water treatment sd
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SOURCES OF WATER Sources of water
In India most of the power plants use River Water.
Hardness of WaterPrevents the lathering of soap Reasons: Presence of certain salts of Ca, Mg and other heavy metal ions like Al3+, Fe3+ and Mn2+ dissolved in water.
2C17H35COONa + CaCl2 2C17H35COONa + MgSO4
+ 2NaCl + Na2SO4
Types of Hardness:Temporary Hardness: Salts responsible: Ca(HCO3)2, Mg(HCO3)2 and carbonate of Iron. Largely removed by boiling of water. Ca(HCO3)2 Mg(HCO3)2 CaCO3 Mg(OH)2 + H2O + CO2 + 2CO2
Permanent Hardness: Salts responsible: CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3
BOILER FEED WATERA boiler feed water should correspond with the following composition: (i) Its hardness should be below 0.2 ppm
(ii) Its caustic alkalinity (due to OH) should lie in between 0.15 and 0.45 ppm. (iii) Its soda alkalinity (due to Na2CO3) should be 0.45 1 ppm.
Excess of impurities, in boiler feed water cause following problems. 1. Scale & sludge 4. Carryover 2. Priming 5. Boiler Corrosion 3. Foaming 6. Caustic embrittlerment
WATER TREATMENT PLANT
The impurities present in river water can be broadly classified into following groups :
Floating solids Suspended solids Colloids Dissolved solids Dissolved liquids, gases etc. For thermal power station the most objectionable impurities are dissolved solids
Depending upon the impurities present in source water pre-treatment plants are designed. Normally following processes are carried out in pretreatment plant Screening Storage of raw water Aeration Chlorination Coagulation and flocculation Filtration
Raw water to Clarified water
Raw water bay RW make up pumps
Clarifi ed water
Lime dosing Alum dosing Flash mixer26 July 2011 PMI Revision 00
SCREENINGCoarse screens are provided at river intake to prevent floating material of fairly large size entering the works. The steel bar forming the screen are normally quite substantial (about 25 mm dia.) and are spaced about 100 mm apart.
The velocity of water through the screen openings should not exceed 0.5 m/s
Fine screens are also normally fitted immediately after the coarse screens, the orifices of which are generally of the order of 6 mm.
RAW WATER STORAGE
The storage provided is for 7- 15 days of the average water demand.
This is sufficient of to reduce pathogenic bacteria, suspended solids and river algae, while at the same time not long enough to encourage other organism to develop.
Chlorine is injected into the raw water soon after it enters the plant.
Dosing rate : 2- 5 mg/L
This destroys the high bacterial count, but also oxidises and precipitates iron and manganise, kills algae, reduce color and slime formation are generally assists settlement.
AERATIONIt is a cheap and valuable means of controlling taste, odor and corrosion Gases are absorbed or liberated from water until equilibrium is reached between the natural content of each gas in the atmosphere and its content in the water. * If water is lacking in dissolved oxygen, it will pickup oxygen from air. * * * Looses excess CO2 or H2S Iron and manganese in solution are oxidized and precipitated out Certain volatile substance is liberated by algae growths or decomposition of organic matter, can be released from water
COAGULATIONCoagulation is a process by which small particles in suspension join together to form large agglomerate. Fine suspended particles and colloids present in water carry charges on their surface and most of the common materials assume negative charge. Obvious way of promoting coagulation is to neutralize or reverse the electrical repulsion effect. In that case, the particles will touch each other or pulled together and agglomerate. This can be done by two ways.
(i) By adding traces of surface- active materials called coagulation aids. (ii) By introducing fresh particles with positive charge.
COAGULATION AIDSLong chain molecule carrying electrically active groups all along the length of the chain. For- example: long chain poly - acryl amides, which carries positive charge.
Most of the very effective aids (poly- acryl amides) are toxic and can not be used in potable water. A common nontoxic aid is activated silica. It is prepared on the site because it is unstable & requirement may be more than ten times than that of poly acryl amides.
FLOCCULATIONFerric and aluminium salts when added in water forms insoluble hydroxides, which come out as floppy flocs, whose surface carry positive charge. This positive charge flocs attaches itself with the negative charge particles in water.
More effective; because Large electropositive surface area Greatly increases particle concentration in water, which increases, the probability of collision, proper pH, good mixing time and presence of nuclei on which precipitate can form, favors good floc formation.
Aluminium and iron hydroxide gives good flocs normally in slight acidic pH Iron : Aluminium: 5 6 pH 6 7 pH
When alum (Aluminium sulphate) is added in water aluminium hydroxide floc and sulphuric acid is formed. Al2(SO4)3 + 6 H2O Al(OH)3 + 3 H2SO4
If turbidity is high alum dosing is also high Here pH may fall below the coagulation pH Lime [Ca(OH)2] is generally used to increase pH and it is also have coagulation property.
Coagulation and flocculation
CLARIFIERSClarifiers are structure/system in which water is cleared from most of the suspended solids with or without addition of coagulant. There are different types of clarifiers: Conventional Type Clarifiers. Solid Contact Unit Clarifier (i) (ii) Slurry re-circulation clarifier Sludge blanket clarifier
Hopper Bottom Clarifier Latest concept. The tank is usually square in plane with upper portion having vertical sides and the lower portion in the form of an inverted pyramid with sides at an angle of 60to the horizontal size of the tank is governed by the turbidity and volume of the feed water.
CONTROL OF FLOCCULATIONA variable velocity valve adjustable from the top of the tank is fitted with inlet pipe to take care exceptional changes of water characteristics or wide variation of flow. Inlet pipe has a fixed diameter outlet to give required discharge velocity.
The effect of increases flow through the tank is the expansion of the sludge blanket until the equilibrium is re-established. The surface of the sludge blanket will rise but carry over will not occur until the increase in flow is excessive.
COLLECTION OF CLARIFIED WATERThe water coming out of the sludge blanket passes up through straight position of the tank and is drawn off by a series of collecting troughs. They have notched side and are provided with adjustable support to enable them to be set for uniform draw off. This eliminates chances of short- circuiting.
SLUDGE DISCHARGEIt is necessary to bleed off a small flow of sludge through concentrating pocket. It may be constructed in the concrete or it may be suspended from the top of the tank. The main discharge pipe is periodically used to remove heavier sludge from the bottom of the tank.
Clarified water to Filtered waterClarified water FW sump Clari-flocculator Cl2 Cl2 House FW pumps Filtered water DM water for Plant purpose To Township for drinking waterPMI Revision 00
Sand filters DW sump
SPECIFICATION OF CLARIFIED WATERThe clarified water should be seen by naked eye without any turbidity. Almost 98% turbidity should be removed.
FILTRATIONFiltration means taking suspended solids out of fluids There are some problems : (i) Suspended matter and water is relatively viscous. This means that very small holes must be used and any reasonable rate of flow, through them will raise a high- pressure loss. (ii) As the filtration process is in progress, the suspended matter slowly blinds the pores of the Screen resulting greater pressure loss and slowly stoppage of the process. Deposition of suspended matter on the pores of the screen is known as formation of filter cake.
There are two kinds of simple screen filter(a) True screens, such as meshes, sieves, sinters, felts, fibrewound candles or cloth held in some device which forces water to flow through. Disposable screens which are created a fresh for every filter cycle. This class includes paper filters and cartridge filter which are used at place where there is used little suspended matter to remove, else filter replacement becomes too costly.
MICRO STRAINERSCylindrical drum covered with a very fine wire mesh. Raw water flows into the middle of the drum and out through the mesh by gravity. The drum rotates continuously and bring the mesh under a high pressure jet.
DEEP BED FILTER
This is a vessel in which filter bed is created from a fine granular medium such as sand etc. through which water is made to flow.
The suspended matter gets caught in the discontinuities which occur within such a bed. When the bed accumulates lot of solids that is practicable, the flow of water practically stops and the bed is back washed for reviving the filtering capacity.
The deep bed filter differ from another filters:(a) (b) They are capable of filtering very fine solids including colloids. Fine sand gives finer filtration because the size of