Chemistry Assighnment

8/2/2019 Chemistry Assighnment

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CHEMISTRYASSIGHNMENT

Submitted By-:

Mrs. Deepshikha Mam



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Submitted By:

Roll no.

2210259 2210250

2210281

2210249

2210253

NAME

RAVI VASDEV PRIYA KANWAR

SUJATA SHARMA

PRATIKSHA

PRIYANKASHARMA



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CONTENT

WATER AND ITS TREATMENT

(PART-II)



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Coagulation and Flocculation

If raw water contains a large amount of fine suspended solids, coagulationand flocculation can be used to remove much of this material. Incoagulation, a substance (usually in a liquid form), is added to the water tochange the behaviour of the suspended particles. It causes the particles,which previously tended to repel each other, to be attracted towards eachother, or towards the added material.

Coagulation takes place during a rapid mixing/stirring process which

immediately follows the addition of the coagulant. The flocculation process,which follows coagulation, usually consists of slow, gentle stirring. Duringflocculation, as the particles come into contact with each other they clingtogether to form larger particles which can be removed afterwards bysettlement (see TB 58) or filtration. A chemical which is often used is alum(aluminium sulphate). Natural coagulants include some types of clay (e.g.bentonite) and powdered seeds of the Moringa olifeira tree. The best type of coagulant and the required dosewill

depend on the physical properties (particularly the alkalinity/acidity) of theraw water and the amount and type of suspended solids.



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Filtration

A number of processes take place in filters, including mechanicalstraining; absorption of suspended matter and chemicals; andparticularly in slow sand filters, biochemical processes. Dependingon the size, type and depth of filter media, and the flow rate andthe physical properties of the raw water, filters can removesuspended solids, pathogens, and certain chemicals, tastes, and

odours. Straining, and settlement (both described in TB 58) are treatment

methods which usefully precede filtration to reduce the amount of suspended solids which enter the filtration stage. This increasesthe period for which a filter can operate before it needs cleaning.Coagulation and flocculation are also useful treatments to precedesettlement and improve still further the removal of solids beforefiltration. If an effective system of removing the flocculated

particles from the filter is feasible, then coagulation andflocculation can also be used before coarse sand filtration. Largerpathogens (e.g. parasitic worm eggs) are more readily removed byfiltration than smaller pathogens (e.g. viruses).



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Sand filtration Slow sand filtration In slow sand filtration the water passes slowly (e.g. flow velocity of 0.1 to

0.2m/h -i.e. m3/m2/h) downwards, through a bed of fine sand. For the filterto perform well there should be no sudden changes in the flow rate and thewater should not be very turbid (cloudy with suspended solids), or the filterwill quickly become blocked. Good slow sand filters can produce good qualitydrinking-water. There are a number of processes which improve the waterquality as it passes through the filter, but pathogens are mainly removed in

the very top layer of the filter bed where a biological film (called the ‘schmutzdecke’) builds up. In a well-designed and well-operated filter this filmstrains out bacteria. Deeper in the sand bed bacteria that pass through theschmutzdecke are killed by other micro-organisms, or they become attachedto particles of sand until they die. The schmutzdecke takes time to becomeeffective, so water needs to flow though a new filter for at least a week beforethe filter will work efficiently. The raw water should contain a fair amount of oxygen to promote the useful biological activity both in the schmutzdecke andfurther down into the filter bed. After a period of use, the material filtered outof the water blocks the surface of the sand and reduces the flow rate to anunacceptable level. When this happens the filter is drained to expose thesand, and the top 15 to 20mm of the bed is carefully removed. When the filteris restarted, it takes a few days before the schmutzdecke builds up again toprovide good quality water so, during this period, the water should not beused for potable purposes. When successive cleaning operations cause thedepth of sand to reach the minimum acceptable value (conventionally 650mmthick (see TB 15)), additional clean sand needs to be added to the bed.



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Rapid sand filtration

In this method the sand used is coarser than for slow sand filtration and the rate off low isfaster (conventionally the velocity of flow is between 4 and 8m/hr).Rapid sand filtration isused for removing suspended solids from water and is particularly effective aftercoagulation and flocculation. No schmutzdecke develops on these filters, so they are noteffective at removing pathogens; the filtered water should subsequently be disinfected orpassed through slow sand filters. There are two main types of rapid sand filter; down flowand up flow.

In a down flow filter the water flows down through a layer of sand, ideally between 1 and2m deep, below a depth of water of between 1.5 and 2.5m (although these depths arerarely practical for household filters which usually have shallower depths and are thereforeless effective). When this type of filter becomes clogged, the flow is reversed to mobilizethe sand particles and wash out the trapped solids. The operation of this type of filter isnormally too complex for household use. In up flow rapid sand filters the water passes upthrough the sand. To clean out debris trapped in the sand, the flow is made to reversesuddenly by opening a fast-acting valve below the filter bed. To prevent the build up of deposits in the sand, backwashing may be carried out every day, although if sufficientwater and pressure is available for backwashing, a longer period will be acceptable. This

type of filter is sometimes used at household level with shallower depths of sand andslower flow rates than for the conventional down flow filter mentioned above. The filterillustrated in Figure 3 is one shown by Heber (1985). It uses a 300mm depth of sand anda filtration rate of 0.5 to 1.5 m/h.



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Charcoal filters

As in the case of the UNICEF filter, granular charcoal can be usedduring filtration. It can be quite effective at removing some tastes,odours and colour. However, there is evidence that sometimescharcoal, particularly if not regularly replaced, can become thebreeding ground for some harmful bacteria.

Some disadvantages of sand filters: good household sand filters are not cheap to construct; owners of such filters need to be well motivated to operate and

carefully clean the filters correctly, and periodically to carry out thetime-consuming task of renewing the sand bed. If any of these tasksare not carried out properly, the quality of the water will beunreliable;

many of the cheaper household sand-filter designs are not able toproduce pathogen-free water;

an alternative source of potable water, or sufficient stored, treatedwater, needs to be available during the days immediately after thecleaning or re-sanding of slow sand filters.



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Ceramic filters



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Ceramic filters

The purifying element in these filters is a porous, unglazed, ceramic cylinder(often

called a candle) which can be locally produced (Heber, 1985), but is usuallymass-produced in factories. Manufactured filter units like that illustrated inFigure 5

(a) Are available but are costly. If filter candles are available they can be

fitted to earthenware pots (b) An alternative arrangement, which avoids the need for watertightconnections through the jars, is to use a siphon pipe

(c) Open porous-clay jars (d) Can also be used. Ceramic filters are appropriate only for fairly clear water because they block

quickly if the water contains suspended particles. Their effectiveness dependson the size of the pores in the clay. Filters with very small pore sizes can

remove all pathogens. The impurities are deposited on the surface of thecandle, so need to be regularly scrubbed off to maintain a good flow rate.Boiling the filter after it has been cleaned is also recommended to kill off thepathogens trapped in the pores, but some filters are impregnated with silverto kill micro-organisms. The scrubbing wears down the ceramic material, soperiodically the candle needs to be replaced before it becomes too thin toguarantee the removal of all pathogens.



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Arsenic removal

Dangerous levels of arsenic can be found naturally ingroundwater and surface water, but can also resultfrom industrial pollution. Excessive amounts are toxicto humans, resulting in various diseases including

cancer. High levels are a growing problem in groundwaters in some countries like Bangladesh. Theeffectiveness of any treatment process depends onthe specific form of arsenic found in the water and thetype of coagulant and filtration material used to purifythe water. Important research is presently being

carried out into arsenic removal to find appropriatesolutions. Treatment processes which include theaddition of lime to soften the water, followed bysettlement, have been in use for some time.



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Combining treatment methods

Methods used to remove chemicals donot necessarily also removepathogens. For this reason,disinfection or filtration using aceramic filter or a well-designed slowsand filter, is likely to be necessary to

produce an acceptable quality of drinking water.



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Fluoride removal

Excessive fluoride (above 1.5mg/l), which is sometimesfound in groundwater, can damage bones and teeth.There are a wide variety of systems for reducingexcessive fluoride and the effectiveness of each

depends on various factors such as the initialconcentration of fluoride, the pH of the water (ameasure of the acidity or alkalinity) and the hardness of the water. The methods most suited to domestictreatment in low-income communities include limesoftening or the use of pretreated bone. One systemwhich seems to have had considerable success in thefield is the Nalgonda system which combines the use of lime (to soften the water) with alum (as a coagulant)followed by settlement; the technique is usedsimultaneously with chlorination to ensure disinfectionof the water.



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LIME-SODA ASH WATER TREATMENT METHOD Lime-soda ash treatment for the reduction of hardness involves the addition

of slake lime [Ca(OH)2] to a hard water supply to remove the carbonatehardness by precipitation with the precipitation being removed by filtration.Non-carbonate hardness is in turn reduced by the addition of soda ash(Na2C03) to form insoluble precipitate which is also removed by filtration.

This particular method of removing hardness a sometimes used by municipalwater plants to reduce the amount of calcium and magnesium in a watersupply. While it is quite effective in reducing hardness, it is not a complete

removal treatment. Often when a city has a raw water source that has 35 to 40 grain hard

water, the local water system will use the lime-soda ash treatment to reducehardness to between 5 and 10 grains.

Lime-soda ash treatment is especially effective if a water containsbicarbonate (temporary) hardness. Where calcium and magnesium areprimarily in chloride or sulfate compounds, this treatment is noticeably lesseffective.

Slaked lime is used to remove calcium bicarbonate from water. In the waterto be treated, the slaked lime ions react with the calcium bicarbonate toform the very slightly soluble calcium carbonate. This precipitated material isusually removed by first settling and then filtering.

Ca(OH) 2+ Ca(HC03 ) 2 --> 2 CaCO3 ¥ + 2 H20 Calcium hydroxide plus calcium bicarbonate reacts to form calcium

carbonate plus water



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Comparison between Cold and hot lime soda process

Cold lime sodaprocess

It is carried out at roomtemperature (25-30°C)

It is a slow process Use of coagulant is a

necessary Filtration is not easy Residual hardness is 60

ppm

Dissolved gases are notremoved It has low softening

capacity

Hot lime sodaprocess

It is carried out at hightemperature (95-100°C)

It is a rapid process No coagulant required Filtration is easy as

viscosity of water is low Residual hardness is 15—

30 ppm

Dissolved gases areremoved It has high softening

capacity



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Zeolite Process



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Zeolite Process

Principal:-



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Zeolite Process

The hard water is percolated through the bed zeolite housed in acylindrical unit. The hardness causing cations like Ca2+andMg2+are retained by the zeolite as CaZ and MgZ and the waterflowing out contains the sodium salts as per the equationsindicated earlier.

When the zeolite is exhausted, it is regenerated using a 10%

sodium chloride solution. Thus, the whole process involvesalternate cycles of softening - run and the regeneration - run. Theregeneration involves backwashing, brining and rinsing of thezeolite bed with water before using it again. The residual hardnessof water from this process is 0-15 ppm.

It is to be noted that any turbidity in feed water should beremoved before sending it through the zeolite bed, as otherwise,the pores will be clogged. Also water containing large quantities of Fe2+and Mn2+, when passed through the zeolite are converted tothe respective FeZ and MnZ, which are very difficult to beregenerated because of their high stability. Mineral acids, if present, in water destroy the zeolite and hence they must beneutralised in advance before feeding the water into the bed.



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Comparison Between Zeoliteand Lime Soda Process

Zeolite process This process produces water of

very low hardness. He cost of the plant and zeolite is

higher. Hence, capital cost ishigher.

The exhausted zeolite bed can beregenerated with brine which isvery cheap. Hence, operatingcost is less.

The plant is compact andoccupies less space. The size of plant depends on the hardness of water being treated

Cannot be used for hot water,acidic water and water havingturbidity and suspendedimpurities.

This process can operate underpressure and can be designed forfully automatic operations.

Lime soda process This process produces water of

hardness of 15-60 ppmdepending on whether it is a hotor cold process.

The capital cost is lower.

The chemicals required areconsumed in this process thusoperating cost is higher.

The plant occupies more space.The size of the plant depends onthe amount of water beinghandled

The process is free from suchlimitations.

This process cannot be operatedunder pressure.



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Types Of Zeolite Process

Natural zeolites

Natural zeolites areamorphous in nature andare derived from green

sands by washing,heating and treating withNaOH. The mostcommonly used naturalzeolite is natrolite(Na2Al2O3.4SiO2.2H2O)and it possesses gooddurability.

Synthetic zeolites

synthetic zeolites areporous and they canbe prepared byheating togethersolutions of sodiumsilicate, aluminiumsulphate and sodiumaluminate. Syntheticzeolites posses higherexchange capacity perunit weight whencompared to thenatural ones.

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