SEWAGE TREATMENT PLANT
Sewage is mostly made up of water, with less than one part of solid stuff per thousand parts of water. In other words, sewage is 99.9% clean water, with the remaining 0.1 consisting of particles, which can be dissolved solids or suspended solids.
The process of eliminating impurities from wastewater and home sewage water is known as sewage treatment. To remove contaminants, it uses physical, biological, and chemical mechanisms. Its goal is to create ecologically acceptable sewage water (effluent) and solid waste (sludge or biosolids) that may be disposed of or reused. Sludge is frequently reused for agricultural uses, but it is also being utilised as a fuel source.
The introduction of contaminating elements lowers the quality of water from the mains, which is utilised in manufacturing, farming, homes (toilets, bathtubs, showers, kitchens, sinks), hospitals, commercial and industrial locations. Pollutants such as organic wastes, suspended particulates, microorganisms, nitrates, and phosphates must be eliminated.
To make wastewater acceptable for reuse or for returning to the environment, the concentration of contaminants must be reduced to a safe level, usually a standard set by the Environment Agency.
Sewage Treatment Plant is the machine that removes contaminants such as organics and in organics from municipal, domestic or commercial sewage wastewater. Depending on the type of STP technology used to treat sewage, physical, chemical, and biological processes are used to remove these contaminants and produce treated effluent that is safe enough for release into the environment as per government regulated norms or can be re-used for applications such as toilet flushing, gardening, and floor mopping.
STAGES OF SEWAGE TREATEMENT
Primary : Physical separation of large contaminants such as textiles, plastics, wood logs, paper logs, and so on. Floating and settleable impurities such as suspended solids and organic waste are removed
Secondary : Removing or lowering the concentrations of organic and inorganic substances
Tertiary (advanced) : The ultimate cleaning step before wastewater is reused, recycled, or released into the environment.
PROCESS DESCRIPTION
PRIMARY TREATMENT :
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Screen Chamber Cum Oil & Grease Trap Unit
Screen chamber Cum Oil & Grease trap helps to remove the floating particles from sewage and Laundry Effluent Screen with clear openings shall be installed to retain extraneous materials such as plastic bags, waste paper etc. from entering the Aeration tank. The frames will be fitted into the channel grooves. Screen will be physically lifted to the ground level for cleaning operations. A Fat Trap shall also be provided in the chamber where oil will be trapped on the surface of fat trap by providing quiescent condition & providing Laminar flow. The recovered oil will be disposed to permitted hazardous waste disposal site. After passing through the Screen Chamber Cum Oil & Grease Trap, Effluent shall be collected in Equalization tank.
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Equalization Tank
Raw Sewage gets collected in the collection tank. The purpose of this tank is to equalize, collect the raw sewage and to cater the peak loads. The Equalization tank is proposed with diffused aeration system, which is required to supply proper oxygen for our cultures. This aeration also helps equalization & mixing the sewage properly and reduces any chance of dilutions. From Equalization tank, the sewage is pumped to Aeration tank.
SECONDARY TREATMENT :
It includes various types of technologies.
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Conventional Activated Sludge Process
Activated sludge is a flocculent culture of organisms generated under controlled conditions in aeration tanks.
The activated sludge process is a multi-chamber reactor unit that degrades organics and removes nutrients from wastewater, resulting in high-quality effluent. The objective is to keep the activated sludge afloat while maintaining aerobic conditions. A steady and well-timed supply of oxygen is necessary to do this. The bacteria use the organic contaminants to grow and change them into energy, water, CO2 , and new cell material in an aeration tank, which provides the requisite detention time and guarantees that the activated sludge and influent wastewater are fully mixed. Agitators or mixers are used in mechanical aeration systems to mix air and mixed liquor. Diffused aeration systems use pressurized air released through diffusers near the bottom of the tank. The flocs can be removed in the secondary clarifier by gravity settling and some of this sludge is recycled from the clarifier back to the reactor. The effluent can then be discharged to final polishing.
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MBBR Technology
Fluidized Media is a bio-carrier used to enhance wastewater treatment. Movable Bed Bio Reactor (MBBR) Media is porous that is ideally suited for immobilization of micro-organisms essential for degradation of environmental pollutants. The concept behind the MBBR reactor is to provide continuously operating bio-film reactor which is non-cloggable, does not require re-circulation of sludge. This is an achieved by making the bio-film grow in smaller carrier elements that move along with beads in the reactor. MBBR Media are kept in constant rotary movement by the air diffusers. The diffuser constantly keeps the bio-media moving at the same time provides the required oxygen to the bio-mass. Apart from making the plant compact, the MBBR reactor employing principle of the attached growth process makes the plant more user friendly because it does not require sludge recycle. In this technology, the sludge generation is small in quantity as compared to conventional treatment process.
The activated biomass developed in the MBBR Reactor will be separated from effluent in Tube Settler. The sludge from Tube Settler is taken to Centrifuge/Filter Press for solid- liquid separation and the Supernatant passes through Filter feed tank.
The sewage overflowing from the outlet launder of settling tank shall accumulate in the Filter Feed Tank. Here the supernatant shall be dosed with Sodium Hypo Chlorite Solution. The dose shall be set based on residual BOD post tube settler and the presence of colour or odour if any in the sewage. From supernatant tank, it is passed through Pressure Sand Filter, Activated Carbon Filter & Softener.
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SAFF Technology
The Submerged Aerobic Fixed Film Reactor (SAFF) is a low-cost wastewater treatment and sewage sanitation system that is commonly used in residential and commercial buildings. It is used to reduce the organic loading of residential and commercial sewage / wastewater, thereby lowering the Biological Oxygen Demand (BOD) and a significant amount of Suspended Solids (SS) that would otherwise contaminate river and sea outfalls if left untreated; in other words, it is used to improve effluent discharge quality significantly.
SAFF reactor is a type of aerobic attached growth treatment process. Small foot print area, stable process, lesser sludge production and modular installations are the key features of SAFF process. Essentially SAFF system is a hybrid reactor where attached growth and suspended growth activity takes place simultaneously.
This technology utilizes an aerobic fixed film process that is a combination submerged attached growth and activated sludge processes.
As with traditional sewage treatment, Submerged Aerated Fixed Films Technology too uses three stages of dealing with commercial and residential sewage / wastewater.
Primary Settlement : Where larger solids settle into the bottom of the primary tank and are removed periodically as sludge, and where other buoyant material which floats upwards is to be removed usually by a scraping/screening method.
Secondary Treatment: Where the biodegradable Organic Matter is degraded by the biomass on the SAFF media. Tubular diffusers are used with the SAFF media giving a good air dispersal and low level of blockage due to growth of biological film.
Final Clarification: Where degraded solids are settled out of the biological treated effluent.
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SBR Technology
The sequencing batch reactor (SBR) is a fill-and draw activated sludge system for wastewater treatment. In this system, wastewater is added to a single “batch” reactor, treated to remove undesirable components, and then discharged. Equalization, aeration, and clarification can all be achieved using a single batch reactor. To optimize the performance of the system, two or more batch reactors are used in a predetermined sequence of operations. SBR systems have been successfully used to treat both municipal and industrial wastewater. They are uniquely suited for wastewater treatment applications characterized by low or intermittent flow conditions.
The activated sludge system is triggered when sewage water is transferred to the SBR tanks. Following that, actions take place in a timely sequence, and the water is cleansed. The convenient sequenced actions are explained here as below :
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Filling : Water purification begins with this step. The influent wastewater is pumped into the tank in this stage. The SBR tank is filled with water. The water must be sufficiently filled to maintain an acceptable food-to-microorganism ratio.
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Aeration : This is the next step in which the reaction occurs. Microorganisms utilize the ammonia, nitrogen and Biochemical Oxygen demand (BOD). The rate at which the treatment works is directly proportional to the sludge mass and time is taken for aeration. Not only this, but it also depends upon the level of impurities and the degree of nitrification.
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Settling : The following stage takes around 60 to 90 minutes. It also depends on how many cycles are performed every day. The aeration ceases and the sludge settles at this stage, as the name implies.
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Decanting : This is the next step which is itself a process. Decanting is when the effluent is removed from the tank without disturbing the sludge settled at the bottom.
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Idling : Idling is described as the waiting time between the two filling cycles.
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Sludge wasting : The sludge accumulates in the tank after all of the procedures are completed. It must be expelled from the system once it reaches the threshold limit.
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MBR Technology
Membrane bioreactors (MBRs) are wastewater treatment methods that combine a perm-selective membrane, such as microfiltration or ultrafiltration, with a biological process, especially a suspended growth bioreactor.
MBRs differ from 'polishing' methods, in which the membrane is used as a tertiary treatment step with no active biomass returned to the biological process. Almost all commercial MBR technologies on the market today employ the membrane as a filter, rejecting the solid debris produced by the biological process and producing a cleared and disinfected product effluent.
A membrane bioreactor is essentially a version of the conventional activated sludge system. While the Activated Sludge process uses a secondary clarifier or settlement tank for solid/liquid separation, an MBR uses a membrane for this function. This provides a number of advantages relating to process control and product water quality.
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ADOX System
ADOX System based sewage treatment plant is used for wastewater treatment through the process of biological decomposition and drainage. It makes use of natural processes & proven technology to treat wastewater from household plumbing produced by bathrooms, kitchen drains, and laundry. A septic tank system has a relatively simple design. It is an underground watertight container (mostly rectangular or round) made of fibre glass, plastic or concrete.
In an ADOX System, sewage enters the septic tank into a settlement chamber, allowing solid waste (sludge) to sink and liquid to rise to the surface. This surface liquid makes contact with oxygen and the organic matter starts to breakdown biologically. This liquid still contains sewage but in small enough particles to be carried through the discharge outlet and into the ground.
Various components of ADOX System are :
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Septic tank : A septic tank will be connected with two pipes (for inlet and outlet). The inlet pipe is used to transport the water waste from the house and collect it in the septic tank. It is kept here long enough so that the solid and liquid waste is separated from each other. It will digest organic matter and separate float able matter (e.g., oils and grease) and solids from the wastewater .The second pipe is the outlet pipe.
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Settling Tank : The effluent pumped from final sump of septic tank to the primary settling tank. The effluent first passes through the flash mixer where the chemical dosing occurs to enhance the coagulation process in the settling tank. The effluent from flash mixer flows by gravity to the primary settling tank. The settling tank removes the suspended particles, inorganic material from the effluent which also reduces the inorganic and organic load partially.
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Filtration : In the second stage, the pre-filter proposed for fine filtration of the effluent coming from the primary settling tank.
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Advanced Oxidation System : After fine filtration the effluent passed through the Advanced Oxidation system. In ADOX System Advanced Oxidation is used to reduce the COD and BOD present in the Sewage Water. ADOX also reduces the organic load i.e. color and odor, also disinfects the sewage treated water which can be easily used for the Horticulture and Flushing purpose.
TERTIARY TREATMENT :
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Sand Filter
After the Filter Feed tank, there are residual suspended matter left in the treated sewage which contributes to the color as well as odor. These residual suspended solids shall be removed in the sand filter by passing through the five layers of graded sand present in the sand filter.
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Activated Carbon Filter
After removing the suspended particles in the sand filter the treated sewage passes through the activated carbon filter. The activated carbon filter contains the granular carbon which adsorbs the residual organic matter i.e. residual color and odor.
SLUDGE HANDLING SYSTEM
The liquid sludge is dewatered in Centrifuge by solid-liquid separation. The filtrate from Centrifuge is recycled back to bar screen chamber & thick sludge can be used as fertilizer. The treated output could be used for various auxiliary applications such as irrigation, gardening, floor washing, machine washing, toilet flushing etc., which in turn reduces on cost of intake water.
COMPARISON OF DIFFERENT TECHNOLOGIES OF STP
| MBBR/SAFF SYSTEM |
SBR SYSTEM |
MBR SYSTEM |
| System is based on attached growth system |
System is based on sequential batch reactor |
System combine a perm-selective membrane with biological growth process |
| System is proven |
System is relatively new |
System is relatively new |
| Low capital cost |
High Capital cost |
Very high capital cost approx. 3 times than others |
| Need approx. 30 % reduction in space than conventional STP |
Need approx. 40% more space than MBBR |
Less space requirement, about 40 % reductions in space |
| Operating cost same as conventional system |
Operating cost is slightly low |
Operating cost is very high |
| Approx. Rs. 15 – 20/m3 |
Approx. Rs. 12 – 18/m3 |
Approx. Rs. 30 – 35/m3 |
| Layman can operate the plant easily. |
Skilled person is required to operate the plant |
Skilled person is required to operate the plant |
| Treated water quality is colourless and odourless up to acceptable level |
Treated water quality is better than MBBR |
Membrane should be replaced within 3 years |
| Operation is easy |
Operation is complicated, need trained persons |
It cannot withstand shock load |
| All equipment and spares are local, so easily available |
Decanting system is imported, not easily available |
All equipment and spares are imported, so are not easily available |
