Movement Standing Behind Purification of Water

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Water is vital for the survival and wellbeing of humans, but many of WA water sources such as groundwater and seawater contain many contaminants. These contaminants can be extremely harmful to the body, this is why we need methods to analyse what contaminants are present in these sources of water. These processes can be ICP-MS (inductive coupled plasma mass spectroscopy) and AAS (atomic absorption spectroscopy. These can detect for many contaminants such as heavy metals which can cadmium or lead. Once these contaminants are detected they must go through a purification and treatment process. These can be desalination through reverse osmosis and distillation, this is mainly used for seawater. In WA groundwater is also another major source of water, so it must go through a different purification and treatment process, the Wanneroo ground water treatment plant currently uses a combination of different purification and treatment methods for groundwater.

Purification of Water

The treatment of water, from different sources is necessary, so that it can be consumed. Deriving potable water from seawater can be done with two methods distillation and reverse osmosis, these can be considered as desalination methods as the processes involve in the removal mineral component from saline water, one of the mineral being salt. As seawater contains more than 35,000 mg L-1 total dissolved solids. In the distillation process water from the sea is evaporated, this works because water is a volatile solvent whereas the salt in the water is non-volatile. So, then the water vapour is condensed to produce distilled water which is free of any salts. This evaporation process is carried out in multiple stages at progressively low pressure, this is called multi-stage flash distillation. This is done so that it is more energy efficient because the heat used in the evaporation process in recycled.

Another method of obtaining fresh water from seawater is reverse osmosis. Osmosis is a natural process in which water diffuses through a semi permeable membrane from a solution which is low in concentration of salt to one which is higher in concentration of salt, until the concentration of salt is equal on both sides of the semi permeable membrane. This process is used by plants through the root hair cells, to absorb water. In commercial reverse osmosis, there are units which force water through a synthetic semi permeable membrane through the use of high pressure.

In this process pressure is applied to the seawater, which is called the feed water. This flows on one side of the semi permeable membrane, the pressure applied causes the water to flow out of the salty feed water and through the semi permeable membrane to produce pure water, as only water can travel through the semi permeable membrane. This results in the feed water becoming saltier as 45% or more of water is extracted. It also results in fresh water which is called the permeate – a substance that diffuses through a SPM. Ions, larger organic molecules, cellular organisms and viruses do also pass through the semi permeable membrane. In Western Australia, the Kwinana and Binninup desalination plants which use reverse osmosis, the fresh water collected is then treated with disinfection by the addition of chlorine and also fluoride. Chlorination, the process of adding chlorine to drinking water, is necessary as chlorine is a bactericide, it destroys various cellular pathogens.

The chlorine inactivates microorganisms, this is done by it damaging its cells membrane, once its membrane is weakened the chlorine can penetrate the cell and disrupt its cellular respiration and DNA activity. This will stop the microorganism from working. When any form of chlorine is added to water it will results in hypochlorous (HOCl) acid and hypochlorite ions (OCl-), a form of chlorine + H2O  HOCl + OCl-. The amount of chlorine added in WA can range from 0.5 to 1.5 milligrams per litre. In some town where there are additional risks because of activities in the catchment, ultraviolet radiation is used, this is effective against all viruses, bacteria and protozoa. It works by altering the DNA in the cells and impending reproduction. For taste and more health reasons some salts are added, this is done by adding both carbon dioxide gas and lime (CaO), this also results in a suitable pH, and also a total dissolved salt level of approximately 200 mg L-1.

Desalination by both process is necessary because drinking seawater is extremely bad for the human body, as seawater contains salt by drinking this water continually it can dehydrate you. Your body will normally discard of excess salt by the kidneys producing urine, but fresh water is required for the salt to be diluted and excreted in urine. If there is too much salt in your body, then your kidneys will fail to get sufficient amounts of fresh water to dilute the salt. Seawater also makes up 48% of WA water supply, so it is a major component to WA water source, giving another source of water. These are the key reasons why the desalination process is needed.

Groundwater is another major source of potable water in Western Australia, ground water currently makes up 40% of the water to IWSS (Integrated Water Supply System) which it supplies to Perth, the South West, the Goldfields and Agriculture regions. Ground water needs to be treated because it contains many harmful microorganisms, it also has bad odours which renders the water unpalatable, it also stains many surfaces, so it cannot even be used for washing. Currently groundwater is treated at six different locations, this is to remove contaminants such as finely suspended solids and various dissolved substances like manganese, iron, hydrogen sulphide, carbon dioxide and organic compounds. Ground water is water that is found between particles of soil which can be sand, silt and clay or found at rocks underneath the grounds surface. This water originates from rainfall which then penetrates the soil. Ground water can be found at aquifers these can be confined (artesian) and unconfined (shallow). Unconfined groundwater is near the surface whereas unconfined groundwater is under a layer of impervious material and usually under pressure. In Perth there are three layers of groundwater sources, the first being the superficial shallow aquifer and then there are confined aquifers in Leederville and Yarragadee. This groundwater cannot be used as drinking, it must go through a combination of treatment to become potable water. The Wanneroo groundwater treatment plant has combination of groundwater treatment, aeration being the first one, then clarification, MIEX, sand filtration, disinfection, fluoridation and the pH balance.

Aeration is the process in which water is sprayed into the air, this is to release any trapped gases such carbon dioxide (CO2) and hydrogen sulphide (H2S). It is essential to release these gases as it increases they increase the acidity of the water and H2S also giving a ‘rotten egg’ stench to the water. The water being sprayed also increases the concentration of dissolved oxygen. Chlorine gas (Cl2) is then mixed with groundwater, by doing this the Cl2 and O2 will act as an oxidising agent and remove any dissolved organic compounds that may be present. The addition of Cl2 and O2 will also decrease the solubility of any dissolved iron and manganese, this also beneficial in the later stages, clarification and sand filtration. The manganese must be removed even though it is not toxic, it gives a black colouration to the water, which may cause staining of clothing and surfaces, this is also the same case with iron but instead it gives a brown colouration. Potassium permanganate (KMnO4) may also be added to this stage as this also oxidises organic compounds, but also hydrogen sulphide (H2S), thus removing the rotten egg smell mentioned before.

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MIEX or magnetic ion exchange is the next stage of the treatment process, it works by the addition of resin beads into the aerated water. These small resin beads have relatively large surface area, these means that large cluster of dissolved organic carbon can from, with attach to the resin beads. These water is then moved to settling tanks for removal of the resin beads. Due to the magnetic characteristics of the resin, this means that beads will settle at a faster rate. Only half of the water is treated with the MIEX process, while the other half has the standard treatment. The MIEX treated water does not require any pH adjustments because it removes most organic carbon, so the standard treated water does require pH adjustment, this is achieved by the addition of lime (CaO), which increases the pH. This pH correction needs to be done because Australian Drinking Water Guidelines specify a lower aesthetic pH value of 6.5 and an upper value of 8.5. As drinking water above or below these values can be extremely dangerous for the body.

The next stage is clarification, this is where the MEIX treated water and the standard treated water all goes to clarifiers, this is where fine particles are removed from the groundwater. These fine particles will not settle down normally and these particles will cause the water to be turbid (unclear). So, in the clarification process water is moved to a large settling tank where ‘alum’ (Al2SO4) and lime (CaO) are added while agitating the water. The alum helps by coagulating all the colour and turbidity particles together to results in the formation if micro flocs. The micro flocs then bind to the alum and become heavier than water. A flocculent aid, called polyelectrolyte, is then added to bind these particles in a size that is able to settle down. These particles then settle out and the clarified water is then drawn off the top off the settling tanks. The clarified water then travels to the filtration segment. This is where water passes into a dual media gravity filters, these withdraw floc particle from the clarification process. The water passes through deeps bed filters of granulated anthracite (coal with up to 98% carbon), coarse sand and blue metal to remove any particles carried over from the clarifiers. Anthracite is effective at absorbing organic materials which can cause water to be coloured.

The water is then transported to clean water tanks where it is treated even further, for disinfection. This is done by chlorination or chloranimation to destroy harmful pathogenic bacteria. Chlorine with ammonia disinfection treatment is a much longer lasting disinfectant, by doing this the less disease will be contracted by people. Fluoridation is the addition of fluorine to the water, this is usually the addition of fluorosilicic acid (H2SiF6), WA fluoride concentration are between 0.6-1 mg L-1, as the ‘Fluoridation of Public Water Supplies Act’ allocated a maximum of 1 mg L-1. This is beneficial for people as it is known to reduce cavity rates because it strengthens the tooth enamel.

The main concern with contamination of water, is the contamination of heavy metals in water, these heavy metals are five times denser in comparison to water, heavy metals is an element that has high density ranging from 3.5 to 7 g cm-1. The danger of the heavy metals is that they can become extremely toxic they begin to build up in the soft tissue of the body. The most common heavy metal contaminants in water supplies is found to be arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and mercury (Hg). The main sources of these contaminants are mining waste, landfill leaches, municipal wastewater and urban runoff. Cadmium is commonly used in the electroplating industry, solders, batteries, TV sets, photography and more electronics, which can all end up in landfills and lead to landfill leaching. Cadmium can come into water sources by metal ore refining, phosphate fertilisers which contain Cadmium and nickel-cadmium rechargeable batteries. Another source is corrosion of galvanised pipes.

If this Cadmium was not filtered out, it could cause renal dysfunction, liver and blood damage and also bone degeneration. Arsenic being another contaminant, is found in natural deposits in the earth’s crust, but in some areas, there may be higher concentration compared to other areas, so this can affect groundwater supplies. Inorganic arsenic can also enter drinking water by wind-blown dust and by runoff from soil, sediments and rocks. Arsenic can also cause darkening of the skin and also small warts may appear on the palms, soles and body, but long-term consumption could lead to skin cancer. Lead a common contaminant, it leaches into water through corrosion, this can occur in the plumbing of houses and buildings, due to the pipes, solders, fixtures and faucets corroding. Another common heavy metal contaminant is copper, humans need a certain amount of copper in their diet, this is because it allows the formation of red blood cells, it also helps prevent many cardiovascular diseases. Although copper is essential for the body, excessive amount of copper may be fatal to the body and cause health issues.

Copper can contaminate drinking water due to the corrosion of copper pipes in plumbing systems. Nickel also being another heavy metal contaminant, can enter water bodies through weathering of rocks and soils and through leaching of minerals. Nickel can also be released into the environment through industrial waste water, this nickel released into the environment usually ends up in soil and sediments, which it then it binds to particles which consists of iron or manganese. When under acid conditions, the nickel will become more mobile in soil, so it can then seep into groundwater, thus contaminating it. The ADWG (Australian Drinking Water Guidelines) outlines that the maximum safe amount of lead in drinking water is 0.01 mg L-1, it also states that the arsenic levels should not exceed 0.007 mg L-1, cadmium levels should not exceed 0.002 mg L-1, chromium (Cr(VI)) at a level of 0.05 mg L-1, copper at 2 mg L-1, nickel at a maximum level of 0.02 mg L-1, and mercury at 0.001 mg L-1. These are all set concentration by the ADWG.

There can be many contaminants in water sources, so thus this water must be tested so that appropriate purification process can be done to remove the contaminant. To test the water inductively coupled plasma mass spectrometry or ICP-MS can be used, this analytical method is a type which can detect metals at a very low concentration. The liquid sample is first nebulised – turned into a fine mist. At the torch of ICP, there is argon which becomes ionised due to a spark produced. The cation and electrons from the argon collide with other argon molecules, this creates high temperatures. The plasma will the reach an equilibrium and remain at a constant temperature of 6000oC.

The fine mist produced enters this high temperature plasma, in which it is dried to a solid and heated to a gas state, this is known as atomisation. These atoms will then continue to travel through the plasma and absorbing energy until the atoms release an electron, thus becoming ionised. These ions travel out of the torch to the interface. The interface is the where the part of ICP is introduced to the MS. Here there is a two-step pressure reduction which allows the ionic gas to enter the MS at an appropriate pressure and temperature. There is also a universal cell, this is to stop any interference which may occur, such as preventing sequences of other reactions. In ICP-MS, quadruple mass spectroscopy is used, this separates the singly charged ions from each other by mass, acting as a mass filter. The quadrupole system works by setting voltages and radio frequencies to allow ions of a given mass to charge ration to remain stable within the rods and pass through to the detector, so ions with another ratio will be ejected. The ions will then go to the detector, where it is struck by the ions. This is then graphed, where on the y axis there is the signal intensity of the ions and on the x axis is the mass to charge ratio, the signal intensity is directly proportional to the concentration of the element in the sample.

There are many advantages of using this method, such the virtually all elements on the periodic table can be detected with this method, and it can also determine isotopic ratios and dilutions due to the MS part of it, only a small sample is also needed which is more convenient, and also it has low detection limits meaning that it can detect elements of small concentrations. There are also many disadvantages to this method which are that, there is high cost for this machine and that lighter elements are not well suited like heavier elements this is because they are more prone to interferences.

Another method that could be used is AAS or atomic absorption spectroscopy, this can be used for the detection of salts in seawater, such as sodium chloride (NaCl), potassium chloride (KCl) and magnesium chloride (MgCl2). These are ionic compounds, so when in seawater they are dissociated to ions, so when using AAS, the detection will be for an individual ion. It can also be used to test groundwater for heavy metal, such arsenic. In this process the sample is sprayed onto a suitable flame, where the sample is atomised. When the sample is in a gaseous state a beam of light is passed through the sample. The light is produced by a hallow cathode lamp, in this lamp a metal of the cathode is chosen, this will be what is being tested, so in this case it may be sodium. The element being tested for will only absorb this light, this is because they will have the same energy levels as the atoms emitting the light, so the other elements present in the sample will not absorb the light due to them having different energy levels. When the light has passed through the sample, it is focused through a slit, so it is able to enter the monochromator, this selects only wavelength of the light, which then it can be analysed. An absorbance value is then given, this is a measure of the degree of light absorbed, with the absorbance value you can then calculate the concentration of the element in the sample. This is done with the use of a calibration curve, this is formed by measure the absorbance of a set of solution with known concentrations.

The advantages of using AAS is that it high accuracy of results given that appropriate standards are used, it is also a relatively inexpensive method. Another advantage is its sensitivity, being that it can measure down part per billion. It is also great for water quality testing as AAS sample must be liquids not solids. With AAS there also many cons to it, one of these as that a large sample is required, one to three millilitres, another is that it can only test for one element at a time.

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