water conditioning lecture print

8/6/2019 Water Conditioning Lecture Print

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Water Conditioning



SO WHAT IS IT ABOUT WATER THAT MAKES IT SO IMPORTANT TO US?

Water is of major importance to all living things.

Up to 60 percent of the human body is Water.

Therefore the quality of Water we drink is very important.

The Drinking Water should be totally clean, pure and free of any diseasecausing MICROBES, and that¶s why it should be properly Treated andDISINFECTED before using it for drinking purposeWater treatment transforms raw surface and groundwater into safedrinking water.

Water treatment involves two major processes: physical removal of solidsand chemical disinfection



Water A polar compound and universal solventColourless

Odourlessand tasteless but a very unique liquid.

Exist in three allotropic forms:

Solid,liquid,gas



Unique property of water

Density:01 gram/cm3 at 4 deg centigradePosses unique property of irregular expansion.Its contracts up to 4 deg and then starts expansionon freezing. It expands about 10 % on freezing.

water can be superheated and super cooled byadding sodium chloride, thus very useful in all the

unit operations and unit processes.



Some uses

As solventModerator

Industrial solventSteam generationMany others



Types of ImpuritiesSuspended impurities . (due to clay or organic material)

Not able to be dissolved and present insurface water and visible and these maybe removed by filtration processDissolved impuritiesthese impurities are due to internal

character of water as water is polar compound and may dissolve polar materials.



What is hardness?Hardness is

± the ability of the water to consume excessiveamounts of soap before foaming

± OR ± the ability of the water to produce scale in water

heaters and boilers where water temperature isincreased dramatically

± hardness is measured in terms of parts per million(PPM) or grains per gallon of CaCO3.

± 01 grain of CaCO3 per gallon = 17.1ppm



Causes of Hardness.

Water hardness is principally caused by:calcium ionsmagnesium ions

Source of calcium and magnesium ionsgeological formations



Causes & Sources of Hardness

CationsCations causing hardnesscausing hardness Anions Anions

CaCa++++ HCOHCO33--

MgMg++++ SOSO44==

SrSr++++ ClCl--

FeFe++++ NONO33--

MnMn++++ SiOSiO33==



Types of hardness

Carbonate( temporary hardness)CO 3, HCO 3

Non-carbonate ( permanent hardness) SO4, Cl, NO3

The maximum level of hardness considered for publicsupply is 300 to 500 mg/l, though many customers

object to water harder than 150 mg/l.



D isadvantages of hardness

excessive soap consumption during launderingscale-formation in hot water heaters and pipes. Quality of product like food industry getsaffected Textile industry gets affected due to impuritiesin water ( in terms of colour)D issolved impurities may cause ion exchangeand less productPharmaceutical industry may get affected by

bad water quality.



Conti«.

Water should be colourless, odourless, tastlessand free from pathogenic bacteria for dairies

water for beverages should not be alkalinewater for cooling systems should be non

corrosive and non scale formingwater should be free from microorganism for

distilleries and sugar industry.Water for laundries should be soft.



Degree of HardnessDegree of Hardness

Hardness Concentration (mg/l)Hardness Concentration (mg/l)6060 ± ± 120 (Moderately Hard)120 (Moderately Hard)

120120 ± ± 180 (Hard)180 (Hard)180 and Over (Very Hard)180 and Over (Very Hard)



softening In precipitation softening, lime (CaO) and sodaash ( Na2CO 3) are used to precipitate calciumand magnesium from water.

Lime treatment can also:kill bacteriaremove ironhelp in clarification of surface water (coagulant)



L ime Lime is commercially available in the forms of:

quicklimehydrated lime

Quicklimeavailable in granular formcontains minimum of 90% CaOmagnesium oxide is the primary impurity

Hydrated L imecontains about 68% CaO

Slurry lime is written as Ca(OH) 2.



Water Water SofteningSofteningThe removal of divalent metal ions

Precipitation/settling or IX resins

Ca(HCO 3 )2 + Ca(OH) 2 2CaCO 3 + 2H 2 O

Mg(HCO 3 )2 + 2Ca(OH) 2 Mg(OH) 2 + 2CaCO 3

Note that the CaCO 3 precipitatedis chemically identical to limestone



Typ ical water treatmentTyp ical water treatmentp rocess with lime softeningp rocess with lime softening



Info««..WaterWater

Seawater contains about 35,000 pp m of dissolved salts thus making seawater unfit for agricultural and domestic use.

Safe drinking water generall y contains less than1000 pp m of dissolved solids (salts).



A SPEC TS A CHEMIS T IS CONCERNED AB OU T:

1. Hardness - relates to the amount of Ca, Mg, andother salts dissolved in water.

2. Salinit y - relates to the concentration of salts inwater. Fresh water should contain 1000 pp m or less of

dissolved solids.

3. p H - relates to the measure of acidit y or alkalinit y of water. p H 6 to 9 p referred

4. Turbidit y - relates to the clarit y of water, the amountof sus p ended matter in water.

5. Color, odor, taste- Organic matter and minerals ma y p roduce a distinct color. Minerals, bacteria anddissolved gases are res p onsible for taste and odor.



WaterWaterFour techniques to soften hard water:

1. DISTILLATION: The water is boiled, and the steam formed is condensed into a liquidagain, leaving the minerals behind in the distilling vessel. Commercial stills are able toproduce hundreds of liters of distilled water per hour.

2. CALCIUM AND MAGNESIUM PRECIPITATION: Calcium and magnesium ions areprecipitated from hard water by adding sodium carbonate and lime. Insoluble calciumcarbonate and magnesium hydroxide are precipitated and are removed byfiltration or sedimentation.

3. ION EXCHANGE: Hard water is softened as it is passed through a bed or tank of zeolite-a complex sodium aluminum silicate. Sodium ions replace objectionablecalcium and magnesium ions, and the water is softened:

Na 2(zeolite)(s) + Ca 2+ (aq) p Ca(zeolite)(s) + 2Na + (aq)The zeolite is regenerated by back-flushing with concentrated sodium chloridesolution, reversing the foregoing reaction.

4. DEMINERALIZATION: Both cations and anions are removed by a two-stage ion-exchange system. Special synthetic organic resins are used in the ion-exchange beds.In the first stage metal cations are replaced by hydrogen ions. Second stage anionsare replaced by hydroxide ions. The hydrogen and hydroxide ions react and you getpure, mineral-free water.



Coagulation

is the destabilization of colloids by neutralizing the forcesthat keep them apart.

Cationic coagulants provide positive electric charges toreduce the negative charge of the colloids.

As a result, the particles collide to form larger particles(flocs).

Rapid mixing is required to disperse the coagulant

throughout the liquid.Care must be taken not to overdose the coagulants as

this can cause a complete charge reversal andrestabilize the colloid complex.



Coagulation removes dirt and other particles suspended in water.alum and other chemicals are added to water to form tiny sticky

particles called ´flocµ which attract the dirt particles.

The combined weight of the dirt and the alums (floc) become heavyenough to sink to the bottom during sedimentation.



conti..Typically, alum is added (aluminum sulfate) or ferric chloride or sulfate to thewater with rapid mixing and controlled pH conditionsInsoluble aluminum or ferric hydroxide and aluminum or iron hydroxo

complexes are formedThese complexes entrap and adsorb suspended particulate and colloidal

material. And separation takes place under the action of gravity.



Flocculation

is the action of polymers to form bridges between theflocs. and bind the particles into large agglomerates or clumps.

Bridging occurs when segments of the polymer chainadsorb on different particles and help particlesaggregate.An anionic flocculants will react against a positivelycharged suspension, adsorbing on the particles andcausing destabilization either by bridging or chargeneutralization.In this process it is essential that the flocculating agentbe added by slow and gentle mixing to allow for contactbetween the small flocs and to agglomerate them intolarger particles.



Conti«F locculation:

Slow mixing (flocculation) that provides for a period of time to promote the aggregation and growth of the

insoluble particles (flocs).The particles collide, stick together and grow larger The resulting large floc particles are subsequently

removed by gravity sedimentation (or direct filtration)Smaller floc particles are too small to settle and are

removed by filtration



ChlorinationChlorine i s t he m o st c ommon c o st- effe ct i v e mean s of d i s infe ct ing w a t er

T he addi t ion of a s mall amo u n t of c hlorine i s highl y effe ct i v e again st mo st b a ct eria, v ir us e s , and p ro t ozoa .

But cysts ( d u ra b le s eed - like st age s) formed by p ara s i t i c p ro t ozoa suc h a s Cr ypt o sp oridi u m and G iardia c an su r v i v e c hlorine .

Chlorine i s a pp lied t o w a t er in one of t hree form s:elemen t al c hlorine (c hlorine ga s) , h yp o c hlori t e s ol ut ion (b lea c h), or dr y c al c i u m h yp o c hlori t e .All t hree form s t o p rod uc e free c hlorine in w a t er



Membrane Properties

Inert & non-biodegradableResistant to p ressure, fouling,Resistant to cleaning agents & tem p eratureUniform p ore distribution & high p orosit yDurable, neutral & h ydro p hilic,Eas y to clean and regenerate



MEMB RA NE PROCESSESMembrane

ProcessOp eratingPressure(B ar)

Pore Size(Micrometer)

A llowableCom p onents

RetainedCom p onents

ReverseOsmosis

(RO)

30 ± 60(440 - 880 psi)

<0.001 Water

Bacteria,Protein,Salts &Sugar

Nano Filtration(NF)

20 ± 40(290 - 585 psi)

0.001- 0.005 Salts & WaterBacteria,

Protein, &Sugar

Ultra Filtration

(UF)

1 -10

(14.7 - 145 psi)

0.005-0.01Salts , Water &

Sugar

Bacteria,

Protein,

Micro Filtration(MF)

<1 (< 14.7psi) >0.01Salts , Water,

Sugar &Protein

Bacteria



Membrane materials

The most commonly used membranematerials are:

Polyvinylidene difluoride (PVDF)Polyethylsulphone (PES)Polyethylene (PE)

Polypropylene (PP)



Reverse Osmosis

Reverse osmosis is a process for desalting water using membranes that arepermeable to water but essentiallyimpermeable to salt.Pressurized water containing dissolvedsalts contacts the feed side of themembrane; water depleted of salt iswithdrawn as a low-pressure permeate.



Terms of Reveres OsmosisFlux ± The number of gallons of permeate produced per square foot of membrane area per day (GFD). Generally, flux rates are determined by thefeed water Silt Density Index.

Fouling ± When gelatinous coatings, colloidal masses or dense bacterial growth form a

compacted crust on membrane or filter surfaces which blocks further flow.

Membrane ± Filtration membranes are thin polymer films that are permeable to water and other fluids.

Microporous membrane filters have measurable pore structures which physically removeparticles or microorganisms larger than pore size.

Ultrafiltration membranes, (sometimes called molecular sieves), also remove moleculeslarger than a specified molecular weight.

Reverse osmosis membranes are permeable to water molecules, and very little else,rejecting even dissolved ions in water.



Conti«.Osmosis ± The diffusion of a solvent through a semipermeable membranefrom a less concentrated solution to a more concentrated solution.

Percent Recover y ± In reverse osmosis, the ratio of pure water output to feed water input. The recovery that the membranes operate at is called internal or membrane

recovery. The recover that the RO machine operates at is called overall or machinerecovery.

Percent Rejection ± In reverse osmosis, the ratio of impurities removed to totalimpurities in the incoming feed water. The formula for calculating percent rejection is100 X (feed ± product)/feed

Permeate ± In reverse osmosis, the water that diffuses through the membrane, therebybecoming purified water.



How RO works,



Membrane Cleaning

A good pretreatment system is essential to achieve along reverse osmosis membrane life, but pretreatmentmust be backed up by an appropriate cleaning schedule .Generally this is done once or twice a year, but more

often if the feed is a problem water.As with pretreatment, the specific cleaning procedure isa function of the feed water chemistry, the type of membrane, and the type of fouling.

A typical cleaning regimen consists of flushing themembrane modules by re circulating the cleaningsolution at high speed through the module, followed by asoaking period, followed by a second flush, and so on.



Conti..chemical cleaning agents commonly used are acids,alkalis, detergents ,Acid cleaning agents such as hydrochloric, phosphoric,or citric acids effectively remove common scalingcompounds.

With cellulose acetate membranes the pH of thesolution should not go below 2.0 or else hydrolysis of themembrane will occur.Oxalic acid is particularly effective for removing irondeposits.Acids such as citric acid are not very effective withcalcium, magnesium, or barium sulfate scale;To remove bacteria, silt or precipitates from themembrane, alkalis combined with surfactant cleaners areoften used .



A pp lications

Approximately one-half of the reverse osmosissystems currently installed are desalinatingbrackish or seawater.

Another 40% are producing ultra pure water for the electronics, pharmaceutical, and power generation industries.

The remainder are used in small nicheapplications such as pollution control and foodprocessing



Brackish Water Desalination

The salinity of brackish water is usually between2000 and 10 000 mg/ L .The World Health Organization (WHO)recommendation for potable water is 500 mg/ L ,so up to 90% of the salt must be removed fromthese feeds.

Early cellulose acetate membranes couldachieve this removal easily, so treatment of brackish water was one of the first successfulapplications of reverse osmosis.

Several plants were installed as early as the1960s.



diagram



What is Electro dialysis

An anionic membrane with fixed positivegroups excludes positive ions but is freelypermeable to negatively charged ions.Similarly a cationic membrane with fixednegative groups excludes negative ionsbut is freely permeable to positivelycharged ions,



Electro dialysis

In an electro dialysis system, anionic andcationic membranes are formed into a multi cellarrangement built on the plate-and-frame

principle to form up to 100 cell pairs in a stack.The cat ion and anion exchange membranes arearranged in an alternating pattern between theanode and cathode.Each set of anion and cat ion membranes formsa cell pair.



A li ti



A pp licationsBrackish Water Desalination

Brackish water desalination is the largest application of electrodialysis.The competitive technologies are ion exchange for very dilute salinesolutions, below 500 ppm, and reverse osmosis for concentrationsabove 2000 ppm.

In the500±2000 ppm range electro dialysis is often the low-costprocess. One advantage of electro dialysis applied to brackish water desalination is that a large fraction, typically 80±95% of the brackishfeed, is recovered as product water.However, these high recoveries mean that the concentrated brinestream produced is five to twenty times more concentrated than thefeed.

The degree of water recovery is limited by precipitation of insolublesalts in the brine.

water conditioning lecture print

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