water treatment 12

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WATER TREATMENT

Water treatment describes those processes used to make water more

acceptable for a desired end-use. These can include use as drinking water ,industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end-use. One such use is returning water that has been usedback into the natural environment without adverse ecological impact.

TREATMENT OF SURFACE WATER

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http://en.wikipedia.org/wiki/Drinking_water

http://en.wikipedia.org/wiki/Contaminant

http://en.wikipedia.org/wiki/Drinking_water

http://en.wikipedia.org/wiki/Contaminant

http://en.wikipedia.org/wiki/Water



The available raw water has to be treated to make it fit, i.e. potable, meanssafe for human consumption. It should satisfy the physical, chemical andbacteriological standards as specified above. The various methods of purification of water are

(i) Screening

(ii) Plain sedimentation

(iii) Sedimentation aided with coagulation

(iv) Filtration

(v) Disinfection

(vi) Aeration

ScreeningScreens are provided before the intake works so as to prevent the entry of big objectslike debris, branches of trees, part of animals etc. Screens maybe of two types, coarsescreen and fine screens. Coarse screens areparallel iron rods placed vertically or at a small slope at about 2.5 cm to 10cm apart. The fine screens are made up of fine wire or perforated metal

with small openings less than 1 cm size. Finer is the screen more are thechances of clogging so generally only coarse screens are used. Thescreens may be manually cleaned or mechanically cleaned dependingupon the requirement i.e. the size of the treatment plant.

Plain SedimentationSedimentation is done to remove the impurities which have specific gravitymore than that of water and are settleable. When water is moving theseimpurities remain in suspension due to the turbulence and as the velocity isreduced they settle down. It is not necessary to stop the motion of water

completely as it will require more volume of the sedimentation tanks. As per the theory of sedimentation the settlement of a particle depend upon thevelocity of flow, the viscosity of water, the size shape and specific gravity of particle. The settling velocity of a spherical particle is expressed by Stoke’slaw which gives the final equation as follows,Vs = g/18 (Ss-1) d2/υWhere Vs = Velocity of settlement of particle in m/sec



d = diameter of the particle in cmSs = specific gravity of the particleυ = kinematic viscosity of water in m2/secKnowing the settling velocity of particle, that is intended to be settled, thedesign of the settling tank is done

Sedimentation Aided with CoagulationThe fine suspended particles like mud particles and the colloidal matter present in water cannot settle down by plain sedimentation with ordinary(lesser) detention periods. Some of the colloidal impurities will not settleeven if the water is detained for long periods in the sedimentation tanks asthe same charge on the clay particles repel each other and do not allowthem to settle down. So the sedimentation is aided with coagulation.Coagulation is a process in which some chemical like alum or ferrous

sulphate is mixed in water resulting in particle destabilization. Operationallythis is achieved by the addition of appropriate chemical like alum andintense mixing for achieving uniform dispersion of the chemical. Thesechemicals are more effective when the water is slightly alkaline. Sometimessodium carbonate or lime is to be added to achieve the suitable pH of water. Flocculation is the second stage of the formation of settleableparticles (or flocs) from destabilized (neutral) colloidal particles and isachieved by gentle (slow) mixing. So in flocculation the alum is first mixedrapidly for dispersion and then slow mixing produces flocks. Both thesestages of flocculation are greatly influenced by physical and chemicalforces such as electrical charge on particles, exchange capacity, particlesize and concentration, pH, water temperature and electrolyteconcentration.

FiltrationFiltration is a physical and chemical process for separating suspended andcolloidal impurities from water by passage through a porous bed made upof gravel and sand etc. Actually the sedimentation even aided withcoagulation and flocculation canno tremove all the suspended and colloidal

impurities and to make water (specially surface water) fit for drinkingfiltration is must. The theory of filtration includes the following actions:• Mechanical straining, the suspended particles present in water that are of bigger size than the voids in the sand layers are retained itself and thewater becomes free of them. The sand layer may get choked after sometime and then it is to be cleaned for further action by washing it back.



• Sedimentation, the small voids in the sand act as tiny sedimentation tanksand the colloidal matter arrested in these voids is a gelatinous mass andthus attracts other finer particles. These finer particles are thus removed bythe sedimentation.

• Biological metabolism, certain micro-organisms are present in the sandvoids. They decompose the organic matter like the algae etc. and thusremove some of the impurity.

• Electrolytic change, according to the theory of ionization a filter helps inpurifying the water by changing the chemical characteristics of water. Thesand grains of the filter media and the impurities in water carry electricalcharge of opposite nature which neutralizes each other and forces thepaticles to settle now by gravity.

DisinfectionThe filtration of water removes the suspended impurities and removes alarge percentage of bacteria but still some of the bacteria remain there inthe filtered water. This bacteria may be harmful bacteria (disease producingbacteria) known as pathogenic bacteria. The process of killing thesebacteria is known as disinfection. There are many diseases like cholera,gastro entities, infectious hepatitis (jaundice), typhoid etc., the bacteria or virus of which transmits through water. It is necessary to make water freefrom any micro-organism before human consumption. Contamination(mixing of pathogenic micro-organism) may take place in the water supplyat any time (because of leakage etc.) so proper measures must be taken tostop it at all levels. Generally the disinfection is done by adding chlorine towater. There should be a residual amount of chlorine after the disinfectionto fight with any probable contamination in the route of water to theconsumer. Following are some of the methods of disinfection• Boiling of water • Treatment with excess lime• Use of ozone

• Treatment with ultraviolet rays• Use of potassium permanganate• Treatment with silver • Use of bromine, iodine and chlorineOut of the above, treatment with chlorine is the most popular andeconomically effective.



• It should be capable of destroying the pathogenic organisms present,within the contact time available and not unduly influenced by the range of physical and chemical properties of water encountered particularlytemperature, pH and mineral constituents.• It should not leave products of reaction which render the water toxic or impart colour or otherwise make it unpotable.• It should have ready and dependable availability at reasonable costpermitting convenient, safe and accurate application to water.• It should possess the property of leaving residual concentrations to dealwith small possible recontamination.• It should be amenable to detection by practical, rapid and simpleanalytical techniques in the small concentration ranges to permit the controlof efficiency of the disinfection process.The factors affecting the efficiency of disinfection are

• Type, condition and concentration of organisms to be destroyed• Type and concentration of disinfectant• Contact time and concentration of disinfectants in water and• Chemical and physical characteristics of water to be treated particularlythe temperature, pH and mineral constituents.Potable water should always have some amount of residual chlorine, asthere are all chances of contamination at all levels. This may be 0.2 ppm. to0.3 ppm. Depending upon the requirement (rainy season or enhancedchances, more Cl2 required). To make sure the presence of chlorine sometests are done out of which Orthotolodine test is the most common one.Orthotolidine Test: In this test 10 ml of chlorinated sample of water is takenafter the required contact period (say half an hour) in a glass tube. 0.1 ml of orthotolidine solution is added to it. The colour formed is noted after 5minutes and compared with the standard coloured glasses. Darker is theyellow colour formed more is the residual chlorine. The test is very simpleand even a semi- skilled employee can perform it satisfactorily.

AerationTaste and odour, both are undesirable in water. Aeration is done to remove

taste and odour. Aeration is done to promote the exchange of gasesbetween the water and the atmosphere. In the water treatment, aeration isperformed for the following purposes• To add oxygen to water for imparting freshness, for example water fromunderground sources may have lesser oxygen.• For expulsion of carbon dioxide, hydrogen sulphide and other volatilesubstances causing taste and odour.



• To precipitate impurities like iron and manganese specially fromunderground water. In aeration gases are dissolved in or liberated fromwater until the concentration of the gas in the water has reached itssaturation value. The concentration of gases in a liquid generally obeysHenery’s law which states that the concentration of each gas in water isdirectly proportional to the partial pressure (product of the volumepercentage of the gas and the total pressure of the atmosphere.) or concentration of gas in the atmosphere in contact with water. Thesaturation concentration of a gas decreases with temperature anddissolved salts in water. Aeration accelerates the exchange of gas. Toensure proper aeration it is necessary to• Increase the area of water in contact with the air. The smaller are thedroplets produced the larger will be the area available.• Keep the surface of the liquid constantly agitated so as to reduce the

thickness of the liquid film which would govern the resistance offered to therate of exchange of the gas.• Increase the time of contact of water droplets with air or increase the timeof flow which can be achieved by increasing the height of jet in sprayaerators and increasing the height of tower in case of packed media.Where oxygen is to be dissolved in water, the concentration or partialpressure of the oxygen may be increased by increasing the total pressureof the gas in contact with water. For this purpose air injected into a mainunder pressure is a reasonably efficient method of increasing the amount of dissolved oxygen.

Ground Water Treatment

1) Extraction, Treatment, and Reinjection

Description of the Technology

The most common method of treating groundwater is to extract the water,

treat it at the surface, and return the treated water to the aquifer. Thisprocess also is known as "pump and treat" technology. There are a varietyof methods to return treated water to an aquifer, one of which is reinjectionwells. Systems using reinjection wells are known as extraction, treatment,and reinjection (ETR) systems. A typical ETR system has three basiccomponents:



1. Contaminated groundwater is pumped out of the aquifer through aseries of extraction wells. This requires locating extraction wells indifferent locations and at different depths in order to capture theplume.

2. The extracted water is then treated at the surface. Carbon has theability to adsorb, or grab onto, passing organic molecules and holdthem in pores within the carbon granule. When contaminatedgroundwater is pumped through a filter of carbon granules, most of the organic contaminants become trapped on the surface of thecarbon. Eventually the carbon fills up and the carbon must bereplaced. Used carbon is sent off-Cape to be recycled.The activated carbon system is housed in a treatment plant and isused to treat groundwater from several extraction wells. Thegroundwater is pumped to the treatment plant from the extraction

well. Safety precautions, such as leak detection monitoring systems,ensure that any leaks are detected immediately.Activated carbon is one way to treat the groundwater after it has beenextracted. Other treatment technologies may be considered.

3. After being tested, the treated water is pumped back into the aquifer through a series of reinjection wells. Reinjection wells are used toreduce the impact of the extraction wells on the groundwater hydrology, to help control plume migration, and to return treatedwater to the aquifer.

4. Design Considerations

ETR groundwater collectionsystems usually consist of a lineor arc of wells placed around thecontaminated area or in the pathof the groundwater flow.

Reinjection wells are generallyplaced beyond or near the edgesof the plume.

Use at Other Locations



ETR is the most commonly used treatment technology for contaminated aquifers. It has been proven effective over a widerange of site conditions and contaminants for hydraulic control of ground water. Activated carbon, an important component of ETRtechnology, is used widely in the treatment of organic groundwater contamination. It is an effective and reliable means of removingorganic contaminants and is suitable for treating a wide range of organics over a broad range of concentrations. It also is used widelyin industry for control of air pollution and odors. It has a long history of use as a treatment for municipal, industrial, and hazardous wastes,and is commonly used in home water purification.

2) Recirculating Well Technology


Recirculating well technology (RWT) is a recently- developed method of treating volatile organic compounds (VOCs) in groundwater. The primarydifference between RWT and ETR technology is that recirculating wellsreturn treated water to the same well, thus minimizing impacts on the water table.

In the recirculating well systems groundwater is drawn into the well near the bottom or from a selected specific zone in the groundwater. It comesinto contact with air which is injected into the well, causing the VOCs totransfer from the water to the air.

The cleaned water then is pumped back out of the well into another zone of groundwater. The combination of contaminated groundwater entering thewell from one zone of the aquifer and clean groundwater leaving the well inanother zone creates a zone of recirculation in the groundwater near thewell. Contaminated water moving through the zone of recirculation is

captured and treated within the well.

The air containing the VOCs is carried up the well to the surface, where itmay be piped to an activated carbon treatment system. Activated carbontreatment removes contaminants from air just as it does from water, asdescribed in the ETR section above. Depending on the design of thesystem, the treated air is either released to the surrounding environment or



directed back into the well for additional removal of contaminants. The air stream is monitored before and after carbon treatment to ensure effectiveremoval of VOCs.

Design of the System

The design of a recirculation well system is determined by the volume of groundwater that must be treated, the concentration of contaminants, thethickness of the plume, the pumping rate, the average groundwater flowvelocity, and the types of soil in the aquifer. The dimensions of the zone of recirculation determine the number of recirculating wells that would beneeded to clean up the plume.


RWT is a recently developedgroundwater treatment technology.It has been used at over 200 sites inEurope and over 80 sites in the U.S.

Preliminary results of therecirculating well pilot tests indicatethat measured cleaning efficiency isconsistent with the pilot test design

and mass removal of contaminantsis being achieved. The pilot testsystems currently are beingevaluted to assess the hydraulicperformance of the systems for consideration as a component of full-scale cleanup alternatives.

3) Natural Attenuation


Natural attenuation refers to the strategy of allowing natural processes toreduce contaminant concentrations to acceptable levels. Naturalattenuation involves physical, chemical and biological processes which act



to reduce the mass, toxicity, and mobility of subsurface contaminationThese processes are always occurring and in many cases may reduce riskto human health and the environment to acceptable levels.

There are several different physical, chemical, and biological processesthat comprise natural attenuation. These include:

• biodegradation—breakdown of contaminants by microorganisms inthe environment, often forming non-harmful byproducts like carbondioxide and water

• chemical stabilization—reduction in contaminant mobility caused bychemical processes

• dispersion—the process of mixing that occurs when fluid flowsthrough a porous medium

•

sorption—attachment of compounds to geologic materials by physicalor chemical attraction

• volatilization—transfer of a chemical from liquid to vapor; evaporation

Natural attenuation, by definition, occurs naturally. However, use of naturalattenuation as a specific treatment method is not a "do nothing" approach.It involves modeling and evaluating contaminant reduction rates todetermine whether it is a feasible method for plume treatment. Theevaluation of whether or not the treatment method is feasible must alsoconsider existing and future potential risk to human health and the

environment.

Furthermore, to use natural attenuation as a cleanup strategy, samplingmust be conducted throughout the process to confirm that degradation isproceeding at rates consistent with meeting treatment and plume cleanupobjectives. Sampling and analysis will determine whether naturalattenuation actually is reducing the mass, toxicity, and mobility of thecontamination.

Design Considerations

Many factors affect whether or not natural attenuation is a feasibletreatment alternative. These include:

• Data must be collected to make modeling as accurate as possible• Intermediate breakdown compounds may be more mobile and more

toxic than the original contaminant.



• Natural attenuation should be used only in low risk situations.• Contaminants may migrate before they are degraded.• Restrictions on use of groundwater down gradient may have to be

enforced until contaminant levels are reduced.


Natural attenuation has been used at hazardous wastes sites across thecountry. Although it is a component of remediation activities at many sites,only in the last few years has it been analyzed specifically as a treatmentstrategy to be considered along with other engineered solutions.

water treatment 12

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