environmental chemistry chapter 13: sustaining the hydrosphere: keeping water green copyright ©...

Environmental Chemistry

Chapter 13:Sustaining the Hydrosphere: Keeping

Water Green

Copyright © 2011 by DBS

Contents

• Water Treatment and Use• Municipal Waste Treatment• Treatment of Water for Industrial Use• Sewage Treatment• Industrial Wastewater Treatment• Removal of Solids Removal of Calcium and Other Metals Removal of

Dissolved Organics• Removal of Dissolved Inorganics• Membrane Processes and Reverse Osmosis for Water Purification• Sludge• Water Disinfection• Natural Water Purification Processes• Water – The Greenest Material: Reuse and Recycling• Water Conservation

Water Treatment and Use


• Treatment of Water -Three Major Categories:

(i) Purification for domestic use

(ii) Treatment for specialized industrial applications

(iii) Treatment of wastewater to make it acceptable for release or reuse

• Type and degree of treatment depend upon the source and intended use or fate of water


• Water for domestic use must be pathogen free but may contain significant levels of calcium and magnesium (hardness)

• Boiler feedwater may contain some bacteria but very little hardness or scale-forming silica

• Wastewater to be recycled in an arid region requires more rigorous treatment than effluent to be discharged into a large river

• Increasing world demand for water will require more sophisticated and extensive water treatment

The World’s WatersContaminants of Concern

Pharmaceuticals

• A


• Emerging Considerations:

–“Exotic” contaminants – pharmaceuticals and metabolites

–Nanoparticles

–Residuals (leftovers)

–Development of new technologies

Municipal Waste Treatment


Figure 13.1. Schematic of a typical municipal water treatment plant

Removes H2S, CO2, CH4 and volatiles

Soluble Fe2+ → insoluble Fe3+

PPTs Ca2+ and Mg2+ as hydroxides

Coagulant to aid settling =Iron (III) or Al sulfate

CO2 lowers pH


• Contamination in water distribution systems

• Iron (“red water”) or copper “blue-green water” from pipe

• Lead from solder formerly used in pipe

• Plasticizers and organic additives from plastic pipe

• Pathogens introduced by accident

Treatment of Water for Industrial Use


• Improper treatment can cause problems:

– Corrosion

– Scale formation

– Reduced heat transfer

– Reduced water flow

– Product contamination

• Leading to:

– Equipment failure

– Increased energy costs

– Higher pumping costs

– Product deterioration


Factors to consider in treatment plant design and operation

• Water requirement

• Quantity and quality of available water sources

• Sequential use of water for applications requiring successively lower water quality

• Water recycle

• Discharge standards


External treatment of all water to a plant

• Removal of suspended or dissolved solids, hardness, dissolved gases

• Aeration, filtration, clarification

Internal treatment for specific purposes may include

• Reaction of dissolved oxygen with hydrazine or sulfite

• Addition of chelating agents to react with dissolved Ca2+ and prevent formation of calcium deposits

• Addition of precipitants, such as phosphate used for calcium removal

• Treatment with dispersants to inhibit scale

• Addition of inhibitors to prevent corrosion

• Adjustment of pH

• Disinfection for food processing uses or to prevent bacterial growth in cooling water

Sewage Treatment

Sewage Treatment

• Usually done in a publicly owned treatment works, POTW

Primary Waste Treatment

• Consists for the most part of removal of solids/grit and surface floating material increasing grease

Secondary Waste Treatment

• Primarily removal of biochemical oxygen demand, BOD, usually by microbial degradation

Tertiary Waste Treatment

• Consists of various measures to bring effluent from secondary waste treatment up to standards suitable for release

• Increasingly to produce water suitable for re-use

Sewage Treatment

Secondary Waste Treatment by Biological Processes

• Where {CH2O} is degradable organic matter (BOD),

{CH2O} + O2 CO2 + H2O + settleable solid biomass

Trickling Filter (Figure 13.2)

• Biomass held on a layer of rocks or other solids covered with MOs degrades organic matter sprayed on it and is in contact with air

• Oldest, but effective means of removing BOD

Sewage Treatment

Rotating Biological Contactors

• Cylinders holding a film of active biomass on a horizontally rotating shaft alternatively contact sewage and air

• Are fixed bed reactors that act similarly to trickling filters

Sewage Treatment

Activated Sludge

• MOs convert OM to microbial biomass and CO2

• Remove BOD

• Produce methane (fuel)

Activated Sludge Process, Figure 13.3

Sewage Treatment

Activated Sludge

Figure 13.4. Pathways for the Removal of BOD in Wastewater Treatment

Sewage Treatment

Figure 13.3. Further Sludge Degradation in the Anaerobic Digester2{CH2O} CH4 + CO2

Activated Sludge

• Removes sludge BOD

• Generates valuable combustible methane gas, CH4

• In best practice of sustainability and recycling

• Disposal of sewage sludge from the anaerobic digester can cause problems, especially in urban areas where land spreading is not an option

• Accumulation of heavy metals can be a problem

• Zinc buildup has been detrimental to farm land that has received sewage sludge over prolonged periods

• Incineration of sewage sludge usually requires supplemental fuel

• Gasification can yield energy in combustible CO and H2

Sewage Treatment

Tertiary (advanced) Waste Treatment

• Applied to secondary sewage effluent

• Removes:

1. Suspended solids

2. Dissolved organic compounds

3. Dissolved inorganic materials including algal nutrients

• Disinfection can be part of tertiary waste treatment

Industrial Wastewater Treatment

Industrial Wastewater Treatment

• Biological wastewater treatment, such as by activated sludge

– Special hazards from industrial wastewater sludges

– Activated carbon/sludge sorption of organics

• Chemical processes:

– Acid/base, precipitation, oxidation/reduction

• Physical treatment

– Density separation and sedimentation, filtration, evaporation, distillation, solvent extraction, air stripping, steam stripping, membrane processes

Removal of Solids

Removal of Solids

• Solids

– Inorganic

– Organic

• Biological including pathogens

• Relatively large particles removed by settling and filtration

• Colloidal particles require coagulation

– Formation of coagulant hydrated Al(OH)3 or Fe(OH)3

– Natural and synthetic polyelectrolytes

• Coagulation/filtration is commonly applied

Removal of Solids

• A

Dissolved Air Flotation, Figure 13.6

Removal of Calcium and Other Metals


• Hardness is due primarily to Ca2+ and to a lesser extent Mg2+

• Calcium present as the HCO3- salt precipitates CaCO3 from boiled water

• Bicarbonate hardness removed by lime, Ca(OH)2

Ca2+ + 2HCO3- + Ca(OH)2 2CaCO3(s) + 2H2O


• When Ca2+ is present as salts other than 2HCO3-, such as the chloride salt, a source of CO3

2- must be added, usually from Na2CO3

Ca2+ + 2Cl- + 2Na+ + CO32- 2CaCO3(s) + 2Cl- + 2Na+

• Mg2+ can be precipitated as the hydroxide,

Mg2+ + 2OH- Mg(OH)2(s)

• Commonly from OH- provided by hydrolysis of CO32- ion from added Na2CO3:

CO32- + H2O HCO3

- + OH-

Recarbonation with CO2 is used to prevent precipitation of CaCO3 or Mg(OH)2 in the water distribution system and to neutralize excessive base


• Removal of Dissolved Salts from Water by Ion Exchange

• Exchange salt cations for H+ over a cation exchanger

H+-{Cat(s)} + Na+ + Cl - Na+-{Cat(s)} + H+ + Cl-

• Exchange salt anions for OH-, which neutralizes H+ produced by cation exchange

OH-+{An(s)} + H+ + Cl - Cl-+{An(s)} + H2O

• Softening water by exchange of Ca2+(aq) for Na+ on a cation exchanger

2Na+-{Cat(s)} + Ca2+ Ca2+(-{Cat(s)})2 + 2Na+

• Ion exchangers used for water softening can be regenerated with NaCl

• • Tends to add salt to wastewater


• Organic Polymer Ion Exchangers

Figure 13.7. Strongly acidic cation exchanger


• Chelation to prevent precipitation of metal compounds such as CaCO3

• Addition of chelating EDTA anion, Y4-,

Ca2+ + Y4- CaY2-

• Prevents precipitation of CaCO3:

Ca2+ + CO32- CaCO3(s)


Removal of Iron and Manganese

• Soluble 2+ ions oxidized to hydroxides or oxides that precipitate

Fe2+(aq) (oxidation) Fe(OH)3(s)

Mn2+(aq) (oxidation) MnO2(s)

Removal of Heavy Metals

• Lime treatment for Ca2+ removal precipitates heavy metal hydroxides or basic salts

• Sulfide precipitates some important heavy metals

Cd2+(aq) + S2-(aq) CdS(s)

• Electrodeposition, such as of Cu2+

Cu2+(aq) + 2e- Cu(s)

• Cementation, deposition of a metal by a more reactive one

Cd2+(aq) + Fe(s) Cd(s) + Fe2+(aq)

Arsenic Removal

• Sorption onto Fe(OH)3 or iron oxides is very effective

Removal of Dissolved Organics


• Removal of Taste, Odor, and Color

• Frequently from microorganisms

• • Geosmin, a cyanobacteria metabolite

• Taste, odor, and color removed by various means, usually those that remove organics

• • Aeration • Oxidation • Adsorption on activated carbon


Biodegradation

{CH2O} + O2 CO2 + H2O

• Activated carbon sorption

– Most common and effective means of organic removal

• Ionizing radiation

– Ultraviolet radiation – photolysis

– May be aided by added oxidants

• Sonolysis

• Ultrasound generates extremely high localized temperatures and pressures by formation and collapse of bubbles (cavitation)

• Generates free radicals, especially H• and HO• from splitting water molecules

• Like photolysis, a “green” reagentless process

Removal of Dissolved Inorganics


• Salts accumulate in municipal water

• Inorganics removal is required for complete water recycle

• Removal of nutrient phosphorus and nitrogen to prevent eutrophication in receiving waters

• Several important methods for removing inorganics from water

• • Distillation (expensive)

• Ion exchange: H+ for cations, OH- for anions leaving H2O, product called deionized water

• • Freezing

• • Membrane processes (see reverse osmosis on a later slide)


• Phosphorus Removal

• Practiced to prevent eutrophication of receiving waters

• Most commonly by treatment with lime, Ca(OH)2

5Ca(OH) 2 + 3HPO42- Ca5OH(PO4)3(s) + 3H2O + 6OH-

• Nitrogen Removal

• Extensive aeration to convert N in biodegraded sewage to nitrate

NH4+ + 2O2

(nitrifying bacteria) NO3- + 2H+ + H2O

• Denitrification by microbially mediated reaction of nitrate with organic matter to evolve N2 gas

4NO3- + 5{CH2O} + 4H+ 2N2(g) + 5CO2(g) + 7H2O

Membrane Processes and Reverse Osmosis for Water Purification


Table 13.1. Membrane Filtration Processes for Water PurificationName Approximate pore size1 Materials removed

Microfiltration 100 nm Particles, single-cell microorganismsUltrafiltration 10 nm Small colloids, virusesNanofiltration 1 nm Dissolved organics, divalent ions

(Ca2+, Mg2+, SO42-)

Reverse osmosis --- Monovalent ions (Na+, Cl-)

1 In nanometers (nm, 10-9 m)

Two problems with membrane processes• Membrane clogging• Disposal of retentate


Reverse Osmosis

• Reverse osmosis is widely used to desalinate water, including sea water

Figure 13.9. Reverse Osmosis


Electrodialysis

• Electrodialysis passes a direct electrical current across membranes alternately permeable to cations and anions giving a desalinated water product and a retentate (brine)

Sludge

Sludge

• Water treatment processes generate sludges

• Solids collected from wastewater constitute sludge

• Organic sludges generated by biological waste treatment

• Treated by anaerobic digestion

• Inorganic sludges come primarily from precipitation of Al(OH)3 and Fe(OH)3 generated primarily during colloid coagulation

• Sludge normally is treated to make it amenable to disposal

• Conditioning with polymers or lime to aid dewatering

• Thickening by settling, filtering, centrifugation to concentrate solids

• Dewatering on sand or gravel beds, with filter presses, with vacuum filters

• Sidestreams of water from sludge treatment can be highly contaminated

• Sewage sludge can be disposed on land as fertilizer

• Problems with metal accumulation

• Sludge can be incinerated

• Supplemental fuel required

Water Disinfection

Water Disinfection

• Disease-Causing Agents Treated by Disinfection

• Primarily viruses and bacteria that cause gastroenteritis

• Millions of children die annually from waterborne diarrhea

• Most notorious bacteria causing explosive diarrhea and vomiting is Vibrio cholerae

• Salmonella typhi causes typhoid fever

• Other bacteria in water that cause gastroenteritis diseases are Salmonella paratyphi, Shigella dysenteriae, and forms of Escherichia coli that have mutated to pathogenic forms

• Protozoans including Entamoeba histolytica and Giardia lamblia can cause severe intestinal disease

• Waterborne viruses include those that cause poliomyelitis, viral hepatitus and rotavirus that cause gastroenteritis

Water Disinfection

• Disinfection With Chlorine and Chloramines

• Most common water disinfection agents

Cl2 + H2O H+ + Cl- + HOCl

HOCl + OCl- are free available chlorine

• HOCl reacts with NH4+ to produce chloramines

• • NH2Cl • NHCl2 • NCl3

• Chloramines are combined available chlorine

• Disinfection with Chlorine Dioxide

• Disinfects without producing potentially toxic trihalomethanes

• Produced on-site: 2NaClO2(s) + Cl2(g) 2ClO2(g) + 2NaCl(s)

Water Disinfection

• Ozone for Water Disinfection:

• Disinfection with ozone is a very “green” process taking raw material from air and leaving no harmful residue

Figure 13.10. Ozone water treatment system

Water Disinfection

• Disinfection by Filtration

• Filtration, such as by simple sand filters

• Cut down diseases such as cholera in the 1900s

• Ultrafiltration can remove even viruses from water

• Disinfection with Ferrate Ion, FeO4-

• Acts as a disinfecting oxidant that produces gelatinous Fe(OH)3

– Oxidizes organics

– Removes humic substances

– Precipitates colloidal impurities

Natural Water Purification Processes

Natural Water Purification Processes

• Most water contaminants can be eliminated by contact with soil

– Impurities sorbed onto soil solids

– Biodegradation by microorganisms held by soil

• Wastewater contains nutrients needed for plant growth

• Soil has physical, chemical, and biological characteristics that enable wastewater

• • Detoxification • Biodegradation • Chemical decomposition

• • Physical and chemical fixation

• Soil provides a system that enables primary, secondary, and tertiary wastewater treatment

• Soil treatment of wastewater is a very green process

• Industrial Wastewater Treatment by Soil

• Soils subjected to industrial wastewater become acclimated to biodegradation of impurities in the water

• Petroleum products are especially acclimated to soil treatment because of decades of acclimation by soil microorganisms to petroleum spills

Water – The Greenest Material: Reuse and Recycling

Water – The Greenest Material: Reuse and Recycling

• The hydrologic cycle is nature’s natural water recycling system

• Reuse is a term applied when water discharged from one source is used as a source of water for another system

• Unplanned reuse of wastewater discharged into a receiving water from which water is taken for another application

• Planned reuse in which wastewater treatment systems are deliberately designed to produce water that can be taken for another application

• Currently most water reuse occurs for

– Irrigation

– Cooling and processing

– Groundwater recharge

• Orange County Water District Groundwater Replenishment System

– Brings sewage to a standard suitable for pumping into groundwater aquifers

– Uses reverse osmosis

– Operation in 2007

Water Conservation

Water Conservation

• Water conservation by simply using less water is the fastest and most effective means of ensuring water supplies

• Important means of water conservation:

1. Indoor and household water conservation practices

2. Water-conserving devices and appliances

3. Outdoor and landscaping water conservation

4. Efficient irrigation practices

5. Conservation in non-irrigation agricultural operations

6. Water-efficient manufacturing

environmental chemistry chapter 13: sustaining the hydrosphere: keeping water green copyright ©...

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