In this article we will discuss about the methods and techniques used to treat and control wastes and waste water from paint industry. Learn about:- 1. Introduction to Paint Industry 2. Waste-Water Generation from Paint Industry 3. Waste-Water Characteristics 4. Reduction of Waste-Water 5. Waste-Water Treatment Methods 6. Waste Generation.
- Introduction to Paint Industry
- Waste-Water Generation from Paint Industry
- Waste-Water Characteristics of Paint Industry
- Reduction of Waste-Water from Paint Industry
- Waste-Water Treatment Methods for Paint Industry
- Waste Generation from Paint Industry
1. Introduction to Paint Industry:
Paint industry uses varied raw material such as resins, solvents, drying oils, pigments and extenders. The major waste generated by the paint manufacturing industry are empty raw material packages containing trace elements, equipment cleaning wastes and spills.
Empty raw material packages are generated during unloading of materials to high speed mixers or mixing tanks. Water solvents are generated from equipment cleaning. Even after distillation of waste solvents for reuse, a residual paint sludge remains.
The paint sludge contains solvents and residual toxic metals such as mercury, lead and chromium. Waste rinse water is generated from equipment cleaning with water and for caustic solutions. Wastes containing undispersed pigments are contained in waste filter cartridges.
The equipment cleaning wastes can be minimised by employing more efficient cleaning methods, like reduction in the frequency of equipment cleaning, use of rubber wipers to reduce the amount of paint left on the walls of a mix tank, use of teflon-lined tanks to reduce adhesion and use of plastic or foam to clean pipes to improve drainage.
2. Waste-Water Generation from Paint Industry:
On close examination of manufacturing process of various paints, it is observed that water is not a constituent in any of the production processes of resins, varnishes, lacquers, etc. However, water is required for the manufacture of stiff or water-based paints and cooling of the ball mills or sand mills in manufacture of oil paints.
Water is used for washing of floors which are littered by spillage of powders, solvents, etc. when a particular batch is finished. In cases where there is vast difference in the colour shade of next batch, the mills and containers, etc. have to be cleaned. This cleaning is done by xylene which is recovered and reused.
Once in a while the vessels are cleaned with caustic soda. This cleaning constitutes the major part of the waste-water generated which is highly alkaline in nature. Other source is the cooling tower blow down. The mills used for grinding powders and the solvents used in mixing have to be maintained at room temperature.
They are cooled by circulating cooling water. Cooling tower blow down generally enters the effluent stream. In summary, it can be stated that waste-waters from the paint manufacturing industries generally tend to be alkaline, contain some oil and grease and Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Suspended Solids (SS). The waste-waters can be assumed to contain small amounts of the products.
The BOD and COD values give only a gross measure of organics in the wastes. Some of the organic and inorganic compounds used in the manufacturing operations are classified as toxic and hazardous. Chromium, copper, lead, zinc, ethyl benzene, di-(2-ethylhexyl) phthalate, tetrachloroethylene and toluene were found in high concentrations.
3. Waste-Water Characteristics of Paint Industry:
1. Waste-Waters from Caustic Cleaning:
It was observed during survey that caustic cleaning resulted in maximum production of waste-water in the paint industry. Vessels used in manufacturing processes have to be cleaned before change of product. This is done using water and alkali. Thus, the waste-water pH is in range of 8.5 to 13.5.
This wastewater consists of suspended solids in the range of 200 to 600 mg/l. BOD varies from 475 to 2400 mg/l and the COD is between 1100-3800 mg/l. The oil and grease is within a range of 32-150 mg/l. The phenolic concentration of this waste-water is 12.5 mg/l.
2. Waste-Water from Resin House:
The waste-water from resin house is that which accumulates from the condenser into the separator. The water layer is disposed off in the waste-water stream and the upper layer of solvents is reused.
The water from separator is generally acidic with pH range from 3.2 to 6.3. The suspended solids range from 240 to 400 mg/l. The BOD and COD of waste-water vary from 225 to 60,000 mg/l and 240 to as high as 78,000 mg/l respectively. The phenolics are between 6 and 86 mg/l. The oil and grease is within a range of 14-25 mg/l.
3. Waste-Water from Stiff Paints:
Stiff paints are water base paints and the main product is distemper. Cleaning of hoppers, grinders and containers consume large amount of water and contributes 400 to 700 mg/l of suspended solids, a large fraction of which is settleable.
As observed during the survey, characteristics of the waste-water stream are COD 1215 to 6000 mg/l, BOD 380 to 980 mg/l and phenolic content varies from 6.4 to 100 mg/l. The oil and grease content of the waste-water is 252 mg/l.
4. Combined Waste-Water:
The characteristics of combined waste-water is presented in Table 17.10.
4. Reduction of Waste-Water from Paint Industry:
Schemes for reducing the generation of waste-water at source should be practised. This is to reduce the effluent load rather than finding methods to treat it. Unnecessary use of water not only adds to the quantity of effluent and the cost of treating it, but also increases the wastage of heat, power and/or product in the effluent.
Steps that can be taken to reduce the generation of waste-water are discussed below:
Good housekeeping reduces generation of both waste-waters and solid wastes. The cooling water is usually uncontaminated and thus, should be collected and reused. It could be used for floor washing or discharged separately into the receiving water bodies rather than mixing with polluted water and discharging into the treatment plant.
Accidental spills and leakages should be reduced to a minimum through proper maintenance of equipment and training of personnel. In case of caustic cleaning, instead of washing away the caustic solution, it can be collected, stored and used for further cleaning.
In case of stiff paints, water from first cleaning should be collected and used later as process water for a similar type of batch. Waste-water volume can also be reduced through reuse of rinse water for preparation of alkali solution. The above procedures can reduce the quantity of generation of caustic cleaning water significantly.
Recovery of Wastes:
A large number of solvents are used in paint manufacturing and a majority of them are recovered and therefore not lost in the waste-water streams. In case of oil paints, solvents are added in grinders which are closed units, therefore, loss of solvents through evaporation is considerably reduced.
High temperature is maintained in resin and varnish manufacture, resulting in evaporation of solvents added. These solvent vapours, along with the water vapours generated through chemical reactions are condensed and collected in a separator. The solvent layer is removed and reused in the next batch.
Combined effluent from paint industries can be satisfactorily treated using the usually physico-chemical and/or biological treatment methods. The treatment consists of coagulant addition and adjustment of pH to an optimum level for maximum precipitation, the precipitated material is removed by gravity separation, either on batch basis or in a continuous flow tank.
Ultra filtration and activated carbon adsorption have also been tried but have not been found to be cost effective. Using physico-chemical processes, removals of 90 per cent or greater can be achieved for some significant toxic pollutants.
Even after this treatment if some toxic pollutants remain, biological treatment can reduce their concentration. Some of the organics which remain in the supernatant after physico-chemical treatment are biodegradable and can be removed through biological treatment.
5. Waste-Water Treatment Methods for Paint Industry:
Considering the practice of waste-water treatment followed in the country and the performance efficiency of each operation, the best alternative is evolved. The treatment system consists of physico-chemical treatment units followed by biological treatment units.
1. Oil and Grease Removal:
Effluents from all units except stiff paint section and caustic cleaning waste are passed through an oil and grease removal device.
Effluent from caustic cleaning operation is highly alkaline in nature and requires neutralisation prior to further treatment. An equalisation-cum-neutralisation tank is provided with an agitator. Effluent from stiff paint is mixed with the neutralised waste-water, dosed with a coagulant and sent to flash mixer. The effluent is then subjected to clariflocculation.
The effluent is clarified in clariflocculator and subjected to biological treatment. Sludge generated in this unit is carried to the sludge drying beds for dewatering.
1. Extended Aeration:
Domestic waste-water from the factory premises is mixed with the supernatant from clariflocculator and is biologically treated by extended aeration process.
2. Secondary Clarification:
Mixed liquor from the aeration tank overflows to the secondary clarifier. The settled sludge is recycled continuously through return sludge pumps to the aeration tank and excess sludge is discharged to sludge drying beds. Effluent from the secondary clarifier is fit for discharge to the environment.
3. Sludge Drying:
Sludge from oil and grease trap, clariflocculator and secondary clarifier is dewatered on sludge drying beds. Filtrate from these beds is returned to equalisation-cum- neutralisation tank.
The above treatment process is expected to achieve 90-95 per cent efficiency of removal of pollutants and thus acceptable to the recipient environment.
The following minimum steps are recommended for waste-water treatment:
1. Adjustment of pH.
2. Removal of oil and grease.
3. Removal of suspended solid.
4. Removal of toxic substance.
The proposed waste-water treatment system should be supported by segregation, reduction, generation and recycling of waste-water. Thus dyes are substances capable of colouring fabrics in such a manner that the colour cannot be removed by rubbing or washing. Every coloured substance is not a dye. For example, azobenzene is of orange red colour, but it is not a dye, because it is not capable of colouring the fibre.
6. Waste Generation from Paint Industry:
The water usage in the industry is mainly for the following purposes:
1. Synthesis of the dyes and dye intermediates.
2. Steam generation and cooling system.
3. Washing and rinsing of reaction kettles, filter press, floors, etc.
4. Domestic and other miscellaneous activities.
The water consumption pattern varies widely from one industry to another. In the same industry the rate of water consumption often changes due to frequent changes of the feed material synthesis reaction and desired products. The change of product pattern needs cleaning and washing which consumes a substantial quantity of water.
Thus water requirement of a dye and dye intermediate industry depends on the following factors:
1. Type of dye produced.
2. Number of products.
3. Gross production.
4. Pattern of working of factory, i.e. continuous or in one shift only.
5. Frequency of change of product pattern, etc.
In-depth study reveals that in general, process water consumption is highest, next to it is the cool and boiler make-up water requirement. The water needs for domestic purposes is the lowest.
It is found that for production of one kg of vat dye, water consumption is the largest (1528 to 10345 l/kg) whereas naphthol dye consumes the least quantity of water (6 to 17 l/kg.). The generation of waste-water follows the trend of consumption of water. In Table 17.11 comparative quantities of water consumption and waste-water generation are provided.
The waste-water quantity varieties according to the number of batches of products manufactured in a day, week or month, the duration of synthesis of the dye in the reactor vessel and the duration of the washing and rinsing operations.
The frequency of discharge may be intermittent in the small dye units, operating one or two batches per day, but in the case of the medium and larger units, which operate a number of reactors simultaneously, waste-water discharge is continuous.
The waste-water generation sources are as follows:
1. Process waste-water including left out mother liquor.
2. Washing and rinsing wastes.
3. Sanitary and other miscellaneous waste-water.
Characterisation of Waste Effluents from Paint Industry:
Some of the effluents while manufacture dye are produced across the world generate 10 per cent effluent, in which 2 per cent is generated from manufacturing, while 8 per cent is from colouring. Colour matter in waste water comes from makers and users of colouring matter i.e. dyestuffs, pigments, textiles, dyeing units and tanneries.
The pulp and paper sector and distilleries which use raw materials with colour as a by-product, also discharge colour matter into waste water. Effluent left after dyeing contains unused dyes in the form of organic and inorganic compounds, toxic metals, suspended and dissolved solids.
Liquid effluents from dye manufacturing are mainly waste-water (biodegradable and non-biodegradable) or high COD waste-water. About 15 per cent of the dye that is manufactured and used by the industry is discharged into the water. Of this, approximately 2-3 per cent is discharged by the dye manufacturing industry and about 14-15 per cent by the textile dyeing industry.
The process waste-water is mainly the mother liquor left over after the product is isolated and separated by filter press. This waste-water is of smaller volume and highly concentrated in terms of pollutants. The vessel washings also contain similar type of pollutants but with lower concentration.
It has been identified that the waste-waters of the industries have the following characteristics:
1. High levels of BOD and COD.
2. High acidity.
3. High TDS.
4. Deep colour of different shades.
5. High levels of chlorides and sulphates.
6. Presence of phenolic compounds.
7. Presence of heavy metals, e.g. copper, cadmium, chromium, lead, manganese, mercury, nickel and zinc.
8. Presence of oil and grease.
The dye industry waste-waters, if derived from naphthalene and anthracene bases are resistant to biodegradation. The colour removal is also not adequate by the conventional chemical and biological treatment.
The characteristics of the combined waste-water of a dye industry is presented in Table 17.12. In Tables 17.13 and 17.14 the characteristics of combined waste-water of two dye industries engaged in the production of other products are given.
Reduction of Waste-Water from Paint Industry:
It is essential to have proper in-plant control measures before going for waste-water treatment.
Some of the relevant measures in-plant are summarised below:
1. Reduction of Waste:
The volume of waste-water can be reduced by proper control of freshwater consumption. The cooling water blow-down may be reduced by raising the concentration factor. Timely maintenance of the units may be done to prevent leakage, spillage, etc.
Minimal usage of water for washing and rinsing may be practised. The last wash water may be recycled for first washing. Application of counter-current washing may also reduce waste-water generation.
Dry cleaning of the floor is preferred. When floor washing is absolutely necessary, the treated waste-waters may be used. The pollution load can be reduced by recovery of chemicals and solvents as far as practicable. Spills, leakages, overflows, etc. may not be allowed to join the waste-water stream.
Storm waters need segregation to reduce the volume of waste-water. Similarly the uncontaminated and less contaminated waste-water streams like cooling water blow-down, boiler blow-down, condensate, etc. are to be segregated and should not be permitted to join the process waste-water stream.
The highly contaminated and coloured mother liquors should be segregated and collected separately. Sometimes strong wastes of the reaction vessels are required to be discharged. These unforeseen strong wastes are to be collected in separate holding tanks and drawn into the waste-water treatment plant at a regulated rate.
3. Process Modification:
The production process equipment should be modified so as to generate less wastes. The raw materials used in the synthesis may be substituted by choice of more readily biodegradable chemicals.
4. Effluent Control:
The waste-water streams originating as mother liquor and strong waste-waters should be segregated and collected in two separate collection tanks. The rest of the process wastewaters may be diverted to the equalisation pond.
Oil and grease and the floating matters present in the waste-water streams are to be arrested by putting up appropriate control measures at the boundary limit of each plant. The waste-water collected in equalisation pond will be monitored to ascertain the concentration of pollutants.
Based on the data obtained, the waste-waters of holdings, ponds are to be diverted to the equalisation pond at regulated rates so that the concentration of pollutants are acceptable to the biological treatment system.
The equalisation of waste-waters is followed by pH adjustment and clarification in primary clarifier. Clarifier may be of sludge blanket type. Ferrous sulphate and lime may be used for precipitation of heavy metals. The overflow of the clarifier after pH adjustment will flow to the extended aeration type biological treatment unit.
The effluent of biological treatment unit will be clarified in convenient type clarifier using coagulant. The reduction of colour may be achieved by segregating and controlled discharge of the mother liquor, which also contains most concentrated form of the chemicals left after dye synthesis.
It is envisaged that substantial colour reduction of the waste streams may be achieved by the three-stage treatment system as proposed above.
However, if the desired limit of colour (400 Hazen unit) is not achieved by the above treatment then one or both of the following steps are to be adopted:
1. Evaporation of the segregated mother liquor and washings of vessels by solar evaporation or by indirect heating using steam.
2. Powered activated carbon adsorption of the treated waste-water.