Water Chemistry and Sewage Water Treatment PDF

WATER CHEMISTRY AND SEWAGE WATER TREATMENT Hard water is one which does not produce lather with soaps or detergents instead a white scum or insoluble precipitate is formed. It is due to the presence of dissolved salts of Ca or Mg. 2C17H35COONa+ CaCl2 -------------->(C17H35COO)2Ca +2 NaCl...

WATER CHEMISTRY AND SEWAGE WATER TREATMENT Hard water is one which does not produce lather with soaps or detergents instead a white scum or insoluble precipitate is formed. It is due to the presence of dissolved salts of Ca or Mg. 2C17H35COONa+ CaCl2 -------------->(C17H35COO)2Ca +2 NaCl Soft water is the surface water that is free from any types of ions or minerals eg:-distilled water and rain water It can be used for cooking ,cleaning etc TYPES OF HARDNESS:- Hardness is of two types ,temporary and permanent TEMPORARY HARDNESS is due to the presence of dissolved bicarbonates of Ca ,Mg or heavy metal ions.So it is also called carbonate hardness.This can be removed by mere boiling of water,as the soluble bicarbonates are decomposed toform insoluble carbonates or hydroxides Ca(HCO3)2 heat CaCO3 (S) + H2O + CO2(g) Mg(HCO3)2 heat Mg(OH)2 (S) +2CO2(g) PERMANENT HARDNESS(non carbonates hardness)is due to the presence of chlorides and sulphates of Ca ,Mg or heavy metal ions.It can be removed by softeningprocess include sodalime process ,zeolite process,and ion exchange process UNITES OF HARDNESS:- 1. Parts per million(ppm); It is the number of parts of CaCO3 equivalent hardness present in 106 parts of water 2. Milligram per litre(mg/L); It is the number of milligrams of CaCO3 equivalent hardness present in one litre of water 1  DEGREE OF HARDNESS :-CaCO3 is chosen as the standard compound for expressing hardness due to the following reasons: 1. The molecular mass of CaCO3is 100, so the calculation is easy 2. CaCO3is found to be the most insoluble salt that can be precipitated during Water treatment 3. CaCO3 is a primary standard substance ie,a standard solution of hard water can be prepared (The important requirements for a primary std substance are,it must be pure and stable) Hardness in CaCO3equivalents:- Hardness =mass of hardness producing substance x 100 M.M of hardness producing substance 0R Hardness =mass of hardness producing substancex 50 Eq. wt.of hardness producing substance Disadvantages of hardwater:- 1. Scale and sludge formation in boilers(due to the continuous evopartion of water),which results in wastage of fuel. 2. Priming and foaming in boilers, leads to deposition of dissolved salts on the turbine blades and super heater which reduces their efficiency 3. It causes boiler corrosion 4. It is not good for dyeing cloth 5. It produces stains in bathroom fittings 6. It causes the wastage of soap in laundry Water softening method –Ion exchange process:- Principle - Ion exchange resins are insoluble ,cross linked ,long chain organic molecules with a porous structure.Ion exchange is a reversible chemical reaction where an ion from water is exchanged for a similarly charged ion attached to the ion exchange resins.Here the functional group attached to the 2  resins are responsible for the ion exchange property. Ion exchange resins are classified into two, 1. Cation exchange resins:- -They contain acidic functional groups like –SO3H,-COOH, -OH (phenolic) they become capable of exchanging their ions with cations (Ca2+ or Mg2+) present inhard water. Eg :-Amberlite -120,Dowex – 50 They are styrene – divinyl benzene copolymers which on sulphonation or carboxylation become capable of exchanging their H+ ions with cations in the water sample. 2. Anion exchange resins:- -They contain basic functional groups like –OH ,capable of exchanging their ions with anions (Cl- or SO42- ) present in water Eg:- Amberlite -400,Dowex-3. -They are styrene-divinyle benzene copolymers containing functional group such as amino or quarternary ammonium groups which when treated with dil.NaOH ,they become capable of exchanging their ions with the anions present in water Procedure:- Hard water is first allowed to pass through the cation exchange column ,so that, all the cations(Ca2+ or Mg2+ )present in hard water are exchanged with H+ ions from the column. 2 R-H + Ca2+ ------------------------- >R2Ca + 2H+ After this process the hard water is passed through anion exchange column,so that all the anions( Cl- or SO 42- ) present in the water are removed and an equivalent amount of OH- are released to the water from the column 2 R-OH +Cl ---------------------- 2 RCl + 2OH- 2 R-OH + SO42- ------------------- > 2R SO 4 + 2OH - 3 H+ and OH- ions liberated from the cation and anion exchangers respectively, combine to form water molecule. H+ + OH- ----->H2O Thus ,water coming out of the exchanger will be free of cations and anions. Hence ,the water is called deionized water or demineralized water. Advantages :-  Highly acidic and alkaline water can be treated by this method  Treated water has residual hardness of 2ppm.  Water obtained by this method is desirable for use in high pressure boilers because of low hardness.  The resins can be regenerated and reused Disadvantages :-  Equipment and chemicals used are costly  Turbid water cannot be treated with this method What problem will occur if water is first passed through anion exchanger then through the cationexchanger ? Ans: when water containing salts like MgCl2 is passed through anion exchanger,theCl- ions are exchanged with OH-, Thus a precipitate of Mg(OH)2 is 4 produced , this results in clogging of resin and the flow is disrupted. How is exhausted resin regenerated from an ion exchange process? The exhausted cation exchange column can be regenerated by passing a solution of dil.HCl or dil.H2SO4 R2Ca +2H+ 2R- H+ +Ca2+ This column is washed with deionized water to take away the SO42- ,Cl- etc and drained The exhausted anion exchange column is regenerated by passing a solution of dil.NaOH. R2SO4 + 2OH- 2R+ OH- + SO42- The column is also washed with deionized water to remove Na+ and drained Reverse osmosis:- The process of removal of common salts (NaCl) from the water is called de- salination The Water containing high concentration(about 3.5% )of dissolved salts called Brackish water eg:- sea water Brackish water is totally unfit for drinking purpose Reverse osmosis is the most commonly used method for the de-salination of brackish water Osmosis is the phenomenon in which flow of solvent takesplace from a region of low concentration to high concentration.When two solutions of different concentrations are separate by a semipermiablemembrane ,flow of solvent takesplace from dilute to concentrated side,due to osmosis.The flow continues till the concentration is equal on both sides.The driving force for osmosis is osmotic pressure. If a hydrostatic pressure greater than osmotic pressure is applied on the concentrated side ,the solvent flow reverses,i.e.solvent is forced to move from concentrated side to dilute side across the membrane.This is the principle of reverse osmosis. 5 In reverse osmosis ,pure solvent(water) is separated from its contaminants instead of removing contaminant from water.This membrane filtration is called “super filtration” or “hyperfiltration”.In this process,pressure of the order 15-40 kg/cm2 is applied to the sea water to force its pure water out through the semipermiable membrane ,leaving behind the dissolved salts. Advantages :-  It is simple and reliable process  It can be used for the removal of colloidal silica,which is not removed by demineralization.  Low capital cost and operating cost.  Replacement of semipermiable membrane is easy. Municipal water treatment ( Domestic water treatment):- Water from rivers ,canals etc cannot be directly used for domestic purpose as they contain various types of impurities. Water used for domestic purpose -  Should be colourless and odourless with a pleasant taste.  PH should be about 8  Free from pathogenic micro organism. Purification of municipal water for domestic purposes involves two steps 6 1. Removal of suspended impurities :- (a) Screening:- Process of removal of large ,floating impurities by passing the water through a big screen with large number of holes. (b) Aeration :- Mixing water with air to remove volatile impurities like H2S ,CO2 that causes bad smell taste to water. (c) Sedimentation:-Process of removal of suspended particles like clay ,sand by allowing the water to stand undisturbed in big tanks for a period of 2-6 hours.so that the suspended particles settle down at the bottom ,due to gravitational force. (d) Sedimentation by coagulation:- Process of removal of colloidal impurities in water by adding coagulants like Alum ,before sedimentation.Then fine colloidal impuritiesare converted into bigger masses ,that settle down easily. (e)Filtration:- Process of removal of harmful colloidal bacteria,micro organismetc by passing the water through a sand filter. 2. Removal of micro organisms:- (Disinfection methods)The process of killing the pathogenic bacteria and other micro organisms from water to make it safe for use is called disinfectionThe chemicals added to water for this purpose are called disinfectants. Disinfection can be done by the following methods :- (a) Using ozone – ozone is an excellent disinfectant used in gaseous form.It is highly unstable and decomposed to form O2 and nascent oxygen.This nascent oxygen is powerful oxidising agent and kills all disease causing bacteria. O3---------------------------------------->O2 + [O] (nascent oxygen) Advantanges:- -It removes colour,taste and odour without leaving any residue. -O3 is harmless and powerful oxidizing agent. Disadvantages :-- It is expensive and hence cannot employed for disinfection of municipal water supply. 7  -O3 is unstable and cannot be stored for a long time. (b)Using UV light – UV rays are produced by passing electric current through mercury vapour lamp.UV rays have very good disinfecting action and kill all the disease –causing bacteria in water. It is mainly employed in swimming pool as disinfection using chemicals may cause harmful effects to the skin of the people. Advantages :- - As no chemicals are used in this treatment , no taste or odour is imparted to water -It leaves no residue in water. Disadvantages :- - Process is very costly and requires technical skill and costly equipments. –This method cannot be used for municipal works. (c )Chlorination:- i.By adding bleaching powder:for a small scale water treatment bleaching powder can be used.Add bleaching powder at a rate of 1Kg /1000 kilo litres of water and keeping undisturbed for several hours CaOCl2 + H2O--------------------------- >Ca(OH)2 + Cl2 H2O + Cl2------------------------------------------ >HCl + HOCl(hypochlorous acid) HOCl is a powerful germicide, bring about the required disinfecting action. ii.By passing Cl2 gas: Chlorine when added to water either in the gaseous form or concentrated solution for ,produces HOCl. H2O + Cl2 ------------------------------------------ >HCl + HOCl(hypochlorous acid) HOCl is a powerful germicides ,kills the microorganism. iii.By using chloramine:when chlorine and ammonia are mixed in the ratio 2:1 by volume ,chloramine is formed. Cl2+ NH3 --------------------------> ClNH2 + HCl 8  ClNH2 + H2O ------------------------- >HOCl + NH3 Now a days chloramine is preferred for disinfection of water,because it does not produce any irritating odour ,more over ,it imparts good taste to the treated water Why chloramine is a better disinfectant than chlorine ?Because- - it is more stable than chlorine -it gives good taste to the treated water. –it does not produce any irritating odour. iv.By break point chlorination: It refers to the addition of chlorine to the water in amounts sufficient to kill all microorganism and destroy impurities like organic compounds ,free ammonia ,reducing compounds etc by oxidation.The water leaves behind some free chlorine to continue the disinfection action for future. The addition of chlorine at the dip or break is called ‘break point chlorination‘. This indicate the point at which residual chlorine begins to appear. Residual chlorine is a powerful disinfectant , which can destroy all types of pathogenic organisms present in water. Thus disinfection becomes highly efficient. Advantages :- -It completely oxidizes the organic compounds,ammonia and other reducing compounds - It removes colour , odour and taste from water. –It destroys all disease –producing bacteria -It prevents the growth of any weeds in water. 9 De-chlorination:- Over chlorination after the break point chlorination produces unpleasant taste and odour in water. This can be de- chlorinated by passing through a bed of molecular carbon or by adding activated carbon to water and removing it by filtration after the reaction period. What are the advantages and disadvantages of chlorination plant? Advantanges:- -It is economical and effective -Storage is easy -Can be used at low and high temperature.Disadvantages:--Chlorine ,when added in excess,produces an unpleasant taste and odour and causes irritation to mucous membrane. -More effective below 6.5 and less effective at higher PH value 10  WATER POLLUTION Sources of water pollution :- 1.Domestic sewage-Bulk amount of domestic and municipal waste released through drains into canals and rivers cause water pollution ,results in many water borne diseases. 2.Industrial effluents- Industrial wastes discharged into water bodies without proper treatment may contain several chemicals 3.Agricultural run-off - Agricultural discharge containing several pesticides ,fertilizers etc cause heavy pollution Sewage treatment:-sewage is liquid waste.Major portion of it is water and the rest is constituted by organic and inorganic matters ,either in dissolved form or colloidal form. Sewage contains both aerobic and anaerobic bacteria,that bring out oxidation of organic matter in it. Difference between aerobic and anaerobic oxidation Aerobic oxidation Anerobic oxidation Takes place in the presence of excess Takes place in the presence of insufficient oxygen quantity of oxygen Oxidation is by aerobic bacteria Oxidation is by anerobic bacteria The products of oxidation are The products of oxidation are NH3, nitrates,phosphates.sulphates etc. H2S,CH4 etc The products are non-offensive smelling. The products are offensive smelling. No bio-gas is produced Bio-gas fuel like ethane is produced Sewage treatment is carried out to reduce BOD of water.If high BOD water is sent to running streams,due to lack of dissolved oxygen aquatic life get destroyed.The various steps involved are 11  1. Preliminary process:- (a) Screening : In this process , large solids and inorganic matter,which are suspended in sewage, are removed. (b) Sedimentation:Most of the suspended solids are removed by this process.Some times chemical coagulants like alum is added to sewage,before Sedimentation. Coagulant forms gelatinous precipitate,making them settle down easily. 2. Secondary treatment or Biological treatment:- The secondary treatment process involves the biological oxidation of organic compounds at aerobic or anaerobic conditions. (a) Aerobic treatment process: During aerobic oxidation process,the carbon of the organic matter is converted into CO2 ,nitrogen into NH3 etceg: Trickling filter method The filter is in rectangular in shape with about 2m depth.It is filled with material like broken bricks ,crushed rocks etc.and fitted with a rotating distributer. The sewage trickles over the filter with the help of a rotating distributer then it moves down through the filtering medium and the aerobic micro-organisms start consuming the organic matter in the sewage. They grow over the surface of the agitates and more or less clear effluent is collected. 12 (b) Anaerobic treatment process:eg- Upflow Anaerobic Sludge Bed reactor (UASB)process :- In this process, the effluent is fed from the bottom of the reactor so that it moves upward through a sludge blanket.The sludge blanket is composed of biologically formed granules. As the waste comes in contact with the granules, gases like methane are formed under anaerobic conditions. The gases are set free and associated particles rise to the top. The degassed granules drop back to the sludge blanket. The gases are collected through the gas collector dome at the top of the reactor. 3. Tertiary treatment:- By this process ,the amount of nitrogen and phosphorous compounds present in the effluent can be reduced. (a) Precipitation - effluent (from secondary process) is treated with CaO so 13 that calcium phosphate is precipitated (b) Nitrogen stripping - Nitrogen is removed as Ammonia gas (c)Chlorination- Disinfection by treating with chlorine Dissolved oxygen : - D.O is the free ,non-compound oxygen present in water. It is an important parameter in assessing water quality.A low dissolved oxygen level can harm aquatic life and affects water quality. Estimation of D.O (Winkler’ s method) – Take a glass bottle having air tight stopper. Fill the bottle completely with water without bubbling.1ml of MnSO4 solution and 1ml alkaline iodide solution is added into the solution. Close the lid and shaken vigorously and allowed to stand for about 15minutes.The precipitate formed gets settled at the bottom. Add 2ml orthophosphoric acid and shaken well till all precipitate get dissolved. 100ml of this solution is then transferred to a conical flask and titrated against standard thiosulphate solution taken in the burette using starch as indicator.The titration is continued until the blue colour disappears. From the titre value the strength of D.O is calculated and hence the amount of D.O in the water sample is calculated. What are the factors which govern the amount of D.O in water? Ans:--(i) Temperature:- The solubility of oxygen in water decreases with increase in temperature. i.e cold water holds more D.O than warm water (ii) Pressure:-Water holds less D.O at higher attitudes due to low atmosphere pressure. (iii) Biologically oxidizable matter :- The level of D.O in water is also affected by the presence of oxidizable pollutants in water like starch or cellulose (iv)Concentration of impurities:-Solubility of D.O in water decreases with increase in concentration of impurities. 14 BIOLOGICAL OXYGEN DEMAND:(BOD) – It is the total amount of oxygen required by aerobic bacteria for complete oxidation of 1Litre of sewage water for a period of five days at 200 C. Significance of BOD:- D.O is an important factor that determines the quality of water. Presence of biologically oxidizable substance increases the BOD of water. During the oxidation of these materials by aerobic micro-organisms, the D.O present in water is consumed.In lakes ,rivers, ponds etc the presence of these substances will be a threat to aquatic life and water gets polluted.Thus, by knowing the value of BOD ,the extent of pollution can be determined. CHEMICAL OXYGEN DEMAND (COD):-It is the amount of oxygen required for the complete oxidation biologically active and inert materials present in 1Litre sewage water using strong chemical oxidizing agents such as K2 Cr2O7 in acid medium. COD values are greater than BOD values. How BOD differ from COD ? -BOD test take five days for measurement where as COD need only 2 hours. – COD values are always higher than BOD values. –Bacteria are essential for the determination of BOD, but not required for COD determination -The value of BOD enables us to determine the degree of pollution in sewage but COD gives the total amount of oxygen required for complete oxidation of sewage. –BOD requires a constant temperature of 200c for incubation where as COD requires a temperature higher than 1000c 15 16