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实验室水净化系统 Water purification for the laboratory

当制定实验室水净化系统的时候,什么应该是技术人员和实验室工作者所需要考虑的关键问题?

一个有经验和值得信赖的供应商所提供的良好的水净化系统可以提供预期的高纯度的纯净水,再加上供应商提供的持续的技术支持,可以保证实验室有一个有效,经济,可靠的纯净水来源。以这个目标作为基础,技术人员和实验室工作者应该如何做到才能满足正确的要求呢?
这个过程的第一步首先应该是确保每一个参与水净化系统设置的人充分了解系统提供的每一个标准化的水纯净度,以及提供这些标准的可行方法和实验室本身的要求。在建立纯净水的纯度级别之后,你需要估计常规定期需要的纯净水量。其次需要决定安装中央式或者独立式单元,这将取决于整体的费用和每一项操作的具体要求。
我们需要在细节上考虑上述的每一步过程,首先从水的纯净度开始。水的纯净度共有三种标准级别:电导率指定级别,当前标准级别,BS EN ISO 3696:1995实验室用水级别。级别1代表了最高的纯净度:0.01mS/m(0.1µs/cm)级别2级别3分别为0.1mS/cm(1s/cm)和0.5mS/m
(5µs/cm)。有若干种方法可以提供上述三种级别的纯净水,而并不是所有的水净化系统都可以同时提供这三种级别的纯净水。
通过沸水产生蒸汽,从而去除那先沸点高于水的杂质,这是一个低效且高温的纯净水制造法,同时需要频繁的维护和高额的运行费用。这个方法已被新的技术超越和代替,并且不能提供最高纯度的纯净水。例如,小型实验室使用的现场蒸馏单元设备,只能提供第二级别纯度的纯净水。
提供纯净水的另一个方法是在主水源处直接连接一次性去离子或离子交换盒。每个交换盒使用树脂的混合物来去除水中的阴、阳离子杂质,并用活性氢和氢氧根离子替换,从而形成水分子。在使用过程中,用于交换杂质和释放活性氢和氢氧根的树脂逐渐消耗并改变颜色,从而指示是否需要更换。这个方法可以提供等级1纯度的纯净水,但如果主水源含有高浓度的溶解杂质而且每天所需的纯净水在10升以上,这个方法同样也是不经济的。
根据供水的质量,反渗透系统可以用来提供大量的纯净水。在这个方法中,水在压力的作用下供到一个包含有半透膜的模块中,此模块可以去除高达98%的无机离子,和几乎所有的胶体微生物,内毒素和有机大分子。半透膜原件日益提高的制作技术不仅可以改善水质,而且有助于提高泵的速度,显著降低相应泵的压力,从而减少能量消耗。
值得注意的是,虽然一个标准独立的RO单元可以提供大量的纯净水,它所提供的水只能符合第三级别的纯度。而通过采用反渗透和去离子的组合,可以实现更高水平的纯度。
纯度可以通过持续将水源暴露在去离子的树脂床中来提高直到达到所需要的纯度。作为一个基于树脂的去离子系统,较大的综合性实验室可以结合电去离子系统(EDI),来进行二次净化然后提供给RO系统中的渗透模块,从而他提供纯度高于10 MΩ.cm.的纯净水。EDi是一种使用了离子交换膜/树脂和电力结合来对水进行去离子的方法。
去离子过程可以将水的纯度提高到1级别,但是同样存在额外的措施可以进一步提供水的质量。例如,当需要具有增强微生物的1级纯度的纯净水时,RO/去离子法获得的纯净水需要进一步处理,使用254nm波段的紫外线照射,并在0.2和0。05微米之间过滤,以除去细菌和细颗粒物。虽然,选择去离子墨盒是相对简单的,同时优化考虑到性能,能耗,和花费,通过RO系统来提高和达到这些要求会是相当复杂的。
虽然上面详述的措施说明了单独标准的反渗透水净化系统可以挺高水的纯净度到1级别,在一些情况下安装中央的系统会更有效和经济。例如,如果纯净水的供应需求来自不同的地点,环形主线中央供应系统会更适合用来提供大量高质量的纯净水,是一个十分节省空间的选择。在中央式系统中,如果将泵通过变速驱动器连接,使每个泵的速度在水处理过程中和输出要求精确吻合,那么可以提高泵的效率。
当你已经分别考虑了单独和中央水净化系统以及半透膜制作工艺上的显著提高之后,是时候该选择一个可以提供你所需质量并同时能够平衡花费和效用的水净化系统了。
在权衡了所有的条件之后,你可以根据实验室现有的技术和需求来确定正确的计划。我们已经考虑了必须要考虑的水的质量,但为了最大化效益,水的质量要求需要满足于整个实验室或是几个特定的工作区域,这也是必须要考虑的。同样,纯净水所需要的量也应该基于长期的使用从而分析一天内用量的高峰和低谷。
一个典型的错误是当最终用户在选取系统的时候没有听取一些经验丰富的供应商例如 Purite 的专业意见,而导致水的纯度过高,因此选择一个能够提供你所需的纯度的纯净水的系统很重要。同样,对于任何时间段将被用到的分接点的数量也需要一个具有现实意义的估计,如果简单假设所有节点在安装后都会同时工作,那么结果会是显著过大的,会变成一个过分昂贵的系统。
为了保护反渗透膜,进入水流的预处理是必须的,特别是在水源中含有高浓度有机污染物,硬物和游离氯的区域,同时通过选择部件容易获得和替换的单元维护也可以成为流线的
为了达到最好的要求,和一个愿意和你在工地上帮助你,根据你的要求为你说明最好的解决方式的供应商是一个明智的选择。 Purite 作为一个值得信赖和有价值的供应商不仅有能力提供系统,而且可以提供计划,设定和安装系统,而不是委托给承包商。在我们正确的帮助和意见下,一个为你们实验室准备的有效,经济,可靠的纯净水供应系统应运而生,并在它的生命周期内保持成本效益。

What are the key issues that technicians and laboratory staff must consider when specifying a water purification system for the laboratory?

A well-specified water purification system from a trusted and experienced supplier will provide an exceptionally high level of purity and, coupled with on-going technical support from an experienced supplier, will ensure that your laboratory receives an efficient, economical and reliable supply of purified water. With this goal on the horizon, how do technicians and laboratory staff achieve the right specification?
The first stage in the process is to ensure that everyone involved in the specification process fully understands the grades of water purity that have been standardised, the available methods of delivering them and the requirements of the laboratory itself. Having established the level of purity required, you then need to assess the volume needed on a regular basis. This is followed by the decision as to whether to install centralised or stand-alone units and this will be dictated by the overall costs and the operational demands of each option.
We need to consider each of the above stages in detail, beginning with water purity. There are three standardised grades, expressed in terms of conductivity, defined in the current standard, BS EN ISO 3696:1995 ‘Water for analytical laboratory use’. Grade 1 represents the highest level of purity, 0.01mS/m (0.1µs/cm), with Grade 2 being 0.1mS/cm (1µs/cm) and Grade 3 at a level of 0.5mS/m (5µs/cm). There are several methods of delivering these grades and not all water purification systems offer the capability to deliver all three.
The practice of boiling water to create steam, thus leaving behind those contaminants with boiling points higher than that of water, is a hot, inefficient system for producing purified water, requiring frequent maintenance and high running costs. It has also been surpassed by newer technologies and is no longer the method that provides the highest purity. For example, on-site distillation units, as used by smaller laboratories, can only supply water typically to Grade 2 standards at best.
Another method of providing water purification is to connect disposable deionisation or ion exchange cartridges directly to a mains water supply. Each cartridge uses a mixture of resins to remove anionic and cationic contaminants from the feedwater, exchanging them with active hydrogen and hydroxyl ions, which combine to form water molecules. During usage, the capacity of the resin to exchange impurities and release active hydrogen and hydroxyl ions is gradually consumed with the resins changing colour as they become exhausted, to indicate when cartridges need changing. This method is capable of delivering a Grade 1 flow of purified water but can be uneconomical if the feedwater contains a high level of dissolved solids and the demand for purified water is more than 10 litres per day.
Depending on the quality of the feedwater available, reverse osmosis systems are able to provide large volumes of purified water. In this method, the   water supply is fed under pressure into a module containing a semi-permeable membrane that is capable of removing up to 98% of inorganic ions, plus virtually all colloids, micro-organisms, endotoxins and organic macromolecules. The ever-increasing refinements in the construction of semi-permeable membrane elements have not only improved water quality but allowed pump speeds, and correspondingly pump pressures, to be significantly lowered, thus reducing energy consumption.
It is important to be aware, however, that although a stand-alone RO unit can produce a large volume of purified water it can only do so at a level of purity that meets Grade 3 standards. It is, however, possible to achieve higher levels of purity with a stand-alone unit by employing a combination of   reverse osmosis and deionisation.
Purity levels can be raised by degrees to the required specification by continually exposing the water supply to a bed of deionisation resin until the required level of purity is reached. As an alternative to resin-based deionisation systems, larger integrated laboratory systems can incorporate an electro-deionisation system (EDi), for secondary purification when fed with permeate from the RO system, producing water with a quality of greater than 10 MΩ.cm. EDi is a purification technology that uses a combination of ion-exchange membranes/resin and electricity to deionise water.
Deionisation can raise purity to Grade 1 level but there are additional measures that can enhance water quality further. For example, where Grade 1 water with enhanced microbial quality is required, the RO/deionised purified water is further processed using UV irradiation at 254nm and sub-micron   filtration between 0.2 and 0.05 microns to remove bacteria and fine particulate matter. However, although the selection of the deionisation cartridges is relatively straightforward, the specification of RO systems that can achieve these enhanced levels of performance can be complex if optimum levels of performance, energy efficiency and operating costs are to be achieved.
While the measures detailed above can raise the purity of stand-alone reverse osmosis water purification to Grade 1 level, there are situations where it is more efficient and economical to install a centralised system. For example, if supplies are required at different locations a centralised system feeding a ring-main may be more appropriate, offering an unobtrusive and space-saving option that can deliver high volumes and high quality. In centralised systems, it may be possible to make gains in efficiency if pumps are linked to variable speed drives, enabling the speed of each pump to be matched exactly to the output demands of the process and water treatment system.
Once you have considered the benefits of stand-alone or centralised water purification systems and the remarkable enhancements to both that have been achieved in refining the quality of semi-permeable membranes, it is time to choose and specify a system that will give you the right level of water quality and the most effective balance between cost and efficiency.
Having weighed up all the available options, you can now make the right specifications based on the available technology and the needs of your laboratory. We have already considered that it is essential to consider the quality of water needed but, to maximise efficiency, it is also essential to   consider whether that quality is required throughout the laboratory, or only at a limited number of work areas. Similarly, the volume of water needed should be analysed based on the patterns of daily use to highlight peaks and troughs in water requirements over extended periods.
A typical specification error that occurs when end-users specify a system without the expert advice of a seasoned supplier such as Purite is oversizing, so it is important to specify equipment that can deliver only the volume of purified water that you need. Likewise, a realistic estimate must be made of the number of take-off points that will be in use at any one time; if it is simply assumed at the installation stage that all points will be in use at once, the result can be a dramatically oversized and therefore unnecessarily expensive system.
Pre-treatment of the feed stream may be necessary to protect reverse osmosis membranes, especially in areas where the feedwater contains high levels of organic contamination, hardness and free chlorine, while maintenance can be streamlined by choosing a unit with parts that are quick and easy to maintain or replace.
To achieve the best specification it is advisable to work with a supplier who is willing to work with you on-site and help you to specify the best solution for your needs. Purite’s strength as a trusted and valuable supplier flows from its capacity not only to supply but to plan, specify and install a system, rather than delegate to outside contractors. With the right help and advice, an efficient, economical and reliable supply of purified water is   available for your laboratory that will remain cost-effective throughout its lifespan.




 

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