Treatment at a stretch

Typically, 100kg of raw wool will contain 65kg of fibre; the rest is dirt, vegetables and wax. Cleaning ­ scouring ­ and processing, entails detergent, oil and grease. Glynn Skerratt of the Centre for Environmental Technology at Staffordshire University, and Robert Skelton, Koch Membrane Systems, consider membrane treatment.

Wastewater from the wool scouring industry is very polluting, and contains substantial amounts of detergent, oil and grease. It can have a chemical oxygen demand (COD) concentration of over 250,000mg/l, and a temperature in excess of 60ºC. Historically, the preferred disposal option for this wastewater was to discharge it to sewer after appropriate pre-treatment, but pressure for water re-use through implementation of environmental management systems and cost reduction measures now makes water recycling increasingly attractive. Membrane treatment systems are helping to make this possible.

Sometimes, scouring also takes place after the yarn has been spun. This is done to remove oily lubricants, which are added to the wool to improve processing during spinning. Both types of scouring use essentially the same kind of scouring machine.

Necessary penalties
A scouring machine typically consists of three or four double bowls, the inner bowls having perforated sides to allow the scoured dirt to pass through. The bowls also feature a series of forks or rakes which move the wool through the length of the bowl to a pair of rollers which squeeze the water and dirt, or the lubricating oil, out of the fibres.

The wool, or yarn, is fed into the first bowl, which contains a solution of hot water and neutral detergent. It then passes to the second, and, if relevant, third bowls, which contain a cooler and weaker scouring solution, before passing to the final bowl for rinsing.

The alternative on-site technologies for the treatment of wastewater from scouring have relied on evaporation and biological treatments. Both of these technologies bring with them necessary penalties in terms of space and cost. However, membrane technology, often in collaboration with evaporation, is now providing a cost-effective and efficient treatment option.

Membranes are permeable or semi-permeable polymeric solids ­ often modified (cellulose-based) natural products such as cellulose acetate, or synthetic polymers such as polyamide, polysulphone, polyethersulphone or polyvinylidene fluoride. The particulate fraction of the waste is separated from the liquid fraction; retained behind the differentially permeable membrane because of its inability to pass through the pores. Exactly what is separated, and how efficiently the process works, is governed by the specific composition of the membrane, the pore size, the particle size distribution in the wastewater and its flow direction and velocity.

There are four main categories of filtration process applicable to the treatment of wastewaters. These are, in order of decreasing membrane pore size, microfiltration, ultrafiltration, nanofiltration and reverse osmosis. At one extreme, microfiltration will separate colloidal solids with a diameter in the range 0.1-1.0 µm, whilst reverse osmosis will separate out most ions and retain ³particles² with diameters down to 0.0001µm.

Ultrafiltration is the mechanism most suited to the requirements of wastewater treatment in the wool scouring industry. Operating at a pressure of between 2-7bar, ultrafiltration membranes have a pore size with a molecular cut-off of around 100,000, making them permeable to most soluble salts.

Youghal Carpet Yarns is based near Cork in Ireland, and produces large amounts of spun and dyed wool yarn for the carpet industry. The first stage of the production process consists of blending the supplied scoured wool to achieve uniformity of quality and colour. Lubricants are then added to the wool to ensure good processing and prevent static electricity build-up, and the wool is spun into yarn. The lubricating oil is then removed in the ³yarn scouring² process and the yarn is dyed before being despatched to carpet manufacturers.

All wastewater produced by Youghal is discharged, after pH balancing, directly into the sea under an Environmental Protection Agency licence. The wastewater comes from two main sources: from the yarn scouring process (incorporating oil from spinning); and from the dye baths. Most of the effluent produced comes from the dye baths and requires only minimum balancing to satisfy discharge requirements. However, without some form of on-site treatment, the total effluent would exceed regulatory limits, which restrict COD to a maximum concentration of 1,150mg/l and BOD to 300mg/l. In order to achieve this quality, the wastewater from the yarn scouring process is treated, using an ultrafiltration plant from Koch Membrane Systems comprising 136, 1in tubular membranes, providing 27m2 of active membrane area, at an operating pressure of 65psi. The system is capable of expansion to 35m2 of active membrane area, simply by adding more tubes, although this extra capacity has never been required in eight years of continuous operation, and the plant is still running on the original set of membrane modules.

COD concentration
In operation, 150m3 of wastewater is generated by the three scouring bowls each week, at a COD concentration of between 70,000-200,000mg/l. It is first pre-filtered to remove any fibres shed from the yarn during the scouring operation ­ minimal because of the efficiency of the 1in tubular membranes. It is then passed, in batch volumes of 60m3, through the membrane treatment plant, which produces, on average, around 1,500l/h of treated permeate. The initial permeate flow rate is around 3,500l/h but, as the batch becomes more concentrated, the permeate rate decreases to around 500l/h. Once the retained volume has reduced to around 7m3, the residual concentrate is then passed through to an evaporator and the remaining semi-solid residue is burnt in the factory¹s own boilers to raise steam.

The treated permeate passing through the membranes has a residual COD concentration of 1,000-2,000mg/l, and by the time this has been diluted by the much larger volume of dye bath effluent, the final total mixture has a COD of 300-500mg/l and is well within regulatory limits. It only needs balancing before discharge to the sea.

Although the cost of installing water-recycling facilities on these sites would currently be too high to justify expenditure, the option is there for the future. Changing economic circumstances might be driven either by potable water cost/availability or by a need to implement a more sustainable water use regime within a company environmental management system. Nevertheless, both of these case studies indicate that membrane treatment systems are able to provide a practical treatment choice for this kind of process effluent.



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