WATER DESALINATION FOR TEXTILE WET PROCESSING
|Abstract Textile wet processing is an all-encompassing term used for bleaching, dyeing, printing and finishing treatments for different types of textile goods like fibres, yarn, tows, woven and knitted fabrics, garments etc. made from both natural and man-made fibres. Wet processing is essential for making the textile materials comfortable and attractive. These embellishing treatments require large quantities of good quality water, a commodity that has acquired the status of a raw material for the textile industry. Unfortunately an important source i.e. sub-soil water is brackish in Pakistan in general and its textile centres of Karachi and Faisalabad in particular. This state of affairs is responsible for the unsatisfactory quality and increase in cost of production of the finished materials.
Quality of brackish water can be improved by different methods but its desalination or demineralization, softening with the reverse-osmosis (R.O.) process is the most feasible. Many textile mills and other industrial units in Pakistan have installed the R.O. plants in the recent past and are very satisfied with the results. This paper discusses the difficulties faced by the industry by using poor quality water and advantages and savings incurred by using the desalinated water. Details about cost of the purifying process for water are also included.
1. Profile of Textile Industry of Pakistan
Textile manufacturing is the major industry of Pakistan and it plays a key role in the economy of the country. The industry exports goods worth more than 6 billion U.S. dollars annually. This is about two thirds of the total foreign exchange earnings of the country. Textile industry also provides at least 40% of the industrial jobs, besides creating vast opportunities for relevant commercial enterprises. These include sales of different textile products, transport of raw materials and the finished goods, supplying of machinery and their spares, packing, forwarding, shipping trades etc. The cotton textile manufacturing consumes more than 9 million bales (1.5 million tonnes) of cotton in addition to 0.5 million tonnes of man-made fibres and 80 thousand tonnes of wool. The industry has not only impressive credentials within the country but is also a major force in the world trade as Pakistan is in the top 10 textile manufacturing and exporting countries.
2. Requirement of Water by Textile Industry
The present day product-mix and manufacturing techniques are a far cry from the pre 1980’s period when Pakistan was exporting raw cotton, yarn, grey cloth and T-shirts, the last named at a ridiculous price of $6 per dozen. Present emphasis is on manufacture of the high-value added finished products and average price of the knitted garments is now $ 45 per dozen. As the name suggests the finished goods need to be bleached, dyed and printed and these processes require plentiful quantities of the good quality water. On the average one Kg of the material requires at least 100 litres of water. Unfortunately the quality-water, found in our perennial rivers, is always in short supply because the first charge on this water is for human consumption and agriculture. The other source is sub-soil water but this is mostly brackish as happens in all semi-arid countries like Pakistan. The saline or brackish sub-soil waters are unfit for bio-consumption and are also unsuitable for most of the industrial uses, including the textile wet processing. The sub-soil waters of the two major textile centres of the country, Karachi and Faisalabad, are highly saline and as such are not suitable for producing high quality finished textile products.
About two decades back, setting up of a textile finishing plant was banned in Karachi only because quality water was not available. Now when the ban has been lifted, all the mills are using sub-soil water for wet processing of their products because water of the rivers ‘Sindh’ and ‘Hub’, that is supplied by the Karachi Water and Sewerage Board (K.W.S.B), hardly meets even 10% of their requirements. The precious KWSB water is, therefore, used mainly for boilers and drinking purposes.
Focussing on the water shortage dilemma of Karachi, it may be observed that this city is the biggest manufacturer of the textile wet processed goods in the country. Karachi has at least 300 registered processing mills that include 200 cloth (woven cotton and man-made fibres), 70 knitting and 30 towel mills. In addition to these about another 50-70 unregistered mills and a large number of factories processing garments, yarns, kitchen towels, mops, laces, jute goods, garment-accessories, etc. also exist that consume fairly large quantities of water. It has been estimated that the wet processing industry of Karachi consumes around 50 million gallons (British) of water every day. This huge supply is met by extracting the sub-soil water either by the mills directly or by the so-called “Tanker” water-suppliers, including the National Logistic Cell.
3. Quality of Sub-Soil Water
The sub-soil water of Karachi is of a poor quality and has high concentration of the dissolved salts or has a high TDS(Total Dissolved Solids). The TDS content varies from 1,500 to 30,000 ppm (parts per million) in different localities of the metropolis and follows no clear-cut pattern of TDS content within a locality. In many factories in the SITE area the TDS ranges between 1,500 to 3,000 ppm but it may go up to 15,000 or even to 25,000 in the nearby mills.
The same pattern of TDS variation exists in the sub-soil waters in Korangi, Landhi and the North Karachi areas. The KWSB water in comparison has a TDS ranging between 250-350 ppm depending on the time of the year, being low in the rainy season and high in winters.
4. Effect of Water Impurities on Wet Processing
High concentration of dissolved salts, especially the hardness-causing calcium and magnesium ones, creates lot of difficulties in wet processing. Nature of the impurities and their effect on the quality of the finished goods are briefly mentioned below.
(a) Organic matter, Turbidity and Color: Turbidity and colour are usually due to presence of organic matter in water and these detract from brightness of the bleached and purity of shade of the dyed goods. The organic matter, whether dissolved or suspended breeds micro-organisms that may develop mildew, fungi etc., which in turn produce coloured spots, foul smell and even holes in the material.
(b) Hardness: Hardness creates many undesirable effects in processing. On heating or coming in contact with alkalis, calcium and magnesium salts are precipitated on fabrics as whitish carbonate and hydroxide particles. Although concentration of these salts is small yet their overall reflection pattern lowers whiteness of the bleached goods and depth of shade of the dyed goods and mars purity and brightness of the hue. In package yarn dyeing, the precipitated particles hinder free-flow of liquor through the packages and tend to cause uneven dyeing. The precipitated salts also impart harshness to the fibres. Excessive presence of these salts also causes uneven dyeing and necessitates addition of expensive sequestering agents. Some processing mills of Karachi have resorted to soften the entire supply of the process water to get more uniform dyeings but softening does not reduce TDS of water and creates some other problems as mentioned in the following Para (c).
– In textile mills, including the spinning and the weaving sections, certain equipments are installed that require circulation of water through pipes. These include boiler, humidification plant and multi-tubular heaters/coolers of the dyeing machines. With passage of time the hardness producing salts in water accumulate to form a hard scale inside the pipes. The scale is a bad conductor of heat and causes wastage of fuel in boilers and lowers efficiency of the other equipment. Removal of the scale from inside the tubes is a time consuming and an expensive proposition and adds to the cost of processing.
(c) Total Dissolved Solids: In addition to the hardness causing calcium and magnesium salts, water contains other dissolved salts that are mainly sodium cations and chloride, sulphate and bicarbonate anions. These sodium salts create certain complications like precipitation of dyes of inherent low solubility.
Presence of excessive amount of sodium ions gives a damp and limp handle to the finished fabric due to their tendency to hold water. The materials dyed in such waters look dull and lack brightness.
High TDS in the boiler feed water causes foaming and carry-over problems that lower efficiency of the boilers and also create difficulties in processing. Further excessive sodium ions in boiler water accelerate corrosion of the iron plates due to their high electrical conductivity. Such waters also require more frequent blow-downs that result in fuel loss.
To sum up the goods processed in the high TDS waters have dull shades, a poor handle and in many cases uneven dyeing. To avoid uneven dyeing expensive sequestering agents are added in the dye bath but still brightness of dyed and printed goods is poor and handle unattractive. These shortcomings lower value of the finished goods.
As discussed above, it appears that the quality-water is going to be in short supply for the industry permanently and this shortage is likely to be progressive. It is, therefore, necessary to consider alternative processes to supplement the existing water sources and the most obvious choice is demineralisation or desalination of brackish sub-soil water and even seawater. There are three major methods that are being successfully used for desalination of water. These are based on the following principles:
1.Vaporization and Condensation.
2. Ion exchange.
3. Reverse Osmosis.
5.1 Vaporization and condensation: In this system, saline water is heated preferably under reduced pressure to boiling point and the water vapours (steam) are cooled to condense to pure water. This process is expensive unless it is made synergistic with power generation. It is also capital intensive and needs high calibre and so expensive expertise to run and maintain the plants. The system has been used in oil producing low fuel-cost countries like Saudi Arabia and UAE but even there the R.O. process has replaced this.
5.2. Ion-exchange Demineralisation Process: This method of water purification is based on the principle of the Water Softening but differs in having two columns of different resins. In the one cations and in the other anions of the dissolved salts are replaced with hydrogen and hydroxyl ions respectively and water of a very high purity or zero conductivity is obtained. After exhaustion, the resins are regenerated: the cationic with a mineral acid and the anionic with caustic soda.
The capital cost of the equipment is lower than the other two systems but cost of the resins and their regeneration chemicals is high and so makes the process uneconomical for the textile industry. This process is mainly used by the pharmaceutical and certain chemical industries where water of ultra-high purity is needed.
5.3. Reverse osmosis process or Hyper Filtration: According to the well-known principle of osmosis when a salt solution is separated from water with a semi-permeable membrane, water molecules tend to move across the membrane towards the salt solution under the concentration gradient. In an enclosed vessel, transport of water molecules through the membrane creates pressure that is known as the “Osmotic Pressure” and is proportional to the difference in concentrations of salt on both sides of the membrane. If pressure is applied on the salt-solution side of the membrane, flow of water is stopped. If pressure exceeds the value of the osmotic pressure, water will start flowing in the opposite direction, i.e. from the salt solution to the waterside. This principle is used in the reverse osmosis process for reducing salt concentration in the brackish water or even the sea water.
The process was known for quite sometime but could become commercially feasible only after robust and long lasting synthetic semi-permeable membranes were developed and became available at competitive prices. The reverse osmosis method of demineralisation of water has now acquired a great commercial significance in the semi-arid countries.
5.3.1 Semi-Permeable Membranes: The semi-permeable membranes are mainly of two types, viz. the spiral and the hollow fibre. Former is a composite of polyamide polymer on polysulphone support membrane. The hollow-fibre module consists of polyamide or cellulose triacetate fibres of 25-250 mm diameter that are sealed on one end. A large number of the hollow fibres are bundled together and placed in a saline water-pressure vessel. Pressure required for making water flow across the membrane depends on salinity of the water, type of membrane and the desired salt removal and it varies from 100 to 400 psi. One square meter of the membrane, whose pore size is of the order of 10-20 Ao, is capable of demineralising about 500 litres or 110 (Br.) gallons of water per day. The pressure pump is usually a multistage type having a throttle valve to control the pressure to the desired level. Power consumption for treating seawater is about 5-9 KWh and for brackish water 2-3.5 KWh per m3 or 220 (Br.) gallons. Life of the membrane depends upon quality of the water and conditions of working. Acidic pH and presence of oxygen, oxidizing chemicals, soil and microbes in water deteriorate the membrane rapidly. The saline water is, therefore, thoroughly pre-treated before passing through the R.O. membrane.
In the following paragraphs information has been collected by a study of two leading textile wet processing mills that were pioneers in introducing the R.O. engineering in their mills. Both the mills use sub-soil water, having a TDS ranging from 3,000 to 6,000 ppm.
The feed water is carefully filtered in 2 stages; in the first or multi-media system particles up to 40 mm are removed and in the second step particles above 5mm are removed in special cartridge filters. The filtering media in the former are gravel and sand and in the latter fine polypropylene fibres. The cartridge filters are replaced with the fresh ones after the filtration pressure reaches a level of 15 psi. Before the first filtration feed water is treated with an oxidizing agent like sodium hypochlorite to destroy any possible microbial growth. Any excess of the oxidizing agent and the dissolved oxygen is next removed by adding a reducing agent that is usually sodium metabisulphite. Before forcing water through the R.O. membranes, it is treated with an anti-scaling agent to minimise formation of scale on the membranes. Hydrochloric acid is also fed at the same time to decompose alkaline carbonates and bicarbonates and a pH of 6.5 is maintained. The R.O. water or the permeate is finally degassed to remove carbon dioxide and its pH is adjusted to 7.5 by adding alkali. The final product may have 2-5% of the TDS of the raw water while TDS of the waste-water is normally not allowed to go beyond 20,000 ppm.
The membranes are periodically back-washed with the R.O. water to which some proprietary cleansing chemicals are also added to remove scale and other impurities. If carefully maintained, the R.O. membranes may last for 4 years but it may be kept in mind that it is very easy to damage these.
6.3 R.O. Process for Sea water: This paper is mainly concerned with desalination of the sub-soil waters but it may be mentioned, for the sake of comparison, that cost of the R.O. plant for sea water is nearly three times and the running cost is higher of the sub-soil waters up to 6,000 ppm TDS.
The net cost of the R.O. water is higher than the KWSB water that is supplied in SITE area of Karachi @ Rs.73/1,000 (Br.) gallons. However, it is lower than the tanker water that is sold, on the average, @ Rs.120/1,000 gallons and has a TDS ranging anywhere from 1,200 to 2,000 ppm. Consideration of regular availability of good and uniform quality of this raw material has encouraged many textile mills in Karachi to install the R.O. Plants. An equally encouraging factor is saving in the cost by at least 30% of the sequestering agent and the printing thickener used in dyeing and printing processes, besides improving the general look and finish of the goods. Many textile mills have installed the R.O. plants and these include M/s Yunus Bros, Siddique Sons, Nakashbandi, Liberty, Afroze, Goodwell, Caravan East, Iqbal Silk, Standard etc. Some prominent non-textile users are M/s Proctor and Gambol, Candyland, Abbot and Knoll Pharmaceutical, Siddique Sons Tin-Plate factory, mineral waters and aerated-drinks manufacturers and many hospitals.
One of the major constraints in setting up of desalination plants in Karachi is insufficient availability of the sub-soil water of a TDS below 6,000 ppm. Some mills have installed as many as 6 pumping wells and still there is shortage of water for their R.O. plant. This situation discourages many mills management interested in the project. In my opinion we have two hitherto unexploited sources of water, i.e. the mills’ own effluent and the municipal sewerage. Effluent of the processing mills is highly colored and has a higher TDS than the sewerage that is about 800-1000 ppm. In case the mill effluent is to be treated arrangements should be made to collect the washings of the bleached and the dyed products separately from the spent bleach and dye liquors. This will reduce the organic constituents (BOD) and TDS of the effluent and make the pre-treatments and R.O. processes more efficient and economical.
The alternative source of the sewerage water has a volume of at least 300 million gallons per day in Karachi. If it is completely got rid of its dissolved and suspended organic impurities, as is done in all the developed countries of the World, it would be an excellent source for textile wet processing. But there is a big “IF” and the purification process can be entrusted only to a private organisation on a commercial basis whose performance be monitored by an independent controller appointed by the consumers. After the usual purification treatments, R.O. desalination should be carried out to bring the TDS of water down to a level of 200-300 ppm and also to ensure complete removal of the organic matter. This suggestion is for a serious consideration of KWSB and the Karachi textile entrepreneurs.