Sewage Treatment: Introduction, Stages, Importance

1 Introduction:

Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through sewer system. sewage consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality. There are two sub-types of sewage such as greywater (from sinks, bathtubs, showers, dishwashers and clothes washers) and blackwater (the water used to flush toilets, combined with the human waste that it flushes away).

Sewage also contains soaps and detergents. Food waste may be present from dishwashing and food quantities may be increased where garbage disposal units are used. In regions where toilet paper is used rather than bidets, that paper is also added to the sewage.

Sewage contains micro-pollutants and macro-pollutants, and may also incorporate some municipal salid waste and pollutants from industrial wastewater.

1.2 Difference between sewage and Wastewater: 

The terms 'Sewage' and 'Wastewater' are regularly used interchangeably, however, there are differences between both. In fact, 'Sewage' is considered a subset of wastewater. Although the terms 'Sewage' usually brings toilets to mind, it is used to describe is considered a all types of wastewater generated from domestic dwellings. There are two types of Sewage: blackwater and greywater.

Blackwater and greywater have different characteristics, but both contain pollutants and disease-causing agents that require treatment.

Wastewater from non-residential sources generally require additional treatment steps that what is needed for sewage. For example, to prevent flooding of treatment plants during bad weather, stormwater should be collected separately. Screens often remove rubbish and other large solids from storm sewers. In addition, many industries produce wastewater high in chemical and biological pollutants that can burden treatment systems. Dairy plants and breweries are perfect example of this. To combat any issues these types of wastewater sources tend to provide their own treatment to preliminary treatment to protect the main wastewater treatment system.

1.3 Importance of Sewage Treatment: 

Sewage treatment is crucial for public health, preventing waterborne diseases by removing pathogens and harmful substances from wastewater. It also protects the environment by preventing pollution of rivers, lakes and oceans, which preserves aquatic ecosystems and wildlife. Furthermore, it enables water conservation by allowing treated water to be reused for non-potable purposes like irrigation and industrial use, reducing reliance on freshwater.

1.4 Global Scenario of Sewage Generation: 

Our domestic wastewater assessment suggests house-hold activities in 2022 generated 267.5 billion m³ of wastewater. This wastewater volume represents 8,019 million people, covering almost the entire global papulation. Domestic wastewater generation per capita worldwide is 33.4 m³ per year or 92 L per person per day. Of the 267.5 billion m³ of domestic wastewater generated, 168.8 billion m³ are 

collected while 98.7 billion m³ remain uncollected, i.e., 63.1% of generated domestic wastewater is collected. The treatment status of domestic waste-water suggests that 54.7% (146.3 billion m³ per year) of the generated wastewater undergoes treatment. In comparison, 45.3% (121.2 billion m³ per year) is released to the environment in untreated form, i. e, as uncollected wastewater (98.7 billion m³ per year) or collected but untreated wastewater (22.5 billion m³ per year).

2 Source of Sewage : 

Sewage, which is a ubiquitous component of contemporary life, is the result of collective human activity waste. There are many sources of sewage such as -

A) Domestic Sewage :

The most prevalent and easily recognisable source of sewage is likely domestic sewage. It includes the following elements and is derived from homes and residential areas:-

a) Sanitary Sewage: 

Sanitary sewage is a byproduct of regular human activity inside of homes. It consists of the water that drains from bathrooms, kitchens, showers, bathtubs and washing machines. Organic materials, detergents, soaps and personal care items are all present in this kind of sewage.

b) Stormwater Runoff : 

Stormwater runoff from rain can contaminate sewage systems and add to domestic sewage, stormwater can gather a variety of contaminants as it travels through metropolitan areas, including trash, oils and chemicals, which further contaminate the Sewage.

c) Greywater: 

Wastewater from all domestic sources such as showers, dishwashers and washing machines ets, except toilet is referred to as grey water. Greywater still Contains organic waste and common home pollutants even though it is less contaminated than blackwater (wastewater from toilets).

B) Industrial Sewage : 

factory, manufacturing and industrial processes all produce industrial sewage. Depending on the industry, its makeup varies greatly, but common elements include:-

a) Chemicals: 

Various chemicals are frequently used and disposed of in industrial processes. It not appropriately managed these substances which may include solvents, heavy metals, acids and hazardous compounds may enter the sewage system.

b) Suspended Solids: 

A variety of industrial operations result in the production of solid waste including plastics, paper pulp, metal shavings and more.

c) Hazardous Products: 

If containment mechanisms fail, industries handling hazardous product such as petrochemicals or pharmaceuticals, may leak poisonous compounds into the sewage System.

C) Agricultural Sewage : 

Agricultural runoff is the main kind of sewage produced by agricultural activities. In rural locations, this sort of sewage is very significant and may consist of:-

a) Animal Waste: 

Large amount of sewage in the form of manure are produced by farms with livestock. Manure management issues can result in runoff that pollutants surrounding water sources by introducing germs and nutrients.

b) Soil Erosion: 

Runoff from agricultural fields may contain silt as a result of soil erosion. This debris can impair water quality, damage aquatic habitats and jam up waterways.

c) Pesticides and Fertilisers: 

Using pesticides and fertilisers in farming might result in the chemicals being washed into neighbouring rivers. This discharge damages aquatic ecosystems and adds to water pollution.

D) Municipal Sewage: 

Municipal Sewage is the wastewater produced by public buildings and institutions such as schools, hospitals and offices of the government. Typically, it includes Sewage from commercial, industrial and home sources. Additionally, transportable sources like recreational vehicles, and boats may discharge sewage into urhan sewage system.

E) Commercial Sewage: 

Sewage generation from commercial facilities such as hotels, restaurants and enterprises, is substantial. Commercial sewage includes substances like :-

a) Food Waste: 

Sewage from restaurant and other food-related businesses contains grease, oils and food scraps. These compounds can cause odours and obstructions in the sewage system making treatment difficult.

b) Chemicals: 

A number of industrial facilities, including dry cleaners and auto repair shops, are prone to dumping chemicals and solvents into the sewage system. Both the environment and general public health may be harmed by these pollutants.

c) Human Waste:

Commercial establishments have restrooms that add to the overall sewage load, similar to how domestic establishments do.

3 Composition of Sewage:

The composition of sewage mainly depends upon per capita consumption of water and varies from place to place and season to season.

The sewage composition can be studied under following two heads:-

3.1 Chemical Composition: 

Chemically, the sewage consists of approximately 99% water and 1% Inorganic and organic matter in suspended and soluble forms. Lignocellulose, cellulose, proteins, fats and various inorganic particulate matter exist in suspended state, whereas sugars, fatty acids, alcohols, amino acids and inorganic ions constitute the soluble forms.

However, on an average, the sewage of towns in our country contains about 350 ppm biodegradable organic matter, 52 ppm N₂, 45 ppm potassium and 16 ppm phosphorus. Salts of several heavy metals such as Zn, Cr, Ni, Pb etc. are also present above permissible levels in sewage.

3.2 Microbial Composition:

The microbial population per ml of sewage may vary from a few lacs to several millions. Various types of micro-organisms, micro-fungi, bacteria and protozoa, collectively called 'Sewage fungus' are known to grow profusely in sewage.

In addition, viruses and many micro-algal genera have also been recorded from sewage.

"Bacteria occuring in sewage are mainly intestinal and soil inhabiting and their common types are coliforms, streptococci, clostridia, micrococci, proteus, pseudomonas and lactobacilli.

4 Need for Sewage Treatment: 

There are many reasons for the need for sewage treatment, such as :-

4.1 Public Health Protection:

• Untreated sewage contains bacteria, viruses and parasites that cause diseases like cholera, typhoid and dysentery.

• Treatment removes these pathogens and harmful chemicals, preventing the spread of water-borne illnesses and ensuring safe water for communities.

4.2 Environmental Protection:

• Discharging raw sewage into water bodies leads to severe pollution, harming aquatic life and destroying delicate ecosystems.

• Treatment prevents this contamination, safeguarding the health of rivers, lakes and oceans.

4.3 Water Conservation and Reuse:

• Treated wastewater can be recycled for various non-potable uses such as irrigation, industrial cooling and toilet flushing.

• This reuse significantly reduces the demand for freshwater, which is a vital and often scarce resources.

5 Stages of Sewage Treatment: 

Sewage treatment generally involves three main stages - the primary, secondary and tertiary treatment stage.

5.1 Primary Treatment Stage: 

In a sewage treatment process, sewage water is first allowed to pass through screen or grit chamber where large solids are removed. This step is followed by aeration or mixing  in a tank and then primary sedimentation where suspended solids settle down. Primary treatment involves addition of a coagulant and aims at removing grits, coarse solids, oil and grease if any present. Preliminary treatment results in sedimentation of solids (sludge) and formation of an effluent that floats above sludge. Effluent from primary treatment consists of 45-50% unstable organic argan matter. The effluent then under-goes secondary treatment where fine solids settle out by means of bacterial activities.

5.2. Secondary Treatment Stage: 

This stage is also known as biological treatment stage. This stage converts organic matter in sewage into stable forms through biological activities, resulting in secondary Sedimentation. Common approaches are trickling filters and activated Sludge method.

Trickling filters comprise an enclosed tank with bed of bricks and a layer of microorganisms. The effluent enters the tank through an inlet and trickles over the bed layer by sprinklers. Microbial activities oxidize the organic matter in the effluent, resulting in the removal of the fine solids, formation of sludge and effluent with less organic solids. 

Activated sludge method involves the addition of mixture active microorganisms to sewage, followed by aeration and agitation. The aerobic conditions and microbial actions oxidize the organic solids resulting in coagulation, flocculation and settling of solids. Activated sludge produces a clear liquid without foul odor.

5.3 Tertiary Treatment Stage: 

The effluent then undergoes tertiary treatment, by incorporation of U.V. radiation or chlorination. Other methods such as sand filters and reverse osmosis may also be used instead for this stage of treatment, depending upon the nature of the sewage and the effluent from secondary treatment.

Sludge Management: 

Sludge is digested in a tank which results in the release of combustible gases: methane (CH4) and carbon dioxide (CO₂), that can be used as fuel. The digested sludge can either be disposed by incineration or used as a fertilizer.

6) Causes of Sewage Pollution: 

There are many causes of sewage pollution such as:-

• The use of toilets as garbage cans : 

Toilets are designated as fixtures for quelling nature's cries. Unfortunately, careless people have turned toilets into deposit banks for waste materials such as papers, sanitary products and some even flush plastics. These waste materials are the root causes of sewage line blockages in the majority of buildings. Blockages cause the toilets served by that sewer to overflow.

These waste materials then clog the waterways further along the sewer lines. Plastics, for example, such as soap wrappers, clog rivers and prevent further flow due to stagnation. As a result, it is home to dangerous organisms and bacteria. The spread of the foul odar from the sewer also contributes to air pollution as a result of the blockages. Sewer treatment plants, in general , face a great deal of difficulty as a result of blockages and foreign materials present in the wastewater, resulting in sewage pollution.

• Fats used in cooking:

There is a lot of fat and oil in kitchen products. Kitchen sinks are also used to wash dirty dishes. These materials accumulate quickly at the walls of the pipes, forming coverings that reduce the diameter of the pipe system that drains wastewater into the sewer. Continued disposal of fats, oils and grease in sinks results in complete clogging of the sewage system. This is even more dangerous than the toilet blockage. No amount of cleaning will result in the removal of the layered cover if there is a blockage. This will necessitate the installation of a new pipe system, which will be costly.

• Wastewater overcapacity: 

Sewers are designed to handle a specific amount of wastewater. Nonetheless, sewers overflow for a variety of reasons. For starters, thene are contractors who, during the construction of a building, end up connecting the new building's sewage system to an existing sewer built for another residential building. This causes sewer overflow, which is hazardous to human health and can result in acute viral, bacterial and parasitic diseases such as giardiasis, typhoid gastroenteritis and Hepatitis A.

• Flooding:

Flooding is another factor that Contributes to an increase in wastewater. When there is a lot of rain, the water seeks a way out and seeps into sewers, mixing with the waster water, resulting in more wastewater in the sewer. If the sewer volume is small, the sewer resulting will most likely be unable to hold the increasing volume of water, resulting in sewer pollution.

7) Effects of Untreated Sewage: 

Untreated sewage pollutes water sources, harming ecosystems by depleting dissolved oxygen and creating harmful algal blooms, which kill aquatic life and reduce biodiversity. It also poses a majar human health risk, spreading waterborne diseases like cholera, typhoid and hepatitis A through contaminated water and seafood leading to several illness and death. Furthermore, the chemicals and pathogens in sewage can degrade soil and pose dangers to both humans and animals that come into contact with contaminated environments.