Water- And Energy-Saving Solutions

Wastewater recycling and heat recovery help to reduce the environmental impacts of textile finishing processes.

Textile World Asia Special Report

I t is difficult for textile finishing to escape the image of an industry that produces emissions. However, there are ways to drastically reduce the difficulties associated with wet chemical processes. This article describes the increasing global problems in the area of water and energy policies, and presents a case study that demonstrates how it is possible to run wastewater-free textile finishing operations.

Within the textile industry, finishing is one of the main sources of emissions. As a supplier of modern, high-performance textile finishing machines that are both energy- and water-efficient, Switzerland-based Benninger AG has now gone one step further to offer a new range of machines that are specifically designed to save even more water and energy. The key to this range is the diaphragm filtration systems that allow water, valuable materials and waste energy to be recovered.

Global Water And Energy Shortages

In the future, water is set to become an increasingly scarce and therefore extremely valuable resource. Demand for water is growing at more than twice the rate at which the world’s population is growing. Over the past 100 years, the world’s population has increased threefold, while water consumption has risen by a factor of seven. Since 1970, the available amount of water per capita has been reduced by 40 percent as a result.

It takes approximately 2,500 to 3,000 liters of water to manufacture a single cotton shirt. The bulk of this water is required to grow the cotton, followed in second place by the wet finishing process. The first consequences of water shortages and wastewater problems are already starting to be felt in the textile finishing industry. For example, new companies in China and India have not been granted approval to set up operations if they have not been able to present a convincing case to the authorities that their approach will help solve issues of water consumption and wastewater. In Europe, companies face closure for the same reason. Textile centers in Asia are reporting rapidly dwindling groundwater reservoirs and heavily salinated groundwater. As a result, many companies face challenges that threaten their very existence.

Holistic Solutions

The global textile industry faces a new challenge. As a consequence of global energy and water shortages, the industry now needs environmentally friendly production methods. In the mid-1990s, Benninger began moving away from its conventional role of machine builder, and has since established itself as a supplier of wet finishing equipment for woven fabrics and knitwear. Now the company for the first time is offering holistic textile finishing solutions under the banner of resource management. These solutions stand for sustainable textile production that is both ecologically sound and ethical. Part of this approach involves adopting a more careful attitude to the use of water, energy and chemicals.

In addition to finishing machines, Benninger supplies the matching diaphragm filtration systems (See Figure 1). These systems can be equipped with a number of functions that enable the following processes:
•    separation of water from the contaminants introduced by the process, with subsequent recycling, whereby the contaminants are concentrated to the point where no liquid waste is generated — zero discharge — and at least 80 percent of the wastewater is reused as process water;
•    recovery of recyclable materials, such as size and caustic soda, from the wastewater and reuse of these materials in the process; and
•    recovery of thermal energy.

Diaphragm Filtration System

In essence, the multi-stage diaphragm system comprises an ultrafiltration stage and downstream reverse-osmosis stage. The ultrafiltration stage is equipped with a special ceramic diaphragm that is used to hold back particulates and long-chain organic wastewater components at temperatures of up to 95°C. In the reverse-osmosis diaphragm that follows, the dissolved dyestuffs and salts are almost completely separated from the water. Afterwards, the process water can be reused directly in all areas of the textile finishing plant without adversely affecting the quality of the end product.

Although this method has long been used in the foodstuffs and pharmaceuticals industries, it is only thanks to the use of back-flushable ceramic diaphragms, which are resistant to chemicals and high temperatures, that this technique can now be applied in the textile industry. With the aid of ultrafiltration, it is possible to protect the reverse-osmosis stage effectively against contamination and thus ensure the operational reliability of the system.

Using a combination of ultrafiltration and reverse osmosis, a recovery rate in excess of 80 percent of the treated wastewater can be achieved. After processing with the diaphragm filtration system, the recycled process wastewater is colorless and can have chemical oxygen demand (COD) values of between 100 and 300 milligrams per liter and a conductivity of around 100 microSiemens per centimeter.

In order to optimize the way the system operates as a whole, Benninger has fine-tuned the design of the diaphragm systems to match the composition of the textile wastewater and the contaminant loads (See Table 1).


In resource management, the first step is always to optimize textile processes. In textile finishing, there are still many ways of doing this, including:
•    continuous dyeing instead of the obsolete method of exhaust dyeing;
•    high-performance washing units instead of simple washing units;
•    optimization of liquor flows with counter-flow and partial-flow water guidance; and
•    freshwater feed according to the level of contamination.

Only after the process has been optimized is a mass balance calculated as the basis for designing a diaphragm system.

Wastewater-Free Textile Finishing Plant

Following are the most important results from a case study performed to understand how a traditional finishing plant performs, and what can be achieved through modernization.

Desizing: Desizing wastewater carries an extremely high COD load. At the same time, recyclable materials can be recovered by using water-soluble size. To do this, an ultrafiltration plant with temperature- and chemical-resistant ceramic diaphragms is required. The results are as follows: water recycling, 85 to 90 percent; size recycling, 75 to 85 percent; and heat recovery, 70 percent.

Bleaching And Scouring: Wastewater from cotton bleaching has a high COD value due to the organic substances that accompany the cotton. This water also usually is yellow, so a combination of ultrafiltration followed by reverse osmosis is needed in order to neutralize the color. The concentrate cannot be reused. It is subsequently concentrated even further and processed into solids or burned. At this stage, the results are highly impressive: water recycling, 80 to 90 percent; heat recovery, 70 percent.

Mercerizing: The mercerizing process generates wash water with a high concentration of caustic soda —around 60 grams per liter (g/l). Ultrafiltration is used initially to clean and concentrate the caustic soda in the wash water and reduce its volume. Afterwards, the caustic soda is concentrated to 35 to 42 g/l by using conventional evaporation methods, which allows the caustic soda to be reused in the mercerizing plant. As the conventional evaporation methods used are very energy-intensive, this is a good approach to concentrate the wash water beforehand via ultrafiltration. The potential savings after this step are high: caustic soda recycling, 75 to 80 percent; water recycling, 80 to 85 percent; heat recovery, 70 percent.

Dyeing: Wastewater from the dyeing process has a high coloration and a high content of electrolytes, so a combination of ultrafiltration and reverse osmosis is required. The results achieved here are: water recycling, 80-90 percent; heat recovery, 70 percent. Figure 3 shows wastewater from dyeing processes that has been treated in diaphragm systems.

Processing The Concentrates: As described above, the concentrates generated in diaphragm systems can be reused only for size recycling and caustic soda recovery. In all other cases, the concentrates are highly contaminated liquors. It is not possible to discharge these concentrates into rivers or lakes. For wastewater-free operation of the plant, all concentrates must be thickened and solidified. The thickened concentrates can be further solidified using evaporation techniques, for example, or they can be burned.

Potential solutions for knitwear finishing plants are based on the same principle as those used for woven fabric finishing with no desizing. However, a lot more needs to be done to ensure the methods used in knitwear finishing can catch up, particularly as the plants often use exhaust processes in jets or softflow machines. The associated water and power consumption is two to three times higher than a more modern open-width finishing system.

Operating Costs: Diaphragm filtration plants run automatically. All of the relevant operating parameters — such as temperature, flow rate and operating pressure — are controlled via PLC. Back-flushing processes and cleaning processes are started automatically.

The initial investment for ceramic diaphragms is higher than for polymer diaphragms. Thanks to their high temperature resistance and chemical resistance, their lifetime is between five and 10 years. Conventional polymer diaphragms are used in the reverse-osmosis stage, and these have a lifetime of two to three years. The operating costs for a two-stage diaphragm system are around 0.60 euros (US$0.90) per cubic meter, which includes both the initial investment costs and the running costs of the plant. The amortization period for a diaphragm system for textile wastewater is around two to three years for water recycling and heat recovery. If the system also is used to recycle size and recover caustic soda, the amortization period is around one to two years.

However, rising energy costs and the increased shortage of water will significantly reduce the amortization period in the future.


Solutions For Emissions Reduction

There is no getting away from the image of textile finishing as a major contributor to emissions. Even in the long term, it is not going to be possible to replace wet chemical methods with physical finishing methods. This is reason enough to start looking for ways to reduce emissions.


Passive protection options against emissions include: process changes such as continuous dyeing instead of jet treatments; optimization of existing processes and recipes; and the use of highly efficient washing and finishing technologies.

The list of active protection options against emissions includes the use of filtration technology to recover water, energy and recyclable materials from the wastewater of wet finishing plants. For the first time, the ceramic diaphragms used in the ultrafiltration stage enable reliable and continuous operation for the treatment of wastewater from textile finishing processes. At the same time, they also act as a protective buffer for the downstream reverse-osmosis stage, which is used for filtration of dissolved dyestuffs and electrolytes. In addition to the recovery of water, energy and recyclable materials, the use of an evaporator and an evaporation plant makes it possible to run textile finishing plants that generate no wastewater at all.

March/April 2008

 

节水与节能方法

对于纺织业中的后整理工序来说，要想摆脱一个排放废水废气工业的形象是很难的。然 而，确实有方法可以大幅降低与湿化学处理相关的难点。本文就描述了在水和能源政策领 域中日益增加的全球问题，同 时提供了一个具体案例分析，以此表明，在纺织后整理运作中 达到"零废水"排放是完全可能的。

在纺织业界内，后整理是主要排水工序之一。作为现代、高性能、节水节能纺织整理机的 供应商，瑞士Benninger公司在技术上又上一层楼，研制出了一系列专为尽可能节水节能而 设计的新型机器。该 系列的关键之处在于横隔膜过滤系统，它可以实现水、有价值的原料 和浪费的能源再利用。

全球水资源与能源短缺

未来，水资源将会变得更为稀缺，并因此而珍贵无比。人们对于水的需求量的增长速度是 世界人口增长速度的两倍多。在过去的100多年中，世界人口已经翻了三番，而耗水量则是 过去的七倍。由 此造成的结果就是，自1970年以来，人均可用水量下降了40%。

每生产一件棉衬衫，就需要用掉约2500到3000升的水，其中主要用于棉花的种植，其次就是 湿整理过程用水。在纺织后整理业界，水资源紧缺和废水排放所带来的危害已经初步显现 出来。例如，中 国和印度的工厂在建厂前，如不能向政府证明自身的运作方式有助于解决 耗水和废水问题，那么将无法获得建厂批准。在欧洲，公司也会因为同样的问题而面临关 门歇业的危机。亚 洲纺织中心正不断地报告地下水存储量的萎缩以及盐碱化问题日益恶 化的趋势。因此，许多公司都面临着挑战，这甚至会关系到它们的生死存亡。


整体解决方案

全球纺织业都面临着新的挑战。鉴于全球能源和水资源短缺，纺织业需要一种环保的生产 方式。在二十世纪九十年代中期，Benninger公司开始从其传统的机器制造商角色中抽身 而出，转 而成为机织面料和针织品湿整理设备供应商。如今，该公司首次提供以资源管理 为主打的整体纺织后整理解决方案。该方案代表了可持续发展的纺织生产，既符合生态要 求，又满足民族特点，其 中的一部分牵涉到在使用水资源、能源和化学用品时所采用的更 为审慎的态度。

除后整理机器外，Benninger 公司还生产与之相匹配的横隔膜过滤系统 (见图1)。此类系 统具备多重功能，并能完成如下操作：

在加工过程中产生的污染物与水分离，达到水资源的循环利用，这样一来就不会产生液体 废料--零排放--并且至少80%的废水可以再利用；

可循环使用原料的恢复，例如浆料和苛性钠，从废水中提出并在加工过程中再次使用；

热能回收。


横隔膜过滤系统

从本质上讲，多极横隔膜过滤系统由一个超滤级和一个下游转换渗透级组成。超滤级装有 一个特殊的陶瓷横隔膜，可以在最高达95 摄氏度的温度下阻隔微粒和长链有机废水成分。 在紧随其后的转换渗透级中，溶解的染料和盐分已几乎完全从水中分离。随后，加工过的 水就可以再次直接投入纺织品后整理设备中所有环节的使用中去，而不会对最终成品的质 量产生任何不良影响。

虽然上述方法已经在食品和医药品领域长期使用，但是在纺织领域仍属新鲜事物，这也多 亏了可以抗高温、抗腐蚀的后冲式陶瓷横隔膜。此外，超滤级可以有效保护转换渗透级， 使其免于污染，也 由此确保了该系统运行的稳定性。

超滤级和转换渗透级两者相结合，可以实现超过80%的处理后废水循环再利用。在经过横 隔膜过滤系统处理后，可利用废水会变得无色，化学需氧量 (COD) 在每升100到300毫克的 范围内，传 导率可达100微西门子/厘米左右。

为优化处理方式，使该系统作为一个整体运行，Benninger 公司已调整了横隔膜系统的设 计，使之可以与纺织品废水和污染物载量 (见表1) 的成分相匹配。

在原料管理方面，第一步始终是要优化纺织品加工。在纺织后整理上，有多种方法可以达 到上述目的，其中包括：

- 以持续染色的方式取代过时的排气式染色；

- 用高性能的洗涤组件取代过去单一的洗涤组件；

- 通过逆流和部分水资源引流完成液体流量优化；

- 根据污染物的水平补充新鲜水资源。

只有在加工过程完成优化后才能获得设计横隔膜系统所需的基本平衡。

零废水纺织品整理厂

以下是案例分析中最重要的结果，可以帮助理解一个传统的后整理设备如何操作，以及现 代化所带来的成果。

脱浆: 脱浆用废水带有极高的COD载量。与此同时，可溶性浆液可作为循环原料再次使用。 为此，需要一个配有耐高温和化学腐蚀的陶瓷横隔膜的超滤设备。结果如下：水资源循环 率，85-90%；浆 液循环率，75-80%；热循环率，70%。

漂白和洗涤: 来自棉漂白的废水因其中所含的有机物质具有高COD值。废水通常为黄色， 所以，就需要一个配有转换渗透系统的超滤装置，以改变废水的颜色。浓缩的液体不能够 循环再用。最终，浓 缩液体的浓度会进一步提高直至成为固体，或被燃烧。在此阶段，其结 果让人印象深刻：水资源循环率，80-90%；热循环率，70%。

丝光处理: 丝光处理中产生的废水具有高浓度的苛性钠--约达60克/升。在洗涤水中使用 超滤级主要用于清除和浓缩苛性钠，以求降低其含量。在此之后，如果使用传统的蒸发方 法，苛 性钠将降至35-42g/l，这会使苛性钠可以在丝光设备中循环再利用。不过，传统方法 对能耗的需求很大，因此，如果能够预先利用超滤装置将苛性钠浓缩无疑是个好方法，它具 有极高的节能潜质：苛 性钠循环利用率，75-80%；水资源循环率，80-85%；热循环率，70%。

染色: 在染色过程中产生的废水颜色浓重，同时具有高浓度的电解质，所以需要将超滤和 转换渗透装置二者并用。可达到的成果是：水资源循环率，80-90%；热循环率，70%。图表3 显示了染色过程中产生的废水经横膈膜系统处理。

浓缩工序: 如上文所讲，在横膈膜系统中产生的浓缩物质仅适用于浆液和苛性钠的循环再 使用。在其他情况下，浓缩物均为高污染液体，不能将其排放到河流或湖泊中。对于工厂 的零废水排放而言，所 有浓缩物都必须提高浓度并最终固化。提高浓度后的浓缩物可以通 过蒸汽技术等手段固化，或通过燃烧的方式最终销毁。

对于针织整理设备而言，其潜在的解决方案与机织面料整理中的方案在原则上相同，但没 有脱浆过程。然而，鉴于针织工厂通常会在喷气机和缓流机中存在排放问题，因此其过程 更为复杂，也 有更多需要关注之处，与其相关的水资源和能源消耗量是一个更为现代的平 幅整理系统的三到四倍。

运作成本: 横膈膜过滤设备可以自动运转。所有相关的操作参数--例如温度、流率和工 作压--都由PLC控制。反冲过程和清洁过程可以自动启动。

对于陶瓷横隔膜的最初投资要高于聚合体横隔膜。鉴于其抗高温和抗腐蚀能力，陶瓷横隔 膜的使用年限在五到十年之间。传统的聚合体横隔膜可以用于转换渗透级，它的使用年限 为两到三年。一 个两级横隔膜系统的运作成本约为每立方米0.6欧元 (约合0.9美元)，其 中包括了初始投资成本和设备运营成本。一个用于纺织品废水处理的横隔膜系统的分期 偿还时间约为两到三年，这 针对水资源循环和热能循环而言。如果该系统统是用于浆液循 环和苛性钠的重复使用，那么时间将缩短至一到两年。

然而，不断增加的能源成本和水资源短缺问题在未来将会大幅缩短分期偿还的时间。

减排方案

纺织品的后整理工序始终无法摆脱"排放大户"的形象。即使以长远目光来看，它也不可能 以物理整理方式取代湿化学处理法。现有理由已足以让我们开始着手寻找减排方法。

消极的减排方式包括：加工程序改变，例如，用持续染色取代喷气处理；优化现有的加工过 程和添加剂配方；使用高效洗涤和整理技术。

积极的减排方式包括：使用过滤技术达到水资源和能源的再利用，以及从湿整理设备中提 取可循环使用的原料。陶瓷横隔膜在超滤级的首次应用使纺织品后整理过程中的废水处 理可以信赖，同 时也可以持续进行。与此同时，它们对下游用于过滤可溶性染料和电解质 的转换渗透级而言，也扮演着一个保护缓冲器的角色。除可实现水资源、能源和原料的循 环利用外，脱 水器和蒸发装置的使用使纺织品整理设备在零废水的情形下运转成为可能。

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