- Introduction
With water being an essential component necessary to many aspects of domestic life, it also represents an integral component in many industries. Due to water’s inherent solvent properties, water extracted directly form freshwater sources such as lakes, rivers, and aquifers are often times full of absorbed minerals and pollutants unfit for human consumption. Due to this, a variety of methods have been implemented in order to purify water from surface and sub-surface sources. However, the world’s water sources vary greatly in purity to begin with, which results in some methods being much more labor intensive in order to purify to appropriate levels.
Though utilized by industrial sectors, the main objective of many water purification methods rests predominately on the need to remove harmful microorganisms and chemicals to insure public health. While protecting public health and the spread of disease, purified water also hold a valuable place in industrial settings by providing the clean water essential to prevent corrosion to pipes, boilers, and other industrial equipment.
2. Conventional Treatment and Advanced Treatments
The Conventional Water Treatment is the most widely implemented form of water purification management by developed and developing countries. As seen below in Figure 1 (Nathanson, 2008), the Conventional Water Treatment is divided into seven parts. Generally speaking, if the water source is sub-surface aquifers, the assumption is there is no need for ‘screening’ water for debris as the water is already screened for large debris when permeating through the soil. However, if the case of the water source originating in surface reservoirs, the first step to the process is roving animal and plant debris.
After removing large plant and animal mater, the next essential step utilizes chemical additives in order to eliminate microorganisms, improve taste and odor, and help settle additional sediments still present. During this process, chlorine, aluminum sulphate (alum), and other chemical polymer compounds are mixed with the water in order to coagulate the remaining sediments. While in the Coagulation and Flocculation Tank, the coagulated solids known as floc, are collected at the basin of the tank effectively separating the remaining solids from the water.
Though successfully removing the remaining solids, the water is still required to be filtered for additional particles. Utilizing layers of sand intermixed with layers of carbon, also known as charcoal, any additional microscopic particles will be caught. It is after this point that the water is considered filtered and additional chlorine and other disinfectants are added to insure any remaining germs are eliminated before the purified water is stored in closed tanks and reservoirs for public use.
Industrial water consumption utilize the same overall style of purification systems that municipalities do (Barnes, 1981). In some cases, industries are able to utilize water sources strait from the municipal water supply, however some particular industries require higher purification levels than municipal standards. In these cases, additional treatment methods like micro and ultra membrane filtration (MF,UF) and reverse osmosis are implemented in order to further separate impurities from the water (Barnes, 1981).
Figure 1 – Schematic of Conventional Water Treatment Plant (Nathanson, 2008)
While a variety of methods are available, the type of method used is generally based upon the quality of the raw water that needs to be refined and the level of purity that needs to be obtained (Montgomery Watson Harza, 2005). As seen below in Figure 2 (Barnes, 1981), the variety of impurities can be broken down into three main classifications; physical from, chemical nature, and biological characteristics.
Figure 2 – Impurities in Raw Water (Barnes, 1981)
These classifications are essential in determining the appropriate treatment method that fits both the quality and operational requirements of the industry (Gary, 2005). Regardless of the method implemented, advanced filtration methods can be defined as membrane filtration systems. Broken down into two distinct physiochemical categories, membrane filtration employs membrane filtration and reverse osmosis as its main method (Montgomery Watson Harza, 2005). As illustrate in Table 1 (Gary, 2005) shown below, the two categories can further be divided based on membrane pore size, which in turn helps to define the physiochemical categories.
Membrane filtration includes micro and ultra filtration methods and both employ the same basic process despite the size difference in the membrane pore used. Using a fine membrane filter, water is forced through the semi permeable membrane in a process known as hydrostatic pressure, which is illustrated below in Figure 3 (Montgomery Watson Harza, 2005). This process uses what is known as ‘cross-flow filtration’ where the water flows across the membrane forcing the water to pass through (Montgomery Watson Harza, 2005). However, micro filtration rarely utilizes extreme pressures in order for the membrane to work resulting in some industry to consider them as none pressure systems.
Figure 3 – Schematic Seperation in a Membrane Filter using Hydrostatic Pressure (Montgomery Watson Harza, 2005)
As illustrated again in Table 1 (Montgomery Watson Harza, 2005) shown above, micro filtration systems are applied in cases where removal of bacteria, fungi, emulsions, or colloids is intended. The method has a two-phase system, which first utilizes a feed stream (untreated water) to pressurize the water before forcing it through the membrane. Mf membrane pore size range in size from 0.1 to 10 μm, which allows considerably high flow rates (Montgomery Watson Harza, 2005) and are usually comprised of polysulfonate, polypropylene, nylon-6, or PVC.
While mainly used in domestic situations to remove unwanted pathogens and bacteria, industrial use is directed towards a high value but low volume implementation of sterile sensitive situations (Montgomery Watson Harza, 2005) like some alcohol, food, and pharmaceutical production. In the case of beer production for example, the use of heat in order to separate yeast is impractical would change the taste resulting in the need to have a sterilization process like micro-filtration so they can employ the widely herd term ‘cold filtered.’ (VA Filtration).
Almost identical to micro-filtration, ultra-filtration employs the same principles while adding a membrane with smaller pores. Ranging in size from 10 to 1000 Å, ultra-filtration requires some pressure in order for the water to effectively permeate through the membrane. However, these systems can capture a great deal more particles than can micro-filtration and are even used in watershed treatment and oil removal vital to many environmental projects today (Water Technology Projects).
In fact, the main utilization of ultra-filtration is in environmental areas. Due to its ability to remove oily substances form water, the system is also employed to remove industrial dyes released into the environment. Dye producers also see value in its ability to prevent environmental fines from contamination while also providing an opportunity to capture and recycle what could have been lost dye (Water Technology Projects).
Regardless of its industrial use, ultra-filtration is often seen as the most economically attainable form of filtration and is often used in order to purify gray water that is fit for human use, but not consumption. But as a result of its small pore size, additional prescreening is often required more so than larger pore membranes like those of micro-filtration.
4. Reverse Osmosis (RO) Membranes
Reverse osmosis is again another system that utilizes the cross-flow system to carry water from one side of the membrane filter to the other. The biggest differences, however is the extreme size difference between the smallest UF membrane and the biggest RO membrane. Typically ranging in size from 5 to 15 Å, or 0.5 to 1.5 nm, RO membranes can be created to have uniform pore sizes meaning that they are able to reject particles on an almost atomic scale.
It has long been an issue of debate as to how to convert salt water into freshwater for consumption. One method is to use a spiral wound module and is the most common reverse osmosis method used for desalinating salt water.
Figure 4 – Scematic of a spiral wound membrane module (Mann and Hummel Water Solutions)
In these modules diagramed above in Figure 4 (Mann and Hummel Water Solutions), the water flows into a space know as the envelope, which lies between the spirals layers, before permeating out through the other side. Despite the high energy cost of reverse osmosis to desalinate water, its versatility in sale concentrations make it a very adaptable system. While systems like ion exchange and electrodialysis works best between salt concentrations of ten to 10,000 ppm and multi-stage distills work best in concentrations between 20,000 and 100,000 ppm, reverse osmosis works best in the middle between 50 and 50,000 ppm (Mann and Hummel Water Solutions). However, despite its applicability, energy demands for flow pressure make it unsuitable for many areas.
Conclusion
Despite the over arching similarities between the various purifying processes, specific treatment processes need to be taken into consideration in order for the required purification standards. This mean taking into consideration the quality of the raw water and as well as meeting the specification required for the successful utilization of the end product. However, meeting quality guidelines is not all the inputs that need to be accounted for as exemplified with desalination systems utilizing reverse osmosis. Capital cost, method specific efficiency, operational costs, and filtration method all play key roles in determining water purification systems effectiveness and utility.
References Cited
Barnes, D. “Water and Wastewater Engineering Systems”, 1981, Chapter 7
Gray, N., “Water Technology, and introduction for environmental scientists and engineers”, 2nd Edition, 2005, Chapter 20
Mann and Hummel Water Solutions. Retrieved March 20, 2011 from
http://www.ultra-flo.com.sg/images/wenton.pdf
Montgomery Watson Harza (MWH), “Water Treatment: principles and design”, 2nd Edition, 2005
Nathanson, J., “Basic Environmental Technology, Water Supply, Waste Management and Pollution Control”, 5th Edition, 2008, Chapter 6
Water technology projects. Retrieved March 20, 2011 from
VA Filtration. Retrieved March 20, 2011.
http://www.vafiltration.com/MFPic.JPG
