Ipsa Tripathy
Bhubaneswar: Every day, wastewater leaves our homes, offices, schools, hospitals and industries through drainage systems. It carries a mixture of water, human waste, food residues, detergents, oils, suspended solids and other substances. Once this water disappears down a drain, its journey is largely invisible. But before wastewater can be safely released into the environment or reused, it may need to pass through several stages of treatment.
A sewage treatment plant is designed to remove contaminants from wastewater using physical, biological and, where required, chemical processes. The exact treatment sequence depends on the wastewater being treated, the plant’s design and the quality of water required at the outlet.
The process begins when wastewater enters the plant. Screens remove larger materials such as plastics, cloth, sanitary products and other debris that could block pipes or damage equipment. This is an important first step because treatment machinery is designed to handle wastewater, not large solid objects.
After screening, wastewater commonly enters a grit removal chamber, where heavier inorganic particles such as sand and small stones settle out. Removing grit reduces abrasion and prevents unwanted material from accumulating in tanks and pipelines.
The wastewater then moves into primary sedimentation tanks. Here, the flow slows sufficiently for many suspended solids to settle under gravity. Oils and grease may float to the surface and be removed. The settled material forms primary sludge, which must be collected and managed separately. These initial stages remove a portion of the suspended solids and organic matter, but the water still contains dissolved and finely suspended pollutants. This is where one of the most important parts of treatment begins: the use of microorganisms.
The role of microorganisms
In many sewage treatment plants, wastewater enters an aeration tank, where it is mixed with air and a community of microorganisms. These organisms, including bacteria, use biodegradable organic matter in the wastewater as a source of energy and material for growth.
As microorganisms break down organic compounds, they convert much of the material into simpler substances, including carbon dioxide and new microbial cells. This process reduces the amount of biodegradable organic matter remaining in the water. Aeration supplies oxygen for aerobic microorganisms and helps maintain the conditions needed for biological treatment.
The mixture then flows into a secondary settling tank. Here, microbial material forms clumps called flocs, which settle and separate from the treated water. Some of this settled biomass may be returned to the aeration tank to maintain the microbial population, while excess sludge is removed for further treatment.
Biological treatment can also be designed to remove nutrients such as nitrogen and phosphorus. These substances are necessary for life, but excessive quantities discharged into rivers and lakes can encourage algal growth and contribute to eutrophication. Not every sewage treatment plant has the same nutrient-removal capacity.
Why treatment does not remove everything
After secondary treatment, the water may look much cleaner, but appearance alone cannot establish whether it is safe for discharge or reuse. Conventional treatment processes are designed primarily to remove solids and biodegradable organic matter and to reduce microbial hazards. Some dissolved chemicals, pharmaceuticals, personal care product residues, metals and other contaminants may remain, depending on their properties and the treatment processes used. Advanced treatment may be required to reduce particular contaminants further.
Disinfection provides another barrier against disease-causing microorganisms. Plants may use chlorine, ultraviolet light or other methods to reduce pathogens. However, disinfection does not automatically remove dissolved chemicals, salts or every contaminant. Its effectiveness also depends on the treatment method and operating conditions.
Water quality must therefore be assessed through appropriate testing, including measurements of organic pollution, suspended solids and microbiological indicators. Additional parameters are needed when nutrient removal, industrial contamination or a particular reuse purpose is relevant.
What happens to the treated water and sludge?
Depending on its quality and the applicable standards, treated wastewater can be discharged into a river, lake or other receiving environment. It can also be reused for purposes such as industrial processes, landscaping, toilet flushing or certain agricultural applications when the treatment and safety requirements for that use are met. Reuse requires appropriate monitoring and management to control health and environmental risks.
The solids removed during treatment do not simply disappear. Primary sludge and excess biological sludge require further processing, which may include digestion to stabilise organic matter, dewatering and appropriate recovery or disposal. Poor management of sludge can create additional pollution problems, so handling these by-products is an essential part of the treatment process.
A sewage treatment plant is therefore more than a series of tanks. It is a managed system that combines physical separation, microbial activity and, where necessary, advanced treatment and disinfection. Its effectiveness depends on suitable design, regular maintenance, trained operation and reliable monitoring.
The water leaving a plant has undergone treatment, but its quality must still match the intended destination. Understanding that distinction is essential to protecting rivers, public health and the water resources that communities depend on.