Water Treatment Key Concepts Flashcards

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Sources of Water

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Natural water on Earth exists mainly as oceans and seas (~97%), polar ice and glaciers (~2%), and freshwater (~1%) in rivers, lakes, streams, and groundwater. These sources determine availability and suitability for treatment and use. Freshwater is the primary target for drinking and irrigation supply.

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Common Impurities

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Water dissolves many substances so it commonly contains dissolved salts and gases, suspended inorganic and organic particles, colloidal silica or clay, and microorganisms. Dissolved impurities change taste, color, and hardness; suspended and colloidal materials cause turbidity; microorganisms can foul smell and cause disease. Treatment strategies differ by impurity type.

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Dissolved Impurities

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Dissolved impurities include ionic salts, dissolved gases, dyes, and small organic molecules that alter the chemical quality of water. These affect parameters like hardness, conductivity, and taste. Removal often requires chemical, membrane, or ion-exchange processes.

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Suspended Impurities

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Suspended impurities are larger particles such as clay, sand, oil, plant and animal matter that make water turbid. They are removable by physical processes like sedimentation and filtration. Controlling suspended solids improves downstream treatment efficiency.

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Colloidal Impurities

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Colloidal impurities are finely divided particles (e.g., silica, colloidal clay) that remain dispersed and cause persistent turbidity. They are difficult to remove by simple settling and often require coagulation or membrane filtration. Proper coagulation/flocculation promotes their aggregation for removal.

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Microorganisms

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Microorganisms in water include bacteria, fungi, and algae that can produce odors and pose health risks. Disinfection methods like chlorination, boiling, ozone, or UV are used to inactivate them. Monitoring biological parameters is essential for safe drinking water.

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Water Quality Parameters

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Important parameters include physical (color, taste, odour, temperature), chemical (hardness, alkalinity, dissolved ions), and biological indicators. Standards such as BIS IS10500:2012 set permissible limits for safe consumption. Resource scarcity and contamination make monitoring critical for public health and allocation.

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Hardness of Water

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Hardness is a property of water that prevents lathering of soap and is due to soluble salts of $Ca$, $Mg$, $Al$, $Fe$, $Mn$, etc. It is often defined as the soap-consuming capacity or the precipitation capacity of water. Hardness causes scaling on heating and impairs cleaning actions.

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Temporary Hardness

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Temporary (alkaline or carbonate) hardness is caused mainly by bicarbonates like $Ca(HCO_3)_2$ and $Mg(HCO_3)_2$ and can be removed by boiling. Boiling converts these to insoluble carbonates, e.g., $Ca(HCO_3)_2$ gives $CaCO_3$, $H_2O$ and $CO_2$. It is reversible by removing bicarbonate species.

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Permanent Hardness

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Permanent (non-carbonate) hardness is caused by sulfates, chlorides, and other non-bicarbonate salts of $Ca$ and $Mg$ and cannot be removed by boiling. It requires chemical or ion-exchange treatments for removal. Examples include $CaSO_4$ and $MgCl_2$.

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Total Hardness

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Total hardness equals the sum of temporary (carbonate) hardness and permanent (non-carbonate) hardness expressed typically as $CaCO_3$ equivalent. It quantifies the overall effect of dissolved multivalent cations on water behavior. Total hardness guides selection of softening and boiler treatments.

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Hardness as $CaCO_3$

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Water hardness is reported as calcium carbonate ($CaCO_3$) equivalents because $CaCO_3$ has molar mass 100 and forms an insoluble precipitate used for standardization. Expressing hardness as $CaCO_3$ simplifies calculations regardless of the actual salts present. Conversions and ppm calculations use the $CaCO_3$ basis.

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Units of Hardness

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Common units are parts per million (ppm) or mg/L as $CaCO_3$, Clark's degree (°Cl), and French degree (°Fr). Conversion examples: $1$ mg/L $=1$ ppm $=0.07$ °Cl $=0.1$ °Fr. Choosing units depends on local practice and reporting standards.

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Soap Composition

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Soap is typically the sodium or potassium salt of long-chain fatty acids, for example sodium stearate $C_{17}H_{35}COONa$. The molecule has a hydrophobic tail that binds oils and a hydrophilic head that stabilizes micelles in water. Soap forms micelles that solubilize and remove dirt and grease.

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Cleansing Action

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Soap molecules arrange into micelles with hydrophobic tails inward trapping grease and hydrophilic heads outward allowing removal by water. Micelle formation suspends dirt for rinsing. Hard water interferes by precipitating soap as insoluble salts, reducing lather and cleaning efficiency.

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Soap Scum Formation

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In hard water soap reacts with $Ca^{2+}$ and $Mg^{2+}$ to form insoluble fatty acid salts (scum), for example sodium stearate with calcium yields $(C_{17}H_{35}COO)_2Ca$ which is insoluble. The reaction consumes soap and forms precipitates that reduce lather. Scum is a primary reason soap performs poorly in hard water.

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EDTA Titration

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Complexometric titration with $EDTA$ quantitatively determines $Ca^{2+}$ and $Mg^{2+}$ because $EDTA$ forms stable 1:1 chelate complexes with divalent cations. The titration is performed in alkaline pH (≈8–10) using a buffer and an indicator. Results convert to $CaCO_3$ ppm irrespective of which cation was chelated.

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Eriochrome Black T

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Eriochrome Black T ($EBT$) is used as an indicator in $EDTA$ titrations; it forms a wine-red complex with metal ions and turns bluish-black when metal ions are sequestered by $EDTA$. $EBT$ is a weak ligand that shows a clear color change at the endpoint. Indicator color progression signals titration completion.

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Complexometric Procedure

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Typical procedure: take a measured water sample, add ammonium buffer to pH ≈9–10 and a few drops of $EBT$, then titrate with standard $EDTA$ until the wine-red color turns bluish-black. The $EDTA$ volume used gives the total hardness in $CaCO_3$ equivalents after stoichiometric conversion. Boiled sample titration yields permanent hardness and subtraction gives temporary hardness.

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Hardness Calculation

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Since $M^{2+}$-$EDTA$ complexes are 1:1, moles of $EDTA$ used equal moles of hardness-causing cations. Convert moles to mass of $CaCO_3$ equivalent using molar mass $=100$ g mol$^{-1}$ to express hardness in mg/L (ppm). This standardizes results regardless of the original salts.

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Alkalinity Definition

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Alkalinity is the water's capacity to neutralize acids, primarily due to presence of $OH^-$, $HCO_3^-$, and $CO_3^{2-}$ ions or salts of weak acids. It acts as a buffer resisting pH changes and is important for aquatic ecosystems, soil stability, and industrial processes. Alkalinity differs from pH but influences acid–base titrations.

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Alkalinity Causes

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Alkalinity arises from hydroxide, carbonate, and bicarbonate ions, salts of weak acids, and buffer-forming salts. The dominant species depends on pH and the nature of dissolved salts. Identifying the species requires titration endpoints and interpretation via the P–M method.

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Phenolphthalein Endpoint

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Phenolphthalein detects the first alkalinity endpoint around pH ≈8.2–10, turning pink in alkaline solutions and colorless when neutralized. Titrating to this endpoint measures phenolphthalein alkalinity (P), which corresponds to $OH^-$ plus half of $CO_3^{2-}$ present. It is the first step in the standard two-endpoint alkalinity titration.

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Methyl Orange Endpoint

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Methyl orange changes color from yellow to reddish-orange over pH ≈3.1–4.4 and is used after phenolphthalein to find the second endpoint. Titration to this endpoint neutralizes remaining $HCO_3^-$, giving total (M) alkalinity. The volumes to P and M endpoints allow partitioning of alkalinity species.

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P–M Alkalinity Table

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The P–M alkalinity framework interprets phenolphthalein (P) and methyl orange (M) titration volumes to identify alkalinity types: if P = M then hydroxide alkalinity dominates, if P = ½M carbonate dominates, and if P = 0 and M > 0 bicarbonate dominates. Combined cases (e.g., P > ½M) indicate mixtures like $OH^-$ + $CO_3^{2-}$. This table provides quick classification from titration data.

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Boiler Sludge

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Sludge is a soft, loose, slimy precipitate formed in boilers from salts like $CaCl_2$, $MgCl_2$, and $MgSO_4$ that collects in cooler, low-flow zones. It is non-adherent, removable by blow-down and mechanical cleaning, but its accumulation reduces boiler efficiency. Sludge is less dangerous than hard scales but still problematic.

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Boiler Scale

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Scale is a hard, adherent deposit (e.g., $CaCO_3$, $CaSiO_3$, $MgSiO_3$) forming on hot boiler surfaces that strongly reduces heat transfer and can cause overheating and failure. Scales are difficult to remove and may require acid dissolution or mechanical removal; severe scaling can lead to boiler explosion risks. Prevention is essential.

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Scale Removal Methods

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Removal methods include mechanical scraping, thermal shock, dissolution with acids (e.g., 5–10% HCl for $CaCO_3$) and chelation with $EDTA$ for $CaSO_4$ as soluble complexes. Softer or loose scales can be brushed or blown down. Selection depends on scale composition and boiler design.

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Internal Treatment

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Internal treatment conditions boiler water chemically to precipitate or convert scale-forming salts into removable forms inside the boiler. Methods include carbonate conditioning ($Na_2CO_3$), phosphate conditioning ($Na_3PO_4$, $Na_2HPO_4$, $NaH_2PO_4$), and calgon (sodium hexametaphosphate) dosing. Correct choice minimizes scale while avoiding caustic embrittlement and other side effects.

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Carbonate Conditioning

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Carbonate conditioning adds $Na_2CO_3$ to feedwater to precipitate calcium as loose $CaCO_3$ sludge (e.g., $CaSO_4 + Na_2CO_3$ forms $CaCO_3$ + $Na_2SO_4$). Excess $Na_2CO_3$ can hydrolyze to $NaOH$ and cause caustic embrittlement. Careful control and monitoring prevents damaging alkaline corrosion.

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Caustic Embrittlement

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Caustic embrittlement occurs when concentrated $NaOH$ penetrates fine cracks in boiler metal, causing localized corrosion and formation of sodium ferroate that embrittles the steel. It is promoted by excess carbonate hydrolysis producing $NaOH$ and evaporative concentration in crevices. Remedies include using phosphate conditioning, sodium sulfate, or tannins to prevent caustic concentration.

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Phosphate Conditioning

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Phosphate conditioning adds sodium phosphate salts (e.g., $Na_3PO_4$, $Na_2HPO_4$, $NaH_2PO_4$) to precipitate hardness ions as non-adherent phosphates such as $Ca_3(PO_4)_2$. The choice of phosphate depends on feedwater alkalinity and pH. Phosphate methods reduce scale and sludge formation when properly dosed.

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Calgon Conditioning

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Calgon is sodium hexametaphosphate, often represented as $(NaPO_3)_6$, used to sequester calcium and form soluble complexes that prevent scale precipitation. It is commonly employed in laundering and some boiler treatments to keep hardness ions in solution. Calgon-based treatments produce removable soluble complexes instead of hard deposits.

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Zeolite Method

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The zeolite (permutit) softening method uses hydrated sodium alumino-silicate ($Na_2O.Al_2O_3.xSiO_2.yH_2O$) to exchange $Na^+$ for hardness ions $Ca^{2+}$ and $Mg^{2+}$. Hard water passes through a zeolite bed and exits softened while the zeolite is later regenerated with concentrated brine ($NaCl$). The method is effective but sensitive to turbid or acidic feedwater and to Fe/Mn fouling.

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Zeolite Regeneration

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Zeolite regeneration uses a concentrated sodium chloride brine to replace accumulated $Ca^{2+}$ and $Mg^{2+}$ with $Na^+$, restoring exchange capacity (e.g., $CaZe + 2NaCl$ yields $Na_2Ze + CaCl_2$). Regeneration flushes out the displaced salts as washings. Proper regeneration recovers softener performance but generates saline waste.

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Ion-Exchange Resins

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Ion-exchange resins are insoluble cross-linked organic polymers bearing functional groups that reversibly exchange ions with water. Cation exchangers (acidic groups like $-COOH$ or $-SO_3H$) swap $H^+$ or $Na^+$ for $Ca^{2+}/Mg^{2+}$; anion exchangers (basic groups like $-NH_2$) swap $OH^-$ for anions. Combined cation-anion treatment (deionization) yields low-conductivity water used in high-pressure boilers and laboratories.

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Resin Regeneration

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Cation resins are regenerated with acids (dilute $HCl$ or $H_2SO_4$) which displace $Ca^{2+}/Mg^{2+}$ and restore $H^+$ form; anion resins are regenerated with alkali ($NaOH$) to restore $OH^-$ form. Regeneration produces waste streams containing displaced ions and regeneration salts. Regular regeneration is needed to maintain capacity.

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Desalination

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Desalination is any process removing salts and minerals from water to produce fresh water suitable for consumption or industry. Leading technologies include reverse osmosis (membrane pressure-driven) and electrodialysis (electrically driven ion transport). Choice depends on feed salinity, energy costs, and desired product quality.

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Osmosis vs RO

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Osmosis is the natural solvent flow from dilute to concentrated solution across a semipermeable membrane driven by osmotic pressure. Reverse osmosis (RO) applies hydrostatic pressure greater than the osmotic pressure on the concentrated side to reverse solvent flow, forcing pure solvent (water) through the membrane. RO therefore rejects dissolved solutes and salts while producing permeate and a concentrated reject stream.

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RO Working Principle

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RO uses semipermeable membranes (e.g., cellulose acetate, polyamide) and applied pressure (typically ~15–40 kg/m^2 or higher) to force water molecules through while retaining ions and larger solutes. It removes particles in the 0.1–1.0 nm range and produces low-salinity permeate. Pretreatment, membrane selection, and pressure determine efficiency and recovery.

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RO Advantages/Disadvantages

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Advantages include improved taste, odor, appearance and removal of a wide range of contaminants; membranes are selective and can produce high-quality water. Disadvantages include concentrate wastewater generation, removal of beneficial minerals, energy and maintenance needs, and fouling tendencies. System design balances recovery, energy, and waste handling.

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Electrodialysis

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Electrodialysis separates ions from water by applying an electric field across alternating cation- and anion-selective membranes causing ions to migrate into concentrate compartments. It is effective for brackish water desalination where ionic species dominate and can be energy-efficient at lower salinities. Electrodialysis does not remove non-ionic organic contaminants.

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Domestic Disinfection

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Common domestic disinfection methods include boiling, chlorination with bleaching powder ($CaOCl_2$), chloramine addition ($ClNH_2$), ozone dosing, and UV irradiation. Each method inactivates microorganisms differently and has trade-offs in byproducts, residual protection, and operational complexity. Effective disinfection ensures microbiological safety of drinking water.