30/12/2016
Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage.
A sewage treatment plant (STP) has to handle the designed quantity of sewage and deliver satisfactory quality of treated water, on a consistent, sustained basis over typically 10-12 years.
This requires proper design and engineering; followed by proper operation and maintenance throughout its life.
We need a STP that-
Achieves the desired results on a consistent and sustained basis.
Is robust and reliable, and lasts for at least 10-12 years without major repairs.
Needs minimum amounts of money, energy and chemicals to achieve the desired treated water quality.
Is easy to operate and maintain.
This blog provides role of units such a STP.
Assured availability of water for various secondary uses, Enormous savings in fresh water costs, Lesser Environmental Degradation and Improved public Health are benefits of a well run STP.
Understanding the STP Stages:-
Bar Screen Chamber
The role of the bar screen is to prevent entry of solid particles/ articles above a certain size; such as disposal cup/glass, polythene bags and sanitary napkins into the STP.
The screening is achieved by placing a screen made out of vertical bars, placed across the sewage flow.
Oil and Grease/Grit Trap
The waste is placed at the discharge point of the kitchen area itself to arrest solid and fatty matter at source. The wastewater output from this unit is taken to the equalization tank.
The solids and fats that are separated in this unit are disposed off along with other and grease from the wastewater at source ensures that the contact time between solids and wastewater is kept to a minimum, so that the wastewater does not absorb additional organic pollutant loads due to leaching of these substances from the solids.
An Oil and grease/grit trap is generally not an essential unit in a typical residential complex. It is however a mandatory unit in commercial and Industrial units with a canteen/kitchen on campus.
Equalization Tank
The sewage from the bar screen chamber and oil, grease and grit trap comes to the equalization tank.
The equalization tank is the first collection tank in an STP.
Its main role is to act as buffer: To collect the incoming raw sewage that comes at widely fluctuating rates, and pass it on to the rest of the STP at an average flow rate.
During the peak hours, sewage comes at a high rate. The equalization tank stores this sewage, and lets it out during the non-peak time when there is no/little incoming sewage.
Aeration Tank
The Aeration tank is at the heart of the treatment system.
The bulk of the treatment is provided here, employing microbes/bacteria for the process. The main role of the Aeration tank is to maintain a high population level of microbes. This mixture is called MLSS (Mixed Liquor Suspended Solids).
The mixed liquor is passed on to the clarifier tank, where the microbes are made to settle at the bottom. The settled microbes are recycled back to the aeration tank. Thus they are retained for a long period within the system
Settling (Clarifier) Tank
There are three purpose and role of the secondary clarifier:
Allow settling of biomass solids in the Mixed Liquor (biomass slurry) coming out of the aeration tank, to the bottom of the clarifier
To thicken the settled biomass, in order to produce a thick underflow
To produce clear supernatant water, in the overflow from the clarifier
The clarifier tank is only a passive device. All the above actions occur due to gravity.
The thick biomass is reticulated back to the aeration tank.
There are three popular design variations in the unmechanized clarifier tank.
They differ in the manner in which the sludge is collected and returned to the Aeration tank.
Air lift pump- Sludge is collected with an air lift pump. By varying the air pressure, the flow rate of the sludge can be adjusted. This version is most prevalent.
Electric pump direct suction – Sludge is collected with an electric pump. Since the flow rate of this pump cannot be varied, the pump is turned off periodically to arrive at a lower net flow rate.
Electric pump and buffer sump – Sludge is allowed to flow in a buffer sump (using gravity). From here, sludge is pumped back to aeration tank using a pump. The net flow rate is adjusted using a bypass pipeline with a valve.
Clarified Water Sump
Overflow water from the clarifier is collected in an intermediate clarified water sump, This sump acts as a buffer tank between the secondary and the tertiary treatment stages in an STP.
In a well-run STP, the treated water quality at this stage is good enough for reuse like perfusion, and water for garden use may be directly taken from this sump, without having to overload the tertiary units.
Filter Feed Pumps
Filter feed pumps are used to take the water from the clarified water sump and pass it through the pressure sand filter and activated carbon filter installed in series.
Activated Carbon Filter
An activated carbon filter is a tertiary treatment unit. It receives the water that is already filtered by the Pressure Sand Filter and improves multiple quality parameters of the water.
Treated Water Tank
The treated water is disinfected to destroy and render harmless disease-causing organisms, such as bacteria, viruses, etc.
The most common methods of disinfection include Chlorination, Ozonation and UV radiation.
Of these, Chlorine finds widespread application. The primary action of the chemical involves damaging the cell wall, resulting in cell lysis and death.
