1. Water Treatment

2. Reflections What are the two broad tasks of environmental engineers?
What is the connection between the broad tasks of environmental engineers and building a water treatment plant?
Why may the water need to be changed/treated?

3. Simple Sorting Goal: clean water Source: (contaminated) surface water
Solution: separate contaminants from water
How?

4. Where are we going? Unit processes* designed to particles
remove ___________
remove __________ ___________
inactivate __________
*Unit process: a process that is used in similar ways in many different applications
sedimentation
filtration
...
particles
dissolved chemicals
pathogens

5. Unit Processes Designed to Remove Particulate Matter
Screening
Sedimentation
Coagulation/flocculation
Filtration
slow sand filters
rapid sand filters
diatomaceous earth filters
membrane filters

6. Conventional Surface Water Treatment
Raw water
Screening
Filtration
sludge
sludge
Alum
Polymers
Coagulation
Disinfection
Cl2
Flocculation
Storage
Sedimentation
Distribution
sludge

7. Screening Removes large solids Simple process
logs
branches
rags
fish
Simple process
may incorporate a mechanized trash removal system
Protects pumps and pipes in WTP

8. Sedimentation the oldest form of water treatment
uses gravity to separate particles from water
often follows coagulation and flocculation
occurs in NYC’s __________
reservoirs

9. Sedimentation: Effect of the particle concentration
Dilute suspensions
Particles act independently
Concentrated suspensions
Particle-particle interactions are significant
Particles may collide and stick together (form flocs)
Particle flocs may settle more quickly
Particle-particle forces may prevent further consolidation

10. How fast do particles fall in dilute suspensions?
What are the important parameters?
Initial conditions
After falling for some time...
What are the important forces?
_________
__________
Gravity
Fluid drag

11. Sedimentation: Particle Terminal Fall Velocity
Identify forces
projected

12. Particle Terminal Fall Velocity (continued)
Force balance (zero acceleration)
We haven’t yet assumed a shape
Assume a _______
sphere

13. Drag Coefficient on a Sphere
Stokes Law
turbulent boundary
laminar
turbulent

14. Drag Coefficient: Equations
General Equation
Laminar flow R < 1
Transitional flow 1 < R < 104
Fully turbulent flow R > 104

15. Example Calculation of Terminal Velocity
Determine the terminal settling velocity of a cryptosporidium oocyst having a diameter of 4 mm and a density of 1.04 g/cm3 in water at 15°C [m=1.14x10-3 kg/(s•m)].
Work in your teams.
Use mks units (meters, kilograms, seconds).
Convert your answer to some reasonable set of units that you understand.
Solution
Reynolds?

16. Sedimentation Basin long rectangular basins 4-6 hour retention time
3-4 m deep
max of 12 m wide
max of 48 m long
Settling zone
Sludge zone
Inlet zone
Outlet zone
Sludge out
We can’t do this in our laboratory scale plants!

17. Sedimentation Basin: Critical Path
Horizontal velocity
Q = flow rate
Outlet zone H
Inlet zone
A = WH
Sludge zone
Vertical velocity L
Sludge out
What is Vc for this sedimentation tank?

18. Sedimentation Basin: Importance of Tank Surface Area
Time in tank W H L
Want a _____ Vc, ______ As, _______ H, _______ q.
small
large
small
large
Suppose water were flowing up through a sedimentation tank. What would be the velocity of a particle that is just barely removed?

19. Lamella
Sedimentation tanks are commonly divided into layers of shallow tanks (lamella)
The flow rate can be increased while still obtaining excellent particle removal
Lamella decrease distance particle has to fall in order to be removed

20. Design Criteria for Sedimentation Tanks
Settling zone
Sludge zone
Inlet zone
Outlet zone
Design Criteria for Sedimentation Tanks
_______________________________
Minimal turbulence (inlet baffles)
Uniform velocity (small dimensions normal to velocity)
No scour of settled particles
Slow moving particle collection system
Q/As must be small (to capture small particles)
This will be one of the ways you can improve the performance of your water treatment plant.

21. Sedimentation of Small Particles?
How could we increase the sedimentation rate of small particles?
Increase d (stick particles together)
Increase g (centrifuge)
Increase density difference
(dissolved air flotation)
Decrease viscosity (increase temperature)

22. Particle/particle interactions
Electrostatic repulsion
In most surface waters, colloidal surfaces are negatively charged
like charges repel __________________
van der Waals force
an attractive force
decays more rapidly with distance than the electrostatic force
is a stronger force at very close distances
stable suspension

23. Energy Barrier Increase kinetic energy of particles
Electrostatic
Energy Barrier
Increase kinetic energy of particles
increase temperature
stir
Decrease magnitude of energy barrier
change the charge of the particles
introduce positively charged particles
Layer of counter ions + + +
van der Waals

24. Coagulation
Coagulation is a physical-chemical process whereby particles are destabilized
Several mechanisms
adsorption of cations onto negatively charged particles
decrease the thickness of the layer of counter ions
sweep coagulation
interparticle bridging

25. Coagulation Chemistry
The standard coagulant for water supply is Alum [Al2(SO4)3*14.3H2O]
Typically 5 mg/L to 50 mg/L alum is used
The chemistry is complex with many possible species formed such as AlOH+2, Al(OH)2+, and Al7(OH)17+4
The primary reaction produces Al(OH)3
Al2(SO4)3 + 6H2O2Al(OH)3 + 6H+ + 3SO4-2
pH = -log[H+]

26. Coagulation Chemistry
Aluminum hydroxide [Al(OH)3] forms amorphous, gelatinous flocs that are heavier than water
The flocs look like snow in water
These flocs entrap particles as the flocs settle (sweep coagulation)

27. Coagulant introduction with rapid mixing
The coagulant must be mixed with the water
Retention times in the mixing zone are typically between 1 and 10 seconds
Types of rapid mix units
pumps
hydraulic jumps
flow-through basins with many baffles
In-line blenders

28. Flocculation
Coagulation has destabilized the particles by reducing the energy barrier
Now we want to get the particles to collide
We need relative motion between particles
Brownian motion is too slow
_________ _____________ rates
__________ shears the water
Differential sedimentation
Turbulence

29. Flocculation Turbulence provided by gentle stirring
Turbulence also keeps large flocs from settling so they can grow even larger!
High sedimentation rate of large flocs results in many collisions!
Retention time of minutes

30. Coagulation/Flocculation
Inject Coagulant in rapid mixer
Water flows from rapid mix unit into flocculation tank
gentle stirring
flocs form
Water flows from flocculation tank into sedimentation tank
make sure flocs don’t break!
flocs settle and are removed

31. Jar Test
Mimics the rapid mix, coagulation, flocculation, sedimentation treatment steps in a beaker
Allows operator to test the effect of different coagulant dosages or of different coagulants

32. Unit Processes in Conventional Surface Water Treatment
We’ve covered
Sedimentation
Coagulation/flocculation
Coming up!
Filtration
Disinfection
Removal of Dissolved Substances

33. Conventional Surface Water Treatment
Raw water
Screening
Filtration
sludge
sludge
Alum
Polymers
Coagulation
Disinfection
Cl2
Flocculation
Storage
Sedimentation
Distribution
sludge

34. Filtration Slow sand filters Diatomaceous earth filters
Membrane filters
Rapid sand filters (Conventional Treatment)

35. Slow Sand Filtration First filters to be used on a widespread basis
Fine sand with an effective size of 0.2 mm
Low flow rates ( cm/hr)
Schmutzdecke (_____ ____) forms on top of the filter
causes high head loss
must be removed periodically
Used without coagulation/flocculation!
filter cake

36. Diatomaceous Earth Filters
Diatomaceous earth (DE) is made of the silica skeletons of diatoms
DE is added to water and then fed to a special microscreen
The DE already on the microscreen strains particles and DE from the water
The continuous DE feed prevents the gradually thickening DE cake from developing excessive head loss
Was seriously considered for Croton Filtration Plant

37. Membrane Filters
Much like the membrane filters used to enumerate coliforms
much greater surface area
Produce very high quality water (excellent particle removal)
Clog rapidly if the influent water is not of sufficiently high quality
More expensive than sand and DE filters

38. Rapid Sand Filter (Conventional US Treatment)
Specific
Gravity
1.6
2.65
Depth
(cm) 30 45
Size
(mm)
0.70
5 - 60
Anthracite
Influent
Sand
Gravel
Drain
Effluent
Wash water

39. Particle Removal Mechanisms in Filters
Transport
Molecular diffusion
Inertia
Gravity
Interception
Attachment
Straining
Surface forces

40. Filter Design Filter media Flow rates smaller Backwash rates
silica sand and anthracite coal
non-uniform media will stratify with _______ particles at the top
Flow rates
m/hr
Backwash rates
set to obtain a bed porosity of 0.65 to 0.70
typically 50 m/hr
smaller

41. Backwash Wash water is treated water! WHY? Anthracite
Only clean water should ever be on bottom of filter!
Sand
Influent
Gravel
Drain
Effluent
Wash water

42. Disinfection
Disinfection: operations aimed at killing or ____________ pathogenic microorganisms
Ideal disinfectant
_______________
inactivating
Toxic to pathogens
Not toxic to humans
Fast rate of kill
Residual protection
Economical

43. Disinfection Options Chlorine Ozone Irradiation with Ultraviolet light
chlorine gas
sodium hypochlorite (bleach)
Ozone
Irradiation with Ultraviolet light
Sonification
Electric Current
Gamma-ray irradiation
Poisonous gas – risk of a leak

44. Chlorine
First large-scale chlorination was in 1908 at the Boonton Reservoir of the Jersey City Water Works in the United States
Widely used in the US
Typical dosage (1-5 mg/L)
variable, based on the chlorine demand
goal of 0.2 mg/L residual
Trihalomethanes (EPA primary standard is 0.08 mg/L)
Chlorine oxidizes organic matter
Pathogen/carcinogen tradeoff

45. Chlorine Reactions Cl2 + H2O  H+ + HOCl + Cl- HOCl  H+ + OCl-
Charges +1 -2 +1 -1
Cl2 + H2O  H+ + HOCl + Cl-
HOCl  H+ + OCl-
The sum of HOCl and OCl- is called the ____ ______ _______
HOCl is the more effective disinfectant
Therefore chlorine disinfection is more effective at ________ pH
HOCl and OCl- are in equilibrium at pH 7.5
Hypochlorous acid
Hypochlorite ion
free chlorine residual
low

46. EPA Pathogen Inactivation Requirements
Safe Drinking Water Act
SDWA requires 99.9% inactivation for Giardia and 99.99% inactivation of viruses
Giardia is more difficult to kill with chlorine than viruses and thus Giardia inactivation determines the CT
Concentration x Time
Enumerating Giardia is difficult, time-consuming and costly.
How would you ensure that water treatment plants meet this criteria?

47. EPA Credits for Giardia Inactivation
Treatment type Credit
Conventional Filtration 99.7%
Direct Filtration* 99%
Disinfection f(time, conc., pH, Temp.)
* No sedimentation tanks

48. Disinfection CT Credits
To get credit for 99.9% inactivation of Giardia:
Contact time (min)
chlorine pH pH 7.5
(mg/L) 2°C 10°C 2°C 10°C
Inactivation is a function of _______, ____________
______, and ___________.
time
concentration pH
temperature

49. NYC CT? Kensico Delaware Pipeline volume =603,000 m3
21.75 km long
5.94 m diameter
peak hourly flow
= 33 m3/s
volume =603,000 m3
5 hour residence time!
Hillview
3.4 x 106 m3

50. NYC CT Problem Hillview Reservoir is an open reservoir
Should the chlorine contact time prior to arrival at Hillview count?
Giardia contamination from Upstate Reservoirs will be decreased, but
recontamination at Hillview is possible

51. Ozone Widely used in Europe O3 is chemically unstable
Must be produced on site
More expensive than chlorine (2 - 3 times)
Typical dosages range from 1 to 5 mg/L
Often followed by chlorination so that the chlorine can provide a protective _______
residual

52. Removal of Dissolved Substances (1)
Aeration (before filtration)
oxidizes iron or manganese in groundwater
oxidized forms are less soluble and thus precipitate out of solution
removes hydrogen sulfide (H2S)
Softening (before filtration)
used to remove Ca+2 and Mg+2
usually not necessary with surface waters

53. Removal of Dissolved Substances (2)
Activated Carbon (between filtration and disinfection)
extremely adsorbent
used to remove organic contaminants
spent activated carbon can be regenerated with superheated steam
Reverse Osmosis
semi-permeable membrane allows water molecules to pass, but not the larger ions and molecules
primarily used for desalination
also removes organic materials, bacteria, viruses, and protozoa

54. Conventional Surface Water Treatment
Raw water
Screening
Filtration
sludge
sludge
Alum
Polymers
Coagulation
Disinfection
Cl2
Flocculation
Storage
Sedimentation
Distribution
sludge

55. Summary Backwash Lagoon Clearwell Filtration Flocculation Rapid Mix
Sedimentation
Rapid Mix

56. Cryptosporidium Oocyst

57. Reynolds Number Check R<<1 and therefore in Stokes Law range
R = 1.1 x 10-6
R<<1 and therefore in Stokes Law range

58. Diatomaceous Earth
Clay DE