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effluent treatment plants

Effluent Treatment Plant or ETP is a wastewater treatment method which purifies industrial wastewater for its reuse. The Primary Aim for an ETP Plant is to release safe water to environment. Industrial effluents contain various materials, depending on the industry. Some effluents contain oils industrial by-products, metals, radioactive wastes and grease, and some contain toxic materials (e.g., cyanide). Effluents from food and beverage factories contain degradable organic pollutants. Since industrial wastewater contains a diversity of impurities and therefore specific treatment technology called ETP is required. 

The effluent treatment plant is designed to treat the effluent coming from different areas of the plant. The treatment of different effluents varies with the type of effluent. Water is recycled from effluent coming from textile & chemical industries using series of operations i.e. coagulation, flocculation, aeration, and filtration techniques mainly reverse osmosis. The effluent produce has high BOD, COD, pH, TSS, TDS and Color material. This study includes characterization of effluent and making of process flow sheet of Effluent Treatment Plant after visit to various locations in industrial areas. Points of optimization were identified in various unit operations involved considering the total cost incurred during the whole process. It was identified that automation and use of highly substantive dyes during coloration stages (dyeing & printing) in a textile mill considerably reduces the amount of effluent produced. Effect of different mesh sizes of coagulating agents was (also) studied in conjugation mixing speed. It was noted that use of polyphosphazene membranes instead of poly-amides for reverse osmosis plants, as they posses better resistance at high pH and temperature.

Nature of Effluent:

Waste generated in textile industry is essentially based on water- based effluent generated in the various processes. Textile industry originates a complex huge volume of waste water containing various chemical used in dyeing, printing and finishing processes. Many dyes which causes intensive color in the waste water. The effluent generated in different step or processes is well beyond the standard and thus it is highly polluted and dangerous.

Water Consumption in Textile Processing:

The production of textile goods involves spinning (fiber to yarn), weaving / knitting (yarn to fabric), chemical (wet) processing, and garment manufacturing. The majority of the water consumption (72%) takes place in the chemical (wet) processing of textiles. The water is required for preparing the fabric for dyeing, printing and finishing operations, Intermediate washing / rinsing operations and machine cleaning.

Other major uses of water in the textile industry

  • Steam generation (boiler feed water)
  • Water treatment plant (reject stream, periodic cleaning of reverse osmosis plant,regeneration and  washing of demineralization, softener plant, back wash of media filters);
  • Cooling (processing machines, cooling tower);
  • Humidification (spinning process); and
  • Domestic purposes (irrigation of lawn and garden, sanitation, cleaning, drinking and miscellaneous uses).

What are the Benefits of Effluent Treatment Plant?

  1. To clean industry effluent and recycle it for further use
  2. To reduce the usage of fresh water in industries
  3. To preserve natural environment against pollution
  4. To meet the standards for emission of pollutants set by the Government
  5. To reduce expenditure on water acquisition
About 97% of the Earth's water is undrinkable. And another two percent is tied up in glaciers and ice caps. And the remaining one percent is left for our own needs.

The Step by Step Effluent Treatment Process:

1. Preliminary Treatment:

The treatment of effluents varies with the type. The Main Objective of Preliminary Treatment is to remove large sized contaminants. The Preliminary method consists of four stages screening (removal of floating solids) , sedimentation(removal of suspended solids using gravitational force), grit chamber(removal of gravel & metal frames to prevent operational difficulties), clarifiers (for Biological waste deposition)

2. Primary Treatment

The Primary Treatment consists of both Physical & Chemicals which includes Coagulation (addition of coagulants), Neutralization (sets the pH Range from 6-9), Primary Clarifiers.

3. Secondary or Biological Treatment

Residual organic effluents are removed in this step using biological and chemical process. Activated Sludge Process, Aeration Pool, Trickling Filters, Biological rotating contactor.

4. Tertiary or Disinfection Treatment

The final treatment including chemical coagulation, filtration (filter blocks are used to ensure high quality of water), Reverse Osmosis, UV Disinfection

1. Screening:

Screening is a mechanical process that separates particles on the basis of size. There are several types, which have static, vibrating or rotating screens. Openings in the screening surfaces range in size depending on the nature of the waste. In the case of textile dyeing industries they should be small enough to catch pieces of cloth, which may damage process equipment, reduce the effectiveness of the ETP or contaminate waterways.

2. Equalization/Collecting Tank:

The many steps in the textile dyeing process (pre-treatment, dyeing and finishing) mean that wastewater quality and quantity varies considerably over time, however ETPs are usually designed to treat wastewater that has a more or less constant flow and a quality that only fluctuates within a narrow range. The equalization tank overcomes this by collecting and storing the waste, allowing it to mix and ensuring that it becomes less variable in composition before it is pumped to the treatment units at a constant rate. The purpose of equalization for industrial treatment facilities are therefore:
To minimize flow surges to physical-chemical treatment systems and permit chemical feed rates that are compatible with feeding equipment.
To help adequate pH control or to minimize the chemical requirements necessary for neutralization.
To provide continuous feed to biological systems over periods when the manufacturing plant is not operating.
To prevent high concentrations of toxic materials from entering the biological treatment plant.
Mixing usually ensures adequate equalization and prevents settle able solids from depositing in the tank. In addition, mixing and aeration may result in the oxidation of reduced compounds present in the waste-stream or the reduction of BOD.

3. Sedimentation and Filtration:

The flocs formed in flocculation (see chemical unit processes for a description of flocculation) are large enough to be removed by gravitational settling, also known as sedimentation. This is achieved in a tank referred to as the sedimentation tank, settling tank or clarifier. Sedimentation is also used to remove grit and suspended solids, to produce clarified effluent, and to thicken the sludge produced in biological treatment. Flocculation and sedimentation should remove most of the suspended solids and a portion of the BOD.

4. Chemical Unit Processes:

Chemical unit processes are always used with physical operations and may also be used with biological treatment processes, although it is possible to have a purely physico-chemical plant with no biological treatment. Chemical processes use the addition of chemicals to the wastewater to bring about changes in its quality. They include pH control, coagulation, chemical precipitation and oxidation. Niagara authority use Fe(SO4), Calcium carbonate(lime),HCL, Electrolyte(polymer).

5. pH Control:

Waste from textile industries is rarely pH neutral. Certain processes such as reactive dyeing require large quantities of alkali but pretreatments and some washes can be acidic. It is therefore necessary to adjust the pH in the treatment process to make the wastewater pH neutral. This is particularly important if biological treatment is being used, as the microorganisms used in biological treatment require a pH in the range of 6-8 and will be killed by highly acidic or alkali wastewater. Various chemicals are used for pH control. For acidic wastes (low pH) sodium hydroxide, sodium carbonate, calcium carbonate or calcium hydroxide, may be added among other things. For alkali wastes (high pH) sulfuric acid or hydrochloric acid may be added. Acids can cause corrosion of equipment and care must be taken in choosing which acid to use. Hydrochloric acid is probably better from an environmental view point but can corrode stainless steel therefore plastic or appropriately coated pumps and pipes must be used.

6. Aeration Tank: 

Aeration is required in biological treatment processes to provide oxygen to the microorganisms that breakdown the organic waste. It may also be applied in the equalization tank to provide mixing and to reduce oxygen demand by oxidizing the compounds present in wastewater. Two main methods are used for this: either mechanical agitation of the water so that air from the atmosphere enters the water; or by introducing air into the tank through blowers (to supply air) and diffusers (to diffuse the air uniformly).

7. Coagulation and Flocculation:

Coagulation is used to remove waste materials in suspended or colloidal form. Colloids are particles over a size range of 0.1 – 1 nm (10-8 – 10-7 cm). These particles do not settle out on standing and cannot be removed by conventional physical treatment processes. In a small sample of wastewater there will be both settleable solids and dispersed solids. A significant portion of these non-settleable solids may be colloidal. Each particle is stabilized by negative electric charges on its surface, causing it to repel neighboring particles, just as magnetic poles repel each other. Coagulation is destabilization of these colloids by neutralizing the forces that keep them apart so that they can agglomerate 2, 3 (come together). This is generally accomplished by adding chemical coagulants and mixing. Figure 2 illustrates how these chemicals reduce the electric charges on colloidal surfaces.

The term ‘flocculation’ may be taken to cover those processes whereby small particles or small groups of particles form large aggregates. Flocculation during wastewater treatment converts finely divided suspended solids into larger particles so that efficient, rapid settling can occur. The term is also used for the dramatic effect when polyelectrolytes are added and large stable flocs are formed very quickly (Figure 3).

8. Secondary Treatment/ Secondary Clarifier: 

The objective of secondary or biological treatment of industrial wastewater is to remove, or reduce the concentration of, organic and inorganic compounds. Biological treatment processes can take many forms but all are based on microorganisms, mainly bacteria. These microorganisms use components of the effluent as their “food” and in doing so break them down to less complex and less hazardous compounds, thus decreasing the BOD and COD. In the process the microorganisms increase in number.The two most common forms of biological treatment are:
Activated sludge plants (ASPs)
Biofilm based systems (often these are trickling filter systems).
Activated sludge (AS) is an aerobic flocculent slurry of micro-organisms which remove organic matter from wastewater and are then removed themselves, usually by sedimentation. Activated sludge is best suited to the removal of soluble organic matter because insoluble organic matter can usually be removed more economically by physico-chemical means. Often, however, wastewaters will contain both soluble and insoluble organic mater.

AS leaves the reactor with the treated effluent but is settled out in a clarifier and returned to the aeration unit to recycle the bacteria. The amount of AS required for effective operation varies according to the design of the ASP and the concentration and nature of the effluent being treated. Suppliers of ASPs should be able to advise on the optimum amount of AS in the system. The ratio of the amount of effluent in terms of BOD and the amount of AS (known as the f/m ratio) is an important design and operating parameter and sometimes ASPs are operating so as to maintain a fixed f/m ratio. If there is more AS in the ASP than desired a portion is removed (this is called wasting) and disposed of.

In the ‘traditional’ trickling filter, the effluent is tricked over the surface of the slime-covered media and the space between the particles of media is occupied by air which passively diffuses through the filter (Figure 3). In the more recent design of the submerged fixed film reactor, the particles of media are submerged in the effluent and the air is blown into the reactor from below.

Both activated sludge and fixed film systems can produce high quality effluent but both have advantages and disadvantages. In the AS process the settling and recycling of AS to the aerobic reactor is vital, but the settling process can be difficult to accomplish. Fixed film systems do not require recycling of biomass and so do not present this problem. Surplus AS needs to be disposed of: this material must be disposed of appropriately so that the pollutants now present in this sludge do not enter the water cycle (see the briefing note “Management of Textile Dyeing Sludge” produced as part of this series for more information on this). The treated liquid is discharged to the environment or taken for further treatment depending on the desired standard of effluent quality or the required use of the wastewater.

9. Sludge collection Tank & Discharge Treated Water:

Sludge is produced from the treatment of wastewater in on-site (e.g. septic tank) and off-site (e.g. activated sludge) systems. This is inherently so because a primary aim of wastewater treatment is removing solids from the wastewater. In addition, soluble organic substances are converted to bacterial cells, and the latter is removed from the wastewater. Sludge is also produced from the treatment of storm water , although it is likely to be less organic in nature compared to wastewater sludge. In Niagara they collect the sludge in the sludge bad and they store it at least 3 to 4 month.After that they put that sludge in small packet and transfer that packet at another place where they had planted these in the ground.

Sludge collection Tank: On another hand they dis charge their final treated water into the local canal. They don’t use the water by recycling . They told us for their lacking of modern technology of ETP they can’t use the water by recycling.

Advantages of Effluent Treatment Plants :

  1. Facilitates ‘economy of scale’ in waste treatment, thereby reducing the cost of pollution abatement for individual SMEs addresses the ‘lack of space’ issue – Effluent Treatment Plant can be planned in advance to ensure that adequate space is available including plans for expansion in future.
  2.  Homogenization of wastewater.
  3. Relatively better hydraulic stability.
  4. Professional control over treatment can be affordable.
  5. Facilitates small scale units, which often can not internalize the externalizes due to control of pollution.
  6. eliminates multiple discharges in the area, provides opportunity for better enforcement i.e., proper treatment and disposal.
  7. Provides opportunity to improve the recycling and reuse possibilities.
  8. Facilitates better organization of treated effluent and sludge disposal etc.

Problems and constraints:

Consistency in compliance to the prescribed standards by the Effluent Treatment Plants is constrained by many reasons, such as:

  1. Operating on ‘one-size-fits-all-basis’.
  2. Lack of access to capital investments, working capitals, specialized technical skills, inconsistent effluent quality from member industries.
  3. Improper management of treatment units at common facility.
  4. Varied nature and scale of the industries, along with the addition of industries in a haphazard manner, without proper planning.
  5. No provision to tackle the fluctuations in the pollution load and quantities, at individual member industries.
  6. No separate treatment units to deal with hazardous and toxic effluents, etc.
  7. Effluent Treatment Plants serving similar kind of industries will have greater operational ease due to their similar nature of characteristics, which will also facilitate in choosing the right treatment scheme with greater certainty. Whereas, the Effluent Treatment Plants serving effluents from heterogeneous industries require greater knowledge and skill for consistent compliance to the prescribed standards.

Austro Water Tech India's Leading ETP Plant Manufacturer

Austro is a Leading Effluent Treatment Plant Manufacturers in India. We have designed and commissioned more than 50 ETP plants in South India Region. We have a proven expertise of textile effluent treatment. An Effluent Treatment plants helps in the removal of contaminants from wastewater & industrial effluents. ETP plant serve for industries like Textile, Medical, Food & Beverage where the water contains waste such organic wastes, suspended solids, bacteria, surfactant, colorant which are collectively called as Effluents and these can be recycled for various purposes. We offer solutions for both standard and customized Effluent Treatment Plants.You can also visit our effluent treatment plant projects.

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