Process Sequence of Effluent Treatment Plant (ETP) in Textile Industry
Maruf Mahfuz Department of Textile engineering World University of Bangladesh Email: maruf.txt@gmail.com
What isEffluent Treatment Plant (ETP)? The effluent treatment plant (ETP) is a specialized facility designed to treat and purify industrial wastewater coming from different areas of the textile industry before its safe discharge into the environment. The treatment of different effluents varies with the type of effluent.
Water is recycled from effluent coming from textile and 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. Effluent also contains grease and oil, which produce foul odor when discharged in water, and it is not easily digested in sludge digestion tanks. 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 and 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 polyamides 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.
Dyeing effluents contain several types of pollutants, such as dispersants, leveling agents, salts, carriers, acids, alkali, and various dyes; wastewater quality is variable and depends on the kind of process that generates the effluent. Most environmental concern relates to the effluents of the dyeing and finishing processes that contain a variety of contaminations of higher concentration of chemical oxygen demand (COD), biological oxygen demand (BOD), suspended solids, organic nitrogen, and some heavy metals.
Effluent Characteristic from Textile Industry: Wet processing of textiles involves, in addition to extensive amounts of water and dyes, a number of inorganic and organic chemicals, detergents, soaps and finishing chemicals to aid in the dyeing process to impart the desired properties to dyed textile products. Residual chemicals often remain in the effluent from these processes. In addition, natural impurities such as waxes, proteins and pigment, and other impurities used in processing such as spinning oils, sizing chemicals and oil stains present in cotton textiles, are removed during desizing, scouring and bleaching operations. This results in an effluent of poor quality, which is high in BOD and COD load.
Highly colored, high BOD, oily appearance, High Suspended Solid, slightly alkaline, low BOD
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.
Figure 1: Water consumption of components of wet processing of textiles
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).
ETP System for Dyeing Industries: Textile dyeing industries need huge quantity of water for textile dyeing, which they normally pump out repeatedly from the ground or natural water sources resulting in depletion of ground water level.
In the dyeing process textile industries generate huge quantity of toxic effluent containing colors, sodium sulphate, sodium chloride, sodium hydroxide and traces of other salts. These are generated after dyeing and after washing of garments / fabrics. After dyeing the waste water produced is called Dye Bath water and after washing the waste water generated is called wash water. Dye Bath contains higher solids in the range 4-5% whereas wash water contains only 0.5-1% solids.
Based on the above mentioned fact “SSP” has developed a technology which can process such harmful toxic effluent water and transform it into reusable water. Thus the textile industries will have the advantage of using the same water in the dying process repeatedly; also the salt used for dyeing can be reused or sold in the market. The technology offered by SSP can overcome all problems pertaining to environmental pollution in respect to textile dyeing industries.
Need of Effluent Treatment Plant (ETP) in Textile Industry: Water is basic necessity of life used for many purposes one of which is industrial use. Industries generally take water from rivers or lakes but they have to pay heavy taxes for that. So it’s necessary for them to recycle that to reduce cost and also conserve it. Main function of this ETP is to clean GCP effluent and recycle it for further use.
The basic thrust of the technology is to convert entire quantity of effluent to zero level by separating water and salt using evaporation and separation technology. The concept and the treatment is based on the removal of the entire COD/BOD and the condensate coming out to meet the fresh water quality requirement in the process.
Effluent Treatment Plant Design: Textile industries (fabric dyeing and chemical treatment industries) are classified according to the Environmental Conservation Rules 1997 as Red category industries, and therefore an ETP must be designed and constructed to treat plant effluent. The effluent from the plant must meet the national effluent discharge quality standards, including the “Quality Standards for Classified Industries”, before discharge to the environment. These quality standards must be ensured at the moment of beginning trial production. The waste discharge standards differ according to the final disposal place of the effluent. The effluent standards are presented in Table of next paragraph. It is the DOE’s mandate to enforce this legislation, and this guide provides the tools required to assess the ETPs proposed by textile industries in the EMP/EIA.
Discharge Quality Standard for Classified Industries: There are various types of ETPs and their design will vary depending on the quantity and quality of the effluent, amount of money available for construction, operation and maintenance, and the amount of land available. There are three mechanisms for treatment which are: Physical, Chemical and Biological. These mechanisms will often be used together in a single ETP.
There are generally four levels of treatment, as described below:
Preliminary: Removal of large solids such as rags, sticks, grit and grease that may result in damage to equipment or operational problems (Physical);
Primary: Removal of floating and setteable materials, i.e. suspended solids and organic matter (Physical and Chemical);
Secondary: Removal of biodegradable organic matter and suspended solids (Biological and Chemical);
Tertiary: Removal of residual suspended solids / dissolved solids (Physical, Chemical and Biological)
There are many ways of combining the operations and processes in an ETP:
A properly designed biological treatment plant, which typically includes screening, equalization, pH control, aeration, and settling, can efficiently satisfy BOD, pH, TSS, oil and grease requirements. However the compounds in industrial effluent may be toxic to the microorganisms so pretreatment may be necessary. Most dyes are complex chemicals and are difficult for microbes to degrade so there is usually very little color removal.
Another option is a physico-chemical treatment plant, which typically includes screening, equalization, pH control, chemical storage tanks, mixing unit, flocculation unit, settling unit and sludge dewatering. This type of treatment will remove much of the color depending on the processes used. It can be difficult to reduce BOD and COD to meet effluent standards and it is not possible to remove TDS.
Most often, physico-chemical treatment will be combined with biological treatment. The typical components of such a plant are screening, equalization, and pH control, chemical storage, mixing, flocculation, primary settling, aeration, and secondary settling. The physico-chemical treatment always comes before the biological treatment units. Using a combination of treatments will generally reduce pollutant levels to below the discharge standards.
Another form of biological treatment is the reed bed, which can be used with a settling tank, or in combination with other treatment processes It presents a natural method of treating effluent which is often lower in capital, operation and maintenance costs. Reed beds can contribute to a reduction in color, a decrease in COD, an increase dissolved oxygen and a reduction in heavy metals, but function best with some form of pretreatment.
As discussed, there are many options for the design of an ETP. The type of plant and the various components of the plant will depend on the characteristics of the effluent. In evaluating an ETP design in an application for an ECC, it is necessary to determine whether the components of the ETP are sized correctly for the flow and to assess whether the effluent is likely to meet the requirements of the discharge standards.
Common Effluent Treatment Methods: Wastewater from the textile processing industry has a huge quantity of pollutants with a high concentration of organic compounds and color matter. Because of the high BOD/COD, coloration and salt load, wastewater drained from the textile processing industry is seriously polluted. Because aquatic organisms need light to develop and be sustained, colored water delivered from the textile dyeing industry destroys properties and leads to an imbalance in the ecosystem.
The preliminary treatment of wastewater decides whether it can be disposed of by surface or subsurface methods or dilution. However, additional treatment is necessary for that wastewater from a preliminary treatment plant to prevent further contamination based on its applications. Typically an effluent treatment plant (ETP) for a textile industry consists of primary, secondary and tertiary treatments to ensure the best performance. Physical, chemical and biological methods are used to remove contaminants from wastewater. To achieve different levels of contaminant removal, individual wastewater treatment procedures are combined into a variety of systems, classified as primary, secondary, and tertiary wastewater treatment. More rigorous treatment of wastewater includes the removal of specific contaminants as well as the removal and control of nutrients. The general classification of different effluent treatment methods used in the textile industry is detailed in Figure 2.
Figure 2: Effluent treatment methods
Physical treatment: Physical treatment, which is considered to be a primary effluent treatment of wastewater, represents the first stage in wastewater purification. Mainly suspended solids are removed during the operation that takes place at this stage. Among the first treatment methods used are physical unit operations, in which physical forces are applied to remove contaminants.
Sedimentation (clarification): This process is used to remove suspended solids from the wastewater with the help of gravity.
Screening: Floating matter such as paper and wood is removed from wastewater by this process with the help of mechanical bar screens.
Aeration: This process is generally applied to water bodies that experience pollution such as sewage mixing, agricultural runoff and overbaiting. It is commonly known as an anoxic condition of water bodies. Air infusion techniques are generally used for the aeration process from the bottom of the lake, pond or lagoon. Aeration can also be achieved by a surface agitation process with the help of a fountain or spray-like device, which allows wastewater to be exchanged with oxygen at the surface and noxious gases such as carbon dioxide methane and hydrogen sulphide to be released.
Filtration: Remaining suspended particles and unsettled flocks from wastewater are removed using a filtration process.
Flotation and skimming: Flotation is a unitary process in which solid particles in liquid suspension become attached to microscopic air bubbles, giving the air-solids agglomerate buoyancy. Under the right conditions, the agglomerate will rise to the surface to join other particles and form a blanket that can be removed by mechanical means. In waste treatment, flotation is used primarily to remove light suspended solids (oils, fats, etc.) that have a tendency to float, as well as waste solids with a wide range of specific gravities.
Chemical treatment: Chemical processes used in wastewater treatment are designed to bring about some form of change by means of chemical reactions. The chemical processes are combined and used with the physical unit operations and biological processes. The chemical treatment of wastewater is an additive process; it is the major disadvantage of a chemical process over a physical treatment. This process increases the net dissolved solid content of the treated water; hence it creates complexity when the wastewater is planned for reuse:
Chlorination: This method of treatment is used in wastewater to disinfect or destroy pathogenic organisms and prevent water from decomposition.
Ozonation: Ozonation introduces an unstable molecule to wastewater and gives up one atom of oxygen to the water molecule, converting it into a powerful oxidizing agent against most waterborne organisms.
Neutralization: Neutralization can be performed for industrial wastewater containing acidic or base (alkaline) substances that need to be neutralized before discharge into water.
Coagulation: The main principle of coagulation treatment is to precipitate ions (heavy metals) and colloids (organic and inorganic) in the wastewater stream by electrical charge or by adding chemicals. In this process, the addition of ions with an opposite charge destabilizes colloids; the coagulation of wastewater can be achieved. Chemicals such as alum [Al2(SO4)3·18H2O] have been used in wastewater treatment for ages for this purpose.
Adsorption: The porous matrix of granular activated carbon is one of the most commonly used adsorbents. In this process, chemicals from wastewater are physically attached to the surface of the solid body by a suitable adhesion method. The effectiveness of the adsorbent is directly related to the amount of surface area available to attract the particles of the contaminant.
Ion exchange: This technique has been used extensively to remove hardness and iron and manganese salts from drinking water. It has also been used selectively to remove specific impurities and recover valuable trace metals such as chromium, nickel, copper, lead and cadmium from industrial waste discharge. The process takes advantage of the ability of certain natural and synthetic materials to exchange one of their ions.
Biological treatment: Biological unit processes are used to convert finely divided and dissolved organic matter in wastewater into flocculent settleable organic and inorganic solids. In these processes, microorganisms, particularly bacteria, convert colloidal and dissolved carbonaceous organic matter into various gases and cell tissue, which are then removed into sedimentation tanks. These processes are also usually used in conjunction with physical and chemical processes, with the main objective of reducing the organic content (measured as BOD, total organic demand or COD) and nutrient content (notably nitrogen and phosphorus) of wastewater.
Aerobic: Under aerobic (O2) conditions, bacteria rapidly consume organic matter and convert it into CO2.The following treatment methods are in this category:
activated sludge
trickling filtration
oxidation ponds
lagoons
aerobic digestion
Anaerobic: This is a bacterial process for domestic sewage in septic tanks which normally retain sewage from 1 to 2 days, reducing BOD by about 35%–40%. The following treatment methods are in this category:
anaerobic digestion
septic tanks
lagoons
Overview of Stages in ETP Assessment Procedure: Shows the ETP assessment procedure. There are 3 stages for reviewing an ETP design and checklists are provided for each. As indicated, in any stage if the information provided for the proposed ETP is found to be inadequate, incorrect or outside the guideline values, the industry must be consulted to provide or correct the information.
Figure 3: Overview of Stages in ETP
Effluent Treatment Plant of a Garment Washing Unit:
Figure 4: Effluent Treatment plant of a Garments Washing unit
Description of Effluent Treatment Plant Process Sequence in Textile Industry:
Cooling and Mixing: After primary filtration, the liquor passes to cooling and mixing tank in which uniform mixing of effluents from various process takes place. A paddle mixer is provided for mixing. Cooling of the effluent may be done with the help of cooling tower.
Neutralization: The effluent is pumped to a tank in which it is neutralized by acid or alkali dozing. The tank has an automatic dosing controller which at automatically control the dose of acid or alkali to maintain the required PH .
Co-Agulation: Then the effluent is pumped to the co-agulation tank. Chemical co-agulation very effective for removal of color and suspended materials, aluminum, ferrous sulphates, ferric chloride, chlorinate dcopper etc. to increase the efficiency of co-agualtion, co – agulation gain may be added for example polyacrylate.
Setting and Separation of Sludge: Some of the soluble organic matter and light suspended solids will form a blanket of flocculent matter with the co-agulants. The blanket is skimmed of to another tank and the remaining solution is moved to pressure filter.
Pressure Filter: For pressure filtration vacuum pumps may be used to force through the filter and suspended flocks are collected in the pressure fine filter.
Discharging to Drain: After filtration the purified water sent to drain which eventually reach to the river or anywhere else.
Process Diagram of Effluent Treatment Plant (ETP):
Figure 5: Process Diagram o ETP
Process Description
1. Inlet launder: The purpose of launder is to flow the effluent of gas scrubber to distribution chamber Inlet channel is designed for a surge flow of 1950m3/hr @ slope of 2% so water flows at 1.5m/s(self cleaning velocity).Self cleaning velocity is that velocity at which if the sludge flows it will not get accumulated in the launder.
2. Distribution on chamber: Purpose of distribution chamber is to divide the flow (design flow of 1140m3/hr) into two equal flows. In case if one of the thickener is closed then there would be no distribution so selection of pipes is done on this criteria. The size of gates is designed such that there is equal distribution always.
3. Flash mixer: There are two flash mixers designed for a flow of 1140m3/hr with a retention time of 60 sec. So its volume must lie around 19m3. In flash mixer alum (coagulant) acts upon sludge so that suspended solids settle down. In addition pH of sludge is also raised by lime as it is required to have a pH of 7-9. Polyelectrolyte (flocculants) also acts upon to fasten the process of coagulation.
Figure 6: Flash mixer
4. Chemical action of alum and lime: Al2 (SO4)3.12H2O.2Al3+ + 3SO42- + 12H2O SO42-+H2O HSO4-+ OH- (Cause pH change) Ca (OH)2 Ca2+ + 2OH– (Cause pH change)
The basic water causes Al(OH)3 to precipitate bringing small particles with them and then making water clear. Fe2O3 is removed mainly by coagulation. The polyelectrolyte makes big lumps of the coagulated particles so they settle down.
Figure 7: Chemical dosing
5. Clarifier: The clarifier separates the treated slurry from clean water. The sludge settles down and cleans water at the top flows down to the cooling tower from where it is cooled and recycled. According to PG the SS content in this water must not be greater than 100 ppm. The clarifier has a racker arm which extracts the sludge out of clarifier. In case if sludge height goes higher than the racker arm then it will automatically lift up and then settle down taking sludge with it. From here sludge is pumped to sludge tank.
Figure 8: Clarifier
A General Structure of Clarifier:
Suspended Carrier Tank: In the first tank, organisms are grown on the inside of special plastic rings. This tank performs most of the treatment. The organisms appear as a thin brown film on the rings.
Figure 9: Suspended Carrier Tank
Sludge tank: In the sludge tank the sludge is continuously agitated in order to prevent settlement of sludge. Each tank has capacity of 224m3 and can hold for 8 hrs. Main purpose of the tank is to hold sludge for transfer to filter press. From sludge tank the sludge is pumped to filter press by filter press feed pump. In the second tank organisms which are suspended in the tank perform the rest of the treatment. The organisms are very small and appear as a fine brown sludge (called Activated Sludge) in the tank.
Figure 10: Sludge tank
Secondary clarifier: The third tank is a clarifier in which the suspended organisms are separated from the treated effluent by settling. The settled organisms are pumped back to the second tank to keep them in the system.
Figure 11: Secondary clarifier
Filter press: Sludge from the sludge tank will be pumped to the Filter Press equipments for dewatering purpose. According to performance guarantee the cake moisture should not be more than 20%. For this purpose different types of filters are used namely- gravity setters, gravity belt filters, centrifuges, vacuum or pressure belt filters and filter press. But among these filter press is most efficient and economical. Other filtration systems offer high pressure filtration, but only the filter press has both high pressure capability and efficient filter cake removal. The filter elements are constructed of lightweight polypropylene. They are extremely corrosion resistant and virtually eliminate plate breakage.
Figure 12: ETP filter press
Filter process:
Figure 13: Filter process
Polishing: The treated effluent from the clarifier is further treated by flocculation with chemicals followed by Dissolved Air Flotation. This step polishes the effluent before discharge to the river.
Figure 14: Polishing
Dewatering: Dewatering is accomplished by pumping a slurry or sludge into chambers surrounded by filter membranes. As pumping pressure is increased the filtrate is forced through the accumulated filter cake and membrane until the chamber is full of solid filter cake. The chambers are formed by two recessed plates held together under hydraulic pressure. The hydraulic ram moves the follower against the stack of filter plates closing the press. The ram continues to apply sealing pressure of sufficient force to counteract the high internal compaction pressures.
Figure 15: Dewatering
The head stock and tail stock are held in place by specially engineered side rail support bars. The filtrate passes through the membrane and is directed by channels in the plates and drain ports to the head stock for discharge. The filtrate typically contains less than 15 PPM suspended solids. The filter cake is easily removed by simply reversing the hydraulic ram, thus opening the press. The lightweight plates may then be moved apart, permitting the compacted cake to fall from the chambers. Higher the internal pressure, the greater the solids compaction. The standard press is constructed to withstand 100 PSI compaction pressure producing a hard dry cake. The special high pressure press can withstand 225 PSI for sludge more difficult to dewater.
Ozone Treatment for Textile Effluent Treatment Plant: The use of ozone in textile effluent treatment appears to be a very attractive alternative with considerable application potential. Ozone is a powerful oxidizing agent when compared with other well knows oxidizing agents. Ozone is capable of causing the degradation of dyes.
Advantages of Ozone Generator in Textile Industry Effluent Treatment Plants:
Ozone reduces COD.
Ozone reduces BOD.
Ozone removes Color.
Ozone eliminates Odour.
Ozonation increases the biodegradation effectiveness.
Decomposes rapidly, leaving no harmful byproducts.
Increase efficiency of Filter.
Benefits of Ozone Generator in Textile Industry Effluent Treatment Plants:
Due to its unstable physical property, it should be generated at the point of application for use in treatment purposes.
After chemical oxidation residual ozone reverts to oxygen.
Environment friendly gas.
Can be retrofitted to existing and new treatment plant.
Low operating cost.
Easy to operate and handle.
Conclusion: The textile industry is one of the leading sectors in the Bangladeshi economy as it contributes nearly 14 percent to the total industrial production. The untreated textile wastewater can cause rapid depletion of dissolved oxygen if it is directly discharged into the surface water sources due to its high BOD value. The effluents with high levels of BOD and COD values are highly toxic to biological life. The high alkalinity and traces of chromium which is employed in dyes adversely affect the aquatic life and also interfere with the biological treatment processes. The quality of such effluent can be analyzed by their physicochemical and biological analysis. Monitoring of the environmental parameters of the effluent would allow having, at any time, a precise idea on performance evaluation of effluent treatment plant and if necessary, appropriate measures may be undertaken to prevent adverse impact on environment. The obtained results were very much useful in identification and rectification of operational and maintenance problems and it can be also utilized to establish methods for improved.
References:
Characterization and Treatment of Textile Wastewater By Himanshu Patel and R.T. Vashi
Sustainability in Denim Edited by Subramanian Senthilkannan Muthu
Pollution Control in Textile Industry By S. C. Bhatia
Manual of Environment and forestry Division Under ministry of environment of Bangladesh
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
