Water treatment

1
Water treatment
2
Outline

• Industrial water treatment
• Drinking water treatment
• Wastewater treatment

3
Industrial water treatment

• Raw water is rarely suitable for industrial use.
• Raw water contains contaminants, which if not
  removed
• cause corrosion and scaling in boilers and heat
  exchangers
• can effect product quality

4
Types of industrial water treatment

• Coagulation, flocculation, and settling
• Filtration
• Demineralization
• ion exchange
• reverse osmosis
• Cooling water and cooling water treatment
• Boiler water and boiler water treatment

5
Coagulation, flocculation, and settling

• Removes suspended solids from raw water.

coagulant
flocculant
effluent
acid or base
raw water
sludge removal
Microfloc continues to coalesce until large
macrofloc appear.
Coagulant neutralizes charge. Microfloc forms.
Microfloc settles in the sedimentation basin.
Clear supernatant leaves the basin.
Surface charges cause particles to repel one
another.
6
Coagulation, flocculation, and settling

• Whats in it for us?

7
Filtration

• Removes suspended solids from raw water.

Membrane filter
Media filter
influent
raw water
Filter media (gravel, sand, anthracite, etc.)
capture suspended particles. Captured particles
are removed by backwashing the filter.
polymer membrane
filtered water
effluent
8
Filtration

• Whats in it for us?

Membrane filter
Media filter
influent
raw water
?
polymer membrane
filtered water
effluent
9
Softening

• Softening is the process of removing hardness
  ions (calcium and magnesium) from water.
• Hardness ions are objectionable because they tend
  to form scale on heat transfer surfaces.
• Two types of softening
• Sodium softening
• Softening by chemical treatment
• Lime softening
• Soda-lime softening

10
Sodium softening
Calcium chloride
Sodium softener removes calcium and magnesium
ions and replaces them with sodium.
Sodium chloride
11
Sodium softening
Nothing!

• Whats in it for us?

Calcium chloride
Sodium softener removes calcium and magnesium
ions and replaces them with sodium.
Sodium chloride
12
Chemical softening
Calcium and magnesium precipitate as carbonates
and hydroxides. The precipitate settles and the
softened water leaves the settling basin.
lime or lime and soda ash
raw water
acid
softened water
sludge removal
13
Chemical softening

• Whats in it for us?

lime or lime and soda ash
raw water
acid
softened water
sludge removal
14
Demineralization

• Remove all or nearly all of the contaminants in
  raw water.
• The contaminants are
• Calcium and magnesium hardness
• Bicarbonate and carbonate alkalinity
• Common methods of demineralization
• Reverse osmosis
• Ion exchange

15
Reverse Osmosis
Pressure pushes water through the membrane.
Dissolved solids remain behind. Rejection of
solids is typically 97 to 99. Recovery (permeate
flow/feedwater flow) is about 50 for a single
stage. As the feedwater travels through the
membrane module, the concentration of solids
increases.
16
Reverse Osmosis Module
permeate
reject
membrane
porous backing
feed
membrane
membrane spacer
17
Pretreatment for reverse osmosis

• Prevent membrane fouling
• Filtration removes suspended solids
• Softening remove calcium and barium to prevent
  precipitation of slightly soluble calcium and
  barium sulfate and carbonates on the membrane.
• pH adjustment increases solubility of
  carbonates preventing them from precipitating.
• Chlorination and filtration remove
  microorganisms to prevent biofouling on the
  membrane.
• Prevent chemical damage to the membrane
• Dechlorination remove chlorine to prevent it
  from oxidizing and damaging RO membrane.

18
Typical RO plant
NaOCl
raw water
filter
micron filter
Na softener
dechlorination
NaHSO3
pre-treated water (RO feedwater)
two-stage RO unit
feedwater
combined permeate
permeate
reject
permeate
reject
19
Typical RO plant

• Whats in it for us?

NaOCl
raw water
filter
micron filter
Na softener
dechlorination
NaHSO3
T
Cl
ORP
or
pre-treated water (RO feedwater)
two-stage RO unit
C
feedwater
combined permeate
permeate
C
C conductivity pH ORP Cl chlorine T turbidity
reject
pH
C
permeate
C
reject
20
Ion Exchange

• Ion exchange resin bead

polymer backbone
polymer backbone
counter ion
functional group
21
Ion exchange

• Cation exchange resin
• Negatively charged functional groups
• Positively charged counter ions
• Anion exchange resin
• Positively charged functional groups
• Negatively charged counter ions
• Any ion in solution having a greater affinity for
  the functional group than the present counter ion
  will exchange with the counter ion.

22
Ion exchange
Cation exchange
Anion exchange
Ion exchange site in the H form
Na ion in solution
Ion exchange site in the OH- form
Cl- ion in solution
Na (aq)
Na
H (aq)
23
Ion exchange demineralizer
Ion Exchange Demineralizer- Service Run
Na Cl-
Mixed bed removes the small amount ion leakage
out of the cation and anion exchangers.
cation
anion
mixed bed
R OH-
R H
product water
H OH-
H Cl-
H2O
24
Ion exchange demineralizer

• Whats in it for us?

Na Cl-
C
cation
anion
mixed bed
R OH-
R H
C
SiO2
C
product water
H OH-
H Cl-
H2O
25
Ion exchange demineralizer
Makeup Demineralizer - Regeneration
acid day tank
dilution water
dilution water
Regeneration is the ion exchange process in
reverse.
H
OH-
anion exchanger
cation exchanger
R Na
R Cl-
26
Ion exchange demineralizer

• Whats in it for us?

acid day tank
caustic day tank
dilution water
dilution water
C
C
concn
concn
cation exchanger
anion exchanger
C
C
27
Cooling Water
28
Cooling water system
warm water return
heat exchangers
cooling tower
blowdown
makeup water
cool water supply
29
Cooling tower

• Evaporation cools the returning water.
• Evaporation also increases the concentration of
  dissolved solids.
• High concentration of dissolved solids leads to
  corrosion.
• If concentration gets high enough solids
  precipitate and accumulate on heat exchange
  surfaces.

fan
return warm water
air
packing
cool water supply to plant
basin
30
Heat exchanger
process liquid

• Efficient heat transfer requires clean tube
  surfaces.

31
Cooling water treatment

• Control scale
• Scale is a hard, tenacious deposit that forms on
  metal surfaces, primarily where heat exchange
  occurs.
• Control corrosion
• Corrosion is wastage of metal caused by reaction
  with substances in the process fluid in contact
  with the metal.
• Corrosion can be uniform or localized (pitting).
• Control biofouling
• Biofouling is the slime produced by
  micoroorganism colonies, their secretions, and
  the debris that gets embedded in the mass.

32
Scale and corrosion

• Solids restrict free flow of liquid through
  pipes.
• Scale forms in high heat transfer zones in heat
  exchange equipment.
• Scale interferes with efficient heat transfer.
• Excessive scale buildup leads to less efficient
  heat transfer.
• Excessive scale buildup also leads to
  under-deposit corrosion and eventually leaks.

scale
tube wall
tube wall
33
Cooling water chemical treatment

• Control scale
• What causes CaCO3 scale?
• Reduce calcium hardness by using softened water
  for makeup.
• Reduce alkalinity and lower pH by adding sulfuric
  acid.
• Keep calcium hardness and alkalinity in the
  cooling water below the concentration at which
  calcium carbonate scale begins to form.
• Control calcium sulfate scaling by adding a
  dispersant.

calcium hardness alkalinity (HCO3-) alkaline
(high) pH
CaCO3
34
Cooling water chemical treatment

• Control corrosion
• What causes corrosion?
• Keep pH in an optimum range for the system
  metallurgy.
• Control total concentration of dissolved solids
  in the cooling water.
• Add corrosion inhibitors to the cooling water.

high concentration of dissolved salts low pH
corrosion
35
Cooling water chemical treatment

• Control biofouling
• What causes biofouling?
• Treat cooling water with a biocide.
• Oxidizing biocides chlorine, bromine, ozone,
  etc.
• Non-oxidizing biocides

microorganisms (bacteria, fungi,
algae) nutrients oxygen warmth
biofouling
36
Cooling water system

• Whats in it for us?

warm water return
heat exchangers
cooling tower
blowdown
makeup water
cool water supply
37
Cooling water system

• Whats in it for us?

warm water return
Cl
pH
C
cooling tower
T
blowdown
makeup water
38
Measuring chlorine in cooling water

• Maintenance
• Open cooling water often has high levels of
  suspended solids which foul the sensor.
• Sensor may require cleaning every few days.
• Filtration???

to FCL or FCLi
cooling water sample
filter
ppm chlorine
chlorine demand across filter
39
Measuring chlorine in cooling water

• Interferences
• The 499ACL-01 and 498Cl-01 sensors are intended
  for use in drinking water.
• Cooling water treatment chemicals can interfere.
• Wrong chemical
• The 499ACL-01 and 498CL-01 sensors are for free
  chlorine only.
• Many cooling water biocides use bromine or a
  mixture of bromine and chlorine.
• The 499ACL-01 and 498CL-01 sensors will not work
  in this application.
• The only choice is the TCL, which measures total
  oxidants.

40
Boiler Water
41
Boilers

• Typical low pressure industrial boiler system...

42
Boiler water circuit
saturated steam
steam separates from water in the steam drum
steam drum
superheater
economizer
steam bubbles form as the water absorbs heat
feedwater
downcomer
water tubes
water drum
43
Boiler water treatment

• Control scale formation
• Control corrosion

44
Scaling and corrosion
tube wall

• As steam escapes, the concentration of solids in
  the boiler water increases.
• When concentration exceeds the solubility, the
  solid precipitates and clings to the tube
  surface.
• Deposition leads to reduced heat transfer
  efficiency and overheating.
• Deposition also leads to underdeposit corrosion.
• Overheating and corrosion eventually lead to tube
  failure.

under deposit corrosion
deposit
water
clean surface
waterwall tube
45
Controlling scale

• Boiler scale has two sources
• Precipitation of contaminants that have
  accumulated and concentrated in the boiler.
• Calcium, magnesium, alkalinity, sulfate, silica,
  and iron
• Deposition of corrosion products formed in the
  condensate and feedwater system

steam
boiler
condensate returns
precipitation of soluble contaminants
makeup water
corrosion products contaminants
46
Controlling boiler scale

• Reduce or eliminate the contaminants that cause
  scale.
• Use soft water for makeup.
• Use dealkalized water for makeup.
• Use demineralized water for makeup.
• Control contaminant concentration in the boiler
• Keep concentration of contaminants low enough
  that precipitation is unlikely to occur
  (blowdown).
• Use treatment chemicals to keep precipitates in
  suspension.
• Use treatment chemicals to chelate (tie up) small
  amounts of hardness and prevent it from becoming
  scale.
• Control corrosion in the condensate and
  feedwater.
• Control corrosion in the steam lines.

47
Controlling corrosion

• Add chemicals to control pH.
• At alkaline pH, a protective oxide film forms on
  mild steel surfaces.
• The film protects the base metal from further
  corrosion.
• As long as the water remains alkaline, the film
  will stay in place and cracks in the film will be
  automatically repaired.
• Control concentration of dissolved salts in the
  boiler water (blowdown).

48
Boilers

• Whats in it for us?

steam
boiler
condensate returns
makeup water
49
Boilers

• Whats in it for us?

pH
C
pH
steam
C
pH
C

DO
boiler
condensate returns
makeup water
C
For systems with deaeraters.
50
Drinking water treatment
51
The goal
customers
raw water
treatment plant
Ensure that the water leaving the customers tap
is suitable for drinking.
Convert raw water into potable water (water
suitable for drinking).
52
To make potable water

• Kill or inactivate harmful microorganisms in the
  raw water.

53
Elements of drinking water treatment

• Filtration
• Disinfection

54
Filtration

• Removes suspended matter
• What does filtration accomplish?
• Improved aesthetics
• Removes some amount of bacteria, viruses, and
  protozoan cysts.
• Removes suspended particles that harbor
  microorganisms and protects them from attack by
  disinfectants.

55
Disinfection

• Kills or inactivates the microorganisms remaining
  after filtration.
• Common disinfectants are chlorine,
  monochloramine, and ozone.
• In the USA a disinfection residual must be
  present in the distribution system. Therefore,
  ozone cannot be used as a final disinfectant.
• The effectiveness of a disinfectant depends on
• Concentration
• Contact time
• Temperature
• pH (chlorine only)

56
How do you know the water is good to drink?

• The turbidity is within the regulatory limit.
• The product of the disinfectant concentration and
  the contact time is equal to or greater than the
  regulatory limit . The limit depends on
  temperature and pH.
• The utility also performs tests for harmful
  organisms, but these take a long time to
  complete.

57
Drinking water filter plant
ozone destruct
coagulant/flocculant
screens
settling basin
raw water
mixer
ozone generator
filters
ozone contact chamber
backwash pump
NaOCl
contact tank
backwash reclaim pond
to screens
58
Drinking water filter plant

• Whats in it for us?

coagulant/flocculant
screens
settling basin
raw water
T
T
OZ
mixer
ozone generator
filters
ozone contact chamber
backwash pump
NaOCl
Cl
pH
Cl
T
T
contact tank
backwash reclaim pond
to screens
59
Distribution system

• Whats in it for us?

chlorine
Cl
Cl
chlorine
In the USA there must be a chlorine residual at
the customers tap.
Cl
60
A few more words about chlorine and drinking
water
61
The good news about chlorine

• Chlorination of public water supplies works.

0.00013
62
The bad news about chlorine

• Chlorine reacts with naturally occurring organic
  compounds in drinking water to produce
  disinfection by-products (DBPs).

Chlorine reacts with precursors to form DBPs in
the distribu-tion system.
chlorine
filter plant
Precursor chemicals occur naturally in many
surface water sources.
63
The bad news about chlorine (contd)

• Whats bad about DBPs?
• Large doses of certain DBPs cause cancer and
  reproductive harm in laboratory animals.
• No conclusive evidence that drinking chlorinated
  water causes cancer or reproductive harm in
  humans.
• EPA has established limits for certain DBPs in
  drinking water.
• THMs (trihalomethanes)
• HAAs (haloacetic acids)

64
So, youve got THMs and HAAs

• Remove the precursor chemicals
• Carefully control chlorine levels, use just the
  minimum chlorine required
• Use a different disinfectant, i.e. monochloramine
  (NH2Cl)

65
Advantages of monochloramine

• Does not form THMs and HAAs.
• Is more persistent than free chlorine in the
  distribution system.

66
Disadvantages of monochloramine

• Monochloramine is a poorer disinfectant than
  chlorine. It requires higher dosages to be
  effective (2 3 ppm).
• When certain precursor chemicals are present,
  monochlor-amine can form potentially harmful
  DBPs.

67
Making monochloramine...
NaOCl or Cl2
NH3
NH2Cl
drinking water
5 ppm chlorine (as Cl2) requires 1 ppm ammonia
(as N).
68
Wastewater treatment
69
The goal
storage reservoir for reclaimed wastewater
wastewater treatment plant
Remove nutrients and harmful micro-organisms from
wastewater so that discharging it to the
environment does no harm.
discharge to receiving water
70
Elements of wastewater treatment

• Remove solids
• Lower BOD (biochemical oxygen demand)
• Reduce nitrogen and phosphorous (optional)
• Disinfect final effluent

71
Wastewater treatment
remove nitrogen and phosphorus
kill pathogens
remove solids
reduce BOD
72
Wastewater treatment
If no BOD removal
untreated wastewater
Untreated wastewater is biodegradable.
Microorganisms in the receiving water use the
wastes for food. The more waste, the greater the
population. Microorganisms consume oxygen as they
grow and multiply (BOD). The oxygen supply in the
lake drops. Fish die. Aquatic plants die. The
lake dies.
73
Aeration basin
secondary sedimentation tank
aeration basin
blower
return activated sludge
Microorganisms (called activated sludge) in the
aeration basin convert organic compounds in the
wastewater into carbon dioxide and biomass, thus
reducing BOD. The aeration system provides the
microorganisms with the oxygen they need for
metabolism.
74
Disinfection step
storage reservoir for reclaimed wastewater
(usually no dechlorination required)
wastewater treatment plant
sodium bisulfite
chlorine
treated effluent
chlorine contact basin
discharge to receiving water (requires
dechlorination)
75
Wastewater treatment

• Whats in it for us?

remove nitrogen and phosphorus
kill pathogens
remove solids
reduce BOD
76
Disinfection step
storage reservoir for reclaimed wastewater
(usually no dechlorination required)
wastewater treatment plant
sodium bisulfite
chlorine
treated effluent
chlorine contact basin
discharge to receiving water (requires
dechlorination)
77
Wastewater treatment

• Whats in it for us?

remove nitrogen and phosphorus
kill pathogens
remove solids
reduce BOD
DO
primary settling
secondary settling
DO
Cl
N and P removal
raw sewage
disinfection
aeration
pH
return activated sludge
solids treatment
C
78
Disinfection step
storage reservoir for reclaimed wastewater
(usually no dechlorination required)
wastewater treatment plant
sodium bisulfite
chlorine
Cl
Cl
treated effluent
chlorine contact basin
discharge to receiving water (requires
dechlorination)
Cl