Water treatment. Outline. Industrial water treatment Drinking water treatment Wastewater treatment. Industrial water treatment. Raw water is rarely suitable for industrial use. Raw water contains contaminants, which if not removed cause corrosion and scaling in boilers and heat exchangers
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Outline • Industrial water treatment • Drinking water treatment • Wastewater treatment
Industrial water treatment • Raw water is rarely suitable for industrial use. • Raw water contains contaminants, which if not removed • cause corrosion and scaling in boilers and heat exchangers • can effect product quality
Types of industrial water treatment • Coagulation, flocculation, and settling • Filtration • Demineralization • ion exchange • reverse osmosis • Cooling water and cooling water treatment • Boiler water and boiler water treatment
Coagulation, flocculation, and settling • Removes suspended solids from raw water. coagulant flocculant effluent acid or base raw water sludge removal Microfloc continues to coalesce until large macrofloc appear. Coagulant neutralizes charge. Microfloc forms. Microfloc settles in the sedimentation basin. Clear supernatant leaves the basin. Surface charges cause particles to repel one another.
coagulant flocculant effluent raw water Sludge removal pH Turb Coagulation, flocculation, and settling • What’s in it for us?
Filtration • Removes suspended solids from raw water. Membrane filter Media filter influent raw water Filter media (gravel, sand, anthracite, etc.) capture suspended particles. Captured particles are removed by backwashing the filter. polymer membrane filtered water effluent
Turb Turb Filtration • What’s in it for us? Membrane filter Media filter influent raw water ? polymer membrane filtered water effluent
Softening • Softening is the process of removing hardness ions (calcium and magnesium) from water. • Hardness ions are objectionable because they tend to form scale on heat transfer surfaces. • Two types of softening… • Sodium softening • Softening by chemical treatment • Lime softening • Soda-lime softening
Sodium softening Calcium chloride Sodium softener: removes calcium and magnesium ions and replaces them with sodium. Sodium chloride
Sodium softening Nothing! • What’s in it for us? Calcium chloride Sodium softener: removes calcium and magnesium ions and replaces them with sodium. Sodium chloride
Chemical softening Calcium and magnesium precipitate as carbonates and hydroxides. The precipitate settles and the softened water leaves the settling basin. lime or lime and soda ash raw water acid softened water sludge removal
turb pH pH Chemical softening • What’s in it for us? lime or lime and soda ash raw water acid softened water sludge removal
Cations Sodium Na+ Potassium K+ Calcium Ca+2 Magnesium Mg+2 Neutral molecules Silica SiO2 Carbon dioxide CO2 Weak organic acids Anions Chloride Cl- Sulfate SO4-2 Bicarbonate HCO3- Nitrate NO3- Fluoride F- Demineralization • Remove all or nearly all of the contaminants in raw water. • The contaminants are… • Calcium and magnesium = hardness • Bicarbonate and carbonate = alkalinity • Common methods of demineralization: • Reverse osmosis • Ion exchange
pressure feedwater reject semi-permeable membrane permeate or product Reverse Osmosis Pressure pushes water through the membrane. Dissolved solids remain behind. Rejection of solids is typically 97 to 99%. Recovery (permeate flow/feedwater flow) is about 50% for a single stage. As the feedwater travels through the membrane module, the concentration of solids increases.
Reverse Osmosis Module permeate reject membrane porous backing feed membrane membrane spacer
Pretreatment for reverse osmosis • Prevent membrane fouling • Filtration – removes suspended solids • Softening – remove calcium and barium to prevent precipitation of slightly soluble calcium and barium sulfate and carbonates on the membrane. • pH adjustment – increases solubility of carbonates preventing them from precipitating. • Chlorination and filtration – remove microorganisms to prevent biofouling on the membrane. • Prevent chemical damage to the membrane • Dechlorination – remove chlorine to prevent it from oxidizing and damaging RO membrane.
Typical RO plant NaOCl rawwater filter micron filter Na softener dechlorination NaHSO3 pre-treated water (RO feedwater) two-stage RO unit feedwater combined permeate permeate reject permeate reject
Typical RO plant • What’s in it for us? NaOCl rawwater filter micron filter Na softener dechlorination NaHSO3 T ORP Cl or pre-treated water (RO feedwater) two-stage RO unit C feedwater combined permeate permeate C C: conductivity pH ORP Cl: chlorine T: turbidity reject pH C permeate C reject
+ + + + + Ion Exchange • Ion exchange resin bead polymer backbone polymer backbone counter ion functional group
+ + + + + + Ion exchange • Cation exchange resin • Negatively charged functional groups • Positively charged counter ions • Anion exchange resin • Positively charged functional groups • Negatively charged counter ions • Any ion in solution having a greater affinity for the functional group than the present counter ion will exchange with the counter ion.
OH- (aq) Cl- (aq) Cl- (aq) OH- OH- Cl- Na+ (aq) H+ H+ Ion exchange Cation exchange Anion exchange Ion exchange site in the H+ form Ion exchange site in the OH- form Na+ ion in solution Cl- ion in solution Na+ (aq) H+ (aq) Na+
Ion exchange demineralizer Ion Exchange Demineralizer- Service Run Na+ Cl- Mixed bed removes the small amount ion leakage out of the cation and anion exchangers. cation anion mixed bed R – OH- R – H+ product water H+ OH- H+ Cl- H2O
Ion exchange demineralizer • What’s in it for us? Na+ Cl- C cation anion mixed bed R – OH- R – H+ C SiO2 C product water H+ OH- H+ Cl- H2O
caustic day tank Ion exchange demineralizer Makeup Demineralizer - Regeneration acid day tank dilution water dilution water Regeneration is the ion exchange process in reverse. H+ OH- anion exchanger cation exchanger R – Na+ R – Cl-
Ion exchange demineralizer • What’s in it for us? acid day tank caustic day tank dilution water dilution water C % conc’n C % conc’n cation exchanger anion exchanger C C
Cooling water system warm water return heat exchangers cooling tower blowdown makeup water cool water supply
Evaporation cools the returning water. Evaporation also increases the concentration of dissolved solids. High concentration of dissolved solids leads to corrosion. If concentration gets high enough solids precipitate and accumulate on heat exchange surfaces. Cooling tower fan return warm water air packing cool water supply to plant basin
Efficient heat transfer requires clean tube surfaces. shell and tube heat exchanger tube shell tubesheet baffle cooling water Heat exchanger process liquid
Cooling water treatment • Control scale • Scale is a hard, tenacious deposit that forms on metal surfaces, primarily where heat exchange occurs. • Control corrosion • Corrosion is wastage of metal caused by reaction with substances in the process fluid in contact with the metal. • Corrosion can be uniform or localized (pitting). • Control biofouling • Biofouling is the slime produced by micoroorganism colonies, their secretions, and the debris that gets embedded in the mass.
cold side hot side scale tube wall under-deposit corrosion Scale and corrosion • Solids restrict free flow of liquid through pipes. • Scale forms in high heat transfer zones in heat exchange equipment. • Scale interferes with efficient heat transfer. • Excessive scale buildup leads to less efficient heat transfer. • Excessive scale buildup also leads to under-deposit corrosion and eventually leaks. scale tube wall tube wall
CaCO3 Cooling water chemical treatment • Control scale • What causes CaCO3 scale? • Reduce calcium hardness by using softened water for makeup. • Reduce alkalinity and lower pH by adding sulfuric acid. • Keep calcium hardness and alkalinity in the cooling water below the concentration at which calcium carbonate scale begins to form. • Control calcium sulfate scaling by adding a dispersant. calcium hardness alkalinity (HCO3-) alkaline (high) pH CaCO3
Cooling water chemical treatment • Control corrosion • What causes corrosion? • Keep pH in an optimum range for the system metallurgy. • Control total concentration of dissolved solids in the cooling water. • Add corrosion inhibitors to the cooling water. high concentration of dissolved salts low pH corrosion
Cooling water chemical treatment • Control biofouling • What causes biofouling? • Treat cooling water with a biocide. • Oxidizing biocides: chlorine, bromine, ozone, etc. • Non-oxidizing biocides microorganisms (bacteria, fungi, algae) nutrients oxygen warmth biofouling
Cooling water system • What’s in it for us? warm water return heat exchangers cooling tower blowdown makeup water cool water supply
Cooling water system • What’s in it for us? warm water return Cl pH C cooling tower T blowdown makeup water
Measuring chlorine in cooling water • Maintenance • Open cooling water often has high levels of suspended solids which foul the sensor. • Sensor may require cleaning every few days. • Filtration??? to FCL or FCLi cooling water sample filter ppm chlorine chlorine demand across filter
Measuring chlorine in cooling water • Interferences • The 499ACL-01 and 498Cl-01 sensors are intended for use in drinking water. • Cooling water treatment chemicals can interfere. • Wrong chemical • The 499ACL-01 and 498CL-01 sensors are for free chlorine only. • Many cooling water biocides use bromine or a mixture of bromine and chlorine. • The 499ACL-01 and 498CL-01 sensors will not work in this application. • The only choice is the TCL, which measures total oxidants.
steam for plant operations boiler condensate returns from plant makeup water condensate tank Boilers • Typical low pressure industrial boiler system...
Boiler water circuit saturated steam steam separates from water in the steam drum steam drum superheater economizer steam bubbles form as the water absorbs heat feedwater downcomer water tubes water drum
Boiler water treatment • Control scale formation • Control corrosion
Scaling and corrosion • As steam escapes, the concentration of solids in the boiler water increases. • When concentration exceeds the solubility, the solid precipitates and clings to the tube surface. • Deposition leads to reduced heat transfer efficiency and overheating. • Deposition also leads to underdeposit corrosion. • Overheating and corrosion eventually lead to tube failure. tube wall under deposit corrosion deposit water clean surface waterwall tube
Controlling scale • Boiler scale has two sources… • Precipitation of contaminants that have accumulated and concentrated in the boiler. • Calcium, magnesium, alkalinity, sulfate, silica, and iron • Deposition of corrosion products formed in the condensate and feedwater system steam boiler condensate returns precipitation of soluble contaminants makeup water corrosion products & contaminants
Controlling boiler scale • Reduce or eliminate the contaminants that cause scale. • Use soft water for makeup. • Use dealkalized water for makeup. • Use demineralized water for makeup. • Control contaminant concentration in the boiler • Keep concentration of contaminants low enough that precipitation is unlikely to occur (blowdown). • Use treatment chemicals to keep precipitates in suspension. • Use treatment chemicals to chelate (tie up) small amounts of hardness and prevent it from becoming scale. • Control corrosion in the condensate and feedwater. • Control corrosion in the steam lines.
protective film (magnetite) water base metal (iron) Controlling corrosion • Add chemicals to control pH. • At alkaline pH, a protective oxide film forms on mild steel surfaces. • The film protects the base metal from further corrosion. • As long as the water remains alkaline, the film will stay in place and cracks in the filmwill be automatically repaired. • Control concentration of dissolved salts in the boiler water (blowdown).
Boilers • What’s in it for us? steam boiler condensate returns makeup water
Boilers • What’s in it for us? pH C pH steam C pH C * DO boiler condensate returns makeup water C *For systems with deaeraters.