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Water technology Precious Commodity: Membrane Technology Against Water Scarcity

Author / Editor: Andreas Hauser* / Dipl.-Chem. Marc Platthaus

Membrane technology, as employed in ultra filtration or reverse osmosis, is becoming increasingly efficient. Water is being treated more and more cost-effectively and with better quality. Thereby, efficiency and safety significantly depend on the membranes used.

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1 In order that water can be recycled efficiently and reliably in the long run, the membranes must be in the focus of planning, operation and management.
1 In order that water can be recycled efficiently and reliably in the long run, the membranes must be in the focus of planning, operation and management.
(Source: TUEV Sued)

Water recycling is increasingly important in order to supply water to the population, to agriculture and to the industry in many regions of the world. In 2030, the global demand for water will surpass the available resources by 40 percent. Initial experiences with water treatment show that the performance of some treatment plants drops after a short time. Quality and quantity of the permeate, of the filtered water, fall short of expectations. To achieve the desired output even then, the use of energy and chemicals has to be increased. This results in not only the increase in operating costs, but also that the components have to be exchanged more often than planned.

Condition of the membrane is crucial

Membranes have been proven successful in treatment processes. They are often used in the last step of the treatment to achieve the high quality of water required for reutilisation. Their condition is thus significant for the efficient and smooth operation and often also the reason for output losses: Membranes can be damaged, for instance due to improper cleaning chemicals. Organic deposits, the so called biofouling also often lower the performance of membranes. In addition the membrane condition can generally be measured only indirectly and cannot be examined during operation. Therefore, it is very rarely documented in a reliable manner. Since the treatment process is intrinsically linked with membranes used, an extensive search for the error is necessary in such cases. A physical examination gives information whether the housing, sealing components (e.g., O-rings) and adhesive joints are intact. Further vulnerabilities are uncovered with challenging analytical processes. These include, for example, flow and salt retention rate is measured under standardized conditions. The results offer indications about damages through fouling, improper handling or chemicals. Imaging techniques such as scanning electron microscopy and atomic force microscopy show changes and deposits on the membrane surface on the smallest level. To understand how the deposits affect the performance of the membrane, they must be precisely characterised. With the help of energy-dispersive X-ray spectroscopy and x-ray photoelectron spectroscopy, conclusions about the composition and distribution are possible. Also, a FTIR-spectroscopy (FTIR = Fourier Transform Infrared), incineration or secondary ion mass spectrometry can show, which group of substances is contained.

The results of these investigations offer a comprehensive picture of the causes for problems. However, they can be used not just to solve existing problems. Operators can and should use these findings also to optimize the treatment process and maintain the performance of the plant as sustainably as possible. Measures that contribute to this involve, for example, the configuration of components, operational and maintenance procedures or pre-treatment steps and the use of chemicals. Adjustments help in reducing biofouling and extending the lifetime of membranes. This also lowers the costs caused by the unscheduled repairs or interruptions.

Overview of standard procedures

In order that water can be recycled efficiently and reliably in the long run, the membranes must be in the focus of planning, operation and management. It starts already in the selection of right membranes. As per the purpose of use, the requirements for membranes used for recycling water also vary. Thereby, the following questions are to be asked, among others:

How is the composition of the wastewater? How consistent is it? Which operational parameters (temperature, pressure) prevail in the plant? How should the recycled water be used? Which quality requirements should be met for that purpose? It is evident herein that water recycling and reutilisation are complex processes that require a lot of experience and know-how. This is aggravated by the fact that the market for membranes is difficult to navigate. Since established norms are missing, the products are hard to compare. However, informal industrial norms and Key Performance Indicators (KPI) exist. The relevant parameters are often measured under different conditions. These include temperature, the pH-value or the composition of the water. However, it is a factor if only the membrane-tissue or the entire module is tested. All this still makes it difficult for the buyer to compare manufacturer specifications and to decide which product is suited for his plant. For certain partial aspects, there are internationally recognized standards. A comprehensive standard is however still missing. Some manufacturers define their own test conditions from case to case. On this background, TUEV Sued is working to standardise membrane technology and has developed its own certification. Existing standards for quality, material, robustness and performance were analysed at first and then brought together. Subsequently, comparison criteria were derived, which are also fundamental for the certification. Standardised test protocols provide information about how efficient are the products, how stable against pressure and chemicals and how susceptible to microbial growth and thus to biofouling. Hence, operators can assess available alternatives easily and choose membranes that are most suitable for their purposes and under the specific conditions. The optimum recycling operation requires an in-depth understanding of the membrane performance about their total life cycle. Analytical processes help in uncovering sources of errors in case of a drop in performance; standards make products comparable. Both factors contribute in optimising processes and to conserve consumable materials, chemicals, energy, labour and save costs. With interdisciplinary expertise and international experience, TUEV Sued Water Services contributes in developing innovative water and sanitary technologies safely and sustainably. As an independent service provider, the company supports operators, developers and manufacturers as well as authorities, non-governmental organizations and investors.

Singapore’s new water program

The Singapore examples shows that water recycling makes a significant contribution in counteracting the growing water shortage. The city state depends on the imports of fresh water from the neighbour Malaysia to ensure water supply. The national reserves are not sufficient due to the high density of population and industry. Singapore has been working for a good 50 years to become independent in terms of supply. Thereby, water recycling plays a decisive role, which is expedited particularly within the scope of NE-Water-Initiative. As of today, reprocessed water covers 30 percent of the national demand. Additionally, the government has issued statutory provisions that obligate the usage of recycled water. It is primarily used in the industrial sector or for air-conditioning systems in the industrial and commercial buildings. Since it exhibits a very high degree of purity, it is used particularly in chip production. Here, the requirements for quality are actually higher than for drinking water. Therefore, recycled water is provided to producers also through a special network. Apart is also used, at least indirectly, also as drinking water. Especially during droughts, it is supplied to reservoirs, treated along with rainwater, for example, and then fed in the drinking water system. High quality standards are maintained here as well. They surpass the requirements defined by the World Health Organisation (WHO). Therefore, this knowledge also increases the acceptance by the population. Thereby, reprocessing also offers clear advantages on the system level: It is an environment-friendly alternative for the disposal of wastewater and reduces nutrient enrichment (eutrophication) of surface water, which can lead to undesired algal formation, among others. Simultaneously, water treatment and reutilization is more and more connected with energy recovery and nutrient utilisation (e.g., ingrain farming).

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* *Dr. A. Hauser: Water Services, TUEV Sued Asia Pacific, 118221 Singapore