The process of extracting contaminated items from waste water is known as sewage treatment. Starting from household sewage to effluents of a manufacturing unit, every waste is treated by sewage treatment plant. Any type of physical, biological and chemical contaminants are removed by including chemical, biological and physical processes. The basic goal of this type of plant is to free the water from contaminants by saving the nature and use the solid waste for suitable reuse or disposal.
The goal of sewage treatment is to process sewage to the point that it will meet the Effluent Discharge Standards of the local authority, or be suitable for re-use, such as use for irrigation. To begin the discussion of the Sewage Treatment Process, it is necessary to know the composition of the sewage. Typical characteristics for domestic sewage are shown in the table below. A brief description of the terms, such as "BOD" and "TSS" is given at the end of this section.
Water and Wastewater Treatment Lecture. Source : NV atCEPImperial
The Sewage Treatment Process
Pretreatment
Pretreatment refers to all the treatment processes done prior to Primary Treatment. Some pretreatment is done right at the sewage source, prior to collection in the sewer system. This includes grease traps at restaurants, oil separators at mechanical shops, lint traps for commercial laundries, and various other types of industrial pretreatment depending on the type of industry.
Pretreatment may also take place at the Sewage Treatment Plant (STP) itself. Depending on the characteristics of the sewage, pretreatment processes at the STP may include grease removal using a grease trap, and screening using a bar screen to remove larger solids such as rags. If there is a lot of sand or grit in the sewage, a grit chamber may also be installed.
Primary Treatment.
A schematic diagram of a typical sewage treatment process is shown in the figure above. After the sewage has been pretreated, it flows into a large tank, called a Primary Clarifier or settling tank, where Primary Treatment takes place. Primary Treatment consists simply of providing a tank with quiet conditions so that any solids in the water are given a chance to settle, and any grease or oil present can float. The size of the tank required to achieve primary treatment can be reduced substantially by installing plates, called lamella plates, in the tank which help to create the necessary quiet conditions.
For smaller STP’s, the tank where the primary treatment takes place usually also serves as the place where the sludge is stored. A common type of Primary Clarifier for small STP’s, for example, is a septic tank.
Primary Treatment reduces the Total Suspended Solids (TSS) in the water by about 50 to 75% and reduces the Biochemical Oxygen Demand (BOD) by about 25 to 35%.
Secondary Treatment.
The readily removable solids are removed in the Primary Treatment process, but lighter solids and dissolved substances remain in the sewage. These are removed in the Secondary Treatment process, in which bacteria are grown which consume these substances. The resulting solids (bacteria) are then removed in a Final Clarifier.
The growth of the bacteria takes place in a tank, and they may be grown either attached to a surface, called a "fixed film" process, or grown in suspension, called an "activated sludge" process. The bacteria which accomplish the sewage treatment require oxygen to be healthy. There are many types of aeration systems for providing air to the bacteria, including blowers, surface aerators, paddle wheels, and pumps with air injection. For most fixed film treatment plants, the media on which the bacteria grow is first submersed in the sewage, and then brought out into the air, which provides air to the bacteria. In other words, the bacteria is brought to the air, not vice versa.
As the bacteria consume the pollutants in the sewage, they grow, and the excess bacteria flow into the Final Clarifier, also called a final settling tank or humus tank. As with the Primary Clarifier, this consists of a tank providing quiet conditions where the bacteria settle. There is usually a weir at the top of the clarifier, where the clear effluent passes over, and out of the STP.
For most small treatment plants, the settled bacteria mass is pumped back to the Primary Clarifier, as shown in the schematic diagram above, where the bacteria settles and is stored along with the primary solids.
Tertiary Treatment
For many sewage treatment applications, the process described above will be sufficient. However additional treatment may be required either to meet the requirements of the local authority, or to treat the water for another purpose such as irrigation.
In some municipalities, for example, if the effluent will be discharged into the ocean, the local authority may require the removal of the nutrients nitrogen and phosphorus to low concentrations. This is because nitrogen and phosphorus could promote unwanted algae growth in the receiving water. The removal of nitrogen is accomplished by bacteria in a two step process. In the first step, called nitrification, organic nitrogen and ammonia present in the sewage are converted to nitrate. This is followed by the second step, called denitrification, in which bacteria convert nitrate to nitrogen gas, under low oxygen conditions. The nitrogen gas is discharged to the atmosphere. Phosphorus removal can also be accomplished by bacteria, but for small plants it is normally removed by precipitating it with a chemical solution containing an iron or aluminum salt.
The addition of nutrient removal processes to an STP will increase the cost of the STP substantially, usually by at least a third, and the operation and maintenance requirements are also increased.
If the effluent will be used for irrigation, the degree of further treatment required will depend on the type of irrigation system, what it is that is being irrigated, and the requirements of the local authority. In order of increasing treatment, the degree of additional treatment required might be: no further treatment, filtration only, or chemical precipitation followed by settling and filtration.
Disinfection
At most small treatment plants, disinfection is accomplished by chlorination, either using chlorine tablets or using a small mixing tank in which dry granular Calcium Hypochlorite is dissolved and then dosed into the effluent using a metering pump. To be effective, there should be a small chlorine contact tank downstream from the point where the chlorine is added, to give the chlorine time to disinfect, before the effluent is discharged.
Ultraviolet (UV) light may also be used for disinfection. It has the advantage of leaving no residual in the effluent, which is beneficial when the effluent will be discharged into a sensitive environment. Disinfection with UV light has a higher capital cost and higher operation and maintenance requirements than disinfection using chlorine.
Sludge Removal and Disposal
Sludge removal and disposal is a key consideration for all STP’s and it is especially important for those in remote areas. The frequency of removal varies for the different types of plants. For activated sludge plants, sludge removal is done every day to every few days, and so the plants are often built with a supplementary sludge storage tank or sludge drying beds, where the sludge removed from the plant itself is stored or dried for later removal. Fixed film plants usually have sludge storage incorporated in the design, and the frequency of removal ranges from one month to a year or more.
Sludge removal is often accomplished with a vacuum tanker truck, but if none is available, a sludge pump, or gravity can be used. Sludge disposal depends on the local facilities available. If there is a dump site nearby, this is a common disposal method. Other methods include application to land, drying followed by application to land, composting followed by application to land, or drying followed by disposal at a dump site. Sludge, in either liquid or dry form, is an excellent soil amendment.
Sewage Recycling Plant
The Sewage Treatment Plant process is similar to the way that a Septic Tank works but mechanical components provide a process to help break down solids to produce a cleaner, more environmentally friendly effluent.
Wastewater and sewage, usually from a number of properties, are fed into the primary settlement tank where solids and liquids separate and the liquor flows into the biozone chamber. In the chamber, a pump aerates the waste and encourages good bacteria to digest the organic matter, breaking it down and purifying it.
Sewage is a resource that can be recycled for various uses like gardening, toilet flushing, car wash, cooling towers. Faced with the alarming fact that over 60% of the sewage generated in our country goes untreated, with its negative impact on human health, the government is focusing on waste management through centralized sewage treatment plants and by mandating large upcoming real estate projects to have their own treatment facility.
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Features
Design allows for quick turnaround time for delivery and installation
Wastewater treatment system is simple to operate and requires low manpower
Effective aerobic operational principal
Pre-engineered, pre-fabricated structures result in lower cost
Unit is easily transported to the project site
User friendly
Low & easy maintenance
Regulatory compliant
Custom design / application specific systems
Long service life
Applications
Mobile home parks
Remote mining, logging, and construction sites
Manufacturing facilities, power plants, military bases
Schools and other educational campuses
Government compounds
Land development / housing / subdivisions
Small and medium sized cities
Recreational areas such as parks, campgrounds, marinas
Low flow / high strength and high flow / low strength applications
Biological waste water treatment for industrial wastewater flows
Specifications
Standard Models Capacity : 10 up to 500 m3/day in single unit
Smaller Plants Capacity : 1m3 per day
STP (Sewage Treatment Plant)
The process of extracting contaminated items from waste water is known as sewage treatment. Starting from household sewage to effluents of a manufacturing unit, every waste is treated by sewage treatment plant. Any type of physical, biological and chemical contaminants are removed by including chemical, biological and physical processes. The basic goal of this type of plant is to free the water from contaminants by saving the nature and use the solid waste for suitable reuse or disposal.
The goal of sewage treatment is to process sewage to the point that it will meet the Effluent Discharge Standards of the local authority, or be suitable for re-use, such as use for irrigation.
We Design, Fabricate, Supply, Erection and Commissioning Sewage Treatment Plants (STP) for treating sewage generated by Industry, large colonies, Hotels, Hospitals, IT Parks and commercial buildings.
Now a days the Sewage Treatment Plant (STP) became statutory requirement of all the State Pollution Control Boards, everybody, whether Hotelier or Industrialist were looking for economical, easy to install and operate compact type Sewage Treatment Plant. Since the land is extremely expensive, very few industries could afford the large treatment units as recommended by most of the Consultants.
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