• Written By Akanksha P John
  • Last Modified 25-01-2023

Sewerage System: Meaning, Types, Sewage Treatment


Sewerage System: The availability of safe drinking water is a basic requirement of our existence. Every day, we use clean water for our everyday activities, yet in order to meet our demands, we pollute the water. Dirty water is black-brown water containing lather and combined with filth and grease that comes from the kitchen sink, washing machines, bathrooms, and drains in our home. Wastewater is the term for used water that may contain faeces.

What is Sewer Sewage?

The wastewater and faeces from homes and other buildings carried by sewers are called sewage. Sewage includes household wastewater, and in addition to a household, sewage is also produced and released by shops, offices, schools, hospitals, factories, industries and all other places where humans work or live.

Sewage also includes rainwater that has run down the streets and roads during rains carrying harmful substances with it and flows into sewer pipes.

Sometimes, sewage contains poisonous chemicals discharged by factories and industries.

Thus, sewage is liquid waste, and most of the sewage is dirty water having dissolved and suspended impurities in it. The impurities present in the sewage are called contaminants or pollutants.

Types of Wastewater

There are three types of sewage in most cases:
i. Domestic Sewage
The utilised water from flats or households is referred to as domestic wastewater. Sanitary sewage is another name for this type of effluent. It could be the water that drips from the shower or even body waste.

ii. Industrial Wastewater
Contaminated water from chemical or manufacturing activities is carried by industrial wastewater. This sewage may contain a variety of harmful compounds that may be poisonous for humans and animals.

iii. Storm Sewage
Storm water refers to sewage runoff from the atmosphere that is gathered in open channels as water particles. This can include rain, drizzle, snow, and other types of precipitation.

Composition of Sewage

Composition of Sewage

Sewage is a complex liquid mixture containing water, dissolved and suspended solids, organic and inorganic impurities, nutrients, and disease-causing bacteria and other microbes. Following are some of the matters that are present in the wastewater.

i. The organic impurities present in sewage are- human faeces, animal waste, urea, oil, fruit and vegetable peels, pesticides and herbicides, etc.

ii. The inorganic impurities present in the sewage are nitrates, phosphates and metals.

iii. The bacteria present includes bacteria that cause water-borne diseases such as typhoid and cholera.

iv. The other microbes include protozoa which cause dysentery.

In addition, sewage contains rubbish such as rags, wooden sticks, cans, plastic bags, napkins, sanitary towels, grits, and sand, etc., which somehow gets into sewers.

Sewerage System Meaning

There are two sets of pipes in a house- one to bring clean water and the other to take away the wastewater. The pipes that carry wastewater are buried under the ground and are called sewers. The provision of drainage at a place by laying sewer under the ground is called sewerage.

Sewerage is an underground network of interconnected pipes that transports or carries the sewage from the place where it is produced to the sewage treatment plant where it is processed.

A manhole is a covered vertical hole in the ground, pavement or road, above the underground sewer pipeline through which a worker can go down up to the sewer pipes for inspection and cleaning, etc. Manholes are provided at every \(50\) metres to \(60\) metres distance in the main sewer pipeline. Manholes are also provided at the junction of two or more sewers and at points where there is a change in the direction of the sewer line.

Sewerage Treatment Plant

A place where wastewater from houses, buildings and industries is brought for processing is called a sewerage treatment plant or wastewater treatment plant. The treatment plant works by treating the wastewater or sewage by a series of physical, chemical and biological processes till it becomes fit to be discharged into the environment. The various processes in the treatment are described below:

i. Screening
Solids in the wastewater are retained via wastewater screening. These solids must be eliminated at the start of the water treatment process since they can reduce the entire system’s efficiency, damage expensive and vital water treatment equipment, or taint the water, creating modest to large-scale natural upsets for a region’s entire ecosystem.

Screens for wastewater treatment can be classified into three categories: coarse, fine, and micro. There are many distinct types of screening within these categories. The size of the screening holes and other mechanical characteristics are used to classify the screens.

ii. Grit and Sand Removal
Sand, gravel, cinder or other heavy solid elements with higher specific gravity than the organic biodegradable particles in the wastewater are referred to as grit.

As water runs slowly through the grit and sand removal tank, sand and grit sink down slowly to the bottom of the tank. The settled sand and pollutants are periodically cleaned from the tanks. Pumps and other plant equipment suffer from increased wear and tear due to grit.

iii. First Sedimentation Tank
The wastewater is then passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the sloping bottom of the sedimentation tank in the form of sludge. Thus, the solid part of the sewage is called sludge. In this way, the first sedimentation tank separates the solid organic sludge from the rest of the wastewater.

(a) The sludge is taken out from the bottom of the first sedimentation tank and put into a large closed tank called a digester tank. In the digester tank, many types of anaerobic bacteria decompose the organic matter present in sludge to produce biogas. The biogas produced here can be used as a fuel directly or indirectly, or it can be used to generate electricity. The digested sludge left after the removal of biogas is taken out, dried and used as manure or fertiliser.

(b) The wastewater left in the first sedimentation tank still has some organic waste in the form of tiny suspended particles, which do not settle at the bottom of the tank, as well as soluble organic matter. There may also be some oil and grease floating on the surface of wastewater in the sedimentation tank. A skimmer removes these floating materials. Thus, mainly watery waste is left in the first sedimentation tank. It is also called clarified water.

iv. Aeration Tank
The mainly watery waste from the first sedimentation tank is passed into the aeration tank. The watery waste already contains aerobic bacteria. Compressed air is bubbled through the watery waste in the aeration tank to provide oxygen to activate aerobic bacteria and make them grow rapidly in this water. The large number of aerobic bacteria produced in this way digest any human waste, food waste, soaps and other unwanted and harmful matter still remaining in the wastewater, leaving behind fairly pure water. In this way, aerobic bacteria clean the clarified watery waste and make it almost harmless.

v. Second Sedimentation Tank
The treated watery waste goes into the second sedimentation tank from the aeration tank and is allowed to stand there. In this tank, the micro-organisms used in the aeration tank settle down at the bottom of the tank, forming a fine sludge called ‘activated sludge’ because active micro-organisms such as aerobic bacteria and oxygen are present. Some of the activated sludge is added back to the aeration tank to increase the population of aerobic bacteria there and speed up the cleaning of watery waste. The remaining activated sludge is sent to the digester tank. The water left in the second sedimentation tank has a very low level of organic matter and suspended matter. This water does not contain many harmful things, so it is quite safe. This water is then discharged into a river or made to percolate into the ground.

Sometimes it may be necessary to disinfect the treated water with chemicals like chlorine or ozone before releasing it into the distribution system. The purpose of disinfection in the treatment of wastewater is to eliminate the harmful micro-organisms completely in the water to be discharged back into the environment. The treated wastewater can also be passed through layers of sand. Sand filtration removes most of the remaining suspended matter from the treated wastewater.

Disposal Methods of Sewage

In order to improve sanitation and prevent water-borne diseases, low-cost on-site and off-site sewage disposal systems are being encouraged for those people who do not have sewage facilities. Some of the following alternative sewage disposals are discussed as follows:

i. Septic Tanks
A septic tank is an underground watertight sedimentation tank that is used to treat wastewater through biological breakdown and drainage.

Two pipes link the tank for the inlet and outlet. The intake pipe gathers the water waste in the septic tank and is long enough to separate the solid and liquid waste. The exit pipe transports preprocessed effluent from the septic tank and distribute it uniformly throughout the land and waterways. The wastewater divides into three layers after a while. The solid waste is broken down inside the tank by micro-organisms from the wastewater. These bacteria quickly break down the solid waste, making it easier for the liquids to separate and drain away.

Disposal Methods of Sewage

ii. Composting Pits
A composting toilet is a self-contained human waste disposal unit that is not connected to a sewer line or a septic tank. A composting toilet breaks down the human excreta with the help of aerobic micro-organisms and dehydrates human waste to compost which can be added to the soil to make it fertile. A composting toilet consists of a toilet seat fixed at the top of a composting chamber which can be further connected to a drying chamber. The drying chamber allows the moisture to escape from compost and make it solid, which can be removed periodically.

Composting Pits

An advantage of these pits is that they supply compost that helps in conserving useful plants and nutrients.

iii. Off-Site sewage System
The development of an off-site sewage system has come from increased urbanisation. Houses are built on plots in metropolitan areas, and each plot is connected to a sewerage line. These wastewater lines gather the waste from numerous residences and transport it to a sewage treatment plant in the community. Once the treatment is complete, the excess water is sent to a neighbouring river or irrigation area.

Methods of Sewage Collection

A sewage system is often made up of underground channels that transport wastewater from various locations. Pipes, manholes, drains, holding basins, catch basins, inlets, and pump stations make up the collection system, which may transport sewage from the collecting point to the disposal point. In metropolitan areas, pipe systems and other appliances are the principal ways of sewage collection.

i. Combined System
Both storm and domestic wastewater are collected in a unified collecting system. This collecting system is usually made up of large-diameter pipelines or tunnels. During rainy seasons, however, this system may collapse because water treatment plants are unable to handle a large volume of storm sewage.

Combined System

ii. Separate System
Separate wastewater systems that handle household and storm wastewater separately can be found in newer communities. Surface runoff sewage or stormwater is discharged into open water bodies such as rivers. For excessive water flow during wet seasons, smallholding basins or catch basins might be built. Domestic wastewater, on the other hand, is sent to a treatment facility.

Separate System


In this article, we studied the various types of sewage waste and different types of wastewater in detail. We also studied the step-wise method involved in the wastewater treatment plant. We know the different alternative sewage disposal like septic tanks, composting pits and off-site sewage disposal, and the different types of sewage collection methods. We should try and maintain sanitation at public places as well as in our homes to avoid any pollution.

FAQs on Sewerage System

Q.1. What are the components of the sewerage system?
Ans: Sewerage is the system that uses sewers to transport sewage or surface runoff (stormwater, meltwater, and rainwater). Receiving drains, manholes, pumping stations, storm overflows, and screening chambers of the combined sewer or sanitary sewer are all included.

Q.2. What is the self-cleansing velocity in the sewerage system?
Ans: To prevent sewers from silting, sewer pipes must be built at a gradient that generates self-cleansing velocities at several possible discharges. This velocity should be maintained at least once a day during peak flow at all portions of the sewerage system. In general, a self-cleansing velocity of over \(0.75\;{\rm{m}}/{\rm{s}}\) is beneficial.

Q.3. What is the sewerage system?
Ans: A sewerage system is a network of pipes, pumps, and force mains used to collect wastewater (also known as sewage) from a community. It conveys sewage from its points of origin to the point of treatment and disposal.

Q.4. What are different systems of sewerage?
Ans: Combined sewerage (also known as conventional sewers) and separate sewerage are two types of sewage systems. Surface run-off and wastewater are carried together by combined sewerage, whereas surface run-off and wastewater are carried separately by separate sewerage.

Q.5. Which system is a suitable sewerage system in India?
Ans: Separate systems for sanitary sewage and stormwater runoff become uneconomical. As a result, in countries like India,  with moderate rainfall, a combined system is used.

We hope this detailed article on Sewerage System proves helpful to you in your preparation. If you have any questions related to the page or in general about the sewerage system, please reach out to us through the comments section, and we will get back to you as soon as possible.

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