Effluent Treatment Mehod

PREFACE

From the very beginning of human civilization nature serves human being a lot. As a mother nature care human. Nature recovers a lot of pollution from the very beginning of earth. As the energy transfer from one stage to another by means of different action, results no wastage of energy, similarly there was no word as wastage to environment. Actually when wastages can not control by the environment then that wastages are termed as pollutants. When a zone’s pollutants concentration begins to high then we called that zone is polluted. Definitely pollution is marched on with the civilization. For this reason some says technology destroys environment but their dark eyes disable to watch that technology protect environment. Technology always runs for hunting solution in a scientific manner. An effluent treatment method is one of its hunts which bring a protection for green environment. Bangladesh will become a developing country in the near future. Textile trade makes a significant contribution in the GDP of Bangladesh. But being an agro based country its environment demands pollutants free environment. As the textile industries discharged effluents directly to the nature Effluent treatment planning is the burning question in almost every industries. This effluent is coming from the wet processing unit of a textile industry. As we will be the future technologist at wet processing unit we have decided gather a clear concept on effluent treatment. We think only technologists from wet processing unit can control the effluent pollution. For this reason we have decide that our project work’s subject should be “STUDY ON EFFLUENT TREATMENT METHOD”. Through this project work we gained a significant knowledge on effluent treatment methods which will help us to supervise wet processing activities.

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Acknowledgment

First of all we are very much grateful to almighty ALLAH . We thank – I QBAL MAH MUD (A GM C OMFI T C OMP OSIT E KNI T L TD.) M ASOO D D AWOOD AKBA NI ( MD ACS T EX TIL ES BAN GLA DES H L TD. ) O VAIS A KBANI (D IRE CT OR A CS T EXT ILE S BAN GLA DES LTD.) for giving the opportunity to work and study in their organizations. For sharing all sorts of information and encouragement, we are indebted to Pr of f. DR . Md. MAH BUBU L H AQUE . Last of all we would like to show gratitude to our departmental head PR OF F.DR . M UST AFIZ UR R AH MAN. for his directions and assistance. Without his supervision it would be impossible for us to finish the study and report successfully. We are extremely enlightened by this project work.

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Aim of the project

The aim of the project is to • Gather knowledge on effluent treatment methods. • Study the effluent treatment methods. • Sort out the advantages of each method. • Study the chemistry of each method. • Study the cost of treatment method. • Study the space consumption of each method.

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Introduction: Textile wet processing unit involves a variety of chemicals comprising a various class of dyes and other chemicals along with huge amount of water the waste water resulting from dyeing operation various threats for aquatic life about forty thousand to eighty thousands tons of dyes and 1500 to 1800 pigments are estimated to discharge in a year by textile processing units due to in complete exhaustion washing operation etc. such waste streams generated from the process house needs suitable treatment prior to there disposal as a legal requirement . now the conservations of chemicals become a most important aspect for environment specially in consideration of the pollution phenomenon and increasing cost of chemicals in order to make the industry much more competitive in the globalize context for that reason liquid waste management of textile wet processing industry approach calls for waste volume reduction by the product recovery and an identification of most appropriate treatment option. In the case of Bangladesh near about 550 wet processing industry present and 80% of these are cotton dyeing industry on the other hand according to world bank report more than 700 wet processing units are needed to feed the fabric in the garments industries as a result it may raised about to 90 percentage will be cotton based wet processing industry so a large amount of effluent will be produced from cotton dyeing industry. For these reasons effluent treatment planning is a burning question for a wet processing unit. In order to solve these problems as a textile technologist of our mother land our present study deals with an application of physiochemical, biological methods for effluent treatments. The project title is study on effluent treatment methods. Basically effluents are those wastages discharging to hydrosphere of environment. It may come from residence and or wet processing industries. Wastewater discharged from a textile wet processing plant contains various types of impurities depending on the type of 4

raw materials, dyes, chemicals, auxiliaries and process used. Some of these impurities are considered toxic while some are not. Off course the toxicity or harmfulness also depends on the amount present in a certain amount of processed or wastewater. Various countries have different standards for acceptable level of toxicity for various purposes. Different types of water application also require different level of acceptable toxicity. For example water is used for drinking purposes, irrigation in the fields, in various types of textile, chemical, food processing, leather processing and pharmaceutical industries, and also to maintain the aquatic life in the canals and rivers. In all these cases different level of purity in terms of toxicity and harmfulness are required.

Effluent’s Character Wastewater discharged from a textile wet processing plant contains various types of impurities depending on the type of dyes, chemicals, auxiliaries and process used. Some of these impurities are considered toxic while some are not. Off course the toxicity or harmfulness also depends on the amount present in a certain amount of wastewater. Various countries have different standards for acceptable level of toxicity for various purposes. Different types of water application also require different level of acceptable toxicity. For example water is used for drinking purposes, irrigation in the fields, in various types of textile, chemical, food processing, leather processing and pharmaceutical industries, and also to maintain the aquatic life in the canals and rivers. In all these cases different level of purity in terms of toxicity and harmfulness is required. This is mainly intended for wastewater generated in the various textile wet processing industries. Therefore, the present discussion will mainly be concentrated on wastewater generated in various types’ textile-processing industries. Table 1 shows the acceptable level of various parameters of wastewater generated from textile wet processing industry. This is Bangladesh Standard. The level of toxicity was considered for water intended to be discharged into river or canals. The wastewater generated in a textile wet processing industry can be recycled back for consumption in the plant or even can be used for drinking purposes but this is expensive enough. So wastewater is generally discharged into the river or canals. Table 1: Characteristics of wastewater to be discharged into the environment. (stipulated by the Dept. of Environment, Government Of Bangladesh) PH BOD

6-9 < 50 PPM or mg/L 5

COD TSS TDS OIL & GREASE COLOUR TEMPERATURE

< 200 PPM or mg/L <100 PPM or mg/L < 2,500 PPM or mg/L < 10 PPM or mg/L CLEAN < 30 0C

The typical characteristic of wastewater generated in a textile wet possessing plant is given in table 2. Parameters have been referenced about a range rather than a particular value of the parameters. This is because the characteristics of textile wastewater for a factory are not always same which may be due to the variation of raw materials, dyes, chemicals and process. For example a factory sometime process 100% cotton and sometime process 50/50 cotton & polyester blend or even 100% polyester. The three different cases will require two different dyes and chemicals of varying quantity. For white goods no dyes are used at all, in that case too the effluent characteristics will be different from that of dyeing effluent. For woven (sized) fabrics the effluent characteristics will be different from that of knit fabrics. The values shown in table 2 are not actual tested values of a particular industry rather they are average of various anticipated wet processing pollution characteristics. The values are assumed on the basis of experience about the contaminants present in a textile wet processing industry. Table 2: Characteristics of wastewater of a typical textile wet processing industry. PH BOD COD TSS TDS OIL & GREASE COLOUR TEMPERATURE

8 –14 400 - 600 ppm 800 - 1,200 ppm 200 - 500 ppm 3,000 - 6,000 ppm 30 – 60 ppm Dark Mixed up to 60 0 C

As was mentioned textile wastewater may contain various types of contaminants but in most cases the toxicity of the above eight parameters are considered important before discharging them into the environment. The parameters are now discussed below Biochemical Oxygen demand (BOD): The strength of the wastewater is often determined by measuring the amount of oxygen consumed by microorganism like bacteria in biodegrading the organic matter. Biochemical Oxygen Demand (BOD) is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it could be considered as an indication of the quality of a water source.

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BOD measures the rate of oxygen uptake by micro-organisms in a sample of water at a temperature of 20°C and over an elapsed period of five days in the dark.There are two recognized methods for the measurement of BOD.

Dilution method To ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionized water. The BOD test is carried out by diluting the sample with de-ionized water saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample (to prevent further oxygen dissolving in). The sample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD the apparent BOD for the control is subtracted from the control result to provide the corrected value. The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called the BOD5. BOD can be calculated by: • •

Undiluted: Initial DO - Final DO = BOD Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor

Manometric method This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxidation is inhibited. The sample is kept in a sealed container fitted with a pressure sensor. A substance absorbing carbon dioxide (typically LiOH) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed and, as ammonia oxidation is inhibited, carbon dioxide is released. The total amount of gas, thus the pressure, decreases because Carbon dioxide is absorbed. Form the drop of pressure, the electronics computes and displays the consumed quantity of oxygen. The main advantages of this method compared to the dilution method are: • • •

its simplicity: no dilution of sample required, no seeding, no blank sample direct reading of BOD value continuous display of BOD value at the current incubation time

Furthermore, as the BOD measurement can be monitored continuously, a graph of its evolution can be plotted. Interpolation of several graphs on a similar water may build an experience of its usual evolution, and allow an estimation of the five days BOD after as early as the first two days of incubation. 7

Microorganisms such as bacteria are responsible for decomposing organic waste. When organic matter such as dead plants, leaves, grass clippings, cellulose components, manure, sewage, organic waste like size, dyes, fats and oils, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. When this happens, the bacteria rob the available dissolved oxygen necessary to survive by the other aquatic organisms like fishes. If there is a large quantity of organic waste in the water supply, a large number of bacteria present in the water body will be working to decompose the waste. When the bacteria consume organic waste they require oxygen. Under this circumstance the demand for dissolved oxygen (DO) will be very high so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline. Nitrogen and phosphates in a body of water can also contribute to high BOD levels. Nitrates and phosphates are plant nutrients and can cause plant life and algae to grow quickly. When plants grow quickly, they also die quickly. This contributes to the organic waste in the water, which is then decomposed by bacteria. This results in a high BOD level. However in some effluent treatment plant TSP & UREA have introduced for the growth of bacteria. The temperature of the water can also contribute to high BOD levels. For example, warmer water usually will have a higher BOD level than colder water. As water temperature increases, the rate of photosynthesis by algae and other plant life in the water also increases. When this happens, plants grow faster and also die faster. When the plants die, they fall to the bottom where they are decomposed by bacteria. The bacteria require oxygen for this process so the BOD becomes high in that location. Therefore, increased water temperatures will speed up bacterial decomposition and result in higher BOD levels. Textile mill wastewater possesses a very high BOD like 400 – 600 mg/l. It is necessary to reduce this BOD value up to a level less than 50 mg/l before discharging them into the environment like canals or rivers. If water of high BOD are discharged into the sea or very large river then off course the concentration of BOD decreases due to dilution and have little or no harmful effect on the aquatic life or environment. Therefore if it is possible to discharge a highly toxic effluent in sea or large river no treatment is necessary. It should be noted that BOD can reduce naturally where effluent density is lower. Say for example shitalaxma a river in Bangladesh having sufficient current and volume of water but it possesses a higher density of effluent so the aquatic life of this zone are threatened by textile effluent. Dissolved Oxygen (DO) The amount of oxygen present in a certain amount of water in dissolved state is known as Dissolved Oxygen (DO). It is normally expressed as mg/l, water may contain DO ranging from 0 to 18 mg/l but in most cases of normal waters, DO lies between 7-9 mg/l. Aquatic lives require certain level of DO to survive in the water and the DO level require to survive in the water varies from one species to another. Even if we talk about fish, some fish require more DO to survive than some other fishes. Chemical Oxygen Demand (COD):

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COD is based on the fact that nearly all-organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. When some wastewater is discharged into a water body, the organic compounds are oxidized by means of the dissolved oxygen present in the water as a result the level of DO falls This is a means of measuring the ability of wastewater to sustain aquatic life, essential for the preservation of the environment. It also enables proper assessment of treatment plant performance. Aquatic organisms and animals require dissolved oxygen to flourish. The Chemical Oxygen Demand (COD) test gives an indication of the impact of discharge waters on aquatic life by measuring the oxygen depleting nature of the discharge water. The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. The amount of oxygen required oxidizing an organic compound to carbon dioxide, ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of ammonia into nitrate. The process of ammonia being converted into nitrate is referred to as nitrification. The following is the correct equation for the oxidation of ammonia into nitrate.

The second equation should be applied after the first one to include oxidation due to nitrification if the oxygen demand from nitrification must be known. Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test.

Potassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by the addition of sulfuric acid.) The reaction of potassium dichromate with organic compounds is given by:

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Where d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25 N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred. In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all redox reactions, one reagent is oxidized and the other is reduced), forming Cr3+. The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample One limitation of COD is that it cannot differentiate between biologically active and those which biologically inactive. One major advantage of COD over BOD is that COD can be measured in just three hours where as BOD measurement takes at least five days. The value of COD is always higher than BOD, this is because BOD accounts for only biodegradable organic compounds while COD accounts for all organic compounds e.g. biodegradable as well as non-biodegradable but chemically oxidized. Total suspended Solids (TSS): TSS is mainly organic in nature, is visible and can be removed from the wastewater by physical/ mechanical means e.g. screening and sedimentation. TSS is measured by filtering a certain quantity of effluent and then drying the filtrate at certain temperature e.g. 1050C followed by weighing. TSS is expressed as parts per million or in milligram/liter. The pore size of the filter paper is very important in estimating the TSS, the nominal pore size 1.58 micro meters. Total Dissolved Solids (TDS): TDS are the solids that are actually in solution, mean a homogenous mixture. Dissolved solids generally pass through the system unaffected. TDS -is the sum total of all of the dissolved things in a given body of water. It's everything in the water that's not actually water. It includes hardness, alkalinity, chlorides, bromides, sulfates, silicates, and all manner of organic compounds. Every time we add anything to the water, we are increasing its TDS if it is soluble. This includes not only sanitizing and pH adjusting chemicals, but also conditioner, algaecides, and tile and surface cleaners. TDS also includes airborne pollutants and other waste as well as dissolved minerals in the fill water. TDS is referred to as the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, and is expressed in units of mg per unit volume of water (mg/L), or as parts per million (ppm). Some dissolved solids come from organic sources such as leaves, silt, plankton, and dyes and chemicals used in processing. Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water forming ions. Ions

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are particles that have a positive or negative charge. Water may also pick up metals such as lead or copper as they travel through pipes used to water distribution. The effectiveness of water purification systems in removing total dissolved solids will be reduced over time, so it is highly recommended to monitor the quality of a filter or membrane and replace them when required. TDS may be the most misunderstood factor in the whole field of chemical processing and public health. In most cases it is misunderstood because no one knows exactly what effect it is going to have on any particular body of water. TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse. Different standards advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million (ppm)) for TDS, however for domestic water suppliers maintain the TDS within 150 ppm. Off course some water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water. At low levels, TDS does not present a problem. In fact, a certain amount of TDS is necessary for water balance. Hardness and Total Alkalinity are both part of TDS. For textile processing the acceptable value of TDS is around 65-150 mg/l. The standard for bath and swimming pool is between 1,000 and 2,000 ppm, with a maximum of 3,000 ppm. For irrigation the acceptable values of TDS are around 1500 ppm. Use of fertilizers increases TDS of the environment. When the water evaporates, it leaves behind all of the solids that had been dissolved in it. This principle is used widely to measure the TDS of a particular body of water. When everything else seems to be all right, and the water still acts unlawfully, check the TDS. High TDS can result in corrosion of metal equipment and accessories, even though the water is balanced. High TDS can cause eye and skin irritation, even though the pH is right and there are no chloramines in the water. High TDS can permit an algae bloom, even with 2-3 ppm chlorine residual. If we drink water of high TDS some of this will stay in the body, causing stiffness in the joints, hardening of the arteries, kidney stones, gall stones and blockages of arteries, microscopic capillaries and other passages in which liquids flow through our entire body. For effluent treatment reducing TDS is expensive so we must follow the minimization of TDS during wet processing. Dyes and the Environment: The loss of dyes to effluent can be estimated to be 10% for deep shades, 2% for medium shades and minimal for light shades how ever the loss of dyes is mainly depending on the

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class of dyes. Dyes are present in the effluent at concentrations of 10 mg/l to 50 mg/l with 1 mg/l being visible to the naked eye. Dyes are complex organic compounds which are refractory in aerobic treatment systems. Some contain metals such as Cr, Cu and Zn. In the aquatic environment, dyes can undergo bio-concentration, ionization, abiotic oxidation, abiotic and microbial reduction and precipitation. The ionic dyes such as acid, direct, basic and metal complex dyes will not volatilize whereas, in principle, solvent, disperse, vat and sulphur dyes have the potential to be volatile. Sorption should also play a major role as dyeing is a sorption process. Hydrolytic reactions are not important because if the dyes survive the biological treatment processes, it is unlikely to degrade rapidly in the environment. Photochemical reactions may be important, as dyes are good absorbers of solar energy. Aquatic plants will not be able to produce food by the process of photosynthesis. As a result their life will be endangered. It is expected that anionic dyes would react with ions such as calcium and magnesium to form insoluble salts and thereby reduce the concentration available for other biological reactions. Redox reactions should also be considered, as in early vat dyeing processes, the dyes were reduced microbially before chemical replacements were introduced. Reduction in the environment would most likely occur under anaerobic conditions, however, the difficulties of working with anaerobic systems has limited research in this area. In general, there is very little literature available on the environmental behavior of dyes. This is probably due to the lack of suitable analytical techniques. Two methods are widely use for discoloration. One is physical sedimentation of dyeing by coagulating and flocculating here dyes remove completely. Ozonization is applicable for discoloration mean breaking pie bond of chromophoeric group of dyes but dyes may remain in discharge water. Temperature: Temperature of water is a very important factor for aquatic life. It controls the rate of metabolic and reproductive activities, and determines which aquatic species can survive. Different aquatic species require different quantity of DO to survive in the water. Temperature inversely affects the rate of transfer of gaseous oxygen into dissolved oxygen. On the other hand at higher temperature the metabolic rate of aquatic plants and animals increases producing an increase in oxygen demand. International regulations related to water temperature and aquatic life classifies water, as "Class 1 Cold Water Aquatic Life" should never have temperatures exceeding 20°C, while waters classified, as "Class 1 Warm Water Aquatic Life" should never have temperatures exceeding 30°C. These regulations also state that temperature for these classes shall maintain a normal pattern of day to day and seasonal fluctuations with no abrupt changes and shall have no increases in temperature of a magnitude, rate, and duration deemed harmful to the resident aquatic life. Generally, a maximum 3° C increase over a minimum of a 4-hr period, lasting 12 hrs maximum, is deemed acceptable. Temperature preferences among aquatic species vary widely, but all species tolerate slow, seasonal changes better than rapid changes. Respiration of organisms is temperaturerelated; respiration rates can increase by 10% or more per 1° C temperature rise. Therefore, increased temperature not only reduces oxygen availability, but also increases oxygen demand, which can add to physiological stress of organisms. Increased

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temperature influence the activity of bacteria and toxic chemicals secreted by the bacteria in water. OIL & GREASES: The term oil and grease, as commonly used, includes the fat, oils, waxes, and other related constituents found in wastewater. Oils and fats are mainly due to the sizing process and also as oils and grease come in contact with the fabric during processing. Apart from this small amount oils is found in the cellulose fibers. These oils and fats are removed during scouring process and finally pass with the wastewater. If the wastewater contains oils and fat, it forms a layer at the top surface of the wastewater. As a result the oxygen cannot come in contact with the water and becomes difficult to increase DO level. Oil & grease discharged into the environment typically has deleterious effects. Oily wastes discharge may have objectionable odours, cause undesirable appearance, burn on the surface of receiving water creating potential safety hazards and consume dissolved oxygen necessary to forms of life in water. Bioassay data indicate that oil is toxic to fish. In greater quantities, it limits oxygen transfer, hindering biological activity. Oils and grease affect respiration of fish by adhering to the gills, it adhere to and destroy algae and plankton. Feeding and reproduction of water life (plant, insect, and fish) is affected by oils and fat. Aesthetics is affected by sheens of oils. Table 7 shows that the level of oil and grease in textile wastewater is above the standard and needs to be reduced. It is not clear about the denim data as additional fats/oils are added during the sizing of denim process so that the oil and grease of denim plant effluent should be very high and must be higher than that of knit dyeing plants

TYPES: There are basically three types of effluent treatment methods. They are, 1. The physico-chemical method. 2. The biological method. 3. The combine method.

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Comfit Composite Knit Ltd. Location: Comfit Composite knit Ltd. is located at Gorai Industrial Area Mirzapur, Tangail. Comfit composite knit LTD. has newly joined for textile service among the world. This is fully export oriented knit composite Textile industry. This manufacturing industry deals with so many known buyers like H&M, G-Tex, AMC, and Hense and so on. This manufacturing industry is well organized with maintaining all the compliance and environmental issue. They established a biological Effluent Treatment Plant with latest technology. They treated 100 meter cube of effluent per hour. Their wet processing unit is of 13 ton/day capacity. For the better quality they use a high liquor ratio than the suggestion of dyeing machine manufacturer. So they have to treat more effluent than the theoretical demand. Effluent character of Comfit Composite Knit Ltd. is discussed; Mainly they use following chemicals for wet processing 1. Detergent (non-ionic). 2. Anti creasing agent. 3. Hydrogen per oxide. 4. Caustic soda. 5. Soda ash. 6. Common salt 7. Glaubers’ salt 8. CLR ( known as organic exhausting agent for dyes) 9. Per oxide killer. 10. Washing off agent. 11. Acetic Acid. 12. Softener. 13. Anti foaming agent 14. Optical brightener. 15. Hydrose

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16. Strong alkaline soap (NSR). Without these they use a lot of chemicals those vary due to order requirement. They use only two types of dyes; reactive dyes & disperse dyes. Dyes and chemicals are soluble in water or in colloidal state. Some of suspended solid like wastages are also flow with raw effluent. They are mainly cotton fibers’ flocks or the yarn. The raw color of effluent is brown/ blue / black. All the liquors are coming through same drain of wet processing unit. So rinsed liquor, dye liquor, soaping liquor, scouring liquor, acid liquor all are getting mixed from the very beginning. The temperature, pH, BOD & COD of raw effluent liquor is given in the following table. In Bangladesh basically these four criteria are controlled for textile effluent.

Basic characteristics of raw effluent Sl NO

CHEMICAL CHARACTERISTICS PARAMETERS

1. 2. 3. 4.

pH Suspended solids Biological Oxygen Demand(BOD) Chemical Oxygen Demand(COD)

UNIT

AMOUNT

mg/L mg/L mg/L

8.2 200-400 180 417

PHYSICAL CHARECTERISTICS 5. 6. 7.

Color Odour Temperature

Brown to black Not distinct 48

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Picture of raw effluent

THE BIOLOGICAL METHOD FOR TREATING TEXTILE EFFLUENT Nature recovers a lot of pollution from the very beginning of earth. As the energy transfer from one stage to another by means of different action, bacteria are very much essential living being of earth. It helps us a lot to prevent pollution. Actually when wastages can not control by the environment then that wastages are termed as pollutants. In textile wet processing basically organic compounds are used to control process. How ever inorganic compounds are also essential. Mainly two methods are applicable to remove such organic compounds one is by coagulation which is followed by flocculation and to sediment. And another is degradation of organic compound by means of bacteria. Here mainly organic compounds are used to be the food of bacteria nothing else. Here sludge are formed which are sediment by means of gravity. Later the sludge are separated and deposited in sludge tank. But bacteria can not control the pH value and can not reduce the chemical oxygen demand (COD). For this manner some of acid addition is required. If acid is not required to neutralize the effluent it must require for providing acidic medium for bacterial action. In biological treatment COD is controlled by aeration by means of blower. The biological method for effluent treating is discussed below according to the ETP of Comfit Composite Knit Ltd. Limited.

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Aeration tank

sludge returning tank

Equalization tank

Screening

Neutralization tank Storage tank Picture: cross section of screening to aeration tank.

Screening: Screen is the synonyms of filtration. Here action is nothing but the filtration or separation of suspended solid from the liquor or raw effluent. Separately three screening chamber is used to filter suspended solid. Drain from two different units has a net of iron having 1 sq. inches of each hole. It separates the different foreign materials like bulk of trees, leaves, polyethylene bag etc. picture will describe better.

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Picture: filtration of suspended solid by iron net To treat 100 cubic meter per hour, here three screening chamber are used. Suspended solid which can pass through the first filter are finally filtered here. The screen has around 250-300 slits per sq inches. This screening system has automatic wiping action with four wipers or brush. Cotton fibers, yarns with the liquor are deposited on the screen and raw effluent passes through the slits. This is a simple filtering method. The screen is curved around 90 degree angle (ie, quarter part of circle). Curved screen provide a strong control of wiping by rotating wiper. Materials deposited here are calculated for a cubic meter effluent load, which is, 276 mg. there is a tray just above the screen in which suspended solids are deposited. Suspended solid

Picture: deposition of suspended solid from the screen Wiper is rotating at an interval of 9 minutes for 3 minutes at 1.5 rpm speed. Its rotation speed duration of pause is regulated on the basis suspended solid’s load. The next picture will provide a clear conception. Tray deposited suspended solid Screen

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Picture: Screening of raw effluent. Raw effluent Wiper After screening of raw effluent raw effluent is stored in storage tank.

Storage tank: Raw effluent is stored after screening in the storage tank. There are two storage tanks. Delivery of raw effluent from storage tank is carried out by means of submergible pump. It has an automatic lifting plan to deliver raw effluent into the next section of ETP. This automation is programmed according to the effluent load in the storage tank which ensures an uniform flow of effluent for every section. The pump gets off automatically after lifting a certain volume of effluent which is varied according to effluent load. In each turn this lifting pump arrangement is allowed to discharge 50 cubic meter of stored effluent. In between two storage tanks there is a bi-pass channel. Through which stored effluent can pass from one store to another. Bi-pass is used when any of the lifting pumps is off for maintenance.

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Pictures: storage tank 1& 2

Picture: delivery from storage tank.

Equalization chamber:

Pictures: equalization tank

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The term equalization means to make equity. Here equalization means to form an identical effluent of different characteristics. The effluent comes from storage tank are mixed homogeneously. The effluents from different stages like scouring, dyeing, soaping etc. are mixed homogeneously here. That’s why it is also termed as Homogenizing chamber. The treatment of equalization chamber is basically depending on character of effluent. Here temperature of effluent is decreased. This chamber equalized with respect to its characteristics, homogeneity, flow and a uniform pollution load as well as to make bacteria acclimatized. In this ETP two vertical agitator and flow jet are used homogenize to mix the effluent. These two agitators are not using as they can not carry any advantage for the treatment. Water jet is carrying out the mixing action continuously and it is placed at the bottom of the chamber. If the mixing is not accomplished homogenously then it will hamper the further treatments resulting inefficient treatment. As the temperature of the effluent is higher than the atmospheric temperature it is necessary to be reduced to meet the temperature demand of the bacterial action as well as the environment. The volume of effluent treated is 2000 cubic meter. Here no cooling mechanism is added as this chamber possess 200 sq meter of open area which is directly in contact with air. Again the effluent is treated here for 20 hours which is another reason for the effluent being cooled. So the cooling action for the effluent is carried out naturally. It should be noted that the movement of molecules provide reduction of their internal energy. Due to the flow jet the effluent keeps on flowing which results the reduction of the thermal energy thereby dropping temperature.

Neutralization Chamber:

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Neutralization refers to preparing neutral. Here neutralization stands for neutralizing the effluent solutions’ pH value. First of all we are discussing about pH. A term used to express the intensity of the acid or alkalinity source. pH represents the effective concentration (activity) of hydrogen ions (H+) in water. This concentration could be expressed in the same kind of units as other dissolved species, but H+ concentrations are much smaller than other species in most waters. The activity of hydrogen ions can be expressed most conveniently in logarithmic units. pH is defined as the negative logarithm of the activity of H+ ions: pH = -log [H+] Where [H+] is the concentration of H+ ions in moles per liter (a mole is a unit of measurement, equal to 6.022 x 1023 atoms). Because H+ ions associate with water molecules to form hydronium (H3O+) ions, pH is often expressed in terms of the concentration of hydronium ions. In pure water at standard atmospheric temperature, H3O+ and hydroxyl (OH-) ions exist in equal quantities; the concentration of each is 1.0 x 10-7 moles per liter (mol/L). Therefore, pH of pure water = -log (1.0 x 10 -7) = -(-7.00) = 7.00. Because pH is defined as –log [H+], pH decreases as [H+] increases (which will happen if acid is added to the water). Since pH is a log scale based on 10, the pH changes by 1 for every power of 10 changes in [H+]. A solution of pH 3 has an H+ concentration 10 times that of a solution of pH 4. The pH scale ranges from 0 to 14. However, pH values less than 0 and greater than 14 have been observed in very rare concentrated solutions. The U.S. Environmental Protection Agency (U.S. EPA) sets a secondary standard for pH levels in drinking water: the water should be between pH 6.5 and 8.5. But the Department Of Environment (BD) suggests the pH value in between 6- 9. Very high (greater than 9.5) or very low (less than 4.5) pH values are unsuitable for most aquatic organisms. Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5 and may die at these low pH values. High pH levels (9-14) can harm fish by denaturing cellular membranes. Changes in pH can also affect aquatic life indirectly by altering other aspects of water chemistry. Low pH levels accelerate the release of metals from rocks or sediments in the stream. These metals can affect a fish’s metabolism and the fish’s ability to take water in through the gills, and can kill fish fry. In cotton dyeing industry effluent is always alkaline then water in nature. So it is required to reduce the pH value. For reducing, acid dozing is essential according to the pH and effluent character. Here in this ETP no acid dozing is required for neutralizing. As the effluent possesses a standard value of pH ie, 8.2 where as the standard value of pH is 6-9 (Recommended by

22

DOE) for treated effluent. But there is a pipe line for acid dozing. If the pH value rises than the limit 8.9 then automated acid dozing will be started. This automation is controlled at panel board. In panel board there is an alarm to inform operator. However from the beginning of ETP they did not need to apply acid for neutralization. This due to their high liquor ratio at every stage of wet processing and washing water, rinsing water, dyeing water etc. are directly discharged to ETP. It should be informed that they use a lot of water for rinsing purpose. In neutralization tank effluent is kept for 6 minutes and volume of effluent is 10 cubic meter.

Sludge return tank: First of all we are discussing about sludge. The settable solids separated from the effluent during sedimentation (clarification). This is simply the degraded part of polymers. The sludge is very toxic in nature and needs to be dealt with very carefully. Under no circumstances it should be mix with the environment again Sludge return tank is not common among biological effluent treatment plans. How ever it brings profit for a knit dyeing unit. We know that effluent discharging from knit dyeing unit possesses low quantity of food for bacteria. For that manner here developer decided to recycle the sludge. Here not only sludge is returned but also food for bacteria is also provided. And Sulphuric acid is also given here. As for bacterial action pH around 6.5.

Picture: sludge return tank

Nutrients delivery Sludge returning Acid dozing

23

In sludge return tank sludge, nutrients and acids are given for the bacterial action. It is about 60 cubic meters in volume. And effluent is treated here for 36 minutes. As nutrient urea is provided, at 2.706 gm per cubic meter of effluent, TSP at 1.875 gm per cubic meter of effluent and 98% pure sulphuric acid at 72 cc per cubic meter of effluent.

Picture: sludge return tank. Effluent flows from neutralization tank to sludge return tank by means of two pumps. This addition of sludge return is a beneficial step for environment. And quality of treatment results best by means of bacteria. If the sludge does not return then a significant amount of bacteria may die due to food, as a result bacteria can not get when it requires. Sludge returning helps bacteria to run its life cycle. It should be noted that manufacturer demanded that sludge is a poor food for bacteria then the polymeric compounds found in effluent. As knit dyeing process possesses less BOD, that’s why sludge retuning is essential. But bacterial treatment can not degrade dyes totally.

Aeration tank: In aeration tank basically aeration is occurred by means atmospheric air. Air from atmosphere is firstly filtered then blower sucks them and flows air to the aeration tank through 860 diffusers. There are three blowers are used to perform their function. Every blower is running for 16 hours in the set of two. And every blower pauses blowing for 8 hours every day. Usually two blowers are running all the time to blow 1500 cubic meter of fresh air per hour. 860 diffusers are uniformly distributed around the 470 sq meter area.

24

air filter blower Picture: blowers for aeration.

Picture: aeration system to tank

Picture: Cross sectional view of aeration tank. Diffusers are kept at bottom of the tank floor. Blower takes air from atmosphere and then it passes though pipe to the diffusers. Then diffusers supply air as well as oxygen to the effluent. These blowers possesses the 25% cost of the plan. Blowers are highly stronger than the normal. These blowers are micro chipped program with the reader JUMO GMBH (it is a dissolve oxygen reader). When the dissolved oxygen comes up to 7.6 mg/L then the inverter of blowers’ motor worked to reduce rpm of blower. These are the description of plan for aeration tank. Now the question how does the function of air? This aeration does nothing but increase the quantity of dissolve oxygen. 4700cubic meter effluent is treated here for 47 hours, it’s a huge time. During this time oxidative chemicals are liked to fill up their oxygen demand from the water. That’s why increasing amount of dissolved oxygen is required to meet the demand. Without this the aquatic life in this tank ie, bacteria demands oxygen which is also filled up by this dissolved oxygen. These are all about air, at aeration tank. During aeration bacteria is also involved to degrade effluents. Next chemical reactions will describe better.

25

The bacterial action: COHNS

+ O2 + Bacteria + TSP & UREA

CO2 + NH3 + Energy + Other end Products

TSP and Urea is used as food for the microorganism COHNS + O2 + Bacteria C5H7NO2 + 5O2

C5H7NO2 (new bacteria) CO2 + NH3 + 2H2O

Apart from the above basics reaction there are some other reactions that take place in the aeration tanks. During aeration the oxygen reacts with C, S and N which is shown below. C + O2 CO2 S + O2 SO2 N + O2 NO2

26

Picture: the aeration tank

Picture: the delivery from aeration tank Oil separation is an important part of an effluent treatment plan. Every where this kind of simple technique is used. In picture the oil separation technique is described. We know that the oil is lighter than water due to density. Here effluent passes through these two pipes. These are pipes having too many holes. Holes are started from the bottom to 8 meter of this tanks’ height. But the height is 10 meter. This results the effluent to pass just below from the liquid surface. So lighter oil cant passes but the liquid water with sludge can pass through the holes. As a result oil is floating at the surface. When a significant amount of oils are layered then manually these are removed from the tank to the sludge tank. Aeration tank of this plan play the chief role for treating effluent. Here polymers are degraded, but only biodegradable polymeric chemical compounds can be degraded. Manufacturer of this plan may appreciate for this simple technique but for environment dyers must need to use selective chemicals. But it is appreciated for knit dyeing industries as the knit dyeing process possesses almost eco friendly chemicals. But for printing like industries, effluent characters can not support this type of effluent treatment plan. 27

Lamella clarifier:

out let of treated effluent Picture: Lamella clarification inlet of effluent (Red line shows the effluent entrance direction & the green shows the treated effluents’ exit direction) This is a special kind of clarification system of sludge from treated effluent. Mainly sludge is denser than the water, which results its sedimentation at the bottom of the chamber. There are eight units for clarification they are separated from each other by thin tin sheets. It has eight clarification chambers. Treated effluent comes out through over flow and the sludge sediment at the bottom of lamella clarification system. From aeration tank effluent with sludge comes through pipe which is joined with the lamella clarification chambers.

28

Cross sectional view of Lamella clarification system Effluent with sludge Effluent without sludge Sludge

Wiper Outlet of sludge Sludge Wiping ground Lamella Filtration tank’s ground

Outlet Drain Tin walls

29

Shaft and motor arrangement for wiping The cross sectional view of the lamella clarification system is representing two chambers only and the rest follows the same principle. Here the figure shows that after the input of effluent from the aeration tank it passed to the chambers through inlet pipes due to gravity. The ground of the chambers is sloped down to ease the sludge through the sludge holes. The flow rate through the sludge hole is much lesser than the inlet flow due to the followings. • • • • •

The hole is much narrow than the width of filtration tank. The layers of deposited sludge at the hole. Very short area between the sludge outlet and the wiping ground. Wiper is rotating at very low rpm (1 revolution/12 minute). Sludge return time from the Lamella sheets is very short.

These reasons cause an effective upward flow of the effluent. The lamella sheets are parallel to each other and inclined with the wall at an angle of 45 degree. This arrangement of lamella clarification provides sludge down ward action to the ground of tank which results passing of effluent without sludge. Thus sludge sedimentation is occurred by the lamella clarification system. Effluent without sludge is over flowed above the tank due to the hydraulic pressure involving in the tank. According to Department of environment Bangladesh this effluent without sludge is dischargeable to environment. But internationally some standard demands colorless effluent. For this manner manufacturer of this plant suggest an ozonization treatment for discoloration. So we may not mention this effluent as treated effluent. Over flowed effluent without sludge is then passed to ozonization chamber. Sludge wiping technology is very simple here. Wiper is rotating at .0833 rpm that is 1 revolution per 12 minute. Sludge deposited on eight different place on the wiping ground. In the circular wiping ground there is a blank space for to fall down sludge which are wiped up. And from there marshy sludge is pumped in to the temporary sludge tank. The following picture will describe better

30

Sludge tank: Treated effluent directly goes to the ozonization chamber and the sludge goes to sludge return tank through pump and pipe arrangement. Here sludge is recycled. If the sludge concentration rises at 70% then the sludge is deposited at sludge tank. The concentration of sludge is measured regularly from the liquid of aeration tank delivery. The flow of sludge is marked at next picture

31

sludge deposite here before returning

Sludge tank

Lamella chambers Picture: Marooned line shows the path of sludge returning to a temporary sludge tank from where sludge is delivering to sludge returning tank or to sludge tank. The passage for sludge is under grounded. Magenta arrow shows the pipe to sludge retuning tank. Green arrow shows the under grounded drain to the ozonization chamber. Sludge concentration measurement is too simple. In sludge measuring cone effluent from the aeration tank delivery is taken and then it is kept on a table for 15 min. if the sludge sedimentations rises up to 70 then its concentration is 70%. The sludge concentration of this plant is recorded 45% as its highest concentration. When the concentration of sludge rises to 70% then sludge is allowed to sediment at sludge tank. In sludge tank there is a sand filtration system in which 2 feet of sand layer is kept and above that sand sludge is

32

deposited. At the ground of tank there is a pipe arrangement with filtration which allows effluent without sludge passing to the ozonization tank. This is all about sludge tank.

Ozonization: It is a new technique for discoloration. Ozone gas is prepared at first then it is passes through the effluent without sludge. Due to business secrecy manufacturers are not like to describe its machine principle. Here we can learn one thing that is ozone is used to discolor effluent. The chemistry of this principle is very simple. We know that pie bond is responsible for hue in dyes. By means of ozone gas this pie bond is broken down. O R-C=C-R + O3

R- C O

C O

Picture: the ozone chamber and the discharged effluent After ozonization the effluent is totally color less which is mentioned as treated effluent. After this treated effluent is directly discharged to the river Hatu bhanga through a cannel.

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Results of Ef flue nt T reatme nt (biological)

SL NO.

Effluent Quality parameters

Concentration present in raw effluent

Concentration present in treated effluent

mg/L

mg/L

Bangladesh Quality Standard at Discharge Public Inland Swear at Irrigated surface secondary land water treatment plant (WASA) mg/L mg/L mg/L

1

pH

8.2

6.5

6-9

6-9

N/A

2

Biological Oxygen Demand (BOD)

180

16

50

250

100

417

42

400

400

3 4

Chemical Oxygen Demand (COD) Time

200 67 hours 12 min

Costing: Chemical cost per day = 6650 tk Power cost per day= 78 tk Man power cost per day= 1000 tk. Total = 7728 tk per day. They processed 1800 cubic meter per day so cost per cubic meter= 3.22 tk

Advantages of biological method:

34

1. 2. 3. 4. 5. 6.

It possesses a low maintenance cost. It can process colorless water. It is highly modernized with the latest technology. Its mechanism is simple enough. Theoretically it is simple as well as practically. It is best for the effluent treatment plan for any kinds of textile industries.

Disadvantages of biological method: 1. It has a very high fixed cost around 50000000 tk rather than the physico chemical method 2. Treatment through this method possesses a huge area around 30 kathas. 3. Non-biodegradable chemicals can not treat here. 4. Its maintenance needs skilled professionals. 5. Dyer needs to select chemicals with respect to its effluent treatment. 6. It possesses relatively high treatment duration around 67 hours 12 mins.

ACS TEXTILES (Bangladesh) Ltd. Location: ACS TEXTILES (Bangladesh) LTD. Is located at Tetlabo, Rupganj, Naryanganj. 35

ACS TEXTILES (Bangladesh) LTD. has newly joined for textile service among the world. This is fully export oriented woven composite Textile industry on 50 bigha of land. This manufacturing industry deals with so many known buyers like Ham-Tex, Springs, Tesco, Nortic form, ladud and so on. This manufacturing industry is well organized with maintaining all the compliance and environmental issue. They established a physico-chemical Effluent treatment plan with latest technology. They treated 75 cubic meter of effluent per hour. Their dyeing unit is of 2 million yard per month and the printing section involved .8 million yard per month. Mainly they use following chemicals for wet processing 17. Detergent (non-ionic). 18. Anti creasing agent. 19. Hydrogen per oxide. 20. Caustic soda. 21. Soda ash. 22. Common salt 23. Glaubers’ salt 24. Washing off agent. 25. Acetic Acid. 26. Softener. 27. Anti foaming agent 28. Optical brightener. 29. Strong alkaline soap (NSR). 30. Synthetic thickener 31. Binders. Without these they use a lot of chemicals those vary due to order requirement. They use pigments and two types of dyes; reactive dyes & disperse dyes. Dyes and chemicals are soluble in water or in colloidal state. Some of suspended solid like wastages are also flow with raw effluent. They are mainly cotton fibers’ flocks or the yarn. The raw color of effluent is brown/ blue / black. All the liquors are coming through same drain of wet processing unit. So washing liquor, dye liquor, soaping liquor, scouring liquor, acid liquor all are getting mixture from the very beginning. The temperature, pH, BOD & COD of raw effluent liquor is given in the following table. In Bangladesh basically these four criteria are controlled for textile effluent.

Basic characteristics of raw effluent Sl NO

CHEMICAL CHARACTERISTICS

36

PARAMETERS 1. 2. 3. 4.

pH Suspended solids Biological Oxygen Demand(BOD) Chemical Oxygen Demand(COD)

UNIT

AMOUNT

mg/L mg/L mg/L

11.8 378.8 416 850

PHYSICAL CHARECTERISTICS 5. 6. 7.

Color Odour Temperature

Brown to black Not distinct 65

Picture of raw effluent

PHYSICO-CHEMICAL METHODS OF EFFLUENT TREATMENT Blower room and Collection tank control room.

Sludge tank Secondary clarifier Primary 37 clarifier clarifier Reaction tank PE FeSO 4 & CaO

Picture: Top view of a Physico chemical process.

According to ACS Textiles the physicochemical method is illustrated below:

Screening: 38

The effluent from the dyeing and finishing is first drained into the screening chamber. This chamber performs few physical filtrations. There is a series of netted filters positioned serially. For removing solid particles including suspended solids, yarn, fibers flocks, thickeners, etc, this process is important. After passing through these filters the effluent is free from solid bodies. This chamber has a capability to keep the flowing effluent for two minutes as it has a volume of 2 cubic meters. The very first netted filter is made of M.S.Bar rod which is then followed by the second filter made of the same material having a hole of 4 square centimeter and the last but not the list is a filter made of stainless still having five holes per square centimeter.

Picture: the screening chamber

Picture Cross section of three filters Picture: X-section of three filters from side and below shows X-section from front side

Oil separation: Oil separation is a major tusk for effluent treatment. In maximum effluent treatment planning oil separation technique is almost same. We know oil is lighter than the water. Here 25.6 cubic meter of raw effluent without suspended solid is flowed for 20.5 minute. 39

It is almost a huge time to layer up oil on surface of effluent around area 16 sq meters. Effluent passes to collection tank under the 1 feet of liquid surface. As a result only liquid water and its colloidal solution can pass to collection tank but not the oil. Special feature of this oil separation chamber is its time of staying in this chamber, which is allowed oil to float on the surface of water even though they were dispersed in the solution.

Picture: Oil separator

Collection tank: Here collection tank is nothing but the proper mixing of effluents of different types. It’s a cylindrical tank of 20 m diameter and 2.5 meter surface water depth. So the volume effluent carrying is 800.75 cubic meter. Effluent is treated here for 10 hour 40 minutes. Here effluent without suspended solid and oil is mixed with air by means blower. Air is filtered first and then blows air at 1260 m3 per hour. 585 diffusers are used to supply air

40

to the effluent for treatment. Here dissolved oxygen of water is increased by the air. The dissolved oxygen’s quantity is increasing to meet the Chemical oxygen demand by the effluent. 3 blowers are used to blow air for 16 hours continuously. And each blower is keeping working off for 8 hours. So 2 blowers of 50 kw/ hr are blowing air to the collection tank. After mixing properly effluent is allowed to pump to reaction tank.

Reaction tank: Effluent from the collection tank is directly passes to reaction tank for treating with chemicals used in physico-chemical process. In reaction tank the main chemistry of physico-chemical method for effluent treating is involved. It bears maximum maintenance cost for the treatment. This is a highlighted demerit for this method. The chemistry is described below.

Picture: reaction tank (left) & X-section of reaction tank This reaction technique can be discussed within two steps one is coagulation and the other is flocculation. In chemical mixing tanks 1.66 ppm of ferrous sulphate solution, 0.5 ppm of lime solution and 0.0016 ppm (1.6 µgm / L) of poly electrolyte solutions are prepared to react with effluent solution. These solutions are flowed to effluent continuously. We saw the oxygen demand for the chemicals are reduced at collection tank. By this reaction tank the biological oxygen demand is solved. Not only the BOD but also the discoloration is also performed. We know dyes are responsible for color. Here in woven wet processing sizes, enzymes, thickeners and other organic materials are responsible for 41

oxygen demand in hydrosphere. If these can not remove from discharge then the aquatic lives of hydrosphere do not have their oxygen to live. Now the question how physico chemical method of effluent treatment reduce the BOD? Principle is so simple coagulation and flocculation then to sediment. CaO leads to form Ca (OH) 2 at first. It is then followed to react with FeSO4 to form Fe (OH) 3. Here this Fe (OH) 3 is working as coagulants. The function of coagulant is to adsorb polymers to form an insoluble salt. As a result all the dyes pigments, chemicals are going to be coagulated. After coagulation these insoluble salts are liked to be precipitated on the ground at a slow rate. To accelerate their precipitation polyelectrolyte is employed here. Polyelectrolyte does nothing but the flock formation of coagulants. The basic idea of adding coagulant is to bring together all the suspended and dye particles so that they can be precipitated out in the reaction The chemical reaction that occurs in the coagulation and flocculation process is shown below; CaO + H2O

Ca (OH) 2

The above reaction take place in lime dosing tank when lime reacts with water and we get calcium hydroxide solution. This solution reacts with the ferrous sulphate solution, which as follows Ca (OH) 2 + FeSO4

CaSO4 + Fe (OH) 3 + FeSO4 (Un-reacted) + Fe (OH) 2

Adequate quantity of polyelectrolyte polymer solution is dosed in the flocculation tank to enhance the process of color removal by the flocculation process. A substantial amount BOD and COD etc. are removed in the coagulation and flocculation process. Flocculation is an essential stage in solid liquid separation process. Particles are too small to be separated by sedimentation filtration or floatation can be caused to aggregate. These aggregates can be easily removed by flocculation. The final structure of the flock is usually a loose, three dimensional network, resulting form the bridging of macro molecular flocculants between particles. Recently polyelectrolyte is widely used to flocculate in industrial use. Polymeric coagulants or polyelectrolyte are high molecular weight organic chains with functional groups at intervals of chain. Since the ionic group may be charged positively or negatively, the polyelectrolyte molecule may carry an overall positive or negative charge. Therefore they are classified as • Cationic • Anionic • Nonionic. There is some specific affinity of polymer segments for the particles surface, so that adsorption and coagulation can occur. Some interactions are: • Hydrophobic bond: responsible for the adsorption of non-polar segments on to hydrophobic surface. 42

• •

Hydrogen bond: polymeric molecules have suitable H-bond sites. Dipole crystal field effects: polar segments of the polymer chain may interact with the electrostatic field at a crystal surface. Some formulas of polyelectrolyte are given bellow: H

Cationic

Amines

N

R +

CL (----CH2----CH2------NH2-------)----X

Polyethylenimine Hydrochloride

R R

Quaternary

N

R +

(----CH2----CH------)----X

Poly ( nmethyl1-4-vinyl pyridinum chloride)

R

N+ Cl CH2

O

Nonionic

Polyamide

C

NH3

(----CH2----CH------)----- Polyacrilamide X C

O

NH2

Polyalcohol

OH

(----CH2----CH------)----X

Polyvinyl Alcohol

OH

43

O Anionic

Carboxylate

C

R O

(----CH2----C------)----X C

Poly(meth)acrylic acid

O

O

O Sulphonic

S O

O

(----CH2----CH------)----X O

S

Polyvinylsulphonate

O

O

Primary clarifier: Primary clarifier is used to sediment sludge and discharge clear water without color. As we discussed above that coagulation and flocculation removed all polis used to sediment sludge and discharge clear water without color. As we discussed above that coagulation and flocculation removed the toxic elements from here the crystal clear water is formed but it is alkaline. Here 2 chambers are used as primary clarifier. Each chamber is allowed to keep 105.625 cubic meter of effluent. And effluent’s treatment duration is about 2 hour 49 minutes.

Picture: X-section of primary clarifier and the surface primary clarifier.

44

This chambers bottom is sloped down around 45 degree to sediment sludge easily. Effluents from reaction tanks are directly pass to this tank. Here sludge sediment at bottom and the clear effluent pass out by means of over flow. Later a significant amount of sludge is pumped out to the sludge tank. Here the delivery effluent is alkaline and a fewer hot than the atmospheric temperature. The over flowed effluent is directly pass to the aeration chamber.

Aeration chamber and secondary clarifier:

Picture: aeration chamber and secondary clarifier and their X-section In aeration chamber acid is given to neutralize the solution with stirring. Though manufacturer suggests using a scrapper but the authority does not as they don’t need to use it. Without stirring relatively a few hot effluents is become cool naturally. Aeration tank has an open area around 490.625 sq. meter and its volume for effluent capacity is 2060.625 cubic meter. Effluent is treated here for 23 hour which is a huge time. So temperature is naturally fall down easily. The inlet pH of the effluent is 11.8. to neutralize its alkalinity around 7 liters of 33% concentrated hydrochloric acid per cubic meter is used. After neutralization its pH comes down to 7.16. If any sludge remains in the effluent they are allowed to sediment again in the secondary clarifier. From aeration tank neutralized effluent is entered to the secondary clarifier. 212 cubic meters this is kept here for 2 hour 52 minutes. Its time is sufficient enough to sediment sludge. The ground of secondary sludge is sloped down around 15 degree. Here treated effluent is also passing by means of over flowing. From here treated effluent is directly discharged to the cannel. Sludge from here is also passes to sludge tank by means of pumping arrangement.

Sludge tank: In sludge tank there is a sand filtration system in which 2 feet of sand layer is kept and above that sand sludge is deposited. At the ground of tank there is a pipe arrangement with filtration which allows effluent without sludge passing to the collection tank. This is again treated.

45

Results of E f fluent T re atment ( ph ysicochemi cal)

46

SL NO.

Effluent Quality parameters

Concentration present in raw effluent

Concentration present in treated effluent

mg/L

mg/L

Bangladesh Quality Standard at Discharge Public Inland Swear at Irrigated surface secondary land water treatment plant (WASA) mg/L mg/L mg/L

1

pH

11.8

7.16

6-9

6-9

N/A

2

Biological Oxygen Demand (BOD)

450

45

50

250

100

750

180

200 50 hours

400

400

3 4

Chemical Oxygen Demand (COD) Time

Costing: Chemical cost per day = 21,600 tk Power cost per day= 156 tk Man power cost per day= 1500 tk. Total = 23256 tk per day. They processed 1800 cubic meter per day so cost per cubic meter= 12.92 tk

Advantages of physico-chemical method: 1. This process posses a very low fixed cost which is around 80,00,000 tk. 2. Discharge effluent is colorless. 47

3. Effluent is 100% pollutant free. 4. Non-biodegradable chemicals can treat here. 5. Its maintenance does not demand highly skill professionals. 6. It is applicable from all kinds of textile industries. 7. It is very simple in technology. 8. Dyer does not need to select chemicals with respect to its effluent treatment. 9. It occupies minimal spaces around 15 kathas. 10. It possesses less treatment duration rather than biological treatment method which is around 50hours.

Disadvantages of physico-chemical method: 1. Treatment cost is very high around 12.92 tk per cubic meter. Cost will be equivalent with its fixed cost within 396 days (14 months) from its installation. Economically it brings loss. 2. This method is not that much modernized. 3. It is chemical based for this reason treatment efficiency is highly depended on its chemicals strength.

Comparisons in between raw effluent character of knit & woven wet processing unit Sl NO

CHEMICAL CHARACTERISTICS PARAMETERS

1. 2. 3. 4.

pH Suspended solids Biological Oxygen Demand(BOD) Chemical Oxygen Demand(COD)

UNIT

mg/L mg/L mg/L

AMOUNT Knit wet processing Woven wet unit processing unit 8.2 11.8 276.8 378.8 180 450 417

750

PHYSICAL CHARECTERISTICS 5. 6. 7.

Color Odour Temperature

Brown to black Not distinct 48

Brown to black Not distinct 65

Here the raw effluent characteristics in between two wet processes pH, BOD, COD and temperature is differing a significant quantity. Let us discuss about this differences. 48

A term used to express the intensity of the acid or alkalinity source. pH represents the effective concentration (activity) of hydrogen ions (H+) in water. This concentration could be expressed in the same kind of units as other dissolved species, but H+ concentrations are much smaller than other species in most waters. The activity of hydrogen ions can be expressed most conveniently in logarithmic units. pH is defined as the negative logarithm of the activity of H+ ions: pH = -log [H+] So according to pH definition it is totally depending on the liquor. As knit wet processing involves relatively high liquor ratio due to the exhaust dyeing method, it must possesses low pH rather than the woven wet processing. BOD from woven wet process is much higher than the knit wet process. Because the woven grey fabric is need to desize and the sizes are strongly responsible for biological oxygen demand by the bacteria in hydrosphere. However knit wet process possesses a significant value of BOD due to the biodegradable chemicals. COD is the chemical oxygen demands by the chemicals present in effluent. In woven wet process it is also higher in value than the knit wet process. Same philosophy of liquor is applicable for the difference between COD value of this two wet process. Chemical concentration in woven wet process is much higher than the knit wet process as woven wet process involves padding method and the knit possesses the exhaust method of pretreatment and dyeing. Basically in textile wet process unused per oxides, alcohols forms during various treatment and reducing agents like per oxide killer are responsible for COD. Temperature of two different wet process units are differing here because in woven wet process a few rinse water is discharged to ETP maximum are like to drain directly. But in knit wet process that we visited for project is like to mix all kinds of rinse water with the dye drop liquor and souring drop liquor. This temperature faults are not the processing faults this is individual treatment plant’s technical fault.

Discussions: Cost comparison: 49

Physico-chemical method possesses a high maintenance cost of chemicals, which is too high and not economically viable. This method can not inspire investor to run this plan. If it is possible then in future we must work for recycle of this water to wet processing unit. This solution may inspire this method from the economical point of view for a long run.

Space comparison: Space in between two methods biological treatment method possesses higher space consumption than the physico-chemical method. How ever space consumption is depending on the plant designer it can vary from person to person.

Minimization of wastewater in Textile Processes

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Attempt has been made in the following sections to describe measures, techniques practices to minimize the pollution and volume of textile effluent. Both pollution and volume of textile effluent is directly related to the cost of operation of the effluent treatment plant. So minimization of effluent flow results best. Reducing pollution Compounds that contribute to the aquatic toxicity of textile effluent include salt, metals, surfactants, toxic organic chemicals, biocides and toxic anions. Some methods of reducing the use of these compounds are to: • Reduce metal content through careful pre-screening of chemicals and dyes for metal content and using alternatives where possible. • Eliminate galvanized plumbing as reactions with brass fittings can take place in the presence of acids, alkalis or salt and lead to the release of zinc. • Reduce the amount of salt in the effluent by optimizing recipes, using low-salt dyes, reusing dye baths and optimizing dyeing temperatures. • Use biodegradable surfactants such as linear alcohol ethoxylates. • Replace chlorinated solvents with unchlorinated alternatives. • Replace the use of biocides with ultraviolet light as a disinfectant for cooling towers. Sizing: As far as environmental pollution is concerned synthetic sizes are better than starch-based sizes. The advantages of this are a reduced pollution load as synthetic sizes have lower BOD levels, and they can be recycled for reuse. Ensure that only the minimum required size is added onto the yarn. This reduces chemical consumption as well as the pollution load to drain during desizing. Here in ACS textile can use pick up% 70 for sizing with minimum wastage. We are also suggesting woven textile industries to use a synthetic size which is recycled again. Pretraetment: Preparation includes desizing, scouring, bleaching and mercerizing. Desizing accounts for more than 50% of the pollution load of preparation while scouring contributes between 10% and 25%. Good preparation is essential for subsequent processing, as any impurities remaining on the fabric will interfere with the dyeing and finishing processes. Some waste minimization options for the preparation department are listed below. •

Desizing: The effluent from desizing will contain the sizes. Using and recycling synthetic sizes in place of starch sizes will reduce the pollution load from desizing.

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Scouring: Incoming raw material should be screened for toxic chemicals, as these will be removed during the scouring process. Detergents must be easily biodegradable. And must avoid the following detergents: linear alkylbenzenesulphonate; nonylphenoletoxylate; dialkyldimthyl ammonium chloride; distearyl dimthyl amonium chloride; di dimithyl ammonium chloride; sulphosuccinates; alkylphenolethoxylates; complexing agents with poor

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biodegradability (e.g. EDTA; phosphonic acid; NTA; phosphonates). Reuse scour washwater for desizing. Recycle continuous scour washwater to batch scouring. •

Bleaching: Replace the use of chlorites and hypochlorites with hydrogen peroxide. Ensure that bleaching is carried out efficiently. Recycle bleach wash water for scouring. It must be noted that recycling the liquor of scoring-bleaching may possible.

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Mercerizing: Recycling of sodium hydroxide through evaporation for reuse in mercerizing or scouring will decrease the pollution load and chemical consumption. However evaporation is expensive it can be stored to reuse with sufficient amount caustic and water again.

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General: Use modern equipment. Replace batch processes with continuous processes. Install counter-current washing. Combine processes such as desizing, bleaching and scouring. Replace harmful chemicals with those of lower environmental impact. Reuse wash-water for cleaning equipment and screens.

Batch Processing: There are a number of waste minimization options for batch processing. These include: Cascading multiple rinsing operations. Reusing of softening baths with reconstitution. Reusing preparation baths (scouring and bleaching) with reconstitution after filtration to remove impurities. Segregating colored effluent streams from clean streams (preparation and rinsing) to ensure that only concentrated effluent is treated. This clean effluent may be used elsewhere in the factory or can discharge. Some suggests two separate outlets drains e.g. (I) more contaminated water drain and (ii) less contaminated water drain. The less contaminated waters are allowed to bypass many stages before mixing together again at certain point effluent treatment process. Dyeing Batch dyeing: Careful selection of dyes is important. Dyes should have high fixation/exhaustion, low toxicity, absence of metals, and be appropriate for the end use. Correct and efficient application procedures must be used and right-first-time production should be achieved. The main areas for waste minimization in batch dyeing include: • Using low liquor ratios. • Using automated dye and chemical dosing systems. Reusing dye baths, rinse water and softening baths. • Optimizing pH and salt for each recipe. • Avoiding the use of auxiliaries that reduce or retard exhaustion. • Using bi-reactive dyes. • Using the newer low-salt reactive dyes. • Avoiding the addition of more chemicals to offset the effects of other chemicals. • Replacing the acetic acid in neutralizing after dyeing with formic acid or dilute hydrochloric acid (acetic acid adds to the COD of effluent). 52

Continuous dyeing: The main waste minimization strategies in continuous dyeing are to: • Maximize dye fixation. • Minimize wash - off. • Minimize the number of times a dye bath has to be dropped and cleaned due to a color change by careful scheduling. • Use automated color kitchens to minimize the working losses and discards. • Improve washing efficiency through the installation of flow restrictors to control water volumes. Use counter current washing procedures. • Optimize dosing of chemicals through monitoring of relevant parameters such as pH, absorbance, turbidity etc. Waste minimization options for dyeing: Operate at lowest possible bath ratio. This leads to a reduction in operating costs, water consumption, chemical use, energy use and less effluent discharge. Table 3 shows the liquor ratios of various dyeing processes. Minimize stripping and / or re-dyeing procedures. Avoid the use of detergents to wash fabric after reactive dyeing; high temperatures are just as effective. Table -3: Liquor ratios for various dyeing processes process liters/kg Dyeing process Dyeing in winches Jet dyeing Package Dyeing Pad Batch

Liquor Ratio 1: 20-30 1: 7- 10 1: 5 – 8 1: 5

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Machine: Now a day’s so many low liquor dyeing machines are available. These can dye fabric even at 1:1 THENCE JET FLOW DYEING machine is an example (from HK). Recently THIES is widely use for exhaust dyeing it consumes M:L is around 1:5

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Minimize auxiliary use. Some auxiliaries interfere with dye fixation and should be replaced with alternatives or removed, as this will reduce the color load of the effluent. Multi-functional biodegradable auxiliaries must choice during recipe. Some auxiliaries are added to compensate for inefficiencies in the process, equipment, or substrate design. Therefore, optimizing these factors will reduce auxiliary use. Recently an organic salt so called CLR is useful to consume half amount of Glaubers’ salt or sodium chloride.

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Right-first-time dyeing. Corrective measures are chemically intensive and have much less chance of achieving the required quality. The greatest costs in reprocessing are associated with the cost of dyes and chemicals - typically, the 53

costs can increase by as much as 30%. Right-first-time dyeing leads to an increase in productivity and more efficient use of resources (e.g. labor, capital). In dyeing polyester, avoid the use of carriers by upgrading dye machinery or replace with less harmful alternatives. Good fabric preparation increases the chance of right-first-time dyeing as fixation is improved. Improved dye fixation. Dye fixation onto cotton can be improved by mercerizing the yarn or fabric prior to dyeing. Pad-batch dyeing is a cold dyeing method used mostly on cellulosics that results in a reduction in pollution, energy use, and costs. The advantages include: no salt or chemical specialties are required, more efficient use of dye, improved quality of dyeing, can be used on woven or knits, and low capital investment results in savings in dyes, chemicals, labor and water. Reuse dye baths, especially those using dyes with high exhaustion such as acid or basic dyes. There are 4 main steps to follow: • Save the exhausted dye bath - this can be done by pumping it to a holding tank (or identical machine doing the same processes) and returning it to the machine for use in the next dyeing procedure. •

Analyze the dye bath for residual chemicals - most auxiliaries do not exhaust in the dyeing process. There is approximately a 10% loss due to adsorption onto the fabric. Others however, are used or lost during the process and must be replaced. Unexhausted dyestuffs need to be analyzed to determine the concentration remaining in the dye bath to ensure correct shading in further dyeing. Dye bath analysis can be performed using a spectrophotometer and specific guidelines for such a procedure.

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Reconstitute the dye bath - water is added to replace that which is lost to evaporation or to the product. Auxiliaries are added in proportion to the water volume (these can be estimated) and finally the dyestuff is added for the required shade.

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Reuse the dye bath - check the temperatures to ensure that the reused dye bath is the right temperature to minimize spotting and unleveled dyeing. If properly controlled, dye baths can be used for up to 15 or more cycles. Use dyes that undergo minimal changes during dyeing (acid, basic, direct and disperse) and reuse dye baths to repeat the same shades. Dye bath reuse is limited by impurity build-up from, for example, the fabric, salt build-up, steam contamination and surfactants. In addition, specialty chemicals may be lost during the dyeing process and these should be screened for their reuse potential. Close scheduling is also required which may limit the flexibility required for bath dyeing.

Water reuse. This can be achieved by dyeing multiple lots in the same dye bath by means of acid dye basically nylon dyeing may proceed in this method. Recently in DEPZ YOUNGONE SPORTS WEAR LTD. is following this method. This is generally possible for those products where high quality dyeing is not essential (e.g. work gloves, hosiery). Install improved machinery that has better controls. 54

Printing: Pollutants associated with printing include suspended solids, solvents, foam, color and metals, and in general, large volumes of water are consumed during the washing-off stages. The main areas of waste minimization in printing include raw material conservation, product substitution, process and equipment modifications, material handling, and scheduling and waste recovery. Other options include: • Waste minimization in the design stages can eliminate the need for dyes containing metals. • Careful selection of surfactants. • Reducing air emissions by replacing solvents with water-based alternatives. • Routine and careful maintenance of printing equipment. • Training employees in the practices of good housekeeping. • Reusing water from washing the print blanket. • Reusing left over print paste if possible. • Removing excess paste from drums, screens and pipes by dry techniques (wiping with a squeegee etc.) before washing with water. This reduces the color load discharged to drain. • Careful scheduling to prevent expiration of print pastes before use. • Investigating alternatives to urea as this increases the nitrogen in the effluent Ethylene glycol may use here. Finishing: There are a number of finishing processes that are carried out on the fabric after dyeing and/or printing. These can be achieved by chemical or mechanical methods. Some waste minimization options are listed as follows: Design fabrics such that the need for chemical finishes is minimized. Use mechanical alternatives to chemical finishes. Use low add-on methods. Minimize volatile chemical use. Install automated chemical dispensing systems. Train employees in good housekeeping practices. Use formaldehydefree cross-linking agents. Investigate the use of spray application of finishes as these have a low add-on and require no residual dumping at the end of a run.

Conclusion: At present wet processing industry in Bangladesh are under pressure to install ETP. For financial and space constraints, it is difficult to meet the deadline of installation. From our findings, we like to state that it may not be necessary to use all methods to meet the permissible limit of the parameters set by the Department of Environment. One suitable method may be good enough to fulfill the requirements.

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