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 Treatment and Use
• 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
Water Treatment and Use
• 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
Municipal Waste Treatment
• 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
Treatment of Water for Industrial Use
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
Treatment of Water for Industrial Use
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
• 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
• 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
• 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 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
Removal of Calcium and Other Metals
• 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 Calcium and Other Metals
• 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
Removal of Calcium and Other Metals
• Organic Polymer Ion Exchangers
Figure 13.7. Strongly acidic cation exchanger
Removal of Calcium and Other Metals
• 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 Calcium and Other Metals
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
Removal of Dissolved Organics
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)
Removal of Dissolved Inorganics
• 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
Membrane Processes and Reverse Osmosis for Water Purification
Reverse Osmosis
• Reverse osmosis is widely used to desalinate water, including sea water
Figure 13.9. Reverse Osmosis
Membrane Processes and Reverse Osmosis for Water Purification
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
• 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
• 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
• 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
• 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 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