Treatment of Industrial Waste Waters PDF

Chapter 1 TREATMENT OF INDUSTRIAL WASTE WATERS 1.1 WATER POLLUTION Natural water is seldom chemically pure. When it rains, organic and inorganic suspended particulate matter, gases, vapours, mists, etc. in the air get dissolved in water, through which it reaches the earths surface. In addition, water carries surface pollutants and contaminants during its flow over the ground. Water, which percolates into the ground, dissolves various salts and becomes rich in total dissolved solids. Thus, it acquires a number of impurities while in its natural state. This necessitates adequate treatment of naturally occurring water before it can be used for domestic, industrial, commercial, agricultural or recreational purposes. The extent of treatment will depend on the end use of the treated water. Use of the treated water even once adds considerably to the amount and variety of pollutants. This necessitates further treatment of the water before it can be reused, although it is not strictly necessary to have water of uniformly high quality for each of the above uses. In view of the limited availability of water for meeting our growing demands, and in the interest of protecting the environment, it is essential to think and act in terms of reducing water consumption, reusing and recycling once-used water, and minimizing the pollutional effects of waste water resulting from a variety of uses. Domestic and industrial uses of water add a number of contaminants and pollutants to it. Contaminants are capable of causing diseases and rendering water unfit for human consumption, while pollutants are substances which impair the usefulness of water, or render it offensive to the senses of sight, taste and smell. Contamination may accompany 1 2 INDUSTRIAL WASTE WATER TREATMENT pollution. Domestic waste water contains contaminants, while industrial waste water may contain both contaminants and pollutants. Industries use water for a variety of purposes, such as for manufacturing goods, heating, cooling, as carrier of raw material, as carrier of waste matter (which constitutes industrial waste water), as a solvent, for firefighting, for lawn sprinkling and gardening, and for use in the canteens and toilets. While only a small fraction of the supplied water is present in the end product, or is lost by evaporation, the rest is converted into industrial waste water. Indiscriminate discharge of these waste water streams into the environment can render soils sick, pollute the receiving bodies of water and cause air pollution by generating obnoxious gases. Treatment of these waste waters within the factory premises, or preferably their elimination at source, should be the aim of every industry. If total elimination of the waste water streams is not feasible, the least that can be done is to reduce their volume and strength, by taking one or more of the in-plant measures such as reducing fresh water consumption, reusing waste water (either with or without treatment), substituting process chemicals for those which contribute to pollution, changing or modifying the manufacturing process, and following good housekeeping practices. Of course, due care must be taken to see that these steps do not adversely affect the quality of the finished product or damage the manufacturing machinery. In addition, proper disposal of residues arising out of recycling and reuse must be provided for, along with the treatment of the waste water streams, which are not to be recycled or reused. 1.2 CATEGORIES OF POLLUTANTS Industrial wastes contain a large variety of pollutants which are categorized as follows: 1. Inorganic pollutants. These include alkalis, mineral acids, inorganic salts, free chlorine, ammonia, hydrogen sulphide, salts of chromium, nickel, zinc, cadmium, copper, silver, etc., anions such as phosphates, sulphates, chlorides, nitrites and nitrates, cyanides; cations such as calcium, magnesium, sodium, potassium, iron, manganese, mercury, arsenic, etc.. 2. Organic pollutants. These include high molecular weight compounds such as sugars, oils and fats, proteins, hydrocarbons, phenols, detergents, and organic acids. Some of these pollutants are resistant to biodegradation and/or others are toxic to aquatic life in the receiving water. Their removal, or at least reduction to a low concentration, becomes necessary in order to be able to treat such waste water by biological means. In addition, industrial wastes may contain radioactive material, which need very careful handling, treatment and disposal. TREATMENT OF INDUSTRIAL WASTE WATERS 3 The characteristics of industrial wastes, which are combined with domestic sewage generated within the factory premises, are somewhat different from those of the industrial wastes alone, on account of dilution offered by the sewage. Further, such mixtures are easier to treat biologically because of the presence of microorganisms in the sewage. If the industrial waste is deficient in nutrients such as nitrogen and phosphorus, these elements are supplied to some extent by sewage, leading to economy in the consumption of chemicals, e.g. urea and DAP, which are commonly used for nutrient supplementation. An added benefit in such a case is that a common treatment plant can be designed for treating both, industrial wastes and sewage. 1.3 TREATMENT AND DISPOSAL OF INDUSTRIAL WASTES The aim of the treatment is to remove pollutants from the waste water and render it fit for safe discharge to the environment. In view of the increasing demand for water, and its decreasing availability, mere end- of-pipe treatment is not the answer to pollution control. Reuse, recycling and where feasible, by-product recovery must become an integral part of the treatment scheme. Experience shows that it is possible to achieve this goal without incurring heavy expenditure. In many cases, the practice of reuse, recycling and by-product recovery has resulted in not only meeting the operating costs, but also offering an attractive payback period to the industry. Some examples of successful reuse and recovery are given in the following chapters. Methods of treating waste water can be classified as follows: 1. Physical methods: These include screening, sedimentation, flotation, filtration, mixing, drying, incineration, freezing, dialysis, osmosis, adsorption, gas transfer, elutriation, etc. 2. Chemical methods: These include pH correction, coagulation, softening, ion exchange, oxidation, reduction, disinfection. 3. Biological methods: These employ aerobic, facultative and anaerobic microorganisms to destroy organic matter and reduce the oxygen demand of the waste water. 4. A combination of the above three methods is also used to treat waste water. Adequate treatment can also be obtained by selecting one or more of the physical, chemical and biological units and arranging them in a logical sequence, so that the effluent of one unit is suitable as influent to the next unit. Selection and sizing of the proper unit(s) is done by (a) flow measurement, sample collection and characterization of the waste water flows, (b) subjecting the waste water samples to treatability studies by employing laboratory-scale models, which may be run on a batch feed basis, semi-continuous feed basis, or continuous basis, (c) deciding which 4 INDUSTRIAL WASTE WATER TREATMENT combination of unit operations and unit processes will be appropriate for the waste water under study, and (d) if necessary, running a pilot plant, which will simulate the working conditions in a full-scale plant. The size of the pilot plant may be chosen such that it can be conveniently incorporated into the full-scale plant. Industries manufacturing a variety of end products can benefit from maintaining a pilot plant, even after the full-scale plant is commissioned, in order to ensure adequate treatment of influents of varying quality. These variations in quality may occur due to change of product, raw materials, or method of production. Pilot plants can play a very important role in the case of industrial estates, in which many different industries are located together and produce waste waters of divergent quality. A common effluent treatment plant (CETP) may be the right technical solution for pollution control in such cases. Pilot plant studies help in arriving at rational design criteria for such estates, choosing the most appropriate treatment train, and avoiding costly modifications to the CETP once it goes on stream. Disposal of treated wastes is an important step an industry has to follow in order to ensure that the delicate ecological balance of the environment is not disturbed. Disposal may be done in a receiving body of water such as a river, lake, or sea. Disposal on land is also practised, taking care to see that the soil is not adversely affected by the residual pollutants in the effluent. Where underground sewerage is available, the treated effluents may be discharged into municipal sewers, provided they meet the quality standards laid down for this mode of disposal. In addition, the quality of the industrial effluents must be such that: (i) they will not endanger the lives of the drainage maintenance crew, who may be required to enter the sewers for maintenance and repairs, (ii) the material of the sewers will not be damaged, and (iii) the effluent treatment plant, if one is provided at the end of the drainage system, will be capable of taking the hydraulic and organic loads imposed by the industrial effluents. Industries wishing to follow this mode of disposal will almost always be required to give some pretreatment to the waste water, its extent depending on: (i) the volume and strength of the waste water, and (ii) the degree of dilution offered by the sewage flowing in the drainage system. A result of treating waste water by one or more of the above discussed means is generation of sludges, which may be organic or inorganic in nature. Sludges constitute a peculiar problem on account of their properties such as viscosity, presence of pollutants in a concentrated form, some of which can be toxic and/or hazardous or difficult to dewater and dispose of. This fact needs careful consideration while designing the waste water treatment plant. The treatment is said to be complete when the solid residues, liquid effluents and gaseous emissions are adequately treated and safely disposed of. TREATMENT OF INDUSTRIAL WASTE WATERS 5 REFERENCES 1. Fair, G.M. and G.C. Geyer (1954): Water Supply and Wastewater Disposal. New York: Wiley. 2. Mahajan, S.P. (1998): Pollution Control in Process Industries, New Delhi: Tata McGraw-Hill. Chapter 2 FLOW MEASUREMENT, CHARACTERIZATION AND TREATABILITY STUDIES OF INDUSTRIAL WASTE WATERS The design of a waste water treatment plant begins with collecting information about the volume of waste water to be treated, its characteristics, and the degree of treatment required in order to meet specified discharge standards. Knowledge about the mode of manufacture, viz. continuous or batch is also useful. A batch process produces an effluent in the form of a slug which lasts for a short time, while a continuous process generates a waste water stream which flows continuously, although at varying rates. Based on this information, one can decide if a grab sample of the waste water would be representative of its quality, or a composite sample would be necessary. Information about the raw materials, chemicals and other ingredients used in the manufacturing process helps one to decide the physical and chemical tests to be conducted on the representative samples for characterizing the effluent. Correct interpretation of the results of the analysis of the waste water samples enables one to choose a proper treatment process. 2.1 MEASUREMENT OF FLOW The measurement of waste water flow can be done either on the outfall channel or pipe carrying the entire waste water flow from the industry, or on the individual waste streams within the industry. The first method is useful in knowing the total flow, but cannot distinguish between the contributions of individual streams. It is useful in the case of industries that discharge a more or less uniform quality of waste water and are not likely to contain toxic pollutants, or valuable ingredients, which can be 6 FLOW MEASUREMENT, CHARACTERIZATION AND... 7 profitably recovered. Measurement of flow from individual streams helps in deciding whether some streams can be segregated, either for giving pretreatment, for recovery of by-products, or for recycling with or without treatment. Such streams, after pretreatment or recovery of by-products, can be mixed with the other effluent streams for further treatment. Measurement of flow rates should be invariably accompanied by collection of waste water samples, either as grab samples or as composite samples. Flow measurement can be done either by measuring the cross- sectional area of the raw waste water channel and multiplying it by the velocity of flow, or by measuring the time required to fill a tank or drum of known volumetric capacity. Readings taken on a calibrated v-notch, a rectangular notch or weir built in the conveying channel can give a fairly accurate estimate of the flow rate. 2.2 COLLECTION, PRESERVATION AND CHARACTERIZATION OF SAMPLES 2.2.1 Collection of Samples Sampling can be done as a grab sample, i.e. a sample which represents the instantaneous quality of the waste stream. Where it is known that the waste water flow rate is continuous but of varying quality, composite sampling is done. This consists of either collecting a fixed volume of sample at equal time intervals, or varying the volume of sample in proportion to the flow rate at the time of collection. In either case, the individual samples are mixed together to give one representative sample. The method of flow-proportionate sampling gives a more realistic sample than the fixed volume-fixed time interval sample. The volume of sample collected should be enough to permit all physical and chemical tests to be carried out on it. The sampling period may range from 8 hours to 24 hours or even longer. Items of information, which should accompany the samples, include location of sampling point, time, day and date of sample collection, nature of sample (grab or composite), duration of sample collection (if a composite sample) and any other relevant information, which will help in the analysis. 2.2.2 Preservation of Samples Samples collected in the field should be conveyed to the laboratory in the shortest possible time, to avoid deterioration in their quality. It is common practice to collect samples in a container surrounded by ice, so that low temperature (about 4°C) is maintained and the samples retain their original quality. Tests such as pH, temperature, colour, odour, etc. are best performed at site immediately after the samples are collected and the observations noted down. If immediate analysis of the samples is not 8 INDUSTRIAL WASTE WATER TREATMENT possible, suitable preservatives should be added to them. Chemical preservatives are to be added only when they are shown not to interfere with the examinations being made. When used, they should be added to the sample bottles initially, so that all portions of the sample are preserved as soon as they are collected. A list of preservatives, which may be used to maintain the quality of samples is given in Table 2.1. Table 2.1 Preservatives for Waste Water Samples Parameter Preservative Maximum holding period Acidity-Alkalinity Preserve at 4°C 24 hours Biochemical Oxygen Demand (BOD) Preserve at 4°C 6 hours Calcium None required Chemical Oxygen Demand (COD) 2 ml/l H2SO4 7 days Chloride None required Colour Preserve at 4°C 24 hours Cyanide NaOH to pH 10.0 24 hours Dissolved oxygen Determine at site Non-holding Fluoride None required Hardness None required Metals, total 5 ml/l HNO3 6 months Metals, dissolved Filtrate, 3 ml/l 1:1 HNO3 6 months Ammonia nitrogen 40 mg/l HgCl2, preserve at 4°C 7 days Kjeldahl nitrogen 40 mg/l HgCl2, preserve at 4°C Unstable Nitrate-nitrite nitrogen 40 mg/l HgCl2, preserve at 4°C 7 days Oil and grease 2 ml/l H2SO4, preserve at 4°C 24 hours Organic carbon 2 ml/l H2SO4 (pH 2.0) 7 days pH None available Phenolics 1.0 g CuSO4 + H3PO4 to pH 4.0, preserve at 4°C 24 hours Phosphorus 40 mg/l HgCl2, preserve at 4°C 7 days Solids None available Specific conductance None required Sulphate Preserve at 4°C 7 days Sulphide 2 ml/l Na acetate 7 days Threshold odour Preserve at 4°C 24 hours Turbidity None available Although the table suggests a number of preservatives, the most effective preservative is maintenance of low temperature (4°C or lower). FLOW MEASUREMENT, CHARACTERIZATION AND... 9 Further, the samples should be analysed as soon as possible after collection. 2.2.3 Characterization of Samples Characterization of the samples is the next step. The samples are analysed using physical and chemical methods. Physical methods include determination of temperature, colour, odour, total solids, suspended solids, settleable solids, dissolved solids, volatile solids, oil and grease. Chemical methods include determination of pH; acidity; alkalinity; biochemical oxygen demand (BOD); chemical oxygen demand (COD); total organic carbon (TOC); cations such as aluminium, arsenic, boron, cadmium, chlorine, chromium, copper, iron, lead, manganese, nickel, zinc; anions such as chlorides, ammoniacal nitrogen, nitrite nitrogen, nitrate nitrogen, phosphates, sulphates, sulphides, etc. In addition to these tests, the samples may have to be tested for cyanides, phenols, detergents, cellulose, hemicellulose, tannin, lignin, etc.tests that are specific to certain industrial waste waters. Procedures to be followed in conducting the analyses of samples should be those specified in the Standard Methods for Analysis of Water and Waste Water. It may be necessary at times to modify the analytical procedure for a certain constituent for which a standard method is not available. In such a case, the results of analysis should be presented with special mention of these modifications. The modified procedure should be subjected to a recovery test, consisting of adding a known amount of the specific pollutant to the waste water sample, determining its concentration by the modified procedure and comparing the results of analysis with the original sample. Results of analysis of the physical and chemical tests are then interpreted so that a preliminary idea of the treatment to be given to the waste water can be had. At the same time, the quality requirements of the treated effluent, as laid down by the pollution control authorities, are studied to enable the designer to narrow down the choice of unit operations and unit processes, and to decide the degree of treatment in order to meet the effluent quality standards. Based on the results of analysis of the raw waste water samples and the quality requirements of the treated effluent, laboratory-scale experiments are then conducted. The samples are subjected to various unit operations and unit processes to find out the suitability of each for treating the waste water. Parameters such as detention time, food to microorganism ratio, surface loading, volume of sludge to be expected, its settleability, types of chemicals required and their quantities are determined, but laboratory-scale studies do not establish achieveable effluent quality or the suitability of mechanical equipment to be used in the full-scale plant. 10 INDUSTRIAL WASTE WATER TREATMENT In addition to the laboratory-scale studies, it may be necessary to run pilot plant studies, in which the actual conditions in the full-scale plant can be simulated. Pilot plants are designed to offer a certain degree of flexibility and enable collection of data which will be used in finalizing the design, the degree of automation, if any is required, the material of construction and, hence, the capital and running costs of the full-scale plant. REFERENCES 1. Greenberg, A.E. (Ed.) (1996): Standard Methods for the Examination of Water and Wastewater, 18th ed., Washington D.C.: American Public Health Association American Water Works Association/Water Environment Federation. 2. Indian Environmental Association (1999): Characterization of wastewaters. Two-day refresher course for Laboratory Analysts working at Wastewater Treatment Plants, Pune Chapter, July 3, 4. Chapter 3 UNIT OPERATIONS AND UNIT PROCESSES 3.1 INTRODUCTION The various methods used in treating industrial waste water are identical to those used in treating domestic sewage. However, the differences between the two modes of treatment arise because of (a) a very high degree of variability in the quality of industrial wastes compared to domestic sewage, (b) large variations in the flow rates of industrial wastes, and (c) the presence of hazardous and/or toxic pollutants in some industrial wastes. In terms of population equivalent, industrial waste water is usually a few times stronger than an equal volume of domestic sewage. In view of the possible presence of hazardous and/or toxic pollutants, it becomes necessary to provide adequate preliminary treatment to an industrial waste water before subjecting it to further treatment. This is achieved by employing various unit operations and unit processes. l Unit operations are those in which physical forces are employed to purify waste water. Some of the important unit operations are screening, sedimentation, flotation, filtration, mixing, equalization, flow proportioning, drying, incineration, freezing, foaming, dialysis, osmosis, adsorption, gas transfer, elutriation, etc. l Unit processes are those in which chemical and/or biological forces are used to purify waste water, e.g. pH correction, coagulation, oxidation, reduction, disinfection, aerobic and anaerobic biological treatment. It is rarely adequate to apply only unit operations or only unit processes to an industrial waste water to obtain an effluent fit for discharge to the environment. Therefore, based on laboratory-scale studies 11 12 INDUSTRIAL WASTE WATER TREATMENT and/or pilot plant studies, a proper choice of unit operations and unit processes is made, and this is arranged in a logical sequence to give an acceptable treatment scheme. A brief description of the various unit operations and processes, and their application in industrial waste treatment is given below. 3.2 UNIT OPERATIONS 3.2.1 Screening Screening is done to remove large suspended and floating solids from waste water in order to protect pumps, pipes and valves from clogging and damage, e.g. rags and pieces of cloth from cotton textile wastes, fine fibres from woollen mills, spent tan bark from vegetable tanning process, leather trimmings from the leather processing houses, bark from the debarking machines in the pulp and paper mills, fruit peelings and fruit rinds from fruit canning and packing industry, and, generally, in industries where good housekeeping practices are not followed. Microscreening may also be used for complete removal of suspended impurities so as to eliminate primary sedimentation. Screens may be coarse, medium or fine, depending on the clear space between bars. Fine screens may be rotary drum type, tangential type, or vibratory type. Screens are manually or mechanically cleaned. In any case, adequate arrangement must be made to treat and dispose of the screened material. 3.2.2 Sedimentation and Flotation Removal of finely suspended and settleable solids is done by plain sedimentation, while chemically aided sedimentation is required for removal of colloidal solids. Flotation, which may be termed negative sedimentation, is done to remove impurities, which are lighter than water and do not settle in a reasonable length of time. Like coagulants in sedimentation, flotation may be done with or without, the aid of flotation agents. Industries employing sedimentation include almost all those producing high suspended organic and inorganic solids, while flotation is particularly useful in woollen mills, slaughter houses, pulp and paper mills, oil refineries and dairies. Both sedimentation and flotation help reduce the solids load on the following treatment units. However, flotation is also used for the recovery of useful material from the waste water streams. 3.2.3 Filtration Filtration of industrial waste water is more often practised downstream of other pretreatment processes than as a stand-alone pretreatment UNIT OPERATIONS AND UNIT PROCESSES 13 method. It is used during neutralization/precipitation of heavy metals and biological treatment to reduce BOD loads. It may also be used to remove lime precipitates of phosphates and as a pretreatment for waste water before it is discharged to an activated carbon column or to a dialysis or reverse osmosis unit. In reuse applications, filtration is used if the treated waste water is to be spread on land for irrigation, groundwater injection, lawn sprinkling and body-contact recreational uses [1, 2]. 3.2.4 Mixing Mixing is an important unit operation in waste water treatment. It is used for mixing of one substance with another, e.g. chlorine or sodium hypochlorite with treated waste water, liquid suspensions such as in the aeration tank of activated sludge process or sludge undergoing aerobic or anaerobic digestion; for flocculation of finely divided suspended solids with coagulants; for heat transfer as in heated digesters; and for mixing neutralizing chemicals with acidic or alkaline waste streams. Continuous, rapid mixing is achieved by providing baffles or hydraulic jumps in open channels, by fixing static mixers or venturi flumes in pipelines. Continuous mixing is achieved by pumping the tank contents and recycling a part of the pumped liquid, by using mechanical mixers, or with the help of compressed air bubbled into the liquid. Compressed air and mechanical mixers also serve the purpose of maintaining the tank contents in a fresh condition. If the waste water contains oily and greasy matter, compressed air helps float a part of this matter, which can be skimmed off. Mixing helps in giving partial treatment to waste water streams of opposite nature, e.g. mutual neutralization of acidic and alkaline effluents or partial cooling of hot streams when mixed with cold streams. Waste water streams, which are highly fluctuating in their quality and flow rates, are best handled by mixing them in equalization tanks. 3.2.5 Equalization Equalization is used to overcome the operational problems caused by variations in quality and flow rates, to improve the performance of downstream processes, and to reduce the size and cost of downstream units of treatment. Equalization helps dilute toxic pollutants. It is also an attractive proposition for upgrading the performance of an overloaded treatment plant. Equalization can be done in-line or off-line. The former is done when equalization of plant loading is desired, while the latter is used when the downstream treatment units, especially biological units, are to be protected against shock loads due to slugs of toxic and/or organic pollutants. 14 INDUSTRIAL WASTE WATER TREATMENT The required volume of an equalization basin is determined by constructing mass flow diagram to represent the inflow and superimposing on it the rate of outflow (which would be by pumping). Two parallel lines, representing the rate of outflow, are drawn tangent to the high and low points of the mass diagram. The vertical distance between the two tangent lines represents the required volume of the equalization basin. An essential requirement of an equalization basin is adequate mixing of the basin contents. This ensures a more or less uniform quality of the outflow, minimizes chances of deposition of solids in the basin and helps keep the waste water fresh. 3.2.6 Flow Proportioning Flow proportioning is not, strictly speaking, a unit operation, but can be used in conjunction with equalization. It consists of storing a waste water stream in a tank of suitable size and discharging it into the other streams (either domestic sewage or industrial wastes), in proportion to the flow in the receiving waste water stream, so that the mixture does not exert an unduly high organic, hydraulic, toxic load on the receiving body of water or the waste water treatment plant. This method is useful in dealing with toxic wastes, or wastes having a high oxygen demand. 3.2.7 Drying and Incineration Drying and incineration are almost exclusively used for handling waste water sludges generated during the various treatment processes. Drying is done to get rid of a large fraction of the moisture entrained with the sludge solids and to reduce its volume. This is done by spreading the sludge in a layer ranging in thickness from 20 cm to 30 cm on sand drying beds. A part of the moisture evaporates and the rest percolates through the sand and gravel layers of the drying beds. This filtrate is recycled to the plant inlet. Sludge so dried can be removed from the beds when its moisture contents are reduced to between 50% and 55%. If the space for constructing the beds is inadequate, mechanical means such as vacuum filtration, centrifugation, plate and frame presses, or belt filters are employed. Incineration of the dried sludge is practised when the sludge contains toxic matter in concentrations which would have an adverse effect on the receiving medium such as soil. It reduces biological sludge into harmless end products such as water vapour and carbon dioxide. Incineration is preceded by heat drying so that the sludge can be burned effectively and economically. Heat drying is necessary when the sludge is to be converted into soil conditioner. Drying permits grinding of the sludge, reduction in its weight and prevention of continued biological activity. The moisture is reduced to 10% or less. Heat drying is achieved UNIT OPERATIONS AND UNIT PROCESSES 15 by using flash dryers, spray dryers, rotary dryers, multiple hearth dryers, or multiple effect evaporators. Care must be taken to ensure that incineration does not give rise to problems of air pollution. 3.2.8 Freezing When impure water (such as an industrial effluent) is frozen, the ice crystals formed are essentially pure water. Three steps are involved in the freezing process. First, heat is removed from the water to cool it to its freezing point. Additional heat is then removed by the vaporization of a refrigerant such as butane in direct contact with the cooled water. This causes fine crystals of ice to freeze out of the solution. When roughly half the water is frozen to ice, the ice-water slurry is transferred to another tank where the unfrozen liquid is drained off and the crystals are washed with pure water. The washed ice is transferred to another tank, where it is melted to form a pure end product. 3.2.9 Foaming Foam separation is particularly useful for waste water containing foaming agents such as detergents and other surface-active pollutants. The process takes advantage of the tendency of surface-active pollutants to collect at a gas-liquid interface. A large interface is created by passing air (or gas) bubbles through the liquid. The foam becomes enriched in the pollutant and the liquid is depleted of the pollutant. The foam is subsequently collected and collapsed to produce a solute-rich liquid product. 3.2.10 Dialysis and Osmosis When an electric potential is impressed across a cell containing mineralized water, cations migrate to the negative electrode and anions migrate to the positive electrode. If cation- and anion-permeable membranes are placed alternately between the electrodes, ions will concentrate in alternate compartments and become dilute in the intervening compartments. If the apparatus is arranged so that the concentrated and dilute streams flow continuously, large-scale demineralization of water can be done. Only partial demineralization is possible by this method. As the phenomenon is specific to ions, application of the process is limited to the removal of soluble ionized contaminants. When solutions of two different concentrations are separated by a semi-permeable membrane, water tends to pass through the membrane from the more dilute side to the more concentrated side and produces concentration equilibrium on both the sides of the membrane. The driving force that impels this flow is related to the osmotic pressure of the system. If the pressure on the more concentrated side is deliberately increased, the flow of water through the membrane reverses, i.e. water 16 INDUSTRIAL WASTE WATER TREATMENT moves from the more concentrated compartment to the less concentrated compartment. This is reverse osmosis. This process can be used for the recovery of caustic soda from the spent caustic resulting from mercerizing of cloth in the cotton textile industry. 3.2.11 Adsorption Organic contaminants in waste water, which are resistant to biodegra- dation and are present in dissolved state, are suitably removed by the process of adsorption. This is a surface phenomenon. It involves collection of the contaminants on a suitable interface, which can be a liquid and a gas, a solid, or another liquid. This process is used as a polishing step for improving the quality of an effluent, which has already received treatment for removal of a bulk of the contaminants. Adsorption occurs in the activated sludge process, when the dissolved and colloidal organic matter, which acts as a substrate for microorganisms, concentrates at the biomass-water interface. In the treatment of dye-bearing textile wastes, powdered activated carbon is used to adsorb the difficult-to- degrade dye molecules on the carbon along with the microorganisms. The return sludge, which contains both the carbon and the adsorbed contaminants, is aerated in a separate tank. The microorganisms in the sludge consume the dye molecules, get desorbed in the process and are returned to the aeration tank. 3.2.12 Gas Transfer Gas transfer is a process by which gas is transferred from one phase to another, usually from gaseous to liquid phase. The functioning of aerobic processes such as the activated sludge process, trickling filtration, aerobic digestion of sludge depends on the availability of sufficient oxygen. Chlorine, used as a disinfectant, must be transferred from gaseous phase to liquid phase. Postaeration of treated effluents depends on gas transfer. One process for nitrogen removal consists of converting the nitrogen to ammonia and transferring the ammonia gas from water to air. Industrial wastes containing volatile solvents can be conveniently treated by aeration to strip off a large fraction of the solvents, which may be recovered, thereby helping to reduce the COD of the waste water to some extent and permitting the reuse of the solvents. 3.2.13 Elutriation Elutriation is a unit operation in which a solid, or a solid-liquid mixture is intimately mixed with a liquid for the purpose of transferring certain components to the liquid, e.g. chemical conditioning of anaerobically digested sludge before mechanical dewatering can be done by washing the digested sludge with water containing low alkalinity. Such a sludge UNIT OPERATIONS AND UNIT PROCESSES 17 contains a high concentration of alkalinity, which consumes a lot of conditioning chemicals. Elutriation transfers the alkalinity from the digested sludge to the wash water. The wash water is returned to the waste treatment plant. Elutriation can be done as a single stage, multi- stage, or countercurrent process. It is seldom used today because the finely divided solids washed out of the sludge may not be fully captured in the main waste water treatment facility [2, 5]. 3.3 UNIT PROCESSES 3.3.1 pH Correction pH correction is an almost universally used unit process required to render a waste water stream fit for further treatment in which pH value plays a vital role, e.g. ammonia removal, biological treatment, nitrification and denitrification, disinfection with chlorine, phosphorus removal and coagulation. Chemicals commonly used are sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid, lime, sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonium hydroxide, etc. This step in treatment requires adequate mixing between the waste stream and neutralizing chemical. As an equalization tank is provided with mixing arrangement, pH correction can be conveniently combined with equalization. Care should be taken to check whether pH correction results in increasing the suspended solids in the waste water, because some solids, which are in solution at low pH may precipitate out due to increase in pH. It is advisable to conduct laboratory-scale studies in order to choose the right type of neutralizing chemical and its optimum dose, as this has a direct bearing on the operating cost of treatment. 3.3.2 Coagulation Coagulation is used to aid the removal of suspended solids from waste water by sedimentation. Chemicals commonly used for this purpose include alum, ferric chloride, ferrous sulphate, ferric sulphate, lime, etc. Sedimentation following coagulation results in an increase in the volume of sludge to be handled. Organic polymers are sometimes used as coagulant aids. Sludge resulting from chemical coagulation may be difficult to dewater, or difficult to biodegrade. Therefore, laboratory-scale trials should be made to select the appropriate coagulant and to provide for handling, treatment and disposal of the sludge generated by coagulation. 3.3.3 Oxidation and Reduction Oxidation-reduction is occasionally used to remove pollutants from industrial wastes, e.g. reduction of hexavalent chromium to its trivalent 18 INDUSTRIAL WASTE WATER TREATMENT form before its removal by precipitation, ozone oxidation to remove dissolved organics and cyanide during pretreatment. Alkaline chlorination is preferred to ozone treatment for destruction of cyanides. Oxidation with chlorine is used for BOD reduction, odour control, as an aid to grease removal, reduction of sludge bulking in the activated sludge process, eliminating ponding and fly nuisance in trickling filtration and wet scrubbing of gas from anaerobic digestion. Chlorine is also useful in minimizing biological aftergrowths in pipelines and conduits conveying treated effluents over long distances. If a biologically treated sewage or industrial waste is to be subjected to tertiary treatment with a view to recycle it, chlorine plays a vital role in keeping down biological growths in the tertiary treatment units as well as in the recycle system. Chlorine has been successfully used in the treatment of cyanide- bearing wastes, textile wastes, phenol-bearing wastes, oil refinery wastes, paper mill wastes, food processing wastes and tannery wastes. In these applications, chlorine acts in one or more of the following ways: (i) as an oxidizing agent, (ii) as a bleaching agent, or (iii) as a disinfectant, depending on the nature of the pollutants present in the waste water. As industrial wastes are highly variable in their quality, it is necessary to determine the proper dose of oxidizing agents by conducting laboratory- scale studies. 3.3.4 Aerobic and Anaerobic Processes Aerobic and anaerobic processes aim at converting non-settleable organic matter into settleable organic matter and to stabilize it. Purification of waste water containing biodegradable organic matter is economically done with the help of microorganisms, which may be inherently present in the waste water, or may be introduced to it in the form of domestic sewage, or in certain cases, as pure cultures of organisms for destruction of specific pollutants. Provision of proper environmental conditions to the microorganisms, such as adequate balanced food, availability of dissolved oxygen (for aerobic systems), total absence of molecular oxygen (for anaerobic systems), absence of pollutants toxic to microorganisms, correct pH value, proper temperature of waste water (depending on whether the organisms are mesophilic or thermophilic in nature), sufficient time for microorganisms to grow and complete the biochemical reactions (which result in destroying a large part of the pollutants) and the presence of inorganic cations and anions in concentrations below the toxic limits for the organisms, results in producing a satisfactory effluent. It is apparent that almost all industrial wastes need some form of pretreatment before they can be subjected to biological treatment. This may take the form of one or more of the above-mentioned unit operations and/or unit processes, which are aimed at creating environmental conditions fit for microorganisms to work in. UNIT OPERATIONS AND UNIT PROCESSES 19 Microorganisms require carbon, nitrogen, phosphorus and other elements in proper amounts, so that their metabolic activities are not hindered, e.g. the C:N:P ratio should be in the range 100:5:1100:20:1 for aerobic treatment. Anaerobic organisms being inherently slow acting, require much less nitrogen and phosphorus compared with aerobic organisms. Inorganic ions required by most organisms, which act as micronutrients, are given in Table 3.1. Table 3.1 Inorganic Ions Necessary for Most Organisms Substantial quantities Trace quantities Sodium (except for plants) Iron Potassium Copper Calcium Manganese Phosphate Boron (required by plants and certain protists*) Chloride Molybdenum (required by plants, certain protists* and animals) Bicarbonate Vanadium (required by certain protists*and animals) Cobalt (required by certain plants, animals and protists*) Iodine (required by certain animals) Selenium (required by certain animals) *Protists include algae, protozoa and fungi. Dissolved oxygen is essential for truly aerobic organisms to survive. Its desirable concentration ranges between 12 mg/l in the activated sludge process, trickling filter process and aerated lagoons. It is maintained by mechanical or pneumatic aeration in activated sludge process and aerated lagoons. Adequate ventilation and limiting the depth of media to about 2 metres in trickling filters using stone media, along with uniform distribution of the waste water over the media, ensures proper aerobic conditions. The depth of the filter can be considerably increased with the use of plastic media, which have large voids (90% to 95%) compared with stone media (40% to 45%) and permit adequate ventilation in spite of great depths (up to 12 metres, against 2 metres for stone media). Oxygen is supplied in large quantities during daytime by the algal activity in waste stabilization ponds. Some organisms use oxygen available from nitrates in waste waters. These are anoxic organisms and are useful in the process of denitrification. Anaerobic organisms cannot tolerate even a small concentration of oxygen. Great care must, therefore, be taken when they are employed in treatment, to see that molecular oxygen has no access to the anaerobic biomass. Toxic and inhibitory concentrations of organic and inorganic pollutants are frequently observed in industrial wastes. These can either slow down the biological process of treatment or stop it altogether if present in large concentrations. Microorganisms adapt themselves to the presence 20 INDUSTRIAL WASTE WATER TREATMENT of these pollutants to some extent. A list of some of these substances, along with their threshold concentrations, is given in Table 3.2 and in Table 3.3 the list of common inhibitors of anaerobic digestion is given. Table 3.2 Threshold Values for Aerobic Biological Treatment [7, 8] Item Threshold for biological treatment pH value 6.58.5 Sulphides 200 mg/l Phenols 500 mg/l (after adequate acclimatization) Chloroform extractables (oil and grease) 50 mg/l Hexavalent chromium 2 mg/l Maleic acid 400 mg/l Oxalic acid 200 mg/l Acrylonitrile 400 mg/l Benzoic acid 500 mg/l Lead as Pb 1 mg/l Nickel as nickel chloride 15 mg/l Zinc 10 mg/l Chlorides 800015,000 mg/l Ammonia 1,600 mg/l Dissolved salts 16,000 mg/l Table 3.3 Some Common Inhibitors of Anaerobic Digestion Substance Toxic threshold Anionic detergents 900 Methylene chloride (CH2Cl2) 100 Chloroform (CHCl3) 0.51.0 Carbon tetrachloride (CCl4) 2.010.0 1,1,1,trichloroethane 2.25 1,1,2-trichloro-1,2,2,trifluoroethane About 10 Monochlorobenzene 900 Orthodichlorobenzene 900 Paradichlorobenzene 1300a Pentachlorophenol 1 to 2. Cyanide 330b Zinc 590c Nickel 530c Lead 1800c Cadmium 1000c Copper 850c Note: Concentrations in mg/l for a digester fed with raw sludge with 4.5% dry solids. aConcentration in sewage entering works. bInitially very toxic, but bacteria acclimatize with time. cToxicity can be controlled by precipitation as non-toxic sulphide salts. UNIT OPERATIONS AND UNIT PROCESSES 21 It should be noted that in waste water treatment, one rarely comes across situations where only pure cultures of microorganisms can be used to treat the waste water. This is mainly because waste water contains a large variety of organic and inorganic pollutants, which microorganisms can use as substrates. As a result, biological treatment takes place in the presence of a variety of microorganisms, all acting on specific substrates either simultaneously or sequentially. This condition can lead to either stimulation or inhibition of reactions, e.g. Table 3.4 shows the stimulatory and inhibitory concentrations of alkali and alkaline earth cations on anaerobic digestion. Table 3.4 Stimulatory and Inhibitory Concentrations Cation Stimulatory Moderately inhibitory Strongly inhibitory Sodium 100200 35005500 8000 Potassium 200400 25004500 12,000 Calcium 100200 25004500 8000 Magnesium 75100 10001500 3000 Note: (a) Concentrations in mg/l. (b) Sodium and potassiumbest antagonists, effective when present in stimulatory concentrations. (c) Calcium and magnesiumPoor antagonists, add to toxicity. They become stimulatory if another antagonist is already present, i.e. 7000 mg/l of sodium and 300 mg/l of potassium decrease retardation by 80%. Add 150 mg/l of calcium and eliminate inhibition. Calcium is ineffective in the absence of potassium. It may be mentioned here that while designing an industrial waste water treatment plant using biological process, the population of microorganisms being dealt with is a mixed culture. They act on different constituents of the waste water at different rates depending on a number of factors such as environmental conditions, presence of inhibitory or toxic substances, nature of the pollutants and their degradability, stage of growth of the organisms, their doubling time, etc. The slowest rate of degradation determines the overall rate of treatment. Adequate hydraulic retention time must, therefore, be provided in the biological reactor to ensure that washout of organisms does not take place. This precaution is necessary especially at the time of startup and commissioning of the plant. The time given to the microorganisms at this stage also helps them acclimatize to the presence of various constituents of the waste water and produce a satisfactory quality of treated effluent. REFERENCES 1. Water Environment Federation (1994): Manual of Practice FD-3, Pretreatment of Industrial Wastes, Virginia, USA. 22 INDUSTRIAL WASTE WATER TREATMENT 2. Metcalf and Eddy (1991): Wastewater Engineering-Treatment, Disposal and Reuse, New York: McGraw Hill. 3. U.S. Department of Health, Education and Welfare (1965): Advanced Waste Treatment Research, AWTR-14S, Report (PHS Publication No. 999-WP-24), Washington, D.C. 4. Porter, J.J. (1971): State of the Art of Textile Waste Treatment, Water Pollution Control Research Series. 12090 ECS 02/71. Washington, D.C.: Environmental Protection Agency. 5. Water Pollution Control Federation (1959): Sewage Treatment Plant Design, WPCF Manual of Practice 8, Washington, D.C. 6. Water Pollution Control (1976): Chlorination of Wastewater, Manual of Practice no. 4, Washington, D.C. 7. Eckenfelder, W.W. Jr. and D.L. Ford (1970): Water Pollution Control- Experimental Procedures for Process Design, New York: Jenkins. 8. Rao, and D. Kantawala (1979): Flow sheets on biological principles and design criteria, Paper presented at the course on Performance of Industrial Effluent Treatment Plants, VJTI, Bombay, May. 9. Mosey, F.E. and M. Foulkes (1984): Control of the anaerobic digestion process, in Sewage Sludge Stabilization and Disinfection. Alan Bruce (Ed.), Chichester, U.K.: Ellis Horwood. 10. McCarty, P.L. (1964): Anaerobic waste treatment fundamentals, Public Works Jour. Manual no. 8, pp. 245249. Chapter 4 STREAM POLLUTION AND SELF-PURIFICATION The effect of discharge of waste water into natural water bodies should be considered from the following two interrelated standpoints: l The effect of waste water on the water environment. l The effect of the receiving water on the waste water. The first effect considered is pollutional effect while the second effect is self-purification. These two factors are discussed in the following sections. 4.1 EFFECT ON THE WATER ENVIRONMENT The effect of waste water on the water environment may be physical, chemical and biological. Physical effect includes increase in turbidity and suspended solids, addition of colour, taste- and odour-producing substances, and formation of sludge banks on the beds and sides of the water bodies. Industrial wastes such as cooling waters from power stations, dyeing and printing wastes from textile industry, spent wash from alcohol distilleries, etc. raise the temperature of water in the receiving body and reduce the dissolved oxygen content in it. These conditions impart an aesthetically unacceptable appearance to the water, create an environment unsuitable for aquatic creatures such as fish, render it difficult to treat, and initiate the chain of chemical and biological effects. Chemical effects include a drastic change in the pH value of the receiving water due to a discharge of acidic wastes such as mine drainages 23 24 INDUSTRIAL WASTE WATER TREATMENT or alkaline wastes such as textile wastes. High chlorides render the water unacceptable as a source of drinking water, high sulphates, under favourable circumstances tend to form hydrogen sulphide and produce malodorous condition; nitrates and phosphates encourage algal and other aquatic growths; toxic and inhibitory substances either wipe out the aquatic life or severely limit its growth, and, most importantly, reduce the available dissolved oxygen in the water. The dissolved oxygen may even become zero in the presence of a slug of oxygen-demanding waste water. It takes considerable time for the receiving water to regain its original quality and re-establish the beneficial aquatic life in it. Thus, one polluter, located upstream, can create serious problems for downstream users. Biological effects due to industrial wastes alone are not very serious because many of them do not contain pathogenic organisms that are present in domestic sewage. An exception to this is the tannery waste which contains anthrax bacilli. When industrial wastes are discharged in combination with domestic sewage, biological effects become significant although a large number of microorganisms in the sewage are killed by unfavourable environmental conditions in the industrial wastes. The physical and chemical effects mentioned above have an adverse effect on the aquatic biological life, e.g. turbidity and suspended solids, along with colour, cut-off penetration of sunlight into the water and reduce photosynthetic activity. Suspended solids can choke the gills of fish and kill them. Organic suspended solids settle to the bottom of the receiving body of water and in the presence of microorganisms, decompose anaerobically. The products of anaerobic decomposition gradually diffuse to the upper layers of water and add to the total oxygen demand. Anions such as chlorides, sulphates add to the total dissolved solids content of the water and interfere with the metabolic process of microorganisms. Nitrates and phosphates encourage enormous algal growth in the water. Dead algal masses settle to the bottom and add to anaerobic conditions. Toxic and inhibitory cations such as mercury, chromium, cadmium, copper, etc. reduce the growth of microorganisms or even wipe out the microbial population, if present in high concentrations. Dissolved oxygen in the water, which is so essential to the survival of micro- and macroorganisms, is reduced and may even become zero under heavy polluting conditions. This leads to either the migration of fish populations or large-scale fish kills, which in turn add to the dead organic matter already present in the water. Toxic heavy metals and radioactive pollutants enter the biological food chain and ultimately reach the human and animal consumers. The net result of all the above effects is a drastic reduction in the usefulness of the receiving body of water. STREAM POLLUTION AND SELF-PURIFICATION 25 4.2 EFFECT ON THE WASTE WATER 4.2.1 Self-Purification 4.2.1.1 Disposal into flowing water Natural bodies of water have the ability to tackle pollution and regain their original quality in due course of time. Thus, when a polluting stream is discharged into these waters, the first step is that of dilution, the degree of dilution depending on the relative volumes of the polluting stream and receiving waters. Due to a decrease in the velocity of the polluted stream, the process of gravitational sedimentation occurs next. This may be compared with primary sedimentation in a waste water treatment plant. Pollutants such as oil and grease, which are lighter than water, float to the surface, as in an oil-water separator. Organic pollutants, which settle to the bottom, undergo anaerobic degradation, similar to the anaerobic digester, while the lighter components remain near the surface of the water and undergo aerobic decomposition. If the pollution load is light, natural aeration, which takes place continuously, provides necessary oxygen to the microorganisms in the water for oxidation of the organic matter. This is similar to aerobic treatment process in the waste treatment plant. In short, a flowing stream is similar to a waste treatment plant, but the steps of purification take place at a slow pace. In a waste water treatment plant, we create conditions similar to those in the flowing stream, but increase the rate of purification by regulating the flow rate through the plant, putting in energy from an external source and separating the stabilized end products, so that the entire process of purification takes place in a reasonable length of time. Natural water is saturated with dissolved oxygen, or nearly so. When polluted waste water is discharged into it, microorganisms by breaking down the oxidizable organic matter use the available oxygen in the water. This results in depleting the dissolved oxygen in the water. On the other hand, atmospheric oxygen is continuously entering the water through the air-water interface, the rate of entry being a function of the difference in partial pressure of oxygen in the air and in the water. Oxygen is also produced during daytime due to photosynthetic activity of algae in the water. The exertion of biochemical oxygen demand (BOD) by microorganisms is called deoxygenation, while replenishment of oxygen is called reaeration. When the pollutant load is small, aerobic conditions prevail even in the presence of the pollutant. The interplay between deoxygenation and reaeration produces the dissolved oxygen profile, known as the oxygen sag curve. The basic differential equation for the combined action of deoxy- genation and reaeration states that the net rate of change of dissolved oxygen (DO) deficit (i.e. the difference between DO saturation value at the temperature of the mixture of waste water and stream water and 26 INDUSTRIAL WASTE WATER TREATMENT the calculated DO of the mixture of waste water and stream water) equals the difference between (1) the rate of oxygen utilization by BOD in the absence of reaeration and (2) the rate of oxygen absorption by reaeration in the absence of BOD. Expressing the rate of oxygen utilization by BOD in the absence of reaeration, the BOD equation by a first-order reaction, d (y ) = k1(La yt) dt t where yt = BOD at time t (mg/l), k1 = deoxygenation rate, t1, La = first stage ultimate BOD, (mg/l) of the mixture of waste water and stream water, and the rate of oxygen absorption by reaeration in the absence of BOD by the equation d (D ) = k2Dt dt t where Dt = DO deficit at time t (mg/l) and k2 = reaeration rate, t1, the net effect of deoxygenation and reaeration can be written as the difference between the two above equations. The negative sign in the reaeration equation indicates that the rate of DO deficit decreases with time. The BOD equation is written as yt = La 1 10 k1t The reaeration equation is written as Dt = Da. 10 k2t where Da = DO deficit at time t = 0. Thus, d (D ) = k1La 10 k1t k2Dt dt t Integrating this equation between t = 0, Da and t = t, Dt, we obtain La (Dt) = k1 (10 k1t 10 k2t ) + Da. 10 k2t k2 k1 This is the equation of the oxygen sag curve. Figure 4.1 depicts the dissolved oxygen sag and its components, viz. deoxygenation and reaeration, with the simplifying assumption that the receiving water is being affected by only one stream of polluted liquid. In practice, this rarely happens. Analysis of an actual case is done by breaking up the stretch of the stream into individual lengths, each length spanning the point of discharge and the point of inflection. As an engineering concept, the oxygen sag curve possesses two characteristic points: (i) a point of maximum deficit, or critical point, with coordinates Dc and tc, and (ii) a point of inflection, or point of maximum rate of recovery, with coordinates Di and ti. The critical point is defined by the mathematical requirement STREAM POLLUTION AND SELF-PURIFICATION 27 dD = 0 dt d2 D and 0 dt 2 Figure 4.1 Oxygen sag curve. Differentiation of the oxygen sag curve equation with respect to t gives the values of critical time and flow time to reach the point of inflection. It also gives the values of critical deficit and deficit at the point of inflection. In addition to the phenomena of deoxygenation and reaeration, solids, which are deposited at the bottom of the receiving body of water, add to the oxygen demand (known as benthal demand) due to decomposition of organic matter in the deposited solids in the presence of microorganisms. If the receiving body is a flowing stream, the extent of this demand depends on its flow regime, e.g. when the stream flows with a low velocity, solids are deposited, while during high velocity flows, the deposited solids are scoured and flow with the water to a location where the velocity of the water is insufficient to keep them in suspension. Under quiescent conditions, gases of decomposition in the benthal deposits, mainly carbon dioxide, hydrogen sulphide and methane, bubble up in sufficient volume to lift rafts of sludge from the bottom. This can occur in streams in which the temperature of the water is high, so that microbial activity is high and the rate of decomposition is also high. If the benthal demand is high, it is incorporated in the equation for oxygen sag curve by replacing k1 by k1 + k3, where k3 represents the deoxygenation rate due to decomposition in the bottom deposit. It is essential to note that the numerical values of the rates of deoxygenation, reaeration and benthal demand are temperature dependent. Therefore, whenever numerical problems on oxygen sag curve are to be solved, the resultant values of the mixture of stream water and waste water, i.e. 28 INDUSTRIAL WASTE WATER TREATMENT temperature, first stage BOD, and DO saturation value must be determined first. This should then be followed by determining the values of k1, k2 and k3 at the resultant temperature. These rates are usually determined at 20°C and should be converted to their corresponding values at the resultant temperature. Formulae for these conversions are k1 at t °C = k1 at 20°C (1.047)(t20) k2 at t °C = k2 at 20°C (1.024)(t20) Generally, the contribution from benthal deposits is considered to be less important than that due to deoxygenation and reaeration. 4.2.1.2 Disposal into lakes and coastal waters The natural dispersal of waste water into lakes and coastal waters is often poor. These receiving waters are usually heavier than the waste water. So, a discharge at the surface results in the waste water overrunning the receiving water. A discharge below the surface results in the waste water surfacing like a smoke plume and on reaching the surface, fanning out radially. Because chemical diffusion is slow, natural dispersion of these waters becomes a function of wind, current and tides. Hydrographic exploration of the receiving waters will show how effective natural dispersion or dilution will be and whether water intakes, bathing beaches, shellfish layings and shore properties will be polluted. In lakes and ponds, displacement- and wind-induced currents as well as temperature and other density effects govern the degree of mixing. In the sea and its estuaries, tidal currents and the volume of the tidal prism are added variables. Mathematical treatment of wind-induced mixing, dispersal by currents and disposal into tidal estuaries is given in. REFERENCE 1. Fair, G.M., J.Ch. Geyer and D.A. Okun (1968): Water and Wastewater Engineering, Vol 2: Water Purification and Wastewater Treatment and Disposal, New York: Wiley. Chapter 5 PRETREATMENT OF INDUSTRIAL WASTES Pretreatment involves some unit operations and unit processes, suitably combined and arranged in a logical sequence, in order to produce an effluent fit for further treatment. If the raw waste contains non- biodegradable pollutants, pretreatment is mostly given by using unit operations. If the waste has toxic and biodegradable pollutants in it, pretreatment aims at removing the former and creating conditions favourable for the microorganisms to work effectively on the latter. It is often observed that a combination of unit operations and unit processes is necessary to achieve the desired degree of pretreatment. Unit operations frequently used are screening, sedimentation and flotation, mixing, equalization, and gas transfer. Unit processes employed include pH correction, coagulation, oxidation and reduction. A combination of the unit operations and processes is required when the above-mentioned unit processes are carried out, because they tend to convert solids in solution or colloidal state into settleable state. A detailed treatability study of the waste water reveals which of the unit operations and/or processes will be useful. Pretreatment is also required when waste water is to be discharged into a common sewer, leading to the common effluent treatment plant, or to the municipal sewer leading to the sewage treatment plant. In the case of wastes which are very strong, the industry may be required to provide even biological treatment and produce an effluent acceptable for discharge to the common effluent treatment plant or the sewage treatment plant. The applications of the pretreatment steps as applied to various industrial wastes are briefly described in the following sections. 29 30 INDUSTRIAL WASTE WATER TREATMENT 5.1 UNIT OPERATIONS Screening: This is used for the removal of floating, large-sized matter such as rags from textile industry, fine fibres of wool in woollen mills, bark and wood chips in pulp and paper industry, rubber bungs, bottles, vials, etc. in pharmaceutical industry, fruit peelings and rinds in the fruit canning industry, spent tan bark, hairs, fleshings, leather trimmings in the tanning industry, dead yeast and other solid agents used in winery and brewery, empty plastic bags, bottles and cartons in dyes and dye intermediate industry, dairies. Sedimentation: This method is used in plain or following coagulation in the ceramic industry, mining industry, ore beneficiation, iron and steel mills, vegetable and mineral oil refining, paper making industry, beet sugar manufacture, lubricant manufacture and engineering industry. Flotation: This method is used in soaps and oil industry, paper industry, detergent manufacture, mining industry, lubricant manufacture. Mixing: This is used in industries producing effluent streams with different pH values, as in chemical manufacture, regeneration and rinse waters from demineralization units, blowdown from boiler houses and cooling towers. Equalization (in-line or off-line): This method is used in batch manufacturing processes such as pharmaceuticals, fermentation products, industries producing different effluent streams with wide fluctuations in quality, toxic chemical manufacturers, e.g. herbicides, pesticides, weedicides. Gas transfer: This method is used in chemical manufacture using volatile solvents, waste water containing large concentrations of ammonia, pharmaceutical industry, waste water containing highly biodegradable pollutants as in the dairy industry. The purpose in this case is to reduce the chances of waste water from becoming stale or septic and creating odour nuisance. Gas transfer, in the above cases, is achieved by using compressed air, which supplies oxygen and helps to keep the waste water solids in suspension. A gas, such as ammonia, can be removed by stripping in a closed tower packed with ceramic rings. 5.2 UNIT PROCESSES pH correction: This method is used where wastes containing high concentrations of oil and grease, with high or low pH values, high in suspended solids which can be coagulated and settled, with high biodegradable contents in addition to high or low pH, containing heavy metals which can be precipitated by adjustment of pH value. PRETREATMENT OF INDUSTRIAL WASTES 31 Coagulation: This is used where all waste streams mentioned under pH correction except those with high biodegradable contents are handled unless the resulting sludge with high organic content can be properly treated and disposed of. Oxidation and reduction: This is used in wastes with chemically oxidizable pollutants, certain organic molecules which become amenable to biological treatment after oxidation, wastes containing heavy metals which can be precipitated, cyanide bearing wastes, phenol containing wastes, etc. An incidental benefit of oxidation is partial satisfaction of the oxygen demand of the waste. Aerobic and/or anaerobic treatment: Aerobic or anaerobic treatment follows pretreatment by the above-mentioned unit operations and unit processes. Subjecting them to anaerobic process, followed by aerobic process, economically treats wastes, high in biodegradable matter. In these cases, anaerobic treatment knocks out a sizeable portion of the oxygen demand from the waste and allows aerobic treatment to be done economically, especially in terms of power consumption for mechanical methods of oxygen supply and land requirement for non-mechanical methods. An added advantage of anaerobic treatment is the availability of methane, a useful component of the result of gases formed during anaerobic degradation. The digested solids from anaerobic process can also be used as soil conditioners. The various methods described above help end of the pipe treatment of the waste water. Further, economy in treatment can be achieved by using one or more of the following measures: 1. Reduction of water consumption in the manufacturing process. 2. Reduction of the strength of waste water. 3. Modifying the manufacturing process. 4. Replacing polluting chemicals by those, which pollute less, or are non-polluting. Some good housekeeping practices are: 1. Recycling and reusing, either with or without treatment, slightly polluted waste water streams. 2. Recovering by-products from the waste streams, where feasible. Execution of the above measures must be done only after ensuring that they do not have an adverse effect on the quality of the finished product. It is possible to economically control pollution due to industrial wastes by following these measures. Chapter 6 TEXTILE WASTES This chapter deals with waste waters generated in the processing of cotton, wool, rayon, semi-synthetics, synthetics, silk and jute. Raw materials used in the manufacturing process are subjected to various physical, chemical and biological changes aimed at removing the natural impurities from the raw material, separating the cleaned portion and modifying its physical and chemical structure to get the desired end product. These natural impurities, along with the chemicals and other cleansing agents used in the process, find their way into the waste water streams and contribute to their polluting characteristics. The ease or difficulty with which these waste waters are treated depends mainly on the nature of the impurities, their concentration, their degradability and amenability to various treatment processes. Methods of disposal of the treated wastes are determined by local conditions and requirements of the pollution control authorities, in addition to the factors mentioned earlier. In general, textile industry offers good opportunities for effective treatment of its waste water, recovery of valuable chemicals and by-products from the wastes, and recycle and reuse of water used in the manufacturing process. 6.1 COTTON TEXTILE WASTES 6.1.1 Raw Material Raw cotton, as obtained from the cotton bush, is pure cellulose, containing impurities such as waxes, gums, pectin, leaves, soil and cotton seed. It 32 TEXTILE WASTES 33 is picked from the fields, the seeds are removed by ginning, the resulting balls of cotton are pressed into bales and sent to the mills for further processing. The manufacturing process starts with the removal of loose dirt, cleaning the fibres and opening them for easy alignment. This step facilitates the operation of spinning. The cotton, thus converted into yarn, is then woven into raw cloth, known as gray goods. Impurities sticking firmly to the cotton fabric are then removed by a number of chemical processes. The fabric is then subjected to bleaching, mercerizing, dyeing and printing. Mills where gray goods are manufactured are called spinning and weaving mills. Further treatment of the gray goods is done in the process house. Mills, which start with raw cotton and end up with finished cloth, are known as composite mills. 6.1.2 Manufacturing Process A general manufacturing flow sheet for cotton textile fabric is shown in Fig. 6.1. Individual operations are explained in the following sections. 6.1.3 Spinning, Weaving and Sizing (or Slashing) This is a dry process and requires the use of water only for maintaining relative humidity in the spinning and weaving section in the range 70%75%. The cotton balls are spun into yarn and subjected to sizing (or slashing). This reduces the chances of the yarn breaking during the weaving process. The sizing agent usually used is starch, or carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) or suitable mixture of these compounds. The agent, especially starch, is applied to the cloth in the form of a paste. The woven cloth is called gray goods and is singed, i.e. passed quickly between two rows of burning gas flames in order to burn the fuzz, or tiny ends of cotton protruding from the surface of the cloth. It is then dipped into a tank containing water to prevent it from catching fire. Thereafter, it is sent to the process house for further treatment. Waste products of spinning and weaving consist mainly of loose dirt accompanying the raw cotton and cotton dust or willow dust, which is pure cellulose. The loose dirt is disposed of with other solid waste from the mill, while the cotton dust is (i) burned as fuel in the mills boiler, or (ii) sold to outside parties as filling material in quilts. A better use of the cotton dust is to digest it with cow dung under anaerobic conditions for the production of methane gas [1, 2, 3], which can be used within the mills canteen or for singeing operations. The cotton dust can also be composted aerobically or anaerobically to give a manure of good quality [4, 5, 6]. Another waste product in the spinning and weaving operations is the starch paste, which may be present in the waste water stream if the vessel in which the paste is prepared is washed with water. This adds considerably to the BOD of the waste water, because starch is a 34 INDUSTRIAL WASTE WATER TREATMENT Figure 6.1 Cotton cloth manufacture in a composite mill. TEXTILE WASTES 35 biodegradable substance. This BOD load can be easily reduced by first scraping dry the vessel containing the starch paste, recovering the paste and then washing the vessel with minimum quantity of water. 6.1.4 Desizing This is done in order to remove the excess sizing agent sticking to the cloth. The desizing compound used may be either the enzyme diastase, or a mineral acid. The cloth is washed after desizing. This is another source of waste water. Water consumption in this (and in almost all washing operations in the textile mill) can be considerably reduced by following the practice of countercurrent washing, i.e. pass the cloth to be washed through a series of tanks containing water which overflows from the last tank in the series of tanks into the next tank, and so on, so that the water travels through this system in a direction opposite to that of the cloth. The cleansing power of the water is thus fully used. (See manufacturing flow sheet.) 6.1.5 Caustic Kiering (or Scouring) This is done either under pressure or at atmospheric pressure. The kier is a tall vessel in which the gray goods are stacked. Chemicals, viz. caustic soda (NaOH), sodium carbonate (Na2CO3), low-BOD detergent and sodium silicate (Na2SiO3) are added and the cloth is subjected to high temperature by injecting steam into the kier. Waxes, gums and pectin, which are firmly sticking to the cotton fibre, are completely removed by this operation. The cloth is removed from the kier and is washed thoroughly to remove the impurities and the chemicals. The waste water from the kier has high pH, dark chocolate brown colour and a frothy appearance. This waste stream should be treated separately and then mixed with the other waste water streams for further treatment. 6.1.6 Chemicking or Bleaching The pale yellow colour of the kiered cloth is removed by using a bleaching agent such as sodium hypochlorite (NaOCl) or hydrogen peroxide (H2O2), followed by water wash. This step facilitates dyeing and printing operations. 6.1.7 Souring The bleached cloth is treated with a dilute acid and washed thoroughly to ensure complete removal of all alkaline chemicals used in the earlier manufacturing operations. 36 INDUSTRIAL WASTE WATER TREATMENT 6.1.8 Mercerizing This operation gives lustre to the cloth and increases its affinity for the dye. It consists of washing the cloth with caustic soda solution of 25%30% strength. The caustic soda solution becomes weak with repeated use. When its strength is reduced, the spent caustic is removed from the mercerizing bath and sent to caustic recovery section, where either dialysis or multiple effect evaporation is done to separate the water from the caustic solution and build up its concentration. The recovered caustic is then topped up to the required strength and is reused. The mercerized cloth is washed with jets of water. This wash water is also collected and sent to the caustic recovery section. 6.1.9 Souring The mercerized cloth is washed first with dilute acid to remove traces of caustic soda and the salt resulting from the reaction between the caustic soda and the dilute acid is removed by a water wash. The cloth is now ready for dyeing, printing and finishing operations. 6.1.10 Dyeing and Printing The bleached cloth is coloured with dyes of various types, and printed with various designs, depending on market demand. The wash waters from these operations are highly variable, depending on the type of dyes, dye chemicals and printing chemicals used. Dyeing and printing chemicals are designed to impart fast colour and non-fading designs to the cloth. As a result, waste waters produced in these sections are difficult to treat, especially from the point of view of colour removal and reduction of oxygen demand. Thorough washing of the dyed and printed cloth is done before it is dried, folded and stored. General characteristics of waste water streams from various sections are given in Table 6.1. Table 6.1 General Characteristics of Waste Water Chemical Pollutants in Nature of process waste water waste water Desizing Starches, fats and waxes High BOD (about 45% of total) Scouring (Kiering) Caustic soda, waxes, greases, Strongly alkaline, soda ash, sodium silicate, dark brown, high BOD fibrous matter (about 30% of total) (Contd.)... TEXTILE WASTES 37 Table 6.1 General Characteristics of Waste Water (Contd.) Chemical Pollutants in Nature of process waste water waste water Bleaching Hypochlorite, chlorine, Alkaline, about caustic soda, acids, 4% of total BOD hydrogen peroxide, sodium silicate Mercerizing Caustic soda Strongly alkaline, low BOD Dyeing Various dyes, auxiliaries, Strongly coloured with chemicals, soap varying hues, about 4% of total BOD Printing Colours, thickeners, Highly coloured, auxiliaries about 8% of total BOD It is apparent from the manufacturing flow sheet that almost every operation involving chemical action on the cotton is followed by water wash, which makes the cotton textile industry a water-intensive industry. It also suggests the possibility of reuse (with or without treatment), recycling and reduction in water consumption. A rough breakup of the water consumption in various operations is given in Table 6.2. Table 6.2 Operationwise Water Consumption Item of consumption Per cent of total Humidification in spinning and weaving 713 Boiler house 1016 Process water (wet processing and finishing) 6080 Sanitary and miscellaneous uses 47 6.1.11 Reuse and Recycling From Table 6.2, it is seen that water used for humidification is lost and cannot be recovered. Boiler blowdown may be used for floor washing, while the condensate can be fed back to the boiler. Waste water from sanitary uses can be subjected to tertiary treatment and made suitable as boiler feed water. The major consumer of water is the process house, in which wide scope for reuse, recycling and reduction of water consumption is available. Some of the steps, which can be taken, are: 1. Reducing the number of washes and using hot water for washing operations. 2. Replacing running dye bath with standing bath. 3. Using countercurrent washing method wherever feasible. 38 INDUSTRIAL WASTE WATER TREATMENT 4. Observing good housekeeping practices. 5. Employing machinery which works on low material to liquor ratio. 6. Recovering exhausted dye bath contents, making up their chemical constituents to the required level and reusing them. This step saves water as well as chemicals. Water from 20%40% can be saved by using the above methods. Studies on the water consumption in the textile mills in Mumbai showed that nearly 15%20% of the water could be reused without any treatment, while about 50% could be reused after chemical treatment and filtration. 6.1.12 Substitution of Chemicals Another way to reduce pollution is substitution of low polluting chemicals for high polluting chemicals, e.g. replacing starch for sizing with CMC or PVA, replacing non-ionic detergent based on nonyl phenol ethoxylate by alfa olefin sulfonate or fatty alcohol ethoxylate in kiering, using formic acid instead of acetic acid in dyeing, employing reactive dyes in place of vat dyes and azo dyes. Another interesting example is that of reducing sulphides in aniline dyeing wastes by using a mixture of molasses and other suitable chemicals. An interesting paper on BOD of textile chemicals was presented by McCarthy during the proceedings of the American Association of Textile Chemists and Colorists. The paper gives an updated list of textile chemicals and expresses their BOD values as per cent rather than milligrams per litre. Multiplying the per cent figure by 10,000 does conversion to mg/l. McCarthy compares the BOD values of starch substitutes with BOD of starch (6001000 mg/l), giving a range 416 mg/l for CMC and 375 mg/l for HPAN + Globe corn starch. 6.1.13 Modifications to Machinery Modifying the existing machinery and/or the manufacturing process in order to reduce the generation of waste water is another way. It is observed that continuous operations require less space and use less water and chemicals than batch operations. Whenever possible, separate operations such as desizing and scouring of cotton fibres should be combined. Use of suitable solvents for desizing and scouring offers a drastic reduction in water consumption, as the process requires very little water. However, complete recovery of the solvent is essential in order to avoid the problem of air pollution. The printing machine is provided with a scraper blade to remove excess printing paste from the rollers. The scraped paste is collected in a trough located directly under the rollers. This paste is reused for producing combination shades. This method is particularly applicable to the printing of thin cotton cloth. A drastic process modification TEXTILE WASTES 39 is not to discharge the effluent at all, but to pump the process liquor to a storage tank, where it is saved to make-up the next similar bath. 6.1.14 Good Housekeeping Practices Good housekeeping practices include the following: 1. Minimum use of chemicals in the process house 2. Prompt attention to leaks of water and chemicals 3. Dry-cleaning of the floor, followed by mopping the floor with a piece of wet cloth 4. Educating the workers on the importance of maintaining clean surroundings 5. Segregating strong waste streams from weak streams, providing separate conveyance system for rain water and, in general, ensuring that only the minimum amount of waste water will have to be handled at the treatment plant. The last step is usually difficult, or in some cases, impossible to implement in existing mills, but should be given serious consideration at the design stage of a new mill. These and the other steps mentioned go a long way in reducing, and in some cases, eliminating pollution, decreasing the consumption of water and chemicals and reducing the overall cost of treating the waste water streams. 6.1.15 Characteristics and Treatment of Raw Waste Water As previously mentioned, waste water contains natural impurities and chemicals used in the manufacturing process. The process of spinning, weaving and sizing produces negligible amount of waste water. Desizing produces a waste stream whose nature depends on the chemicals used in the process. Kiering generates waste water high in temperature, total dissolved solids, suspended solids, pH value, colour, BOD and, in general, has a frothy appearance. Waste water from bleaching section contains the unreacted bleaching agent and can be conveniently mixed with the remaining waste streams. Souring produces an effluent low in pH and can be used to partially neutralize the high alkalinity of the other waste streams. Mercerizing discharges waste water high in pH, low in suspended solids and BOD. Dyeing and printing are the two operations from which highly variable quality effluents are produced. Typical polluting loads of cotton processing wastes are given in Table 6.3. 40 INDUSTRIAL WASTE WATER TREATMENT Table 6.3 Polluting Loads Process pH value BOD Total solids BOD load Total (mg/l) (mg/l) (kg/ solids 1000 kg load goods) (kg/ 1000 kg goods) Desizing 17005200 1600032,000 14.816.1 6670 Kiering 10.013.0 6802900 760017,400 1.517.5 1947 Scourung 50110 1.363.02 Bleaching 8.59.6 901700 230014,400 5.014.8 38290 Mercerizing 5.59.5 4565 6001900 10.513.5 185450 Dyeing Aniline black 4055 6001200 510 100200 Basic 6.07.5 100200 500800 1550 150250 Developed 5.010.0 75200 29008200 1520 325650 Direct 6.57.6 220600 220014,000 1.311.7 25250 Naphthol 5.010.0 15675 450010,700 25 200650 Sulphur 8.010.0 111800 420014,100 2250 3001200 Vat 5.010.0 1251500 17007400 1230 150200 Note: Numerical figures indicate the pollution loads which the various processes generate. They can be converted into corresponding concentrations if the volume of waste water produced is known. Chemical analysis of the waste water produced in a typical cotton textile mill is given in Table 6.4. Table 6.4 Chemical Analysis of Composited Waste Water Item of analysis Concentration pH value 11.2 Suspended solids 1000 5-day 20°C BOD 1000 Chemical oxygen demand 2200 Oil and grease Nil Phenolphthalein alkalinity as CaCO3 480 Methyl orange alkalinity as CaCO3 1200 Dissolved oxygen 1.0 Note: All values except pH are in mg/l. It is seen from Table 6.4 that the waste water is high in pH, suspended solids, BOD and COD. It is essential to treat it before discharge either to the municipal sewer, or to an inland receiving body of water, or the sea, or on land for irrigation. Fortunately, the organic pollutants in the waste water (except some of the dyes) are biodegradable. So, the waste water can be conveniently mixed with domestic sewage. As the waste is TEXTILE WASTES 41 deficient in nitrogen and phosphorus, nutrient supplementation is necessary, although domestic sewage supplies a part of these elements. Process houses, which produce kier waste in large volumes may be required to treat this waste separately before mixing it with other waste water streams for further treatment. Kier waste treatment usually consists of neutralizing the high alkalinity and pH value with a suitable acid such as sulphuric acid, or passing flue gas into the waste. Coagulating the suspended solids in the waste with lime, settling overnight and discharging the supernatant from the reaction tank for mixing with the rest of the waste give further treatment. Alternatively, the waste can be subjected to aerobic biological treatment after reducing the pH to 8.5 or lower. Nutrient supplementation with urea and diammonium phosphate is necessary for successful biological treatment. In either case, the generated sludge is dewatered and disposed of on land. As already mentioned, the pollutants in cotton textile waste water are generally biodegradable and may be suitably mixed with domestic sewage before treatment of the mixture. It is a usual practice to use activated sludge process or its modification for this purpose. Pretreatment in the form of fine screening, followed by equalization and pH correction ensures that the biological treatment is not adversely affected by fluctuations in quality and flow rates of the raw waste water. Nemerow recommends dispersed growth aeration for cotton finishing waste water. Laboratory and pilot plant studies have shown BOD reduction of 46.8% after 12 hours of aeration at reduced rate of air supply. This step had the advantage that high concentrations of sensitive sludge did not have to be handled during treatment. Biological activity during aeration produced carbon dioxide, which helped to reduce the excess alkalinity in the waste water and reduce the cost of neutralizing chemical to some extent. A BOD removal of 71% was achieved when the waste water was neutralized prior to biological treatment. Williams and Hutto, Jr experimented with aerated lagoons followed by secondary settling of textile wastes. They got a BOD removal efficiency of 75%80% after 48 hours of aeration. Sihorwala and Reddy studied the effectiveness of polyelectrolytes on the treatment of cotton textile wastes. They subjected waste water samples from two separate channels in the textile mill, one carrying waste from desizing, mercerizing and kiering while the other carrying waste from dyeing, printing and finishing operations. A mixture of the two waste streams was also studied. They found that waste water from the first stream was less amenable to coagulation than that from the second stream, while the mixture showed amenability which was between that of the two streams. Effectiveness of the treatment was assessed on the basis of colour removal, COD removal and suspended solids removal. The coagulants used were alum, ferric chloride and Catfloc-T. 42 INDUSTRIAL WASTE WATER TREATMENT 6.2 WOOLLEN MILL WASTES Woollen fibre, unlike cotton (which is a fibre of vegetable origin), is a fibre of animal origin. Wool, as removed from the sheeps back, contains considerable quantities of dirt, grass, burrs picked up during grazing, excreta, the dried perspiration of the sheep called suint, and the wool grease discharged from the animals glands, which protects its skin during growth. Grease wool may contain as little as 30% fibre and 70% foreign matter, which must be removed before the fibre can be used in textile manufacture. The sheeps perspiration consists largely of potash salts and water-soluble organic acids. The wool fat, or yolk, which is impure lanolin, is insoluble in water, but soluble in certain organic solvents. For every 100 kg of finished wool, 250 kg of raw wool has to be processed. In extreme cases, raw wool may contain as little as 30% fibre, 70% foreign matter, of which 45% is grease. Contribution of wastes comes from the operations of scouring, carbonizing, dusting, bleaching, dyeing and finishing operations. The manufacturing operations are defined as follows: 1. Grading: The raw wool is graded according to its quality. 2. Dusting: Impurities are removed by mechanical means by subjecting raw wool to a dusting/opening operation, whereby the wool is opened up and is ready for the scouring process. Considerable dirt, amounting to 5%15% of the total impurities, is removed in this step and is disposed of in a dry condition. 3. Desuinting/Scouring: Desuinting is done by washing the wool in water at a temperature 50°55°C. Scouring with alkali, soap and soda ash follows this. This step contributes from 55% to 75% of the total BOD load and removes almost all natural and acquired impurities from the wool fibre. Water used ranges 7000 to 10,000 litres per 100 kg wool fibre. 4. Drying: Passing it through a series of driers dries the scoured wool. 5. Carbonization: It is a process in which hot concentrated sulphuric acid is used to convert the vegetable matter in the wool into loose, charred particles, which are mechanically crushed and then taken out of the wool in a machine called duster. This step produces some solid waste but little liquid waste. 6. Dyeing: During this process, a hot dye solution is generally circulated by pumps through the wool, which is packed in a removable metal basket suspended in a kettle. Waste water generated by dyeing is highly coloured and contains many toxic substances. BOD contribution from this stream is 1%5% of the total BOD load. 7. Oiling: The traditional oiling agent used is olive oil. It is mixed with water and sprayed on the wool. Oiling increases cohesion of the fibres and aids in spinning operation. The amount of oil used varies from 1% to 11% of the weight of wool. All of this oil is removed from TEXTILE WASTES 43 the woven cloth later in the finishing operation. The percentage contribution of BOD from this waste varies with the type of oil used. 8. Spinning and weaving: The wool fibres are spun and loosely woven into cloth. 9. Fulling: The loosely woven wool from the loom is shrunk into tight, closely woven cloth. There are two common methods of fulling, viz. alkali fulling and acid fulling. In alkali fulling, soap or synthetic detergent, soda ash and sequestering agents are used. In acid fulling, the fabric is impregnated with an aqueous solution of sulphuric acid, hydro

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