Water Purification In The Coming Decades Environmental Sciences Essay

This article was written by six academicians related to a work for The Center of Advanced Materials for the Purification of Water with Systems (NSF STC WaterCAMPWS, University of Illinois, USA). They are working for various departments which are suitable for this research at different universities. The WaterCAMPWS works for new technologies for water treatment. (1)

This article focuses on recent technology for disinfection, decontamination, re-use and desalination methods to improve water quality. It describes the importance of water and water problems, moreover it gives information about the water treatment systems using today and will be used in the future. It also makes comparisons to identify the advantages and the disadvantages of water treatment systems. It is suitable for anyone who wants to be informed about water purification techniques. A person who has the technical infrastructure about water and water treatment can understand more easily this article. While reading sometimes difficulties may be encountered to understand this article but in general language of the article was clear.

At the first part of the article authors mention the importance of the water, pollutants water problem and new water treatment technologies. Water problem is explained clearly by using remarkable numbers but they do not give the date and sources about numbers. In addition to the situation at the article, developed and developing countries are provided the examples of the water situations to make the water problem more understandable.

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Water is needed for living organisms to survive and there is life on the Earth thanks to water.(2) According to the World Health Organisation (WHO) water may contain bacteria, suspended particles, parasites, viruses, organic chemicals, heavy metals and solvents above the allowed amount for example, Escherichia coli and arsenic level limit is should be less than 10 μg/l (WHO report, 2008) (2-3) The elimination of unwanted materials, chemicals, contaminants, impurities, micro-organisms is called water purification which is a cleaning process. (4) Clean water demand is rising day by day because of increasing in population, water shortage and users competition. (5) If people accessed clean potable water easily, it would be a completely positive effect on health. Unfortunately, WHO report in 2007 shows that 1.1 billion people cannot reach the clean water and 1.8 million people die dehydration from diarrheal because of safe water problem each year. Hearth problems, brain stokes, HIV/AIDS, acute respiratory diseases, chronic lung diseases are some health problems which are caused by diarrheal.(3) The recent water treatment activities are not safe, sustainable and sufficient enough but they can be develop such as energy efficient systems, renewable purification.(2) New generation water treatment systems, which are based on such as disinfection, flocculation, sedimentation and decontamination technologies, should be developed to remove bacterial, viral and parasitic pathogens more effectively.(9)

At the beginning of 4000 B.C. people tried to make drinking water better in taste and smell. Some treatment methods were developed such as sand filtration, disinfection by sunlight and boiling after 1500 B.C. (9) At 1600’s desalination experiments began and around 1800’s the first water purification plant was built in Scotland. During the 20th century, people focused on drinking water quality especially removing the pathogens. In 1914 The U.S. Public Health Service prepared standards about drinking water and which were revised by the Public Health Service in 1925, 1946 and 1962. (9-3)

Disinfection is the process of the deactivating or destroying of pathogenic microorganisms present in water. (10) In the paper, extensive information about disinfection methods and the comparison between disinfectants were written. Authors give information about how the disinfection processes can control the viruses but I think this is unnecessary detail about the subject. Using sunlight (solar) was the first application of disinfection. (10) The most common disinfection methods are free chlorine and chlorine related chemicals because of cost and easy accessibility, they eliminate the living materials by oxidation of cell wall compounds. Free chlorine (Cl2) is so effective method but chlorine is a very dangerous chemical and it produces toxic disinfection by-product DBPs for example trihalomethanes. (11) The method of using solid calcium hypochlorite (Ca(ClO)2.4H2O – 65% chlorine) as a disinfectant has a lot of limitations such as corrosion and taste problems. Another disinfectant is sodium hypochlorite (NaClO) solution (5-15% chlorine) which cannot be stored easily. (11) Viruses, yeast, spores, bacteria, and molds can be killed by ozone disinfection method. Ozonation method is more effective than chlorine method and it does not produce any danger by-products. (10) There are some disadvantages of this method for example, it is not easy technology, low dosages are not enough to kill pathogens and it is costly. (11) Ultraviolet light (UV) can be used for inactivating pathogens by photochemical reactions in the RNA and DNA which is the result of light absorption. (10) It is effectively and for this method the contact time is short, there are not toxic or non-toxic by-products but it cannot use water which includes suspended material. The new method is use chlorine with UV and ozone which is very effective method to eliminate bacteria, protozoa cysts and some vital pathogens. The solar photocatalysis disinfection which means killing of pathogens by using the photocatalysis of TiO2 is the future method for water disinfection.(12) The use of nanoscale science in the water treatment systems, which includes nanocatalysts, nanostructure membranes, bioactive nanoparticles, nanoparticles filtration, nanosorbants, is the futuristic disinfection method. Nanoparticles which have large surface areas will be the best materials for water purification systems in the near term. Today, the major challenges of nanomaterials are the integration of water purification process, cost effective and design problems. (5)

In the article another method is described called, the water decontamination which is the process of eliminating the harmful substances (chemicals, organisms, heavy metals, radioactive materials) from the water. In this section of article, authors try to explain some problems about measuring and detection of compounds concentrations, modelling, and remediation strategies. Reverse osmosis, synthetic resins, activated carbon, sand filtration are the some methods to remove contaminants from the water.(13) The best treatment method which supplies the most purified water is not clear. The determination of contaminant type and concentration are as important as the decontamination technology. The advanced laboratory technology is necessary for measuring of low concentration of toxic compound for example Hydride Generation Atomic Absorption Spectrophometery and UV-Spectrophometry can be used to analyse arsenic. (13) Chromatography and mass spectroscopy which have the drawbacks such as cost, sensitivity and efficient; are the old but the most common technologies. Catalytic DNA is the recent system for testing heavy metals in the water and the traditional methods for this are X-Ray Fluorescence, some chemical tests and Inductively Coupled Plasma.(13) The biosensoring technology is defined as the usage nanoscale or microscale biological sensors to detect contaminants with high sensitivity. The technological development about biosensors is necessary to determine the toxicity of water treatment process.(14)

Highly treated water can be called reclaimed water which is the low cost alternative of potable water. If we consider the amount of consumed water per day, remained water on the Earth and the growth of population, the reuse of reclaimed water is absolutely necessary. Some of the uses of reclaimed water are irrigation, industrial activities and groundwater recharge.(15) Since historical times wastewater has been used but it became attractive two or three decades ago. (16) There are several methods for reclaimed water, such as chlorination, filtration and biological systems which have been used since early 1800s. (15) Authors give information about some recent methods such as membrane bioreactors and filtration. For the biological treatment ultrafiltration and microfiltration technologies have been understood to provide high quality purified water. (17) Today activated sludge process, which is another biological treatment method, are designed for high performance. (18) The membrane bioreactor (MBR) method is relatively new treatment technology which is based on liquid-solid separation and its applications are becoming acceptable day by day for small scale with high quality of water. (17-18) Membrane bioreactor process has some advantages compared to the activated sludge process such as more compact reactor, smaller footprint for high concentration, higher mixed liquor suspended solids concentration (for MBR process average concentration 8-12 g/l). The most important drawback of MBR method is some terms such as flow issues should be developed. (18) The cleaning of membrane and dynamic effects such as aeration loss, saline intrusion and backflush loss are the two parameters which affects the performance of MBR technology. The improvements about material characteristics, permanent fouling, cost and membrane cleaning process should be done for this process. The average annual growth rate 10.9 % is estimated for the global MBR market. (19) Reverse osmosis (RO) is a liquid membrane method which removes dissolved materials. The combination of MBR and RO is highly efficient which provides 67% water recycling at the lowest cost. (20)

Desalination is a method that separates the salt from water. The oldest process to remove salt from water is boiling. Thermal distillation method and natural the hydrologic cycle work same way and in the early 1900s, membrane system was developed. There are three types of thermal methods namely; multi-stage flash distillation, multi-effect distillation and vapour compression distillation. (21) The advantages of desalination by distillation are low operating and maintenance cost, minimal environmental effect and the highly purified water production, the disadvantages are high level knowledge and large capacity plants.(22) Electrodialysis, Electrodialysis reversal and reverse osmosis are the three main membrane processes for desalination. (21) Simple systematic structure, removing organic and inorganic contaminants and high production/capacity ratio are the advantages on the other hand the necessity of pre-treatment step, the difficulty of cleaning membrane and decreasing in flow rate are the drawbacks of membrane processes. (22) Total number of desalination plant is nearly 1400 (80% Membrane desalination, 20% Thermal desalination). Reverse osmosis desalination process includes four parts; pre-treatment, high-pressure pumps, membrane systems and post-treatment. (21) Hybrid desalination method can be defined as the comparison of thermal and membrane desalination process which generates power. The advantages of this process are using less energy, low cost of construction and high plant efficiency. Nowadays the RO and simple hybrid multistage flash configuration are very popular. The advantages of integrated systems are good thermal efficiency and high quality purified water. (23) The Affordable Desalination Collaboration process is newly design of reverse osmosis process which demonstrates the water with a good design. (21) Carbon nanotubes have the specific electrical, chemical and mechanical features. In the future, nanotubes will allow us to desalinate cheaply but there are some challenges, we have to face about this method like capital cost and difficult mechanism. Carbon nanotubes, Biomimetic membranes and forward osmosis are next generation systems for desalination. (8-21) In this section of the article the figure about reverse osmosis and activate desalination (figure 5) makes understandable the processes. The information in the book related to entropy and energy calculation is the high-level information about the subject.

This article contains extensive knowledge about the water treatment systems. Authors clearly conclude solutions of water treatment systems. I have some knowledge about water purification before preparing this report but while writing I learned lots of information about new water treatment techniques from article and references. This research was made in 2008, it is a technological subject because of this when research is carried out is very important. Some current water treatment techniques need intensive energy and chemicals. We should improve the not only water treatment systems but also monitoring, measuring, modelling and management systems to deliver safe and clean water to everybody. As a result, we need new, sustainable, environmental and inexpensive purification methods.

Science and technology for water purification in the coming decades by M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Giorgiadis, B.J. Marinas, A.M. Hayes, Nature 452 (2008) 301-310ö