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Advances in wastewater-treatment technology have arguably been driven to a large extent by the imposition of strict guidelines and directives on pollution control over the last few years. But, with the potential fall-out from a global economic downturn weighing heavily on the minds of most companies in the textile industry, solutions that push down costs and maximise efficiency are set to take on a new dimension as the sector comes under increasing pressure to adapt.

One such treatment technology involves German company Körting, which has found a way of recovering diluted caustic soda (weak lye) by evaporating water. The Caustic Soda Recovery System (CSR-System) for mercerising lye separates the weak lye into strong lye and vapour condensate. The strong lye, which is recovered, can then be reused for mercerising.

However, depending on the quality of the fabric, it might be advisable to clean the lyewith hydrogen peroxide. So, Körting has developed a lye-cleaning system with peroxide to ensure that the recovered lye meets high quality standards during production. The vapour condensate is slightly alkaline, soft water, without any hardness, and has a temperature of approximately 80°C. It can be used for washing, for example, in a mercerising or bleaching machine, or in other pretreatment.

With the Caustic Soda Recovery System requiring steam and cooling water, almost the same amount of steam used for the recovery of the mercerising lye can be saved in the hot-water generation, where cooling water is heated up to 60 – 80°C. The company claims the system is very energy efficient, especially when the hot-water generation is integrated into the central hotwater system, but there is no direct contact between the heating steam and the lye, allowing the heating-steam condensate to be re-used as boiler feed water without additional treatment.

Körting also adds that during textilefinishing processes, especially the mercerisation of cotton fabrics, a quantity of weak caustic lye (3 – 7 % NaOH) coming from the rinsing water leaves the stabilisation part of the mercerising machine. Companies operating without lye recovery can re-use a small quantity of the weak lye (rinsing water enriched with NaOH) from the stabilisation bath in other production departments, including boiling and pre-washing processes, but most of it is waste liquid which, Körting says, not only results in financial losses but also breaches laws governing the discharge of waste fluids.

For the recovery of mercerising lye, its solution is through the use of specially developed evaporation plants that concentrate the weak lye by partly evaporating water. Such plants are said to have following characteristics and advantages:

● Lye is recovered, enabling it to be used again in the finishing process and therefore saving the costs for fresh lye

● Without the use of neutralising agents the pH value of the wastewater is reduced considerably

● The correct integration of the evaporation plant within the complete lye cycle (lye balance) eliminates the problem of socalled surplus lye for the wet-on-wet mercerisation process

● Only steam is used to run the evaporation plant. The total heat capacity of this steam, when leaving the evaporation plant, is used to generate hot water. The evaporation plant has no energy consumption

● The evaporated, slightly alkaline vapour condensate is available for washing purposes, for example, in the stabilsation bath directly after the impregnating bath or in the pre-washing area, free from any water hardness and already hot. Sometimes this water will be used as boiler feed water after a specific treatment

● The hot steam condensate resulting from the first evaporation stage can directly be used as boiler feed-water

Apart from the water-evaporation rate, a mill’s total energy usage is reputed to be the most important factor in choosing a suitable evaporation plant, as the number of stages, pre-heaters and additional heaters determines how much hot water is required.After the evaporation process its heat is used to generate hot water. However, additional steam is not needed when the evaporation plant operates within the mill’s hot-water requirements. To achieve this, the evaporation plant must be designed accordingly.

In principle, Körting says it is better to generate a smaller quantity of hot water at 85°C, for example, than a larger quantity at 60°C, which then needs to be heated up with direct steam just for the machines. If there is already a surplus of warm water in the factory, heating up the existing warm water via the evaporation plant will result in economical energy usage.

To have the correct number of stages, a complete cost calculation is needed, which should take into account the mill’s total heat usage and the actual evaporation plant itself. The necessary investment costs must be set against the savings for the mill’s total energy requirements, fresh lye, the reduced quantity of waste liquid and the expenditure for neutralising agents.

With a rising number of stages, the specifically required heating steam (kg/h heating steam per kg/h water evaporation) is reduced. Investment costs, however, increase with the rising number of stages, therefore it is usually not economical to have more than four stages. A waterevaporation rate of 5,000kg/h is believed to demand a two- or three-stage plant, with a water evaporation rate of 15,000kg/h requiring a three- to four-stage plant. As only water is removed from the weak lye during the evaporation process, dirt, dyes, fibre and sizing residues, along with other foreign substances resulting from pretreatment, remain in the recovered lye.

The degree of pollution depends on the condition of the materials during mercerisation, influenced by the method of pretreatment of the fabric. It is claimed that a singed, desized and well-washed material will not contaminate the lye too much, while bleaching and rinsing before mercerisation is also thought to be a good method to retain a clean lye cycle. But cleaning the strong lye by passing it through a sufficiently dimensioned strong-lye sedimentation tank, combined with a special designed H2O2 (Peroxide) cleaning system, has proved to be the best cleaning method within the recuperation process, according to Körting.

Joseph Egli Italia Srl, on the other hand,has developed a system that depurates textile wastewater. The company claims the treatment can recover up to 98% of water and 80% of brine. The system consists of the following steps:

● Biological wastewater treatment at outlet with water quality suitable for discharge to surface recipient or for tertiary treatment to be re-used

● Water reuse system

● Filtration at 100 micron

●Ultrafiltration

● Multistage Reverse Osmosis, with production of a permeate back to textile process

● Nanofiltration, permeate back to production, recovered brine back to textile process

● Mechanical evaporation, eventually followed by solar evaporation, for nonrecyclable matter concentration, that finally undergoes a landfill disposal

The company has also recently developed and tested systems for dyehouse wastewater, using a Brazilian dyehouse that processes cotton and poly-cotton blends as a case study. It performed pilot tests, simulating anaerobic treatment (AMMBR), which reportedly showed that the water colour was reduced significantly after eight hours.

Results:

● Colour range in the homogenisation = 1650 – 2750 U Pt/Co

● Average colour after 8 hours in the AMMBR phase = 105 – 180 U Pt/Co