What is Sewage Water and its Treatment Process?

Biology Topics related to disease and health provide critical insights into human physiology and medicine.

Wastewater, Sewage and Sanitation

Clean water is a basic necessity of our life. We use clean water everyday for drinking, bathing, washing clothes, cooking food, washing utensils, and flushing toilets, etc. When we use water for various purposes, we make the water dirty. The black-brown water containing lather (jhaag) and mixed with dirt and grease, which comes from the sink of a kitchen, a washing machine, a bathroom shower or a bath tub, and goes down the drain in our house is dirty water. This used water is called wastewater.

Again, when we flush the toilet, then faeces and urine go down the drain with water. So, the filthy water which comes out from the toilet and goes into drain is also dirty water. Thus, the wastewater may also contain faeces (or human excreta). From this we conclude that in everyday life, we convert clean water into dirty water or wastewater. This is essential to keep cleanliness and remain healthy.

We have already studied that there is severe shortage of freshwater due to increasing population, pollution, industrial development, mismanagement and various other factors. So, the used water should not be wasted. Used water or wastewater could be re-used after cleaning it. We must clean the used water by removing various impurities from it. The cleaning of water is a process of removing pollutants (harmful substances) from it before it is re-used or released into the environment (such as rivers, lakes, sea or ground). We will study the cleaning of wastewater in detail, after a while.

The wastewater and faeces (human excreta) from homes and other buildings which is carried away in sewers, is called sewage. Sewage includes household wastewater from kitchen sink, washing machine, washbasin, bathroom shower, bath tub and faeces (human excreta) from the toilet (see Figure). In addition to homes, 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 harmful and poisonous chemicals disharged by factories and industries. Sewage is a liquid waste. Most of the sewage is dirty water having dissolved and suspended impurities in it. The impurities present in sewage are called ‘contaminants’ or ‘pollutants’. Please note that when we talk of ‘wastewater’, it means ‘sewage’.

Composition of Sewage

Sewage is a complex liquid mixture containing water, dissolved and suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria, and other microbes (such as protozoa), etc.

1. The organic impurities present in sewage are: Human faeces, Animal waste (like Animal dung), Urea (as Urine), Oil, Fruit and Vegetable wastes, Pesticides, and Herbicides, etc.
   
2. The inorganic impurities present in sewage are : Nitrates, Phosphates and Metals.
3. The nutrients present in sewage are : Nitrogen and Phosphorus.
4. The bacteria present in sewage include those bacteria which cause water-borne diseases such as cholera and typhoid.
5. The other microbes present in sewage are protozoa which cause a water-borne disease called dysentery.

In addition to the above components, sewage that reaches sewage treatment plants may also contain rubbish such as rags, wooden sticks, cans, plastic bags (polythene bags), napkins, sanitary towels, grit (small pieces of stone), and sand, etc., which somehow get into the sewers.

The Sewerage System – Wastewater treatment

In a house (or a public building) generally there are two sets of pipes : one set of pipes brings clean drinking water into the house and the other set of pipes takes away wastewater (or sewage) produced in the house. The clean ‘water supply’ pipes are narrow whereas the wastewater carrying pipes are much wide. The pipes which carry away waste water (or sewage) from houses and other buildings are buried under the ground. An underground pipe which carries away dirty drainage water and waste matter (such as from toilets) is called a sewer. The provision of drainage at a place by laying ‘sewers’ under the ground is called sewerage.

If somehow we could see through the ground, we would see a network of big and small interconnected pipes called sewers forming the sewerage. Actually, sewerage is an underground network of interconnected pipes (called sewers) that transports or carries the sewage from the place where it is produced (homes and other buildings), to the sewage treatment plant (or wastewater 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 metre to 60 metre 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 sewer line.