1. Introduction to Environmental Engineering and Science (3rd ed.)
Chapter 6. Water Quality Control

2. Municipal Water and Wastewater Systems

3. Drinking Water Standard
Primary Standards: enforceable and based on health-related criteria: Maximum Contaminant Levels (MCLs)
1) chemical standards, 2) radionuclides, 3) microbiological standards (Table 6.1)
Secondary Standards: nonenforceable and intended to protect welfare (Table 6.2)

4. Schematic of a typical surface water treatment plant
Figure: 06-02

5. Schematic of a typical groundwater treatment plant
Figure: 06-03

6. Forces acting on a particle settling in water
FB = vpρg
Figure: 06-04
FG = vpρpg
FD = 3πμvsdp

7. Sedimentation: gravitational settling
Stokes’ law
vs = g(ρp-ρ)dp2/18μ (6.5)
only valid at laminar region (Re < 1)
Rectangular settling basin (Figure 6.5)
hp = vsθ = vsVb/Q (6.6)
Critical settling velocity (vo)
vo = h/θ =hQ/Vb = hQ/hAb = Q/Ab (6.7)
(Example 6.1)

8. Trajectory of a particle settling
Figure: 06-05
To design a clarifier to remove all particles of a size dp at a flow rate Q,
Surface area, Ab = 18Qμ/g(ρp-ρ)dp2 (6.8)

9. Coagulation and flocculation
Coagulation: a chemical treatment process that destabilizes particles (sticky)
Coagulant: alum, Al2(SO4)3·18H2O
Al2(SO4)3·18H2O + 6HCO3- →
2Al(OH)3(S) + 6CO2 + 18H2O + 3SO42-
Al(OH)3: a light, fluffy floc that adsorbs destabilized particles on its surface as it settles.
Rapid mix/coagulation: less than one-half min.
Flocculation: gentle agitation for approximately one-half hour

10. Flocculation and sedimentation tank
Figure: 06-06

11. Filtration & Disinfection
Filtration: removing small particles from water
Primary disinfection: to kill any pathogens in the water. Dinfectant: hypochlorous acid (HOCl) and hypochlorite (OCl-)
Secondary (or residual) disinfection: to prevent pathogen regrowth in the water during the period before it is used. Disinfectants: chloroamines (NH2Cl, NHCl2, NCl3)

12. A disadvantage of free chlorine disinfection
Formation of halogenated disinfectant byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs)
Alternative disinfectants: ClO2, O3
The rate of inactivation of pathogens: Chick-Watson model
r(N) = dN/dt = -kCnN

13. Hardness
Water hardness: the presence of multivalent cations, mostly Ca2+ and Mg2+ (unit: mg/L as CaCO3)
Temperature increase results in precipitation of CaCO3 and Mg(OH)2
Equivalent weight (eq)= molecular weight / n
Normal concentration = eq/L = N
mN = meq/L = (mg/L)/eq
1 meq/L = 50 mg/L as CaCO3 (Example 6.4)
Hardness classification: Table 6.4

14. Alkalinity Alkalinity = (HCO3-) + (CO32-) + (OH-) - (H+)
Unit: meq/L or mg/L as CaCO3
Alkalinity (meq/L) = (HCO3-) for 6 ≤ pH ≤ 8.5
Total hardness = carbonate hardness + noncarbonate hardness
Example 6.6

15. Hardness & Alkalinity (Example 6.6)
Figure: 06-09

16. Softening Lime-soda ash process
Add hydrated lime (Ca(OH)2) to increase pH above 10: CaCO3 and Mg(OH)2 precipitation
Add soda ash (NasCO3): promote CaCO3 precipitation
Recarbonation by adding CO2(g)
Ion-exchange process using ion-exchange resin (must be regenerated!)

17. Size ranges and types of contaminants
Figure: 06-11

18. Schematic of a typical wastewater treatment facility
Figure: 06-12

19. Primary and Secondary Treatments
Primary treatment: removal of 50-65% suspended solids and 25-40% BOD (detention time: hrs)
Secondary (biological) treatment
Suspended growth treatment: a) activated sludge, b) membrane bioreactor, c) aeration lagoons and oxidation ponds
Attached treatment process: a) trickling filters, b) rotating biological contactors (RBC)

20. Activated sludge wastewater treatment process
Figure: 06-15

21. An oxidation pond
Figure: 06-17

22. A trickling filter
Figure: 06-18

23. Rotating biological reactor
Figure: 06-19

24. Sludge treatment: anaerobic digestion
Figure: 06-20

25. Schematic of a two-stage anaerobic digester
Figure: 06-21

26. Nutrient removal Nitrogen removal: nitrification/denitrification
NH4+ + 2O2 → NO3- + 2H+ + H2O
2 NO3- + organic matter → N2 + CO2 + H2O
Phosphorous removal: by adding coagulant alum (Al2(SO4)3) or lime (Ca(OH)2)
Al2(SO4)3 + 2PO43- → 2AlPO4(S) + 3SO42-

27. Hazardous waste
A listing of substances and industrial process (Code of Federal Regulations) (Table 6.9)
Any of the following characteristics
Ignitable: vapor ignites at low temperature (<60oC)
Corrosive: pH <2 or pH >12.5
Reactive: unstable under normal conditions
Toxic: Toxicity Characteristics Leaching Procedure (TCLP) test

28. Hierarchy of priorities in hazardous waste management
Figure: 06-23

29. The Superfund remedial process
Figure: 06-24

30. Hazardous waste treatment technologies: Remediation technologies
Treatment technologies vs. hazardous wastes (Table 6.11)
Chemical, biological and physical wastewater treatment processes
Thermal destruction processes
Fixation/stabilization

31. Physical treatment Sedimentation
Adsorption: granular activated carbon (GAC)
Aeration
Reverse osmosis
Ion exchange
Electrodialysis
Monitored natural attenuation (MNA)

32. Vacuum filter
Figure: 06-25

33. Air stripping tower
Figure: 06-26

34. Chemical treatment Neutralization: lime (Ca(OH)2) or acid
Chemical precipitation: lime, caustic soda (NaOH)
Chemical reduction-oxidation: reductants/oxidants
UV radiation/oxidation: AOPs
O3 + hv + H2O → H2O2 + O2
H2O2 + hv → 2OH·

35. Neutralization
Figure: 06-27

36. Chemical Precipitation of metals vs. pH
Figure: 06-28

37. Destruction of chlorinated organics using O3 with or without UV
Figure: 06-29

38. Biological treatment: Aqueous waste treatment

39. Rotary kiln hazardous waster incinerator
Waste incineration:
Rotary kiln hazardous waster incinerator
Figure: 06-31

40. Land disposal Landfills Surface impoundments Waste piles
Injection wells
Land treatment facilities
Salt domes or salt bed formation
Underground mines or caves
Concrete vaults or bunkers

41. Landfill
Figure: 06-32

42. Hazardous waste injection well
Figure: 06-33