When thought of wastewater, what usually comes up in one’s mind is domestic wastewater and sewage. But more than that, wastewater includes any used water and water-carried solids from a community, which flows to a treatment plant, like stormwater, surface water and industrial water. In all these types of wastewater, only around a percent of water is solids, chemicals, nutrients, fats and other pollutants.

Here is a walk through of how the basic treatment of domestic wastewater takes place.


The sewer systems function by gravity flow, which pulls wastewater towards the treatment plant. The wastewater enters the plant and passes through the preliminary treatment called ‘screening’, where substantial objects are trapped and removed for landfill disposal. After the screening, the water is pushed towards a grit chamber to skim out heavier solids like rocks, sand, gravel, and other materials, which are also sent to the landfill for disposal. Following the preliminary treatment, the water flows to a primary sedimentation basin where the left out solids from grit chamber are separated from the water. At this stage, the small yet heavy particles like vegetables or fruits, settle at the bottom (sludge), and lighter particles like fats and oils stay afloat, which are removed by skimming.

Aeration & Sedimentation

The wastewater after the screening, grit chambers and preliminary treatment, enters the secondary treatment process, the two step process

The first step is called the ‘aeration’. Here, the wastewater is mixed with air and microorganisms which consume suspended organic matter such as food, human faeces and other organic matter. In the second phase, the wastewater undergoes a yet another sedimentation process where the microorganisms - the activated sludge, are removed from the fairly clean wastewater. The activated sludge is returned to the oxidation ditches to absorb more organic matter or is diverted to the digestion process for the waste is treated and dewatered to reduce pathogens and organic attraction like flies, mosquitoes, rodents etc. This sludge is called biosolids. This is mostly good for land application for farmland or reclamation sites or disposed at the landfill.

Disinfection The wastewater after sedimentation process is relatively clean, and is sent to filtration process to remove fine particles, afterwhich the water is ready for disinfection. The common form of disinfection process is adding chlorine to the wastewater. Other disinfection methods like ozone, UV light, and peroxide. The water is sent through a chamber or series of basins to disinfect and kill the bad microorganisms. This wastewater is called ‘reclaimed water’ and is released in the environment. This water can be recharged into the groundwater and is often used for farms and non-edible crop irrigation, dust control and construction activities. This controls the harm caused to the environment by releasing untreated water.

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