.
- The secondary stage removes suspended solid particles and biodegradable organic materials through various chemical treatments from the primary stage and biological processes like the suspended-growth method.
- The tertiary stage combines preliminary, primary, and secondary steps to remove left-over traces of contaminants.
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Application: Effluent treatment plants are majorly used in pharmaceutical, chemical, textile, and dyeing industries to treat industrial wastewater containing extensive amounts of chemical contamination.
- Common and combined effluent treatment plants (CETPs)
Common and combined effluent treatment plants work on the same principle as the effluent treatment plant. They provide water treatment services to small wastewater generators like small-scale manufacturers and tanneries who cannot afford to run individual wastewater treatment plants. Manufacturers send the wastewater to a centralized water treatment plant and share the operational and maintenance expenses to get clean water.
Application: To treat wastewater of many small-scale industries and commercial projects.
- Reverse Osmosis (RO) water treatment plants
Reverse Osmosis works on the principle of membrane separation and uses external pressure to carry out the filtration process. The wastewater passes through a semi-permeable membrane which removes most of the salt from the water along with the solid particles and chemicals. In addition, it removes most of the microorganisms and produces 99% clean water.
Application: It is widely used in the chemical, pharmaceutical, and food and beverage industries. It is also used in mechanical, semiconductor manufacturing, metal furnishing, and boiler feed industries.
- Demineralization (DM) treatment plants
Demineralization removes dissolved components and minerals entirely by using the ion-exchange principle. Two types of ion-exchange resins, viz.- cation resin and anion resin, release hydrogen and hydroxyl ions, respectively. The cation exchange results in softening of water by removing salts, and the anion exchange removes nitrate from water. The cation and anion exchange together removes all of the ionic compounds from the water and produces highly pure water.
Application: DM water is used as feedwater for boiler, chemical, pharmaceutical, food and beverage industries.
With water scarcity as a backdrop, wastewater treatment is no more an option but a necessity. In the modern age, different types of wastewater treatment plants are available to treat every type of wastewater and keep the natural water resources healthy.