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A survey of traditional and innovative methods of water purification. Aeration of Water. Removes dissolved gases (e.g. H 2 S), VOCs, oxidizes easily oxidizable organics to CO 2 and Fe +2 to Fe +3 (which is precipitated as Fe(OH) 3. Removal of Calcium & Magnesium.

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A survey of traditional

and innovative methods of

water purification

Env. Chemistry, Baird & Cann


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Aeration of Water

  • Removes dissolved gases (e.g. H2S), VOCs, oxidizes easily oxidizable organics to CO2 and Fe+2 to Fe+3 (which is precipitated as Fe(OH)3

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Removal of Calcium & Magnesium

  • Water from wells with limestone bedrock are generally high in Ca+2 & Mg+2 & these ions can be removed by precipitation with phosphate or more commonly carbonate ion; under sufficiently basic conditions Mg+2 is precipitated at Mg(OH)2

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Disinfection to Reduce Illness

  • The elimination of microorganisms (many of these are from human & animal feces) many of which cause illnesses

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Filtering of Water

  • Filtering through sand removes suspended solids, including bacteria, down to 10 µm in size

  • Forcing water through filters with small openings can remove bacteria, viruses and some chemicals

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Removal of Colloidal Particles by Precipitation

  • Larger particles are allowed to settle but smaller insoluble particles are suspended in the form of colloid particles

  • Particles from 0.001 to 1 µm group together & generally have charged surfaces which repel other similarly charged particles

  • Colloids are precipitated by addition of Fe2(SO4)3 or Al2(SO4)3 –produce gelatinous hydroxides

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Disinfection by Membrane Technology

  • Membranes pores 0.002-10 µm can remove ions, molecules, small particles including viruses and bacteria –microfiltration and ultrafiltration Figure 10-2

  • Nanofilters allow water molecules (tenths of nanometer in size) but filter out Ca+2 & Mg+2 ions

  • Reverse osmosis (hyperfiltration) –water under high pressure is forced through semipermeable membranes composed of organic polymeric material (cellulose acetate or triacetate) –molecules down to 0.001 µm filtered –used to desalinate water & producing water exceptionally clean of ions for renal units

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Disinfection by UV Radiation

  • Hg vapor lamps 254 nm (UV-C) immersed the water flow

  • 10 seconds of irradiation are sufficient to remove toxic microorganisms including Cryptosporidium –disrupts DNA interrupting replication

  • Dissolved iron along with humic substance absorb UV light and interfere with the disinfection

  • Small units are possible for small populations

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Disinfection by Chemical Methods: Ozone

  • Oxidizing agents more powerful than oxygen –needed to rid water of bacteria and viruses

  • O3 produced by electric discharge, about 10 minutes contact time sufficient –decomposed rapidly leaving no residual O3

  • Leads to oxidation of organic compounds containing O –leads to carbonyl compounds; also reacts with Br – to produce BrO3-, a probable carcinogen in humans; BrO3- brominates organic compounds (disinfection by-products or DBPs)

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Disinfection by Chemical Methods: Chlorine Dioxide

  • ClO2 (used in thousands of European communities & 300 in N. Am.) not a chlorinating agents –does not lead to significant amounts of toxic chlorinated organics

  • Produced on the spot from sodium chlorite –concerns about toxicity of chlorite & chlorate residuals

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Disinfection by Chlorination: History

  • ½ US population uses surface water; ¼ groundwater disinfected by HOCl

  • Kills bacteria as it readily passes through cell membranes

  • Inexpensive and residual

  • Began in early 20th century in US, Canada & GB

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Disinfection by Chlorination: Production of HOCl

  • Large scale

  • Small scale (pools) from NaOCl, Ca(OCl)2

  • If alkaline conditions then strong eye irritant is formed

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Disinfection by Chlorination: Production of HOCl

  • Chlorine must constantly be replenished in pools

  • HOCl can also be produced from isocyanuric acid –inert to UV-A

    light

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Disinfection by Chlorination: By-Products & Their Health Effects

  • HOCl not only an oxidizing agent but also a chlorinating agent –forms chlorinated organics some of which are toxic e.g. chlorinated acetic acid, chlorinated phenols (offensive odor & toxic), trihalomethanes (THMs) of particular concern is trichloromethane or chloroform (suspected liver carcinogen & negative effects on reproduction & development)

  • Low levels (30 ppb) may pose a health threat

  • THM limits in drinking water 80 ppb in US & EU

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Disinfection by Chlorination: By-Products & Their Health Effects

  • Level of THM depends on levels of organic compounds in boggy areas concentrations 250-400 ppb

  • Epidemiological studies indicate a connection between THM levels & bladder & rectal cancer –shift to O3 and ClO2

  • Some concerns about THMs and adverse reproductive outcomes

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Disinfection by Chlorination: Advantages over Other Methods Effects

  • Saves many lives (typhoid, cholera) –over 20 million people (mostly infants) die/year in developing countries

  • Residual chlorine left to continue disinfection –chlorinated amines products are also good disinfectants and are longer-lived than HOCl

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Groundwater: Its Supply, Chemical Contamination, and Remediation

  • The Nature & Supply of Groundwater

  • Underground water supplies range in age from a few years to millions of years

  • When water table at the surface of the soil gives rise to swamps

  • Water in highly fractured rocks bounded by at the bottom by clay or impervious rock is an aquifer –water supply for ½ of N. America & 1.5 billion worldwide

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Contamination of Ground Water are being drained faster then they can be replenished

  • Traditionally considered pure water –generally less organic matter, fewer disease carrying organisms

  • Contamination from disposal of organic wastes, discharges of septic systems, municipalities, industries, and farms

  • Harder to cleanup than surface water

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Nitrate (NO are being drained faster then they can be replenished3-) Contamination of Ground Water

Nitrate in groundwater originates mainly from four sources:

• application of nitrogen fertilizers, both inorganic and animal manure, to

  • cropland,

    • atmospheric deposition,

    • human sewage deposited in septic systems, and

    • cultivation of the soil.

    12 millions tons of nitrogen/year from fertilizer in US; manure 7 million tons; nitrate from atmosphere formed form NOx

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Maximum contaminant level (MCL) is 10 ppm for nitrate are being drained faster then they can be replenished

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Nitrates in Water are being drained faster then they can be replenished

  • Rising levels of nitrates in drinking water (highly soluble in water) –runoff from agricultural lands (fertilizers, manure); urban areas –lawns, golf courses, parks etc.

  • Runoff into seas such as the Baltic, Black, Adriatic causes algae blooms

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Health Hazards of Nitrates in Drinking Water are being drained faster then they can be replenished

  • Methemoglobinemia –bacteria in a baby's stomach convert some nitrate to nitrite

    ion, which combines with & oxidizes hemoglobin –interferes with oxygen uptake/transport (“blue baby syndrome”)

  • Link to non-Hodgkin's lymphoma; stomach cancer?

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Nitrosamines in Food & Water are being drained faster then they can be replenished

  • Nitrites (formed the stomach; nitrates & nitrites used as preservative in food) react with amines to form N- nitrosamines

  • NDMA strong methylating agent (DNA) & probable human carcinogen

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Groundwater Contamination by Organic Chemicals are being drained faster then they can be replenished

  • Sources: municipal landfills & industrial sites (leachate)

  • Typical contaminants

    • Chlorinated solvents, trichloroethene (TCE) perchloroethene (perc) –metal degreasing, dry cleaning

    • Benzene, toluene, xylene (BTX) -gasoline

    • MTBE

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The Ultimate for Organic Contaminants in Groundwater are being drained faster then they can be replenished

  • Less dense than water (hydrocarbons) float on top, more dense (chlorinated) sink to the bottom

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Decontamination of Ground Water: Pump-and-Treat are being drained faster then they can be replenished

  • Water is pumped from the ground treated to remove organics and pumped back into the ground –or a fine mist is sprayed into the air to remove VOCs and then use for agriculture

  • Volume of water is large and systems must operate for a long period of time

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Decontamination of Ground Water: Bioremediation & Natural Attenuation

  • Bioremediation generally uses bacteria to decompose organic pollutants

  • Natural attenuation (usually biotransfomation) – the following are treated effectively this way (Figure 10-2)

    • BTEX hydrocarbons (i.e., BTX hydrocarbons plus ethylbenzene),

    • low-molecular-weight oxygen-containing organics, and

    • methylene chloride.

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Decontamination of Groundwater: In Situ Remediation Attenuation

  • Mainly for volatile C1 & C2 chlorinated organics

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The Chemical Contamination & Treatment of Wastewater & Sewage

  • Some communities have a sanitary sewer system (to collect & treat household, industrial, etc. wastes) as well as a storm sewer system (which directly deposits storm water runoff into a body of natural water) while some have a combined sanitary & storm system. During heavy rains the combined systems are overwhelmed and the water is deposited untreated into waterways through combined sewer overflows (CSOs).

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Sewage Treatment (Figure 10-8) Sewage

  • Primary (mechanical) removes larger particles using screen & settling basin/lagoon forming bottom sludge & floating grease which is skimmed off –this removes about 30% of the BOD

  • Sewage now is clarified but still very high BOD (several hundred ppm) due to suspended colloidal organic particles

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Sewage Treatment Sewage

  • Secondary –biological oxidation by microorganisms to CO2 & water or a sludge. Sewage is kept well aerated

  • BOD down to <100 ppm or about 10% of the BOD of untreated sewage

  • Some municipalities now treat with chlorine or UV light before pumping into a waterway

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Sewage Treatment Sewage

  • Tertiary or advanced chemical treatment may produce water of sufficient quality to be recycled –prevalent in Europe

    • further reduction of BOD by removal of most of the remaining colloidal material using aluminum

    • removal of dissolved organic compounds (including chloroform) and some heavy metals by adsorption onto activated carbon

    • phosphate removal (as discussed in the next section)

    • heavy-metal removal by the addition of hydroxide or sulfide ions, to form insoluble metal hydroxides or sulfides

    • iron removal by aeration at a high pH to oxidize it to its insoluble Fe+3 state

    • removal of excess inorganic ions.

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The Origin & Removal of Excess Phosphate Sewage

  • Lake Erie major phosphate (from detergents, fertilizers) problem in the 1960-70s leading to copious numbers of dead fish & weeds lining the shoreline; PO43- is usually a limiting nutrient for algal growth –dead algae decompose usurping oxygen & killing fish

  • The series of changes, including the rapid degradation and aging, that occur when lakes receive excess plant nutrients from their surroundings is called eutrophication

  • When the enrichment arises from human activities, it is

    called cultural eutrophication.

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Phosphates in Detergents Sewage

  • Used as a builder –complexes (chelates) with Ca2+ & Mg2+ to prevent detergent from complexing with these ions, also provides an alkaline environment; slowly hydrolyzes to PO43-

STP

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Phosphates Sewage

  • Removal can be achieved by the addition of Ca(OH)2 to for insoluble calcium phosphate precipitates

  • Phosphates are used sparingly –replacements: sodium citrate, sodium carbonate, sodium silicate, zeolites, NTA

NTA

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Point & Nonpoint Sources Sewage

  • Point sources are specific locations such as factories, landfills, and sewage treatment plants that discharge pollutants

  • Nonpoint sources are large land areas such as farms, logged forests, septic tanks, golf courses and individual domestic lawns, stormwater runoff, and

    atmospheric deposition.

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Green Chemistry Sewage: Sodium Iminodicuccinate (IDS)-A Biodegradable Chelating Agent

  • Most chelating agents are not biodegradable or are slowly biodegradable they must be removed in sewer treatment plants

  • IDS is biodegradable, nontoxic & synthesized via a process using water as a solvent & producing very little pollution

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Reducing the Salt Concentration in Water (Desalination) Sewage

• Reverse osmosis As previously mentioned, this technique is also used to produce drinking water from salt water, such as seawater.

• Electrodialysis: Here a series of membranes permeable only to either small inorganic cations or small inorganic anions are set up vertically in an alternating fashion (see Figure 10-12) within an electrical cell. Direct current is applied across the water, so cations migrate toward the cathode and anions toward the anode. The liquid in alternating zones becomes more concentrated (enriched) or less concentrated (purified) in ions; eventually the ion-concentrated water can be disposed of as brine and the purified water released into the environment. This technology is also used to desalinate seawater for drinking purposes.

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Reducing the Salt Concentration in Water (Desalination) Sewage

• Ion exchange: Some polymeric solids contain sites that hold ions relatively weakly, so one type of ion can be exchanged for another of the same charge as it passes by it. The exchange sites of a cationic resin are initially occupied by H ions and those of an anionic exchange resin are occupied by OH ions. When water polluted by M and X ions is passed sequentially through the two resins, the H ions on the first are replaced by M, and then the OH ions on the second are replaced by X. Thus the water that has passed through contains H and OH ions rather than those of the salt, which remain behind in the resins.

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The Biological Treatment of Wastewater & Sewage (Alternatives)

  • Artificial marsh –decontamination is accomplished by bacteria & other microbes; plants absorb metals

    • Decaying vegatation increases BOD & marsh requires large land area

  • Septic tanks –solids sinks, oils & greases rise & periodically removed; bacteria feed on the sludge liquefying the waste, partially purified liquid flows out into underground fields

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Disposal of Sewage Sludge (Alternatives)

  • Spread on golf courses, farm fields, residential areas as low grade fertilizer called bioslid

  • Controversial because sludge may contain toxic materials such as heavy metals

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Modern Wastewater & Air Purification Techniques (Alternatives)

  • VOCs

    • The VOCs are removed from the wastewater by air stripping.

    • The resulting VOCs, now present in low concentration in a contained mass of humid air, are destroyed by a process of catalytic oxidation -air, heated to 300–500°C, is passed for a short time over platinum

  • Low level VOCs removed by absorption on activated carbon

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Advanced Oxidation Methods for Water Purification (Alternatives)

  • Organics (such as chloroorganics) are destroyed using Advanced oxidation methods (AOMs) –mineralization (CO2, H2O, mineral acids) -AOMs generate strong oxidizing or reducing agents e.g. OH –expensive –UV light employed

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