The knowledge about "civil engineering" provided by Er. Md Faiz Ali through this plateform is absolutely correct and verified based on practical knowledge and good experiences.

Thursday 16 February 2017

Sewage Treatment

  9:23 pm

 What is sewage treatment :-
Municipal waste water, including sewage, is treated in a multi step process before the treated water is released into the environment.

Why we do this :-
  • Water pollution can be reduced when water is treated.
  • To control/recycle utility water.
The earth has an abundance of water, but unfortunately only a small percentage (about 0.3%) is even usable by humans. The other 99.7% is in the oceans, soils, icecaps, and floating in the atmosphere.
Earth contains as
Sea+Oceans    = 96.5%
Ground water  = 1.7%
Glaciers           = 1.7%
Vapor+Air       = 0.001%
Only  2.5% of this water is freshwater.

How to treat water to make usable :-

  1. Source control.
  2. Preliminary treatment.
  3. Primary treatment.
  4. Secondary treatment.
  5. Biological treatment.
  6. Tertiary treatment.

What is waste water ?

Wastewater, also written as waste water, is any water that has been adversely affected in quality by anthropogenic (انسانی  ) influence. Wastewater can originate from a combination of domestic, industrial, commercial or agricultural activities, surface runoff or storm water, and  from sewer inflow or infiltration.

Types of waste water :-

Waste water comes in three main types.
  1. Black water
  2. Gray water
  3. Yellow water     

Black water

This is wastewater that originates from toilet fixtures, dishwashers and food preparation sinks. It is made up of all the things that you can imagine going down the toilets, bath and sink drains. They include poop, urine, toilet paper and wipes; body cleaning liquids, anal cleansing water and so on. They are known to be highly contaminated with dissolved chemicals, particulate matter and is very pathogenic.

Gray water

This is waste water that originates from non-toilet and food fixtures such as bathroom sinks, laundry machines, spas, bathtubs and so on. Technically it is sewage that does not contain poop or urine. Gray water is treated very differently from Black water and is usually suitable for re-use.

Yellow water

This is basically urine collected with specific channels and not contaminated with either black water or gray water.

Source of waste water :-

  1. Domestic.
  2. Industrial.
  3. Commercial. 
  4. Agricultural activities.
Necessity or proper of waste water :-

We consider wastewater treatment as a water use because it is so interconnected with the other uses of water. Much of the water used by homes, industries, and businesses must be treated before it is released back to the environment.

 If the term "wastewater treatment" is confusing to you, you might think of it as "sewage treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but it would be overwhelmed if we didn't treat the billions of gallons of wastewater and sewage produced every day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level nature can handle.

Wastewater is used water. It includes substances such as human waste, food scraps, oils, soaps and chemicals. In homes, this includes water from sinks, showers, bathtubs, toilets, washing machines and dishwashers. Businesses and industries also contribute their share of used water that must be cleaned.

  1. Fisheries
  2. Wildlife Habitats
  3. Recreation and quality of life

What is Sewage ?


Water that is discharged after home or industrial use, and cleared away via a system of pipes leading to a waste water treatment plant.

For what purpose are sewerage systems required in the local authority area?

To prevent pollution and hazards that potentially can cause diseases, and to prevent ground water pollution.
As opposed to closed water systems that are not accessible to the public, except  when opening the tap. The sewerage systems are open and allow the entry of various objects and materials via lavatories (شوچالیوں) and other systems for discharging water after home or industrial use.
These objects and materials that are transferred together with the sewage to the treatment plants, make the treatment work difficult and cause substantial disruptions (رکاوٹ ) in the process and affect the effluent (دھار ) quality.

Catchment  of sewage :-



A catchment is the area of land surrounding a dam and the river that feeds it. Rain falling in the catchment area that isn't absorbed into the ground runs into the river and dam. The quality of the water is determined by how 'clean' the catchment area is. It is important that we take care of our catchment areas, as good water quality is important for healthy and successful communities.

Population of forecast water :-
Population is one of the most important factors for design of the water  systems, so it should be estimated, so as to know the increasing demand and ensure continuous supply to them.

population data is obtained by previous records and the rate of increase is found out and this used for further analysis, which may be by using the methods described here.

Population data can obtained by using described below :
  1. Arithmetic growth method.
  2. Geometric growth method.
  3. Curve linear method.
  4. Decline growth method.
  5. Ratio growth.
  6. Logistic method
  • Mathematically/Arithmetic growth method :
It is based on the assumption that the rate of growth of population is constant. It means that the each year population increase by the same increment.

dp/dt = Ka

Where, dp/dt is the rate of change of population.
              Ka is the constant arithmetic increment.
Ka can be determine by finding the slop of the graph of population against time. The population in the future is thus estimated.

              dP /dt ∝ P => dp / dt = Kg 

where,   Kg = Geometric Growth constant.

If P0 is the population at any time t0 and Pf is the population at time tf then,

                 ∫Pf P0 dp/p = Kg ∫ tf t0 dt = Ln (Pf/P0 = Kg (tf/t0)

           => Ln (Pf/P0 = Kg Δt

          => (Pf/P0 = (e) Kg Δt and 

                Pf = P0 (e) Kg Δt

  • Logistic method :

When the growth rate of population due to birth, death and migration are under normal situation and not subjected to extraordinary changes due to unusual situation like war, epidemics earth quakes and refugees etc. Then this method is used:

According to this method

P = P sat / (1+ ea+ bΔt), where P sat is the saturation population, of the community and a, b are constants. P sat, a and b can be determined from three successive census populations and the equations are

P sat = 2 P0 P1 P2 - P12 (P0 + P2) / (P0 P2 - P12)

  To know more about STP Step by step click HERE

Estimation of water demand :-


While estimating the water demand some factors should be considered.
As for example:- 
  1. Type of area/city.
  2. Size of the area/city.
  3. Population of the area. 
  4. Industries.
  5. Average daily water consumption.
  6. Maximum daily consumption.
  7. Maximum weekly demand.
  8. Fire water demand / fire demand.