1. Environmental Chemistry TIP 2011
Wastewater Treatment
Environmental Chemistry
TIP 2011

2. What is Wastewater Treatment?
Wastewater treatment is also referred to as sewage treatment
Process of removing physical, chemical, and biological contaminants from wastewater and household sewage
Goal is to separate wastewater into:
Environmentally-safe fluid waste stream
Solid waste to be disposed or reused

3. Wastewater Sources Wastewater comes from: Homes Businesses
Sinks, showers, toilets, washing machines, dishwashers
Businesses
Industrial facilities
Storm runoff
From roads, parking lots, roofs
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4. What Needs to be Removed?
Wastewater may contain a variety of substances:
trash and debris
human waste
food scraps
oils
grease
soaps
chemicals (cleaning, pesticides, industrial)
pharmaceuticals and personal care products

5. What Needs to be Removed?
Wastewater from both domestic and industrial sources may contain a variety of potentially harmful contaminants, including:
Bacteria
E. coli (right), Giardia, Hepatatis A
Viruses
Nitrates
Metals
mercury, lead, cadmium, chromium, arsenic
Toxic materials
Salts

6. Example 1: Concentration
A 4.2 mL wastewater sample was tested and found to contain 7.6 ng of lead (II) ions. What is the molarity of lead (II) ions in this solution?

7. Example 1 Solution 7.6 ng Pb x 10-9 g x 1 mol Pb x 1 mL soln
4.2 mL ng g Pb L
= 9.1 x 10-9 mol/L
= 9.1 x 10-9 M Pb

8. Why Treat it?
The environment is able to naturally dilute and degrade water contaminants, but only in small amounts
Wastewater treatment reduces pollutants to levels that the environment can safely handle and process

9. Why Treat it?
Decaying solid matter left in water consumes dissolved oxygen from the water
Known as Biochemical Oxygen Demand (BOD): the amount of dissolved oxygen needed by aerobic organisms to break down organic matter
Lack of oxygen can kill plants and aquatic life
Excessive nutrients (nitrogen and phosphorous) can also lead to deoxygenation
Increased plant and algae growth, which eventually die and decompose, lead to an increased BOD

10. Process of Water Treatment
Water which enters a water treatment facility undergoes a series of steps to cleanse the water using physical, chemical, and biological processes
Upon exiting the water treatment facility, the decontaminated water is released into rivers or streams, entering again into the environment
Sometimes used specifically for agriculture and irrigation
Possible to purify into clean drinking water again

11. Steps of the Wastewater Treatment Process
1. Pretreatment
2. Primary Treatment
3. Secondary Treatment
4. Tertiary Treatment
5. Sludge Processing

12. Water Treatment Process
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13. Treatment Process Step 1: Pretreatment
Prepares waste water for entering the treatment plant
Removal of larger debris by screening (shown right)
Trash
Tree limbs
Removal of grit and gravel by screening and settling
Gravel must be removed early as it can damage machinery and equipment in the treatment plant

14. Treatment Process Step 2: Primary Treatment
In Primary Treatment, as much solid material is removed as possible by relying on gravity
Removes most of the sludge and scum
Sludge: Organic and inorganic materials which will naturally settle
removed by sedimentation
Scum: Materials which will float (oil, grease, soap)
removed by skimming
This step successfully removes 50 to 70% of suspended solids and up to 65% of oil and grease
Colloidal and dissolved materials are not affected by this step

15. Separation of Oil and Grease
Oil and grease will naturally separate from water due to differences in polarity
This is also known as the hydrophobic effect
Water is considered a polar substance, while oils and grease are considered nonpolar substances
A polar molecule is one in which electrons are unevenly distributed within the molecule due to differing electronegativities
Nonpolar molecules generally have evenly distributed electrons andhave no areas of partial charges

16. Separation of Oil and Grease
Water molecules have regions of differing electron density, making one end of the molecule have a partially negative side, while the other is partially positive
Water molecules are attracted to one another due to attractions between these positive and negative regions (hydrogen bonding)

17. Separation of Oil and Grease
Oil and grease are typically long chains of hydrocarbons, making them nonpolar, hydrophobic substances
Mixing a hydrophobic substance such as oil into water disturbs the attractions between polar water molecules
Hydrophobic substances tend to aggregate together in water in order to minimize the surface area that contacts the water which minimizes the disturbance
Oils and grease rise to the top of water due to a difference in density

18. Primary Treatment: Physical Separation
Sewage flows through large tanks known as primary clarifiers or primary sedimentation tanks
Round or rectangular basins, 3 to 5 meters deep
Water retained here for 2 to 3 hours
Sludge will settle toward the bottom of tanks, while scum will rise to the top. Both are removed and pumped to sludge treatment tanks
Mechanical scrapers continuously drive sludge into a well at the bottom of the tanks to be removed
Mechanical skimmers or rakes remove oils and grease from the surface.
May be recovered to use in saponification

19. Saponification
Saponification is the base hydrolysis of fats and oils to produce glycerol and a crude soap
triglyceride (fat) glycerol

20. Primary Treatment: Aeration
Another process during primary treatment is aeration
Water is agitated and exposed to air, which serves two purposes:
Allows some dissolved gases to escape, such as foul smelling hydrogen sulfide gas
Allows more oxygen to be dissolved into the water. Oxygen may be bubbled into water at this point.
Increasing dissolved oxygen in water compensates for the increased BOD and helps with the sludge settling process

21. Example 2: Concentration
9 ppm is considered a healthy dissolved oxygen concentration in water. What is this concentration expressed in molarity?

22. Example 2 Solution 9 g O2 x 1000 g H2O x 1 mol O2
g H2O L H2O g O2
= 3 x 10-4 M O2

23. Treatment Process Step 3: Secondary Treatment
Secondary treatment is designed to remove residual organic materials and suspended solids that were not removed during primary treatment
Works to degrade the biological content of the sewage that comes from human waste, food waste, soaps and detergent.
Removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment and flocculation

24. Secondary Treatment: Aerobic Biological Treatment
performed in the presence of oxygen by aerobic microorganisms
Aerobic = in presence of oxygen
principally bacteria and protozoa
metabolize the organic matter in the wastewater, including sugars, fats, and short-chain hydrocarbons
Results in production of several inorganic products, including CO2, NH3, and H2O, as well as reproduction of more microorganisms

25. Secondary Treatment: Flocculation
Process in which colloids come out of suspension to form flakes, or floc
Differs from precipitation!
Precipitation involves particles which are dissolved in a solution
Flocculation involves particles that are suspended within a liquid, not dissolved

26. Colloid Properties
Colloids contain microscopic particles dissolved evenly throughout a substance
Particles finer than 0.1 µm in water remain in constant motion because they often carry an electrostatic charge which causes them to repel each other.

27. Colloids and Flocculation
If the electrostatic charge of colloid particles is neutralized, the finer particles start to collide and combine together into larger groups of particles
Due to the influence of Van der Waals forces:
These larger and heavier particles are called flocs
Floc can either be filtered out of wastewater or left to settle out as sludge

28. Colloids and Flocculation
Flocculants, or flocculating agents are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to combine, forming a floc.
Many flocculants are multivalent cations such as aluminum, iron, calcium, and magnesium
Often, colloid particles carry a negative charge
These positively charged flocculant molecules interact with negatively charged colloid particles and molecules to reduce the barriers to aggregation.

29. Colloids and Flocculation
Many flocculating agents under appropriate conditions (such as pH, temperature and salinity) will react with water to form insoluble hydroxides
These hydroxides will precipitate from solution and link together to form long chains or meshes
physically traps small particles into the larger floc
Polymers can also be used as flocculants

30. Common Flocculants Chemical Flocculating Agents:
Alum
Aluminum chlorohydrate
Aluminum sulfate
Calcium oxide
Calcium hydroxide
Iron (II) sulfate
Iron (III) chloride
Polyacrylamide
Sodium silicate
Natural Products Used as Flocculants:
Chitosan
Isinglass
Horseradish tree seeds
Gelatin
Guar Gum
Alginates (from brown seaweed)

31. Treatment Process Step 4: Tertiary Treatment
Tertiary treatment (also known as advanced treatment) includes the remaining processes necessary to remove the following from wastewater:
Nitrogen
Phosphorus
additional suspended solids
remaining organics
heavy metals
dissolved solids
Final treatment stage before water is released into rivers, lakes, or groundwater

32. Example 3: Dilution and Concentration
Nitrogen is usually present in wastewater as ammonia million L of wastewater entering a treatment plant have an initial ammonia concentration of 0.75 mM. By the time the wastewater reaches the tertiary treatment phase, the volume has been reduced to 2.9 million L. What is the concentration of ammonia at this point?

33. (0.75 mM NH3)(3.5 million L)=(M2)(2.9 million L)
Example 3 Solution
(M1)(V1) = (M2)(V2)
(0.75 mM NH3)(3.5 million L)=(M2)(2.9 million L)
M2 = 0.91 mM

34. Tertiary Treatment
Depending on the types of contamination and the desired end use, one or more processes may be used in tertiary treatment:
Sand filtration
Nutrient removal (nitrogen and phosphorous)
Odor removal
Disinfection (via chlorination, ozone, or UV radiation)

35. Tertiary Treatment Sand filtration
Removes any remaining suspended solids not removed by sedimentation and flocculation
May be combined with filtering over activated carbon to remove toxins and odors

36. Tertiary Treatment Nutrient Removal
Excessive release of nitrogen and phosphorous leads to a condition known as eutrophication (presence of excessive nutrients)
Eutrophication encourages excessive algae and weed growth
Leads to deoxygenation of water
Some algae can release toxins into water

37. Tertiary Treatment
Nutrient removal may be accomplished through biological processes by passing wastewater through 5 different chambers:
Anaerobic fermentation zone
very low dissolved oxygen levels and the absence of nitrates
Anoxic zone
low dissolved oxygen levels but nitrates present
Aerobic zone
Secondary anoxic zone
Final aeration zone

38. Biological Nutrient Removal Zones

39. Nutrient Removal: Nitrogen
The majority of nitrogen in wastewater is in the form of ammonia, NH3
Nitrogen removal takes place in two parts:
Nitrification: oxidation of ammonia to nitrate
Denitirication: reduction of nitrate to nitrogen gas
Nitrogen gas is then released into the atmosphere

40. Nutrient Removal: Nitrogen
Nitrification
Nitrification occurs in the 3rd zone (aerobic zone)
Two step process
Each step carried out by a unique bacteria
Step 1: oxidation of ammonia to nitrite (NO2 -1)
NH3  NO2 -1
Step 2: oxidation of nitrite to nitrate (NO3 -1 )
NO2 -1  NO3 -1

41. Nutrient Removal: Nitrogen
Denitrification
After nitirification in the 3rd zone, wastewater rich in nitrates is recycled back to the 2nd zone (first anoxic zone)
The recycled nitrates, in the absence of dissolved oxygen, are reduced by bacteria to nitrogen gas
NO3 -1  N2
Incoming organic carbon compounds present in this zone act as hydrogen donors

42. Nutrient Removal: Nitrogen
Denitrification
In zone 4, the second anoxic zone, any nitrates not reduced in zone 2 are reduced by the respiration of bacteria present
In zone 5, the re-aeration zone, oxygen levels are increased to stop the denitirication process
Stopping denitirification prevents problems with settling

43. Example 4: Redox Reactions
Redox Reactions: Balance the three redox half reactions associated with nitrogen removal:
Nitrification Step 1
Nitrification Step 2
Denitrification

44. Example 4 Solution NH3  NO2-1 NH3 + 2H2O  NO2-1 + 7 H+
Nitrification Step 1
NH3  NO2-1
NH3 + 2H2O  NO H+
NH3 + 2H2O  NO H+ + 6 e-
Nitrification Step 2
NO2-1  NO3-1
NO2-1 + H2O NO H+
NO2-1 + H2O NO3-1 +2H+ + 2e-
Denitrification
NO3-1  N2
2 NO3-1  N2
2 NO H+  N H2O
2 NO H e-  N H2O

45. Nutrient Removal: Phosphorous
Phosphorous may occur as organic or inorganic forms
Of the 5 to 20 mg/L total phosphorous content in wastewater, 1 to 5 mg/L is organic
Phosphorous is typically present in the form of phosphates
Typical forms include:
Orthophosphates: easily used in biological metabolism
Polyphosphates: contain two or more phosphorous atoms in a complex molecule. Can slowly undergo hydrolysis to orthophosphates
Phosphorous may be removed biologically or chemically

46. Nutrient Removal: Phosphorous
Biological Removal:
Biological phosphorous removal takes place in Zones 1 and 2 of the five zone system mentioned earlier
In Zones 1 and 2, the anaerobic fermentation zone and first anoxic zone, bacteria are stressed by the low oxygen conditions and release phosphorous to maintain cell equilibrium
When these bacteria reach later zones with higher oxygen supplies, they rapidly accumulate phosphorous in excess of what they normally would
Removed along with sludge

47. Nutrient Removal: Phosphorous
Chemical Removal
Phosphorous can be precipitated out of the wastewater mixture using salts of iron, aluminum, or calcium
Some of this is accomplished during flocculation
Produces more sludge due to precipitate formation
More expensive than biological removal (added cost of chemicals)
Usually more reliable and more effective than biological removal

48. Chemical Removal of Phosphorous Using Calcium
Usually added in the form of lime, Ca(OH)2.
Reacts with the natural alkalinity in the wastewater to produce calcium carbonate
Ca(HCO3)2 + Ca(OH)2  2CaCO3 + 2H2O
As the pH value of the wastewater increases past 10, excess calcium ions will then react with the phosphate, to precipitate in hydroxylapatite:
10 Ca PO OH- ↔ Ca10(PO4)*6(OH)2 (s)
Amount of lime required depends on pH of water rather than amount of phosphate present
Neutralization may be required to lower the pH before further treatment or disposal, typically by recarbonation with carbon dioxide

49. Example 5: Acids, Bases, and pH
The pH of domestic wastewater is about 7.2.
What are the concentrations of hydronium and hydroxide ions in water at this point?
Lime is only effective in removing phosphorous at a pH higher than 10. What is the pH of 9800 L of water treated with 50 g Ca(OH)2?

50. Example 5 Solution Part 1: [H3O+] = 10 -pH = 10 -7.2 = 6.3 x 10-8 M
[H3O+] [OH-] = 1 x 10-14
[OH-] = 1 x 10-14
6.3 x 10-8
= 1.6 x 10-7M
Part 2:
50.0 g x 1 mol Ca(OH)2 x 2 mol OH
9800 L g mol Ca(OH)2
= 1.38 x M OH-
[H3O+] = 1 x 10-14
1.38 x 10 -4
= 7.25 x M H3O+]
pH = -log[H3O+]
= -log (7.25 x M)
= 10.1

51. Chemical Removal of Phosphorous Using Aluminum
Typically use alum or hydrated aluminum sulfate to precipitate aluminum phosphates (AlPO4).
Al3+ + HnPO43-n ↔ AlPO4 + nH+
Reaction affected by pH, equilibrium of competing reactions, and presence of trace elements in wastewater
Aluminum may adversely affect some of the bacteria used in sludge and digestion and should be used carefully

52. Chemical Removal of Phosphorous Using Iron
Iron (III) chloride or sulfate or iron (II) sulfate can be used to form iron phosphate precipitates
Fe3+ + HnPO4 3-n ↔ FePO4 + nH+
Lime is usually added to raise the pH to enhance the reaction

53. Example 6: Solubility and Net Ionic
Write the full balanced equation (including states) and the net ionic equation for the reaction of iron (III) sulfate with sodium phosphate.

54. Example 6 Solution Balanced Reaction: Net:
Fe2(SO4)3(aq) + 2Na3PO4(aq)  2FePO4(s) + 3Na2SO4(aq)
Net:
Fe3+(aq) + PO43-(aq)  FePO4 (s)

55. Tertiary Treatment: Disinfection
Disinfection of wastewater reduces the number of microorganisms in water that may lead to disease before discharging back into the environment
Usually the very last step before discharge
Effectiveness depends upon conditions of treated water at this point, including cloudiness and pH
Three major strategies: chlorination, ozone, and UV radiation

56. Disinfection: Chlorination
Most commonly used form of disinfection due to low cost and high effectiveness
The exact mechanism by which chlorine disinfects is not fully understood. It likely involves oxidative damage to microbial cell membranes and vital protein systems
Chlorination also helps to reduce any odors in the water
Drawbacks:
may create chlorinated organic compounds that may be carcinogenic
Residual chlorine is toxic to aquatic life
May be necessary to dechlorinate water before release

57. Disinfection: Chlorination
When chlorine (Cl2) is injected into water, it forms hypochlorous acid and hydrochloric acid in a pH dependent equilibrium
Cl2 + H2O → HOCl + HCl
Depending on the pH, the hypochlorous acid will partly dissociate to hydrogen and hypochlorite ions:
HClO → H+ + ClO-
In acidic solution, the major species are Cl2 and HOCl while in basic solution only ClO- is present.
Very small concentrations of ClO2-, ClO3-, ClO4- are also found

58. Disinfection: UV Radiation
Ultraviolet radiation damages the genetic structure of bacteria and viruses which makes them incapable of reproduction
Since no chemicals are used, UV disinfection poses no risk to organisms which will later encounter the treated water
Requires highly treated water with little cloudiness. Suspended solids in the water may block out the UV rays
Maintaining UV lamps can be costly

59. Disinfection: Ozone
Ozone (O3) is generated by passing oxygen gas (O2) through a high voltage potential. Voltage breaks O2 into oxygen atoms which will recombine as O3 gas
O2 + electricity  O3
Ozone is very unstable. Generated as needed rather than stored
Produces fewer by-products than chlorination, but much more costly

60. Example 7: Reaction Stoichiometry
Write the balanced equation for the synthesis of ozone from oxygen
If 56.8 g of ozone must be synthesized, how many moles of oxygen gas are required?

61. Example 7 Solution 3 O2  2 O3 56.8 g O3 x 1 mol O3 x 3 mol O2
48 g O mol O3
= 1.78 mol O2

62. Disinfection: Ozone
Ozone is very effective in destroying viruses and bacteria and may act by several mechanisms:
Direct oxidation and destruction of the cell wall with leakage of cellular components
Reactions with radical by-products of ozone decomposition
Damage to the constituents of the nucleic acids (purines and pyrimidines)
Breakage of carbon-nitrogen bonds leading to depolymerization

63. Tertiary Treatment: Odor Removal
Odor in waste water typically form as a result of anaerobic conditions
Most common odor is hydrogen sulfide gas
Odor is eliminated along the way by aeration, chlorination, biological degradation, and circulation of fluids
Other methods to eliminate hydrogen sulfide are by adding iron salts, hydrogen peroxide, or calcium nitrate

64. Treatment Process Step 5: Sludge Treatment
Sludge consists of all the solid material removed from wastewater during the water treatment process
While the water in treatment is ready for release into streams and groundwater, sludge requires further treatment before it can be disposed or used
Must reduce the amount of organic matter
Must reduce the number of disease causing microbes
Remove as much remaining liquid as possible
Sludge treatment options include:
Aerobic digestion
Anearobic digestion
Composting
Incineration

65. Sludge Treatment
Sludge is most often processed by biological anaerobic digestion
Bacteria metabolize the organic material in the sludge
Occurs over a period of 10 to 60 days, depending on the capabilities of the digesting tanks
Reduces the volume of sludge that requires disposal
Makes the sludge more stable
Improves the dewatering characteristics of the sludge
Shorter retention time and smaller tanks required
Requires higher temperatures, resulting in a higher energy cost

66. Sludge Treatment
One byproduct of anaerobic sludge digestion is the production of biogas
Biogas contains about 60 to 65% methane (CH4) and can be recovered as an energy source.
Methane is a combustible, renewable fuel
CH4 + O2  CO2 + H2O

67. Sludge Treatment
In small sewage treatment plants, sludge is processed using aerobic digestion
Under aerobic conditions, bacteria will consume organic material and convert it into carbon dioxide
Energy cost associated with adding oxygen to process and blowers to remove CO2

68. Sludge Treatment
Composting of sludge is similar to aerobic digestion, except other organic materials such as sawdust are mixed in with the sludge
Incineration is the least used method of sludge treatment.
Sludge burns poorly due to low calorific value, so extra fuels must be added
Worries of emissions associated with sludge
High energy cost to vaporize residual water present in sludge

69. Sludge Treatment
Sludge that does not originate from highly industrialized areas and is for the most part free of toxic chemicals can be used as fertilizer
Water is removed from sludge by centrifugation and addition of chemicals that aid in polymer formation
Dried sludge can be converted into fertilizer pellets which are usually rich in phosphorous

70. Water Treatment View the entire process in action

72. Sources
Severn Trent Water. “The Water Treatment Process” Online. 9 July 2011.
United States Geological Survey. “Wastewater Treatment: Water Use” Online. 9 July
South Carolina Office of Regulatory Staff. “Overview of Basic Wastewater Treatment Process” Online. 9 July
Author Unknown. “Sewage Treatment” Online. 8 July
United States Geological Survey. “A visit to a wastewater-treatment plant: Primary treatment of wastewater” Online. 9 July
Natural Resources Management and Environment Department. “Water Treatment” Online. 10 July
Environmental Protection Agency. “Water Treatment Process” Online. 8 July
Environmental Protection Agency. Wastewater Technology Fact Sheet: Ozone Disinfection. (1999) Online. 11 July
Author Unknown. “Chlorination” Online 11 July
Lenntech Water Treatment Solutions. “Phosphorous removal from wastewater”. Online 10 July
Author Unknown. “Flocculation” Online 9 July