Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
0
Global water aspects
1
Introduction to urban water management
2
Basics for systems description
3
Water transport
4
Matter transport
5
Introduction to water supply
6
Water extraction
7
Water purification
8
Water distribution
9
Introduction to wastewater disposal
10 Urban drainage
11 Wastewater treatment
12 Sludge treatment
Peter Krebs
Dresden, 2010
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 2
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 3
Two major steps of water purification
Turbid raw
water
Removal of
particles
Urban Water
Pre-treated
water
Drinking
water
Enhanced treatment, stabilisation,
disinfection
Chapter 7 Water purification
© PK, 2010 – page 4
Tasks and processes; overview
Task
Conventional Process
New approaches
(partly in development)
Removal of particles
Flocculation, sedimentation,
flotation, filtration, bank filtration,
groundwater enrichment
Micro-filtration, ultrafiltration, ultrasonic Plankton
Removal of dissolved
inorganic substances
Decarbonisation,
flocculation/precipitation, ion
exchange, Gas exchange
Reverse osmosis, nanofiltration, special ion
exchange processes, specific
adsorption technologies
Removal / destruction of
organic substances
Flocculation, ozonisation, active
carbon filtration, biological
filtration, sub surface passage, gas
exchange
Nano- and ultra-filtration,
enhanced oxidation processes
Removal / inactivation /
destruction of microorganisms
Disinfection with ozone, chlorine,
chlorine dioxide; sub surface
passage and particle removal
UV-radiation, Micro-filtration,
ultra-filtration
(DVGW Handbook of water supply, Vol. 6)
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 5
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 6
Goals of gas transfer
• Export of CO2, for de-acidification
• Export (purging) of volatile substances, such as H2S, CH4
• Stripping of volatile organic compounds
• Import of oxygen for oxidation of dissolved compounds
(e.g. ferric substances, manganese, ammonia) and to improve
taste and development of protection layers
• Introduction of ozone
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 7
Transfer process description
HENRY-DALTON law
cS =  · cG
cS
cG

Import flux
Fin  DcS  c 
Urban Water
cS  c
Gas saturation concentration in water
Gas concentration in air
Solubility coefficient = f (t, pG)
Export flux
Fex  DcS  c 
Chapter 7 Water purification
cS  c
© PK, 2010 – page 8
Aeration with free surface
(® Aquadosil)
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 9
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 10
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 11
Removal of particles: overview
Sieves
Removal of coarse particles
Sedimentation
Removal of easy-to-settle and flocculated particles
quiescent, undisturbed flow conditions
Filtration
sand layer passage
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 12
Removal of coarse particles
Rack
only with surface waters
wide and narrow slots
Micro sieves
Steel or textile
grid size < 0,1 mm,
continuous back rinsing







Urban Water
Raw water
Clear water tank
Effluent
Micro sieve
Rotating cylinder
Rinsing
Driver, controlled
Chapter 7 Water purification
© PK, 2010 – page 13
Micro sieve
Source: Bodenseewasserversorgung
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 14
Particle concentration (gTSS/m3)
Processes to remove particles
1000
Rack
Sedimentation
Flocculation
10
Sieves
Filtration
0.1
10-5
Viruses
Urban Water
10-3
Bacteria
10-1
Algae
Chapter 7 Water purification
10
Particle size (mm)
© PK, 2010 – page 15
Flocculation
Mixing
Flocculation
De-stabilisation,
Aggregation and
generation of micro-flocs generation of macro-flocs
Separation
Sedimentation, flotation,
filtration
Enhanced treatment
Filtration
ev. flocculation aid
Flocculant
Sludge Treatment
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 16
Sedimentation
Q
Sedimentation Length LS
Q
Length of tank
 Inlet region
 Sediments
Urban Water
 Effluent region
 Sedimentation region
Chapter 7 Water purification
© PK, 2010 – page 17
Fast filtration
·
often layered (two or more layers)
top:
coarse, light material (e.g. pumice stone 1,4 – 2,5 mm)
bottom: fine, heavy material (e.g. sand, 0,71 – 1,25 mm)
·
hydraulic load 4 – 30 m/h
·
Pressure head 2 – 5 m
·
Quick increase of resistance an pressure head decrease;
clogging of fine material is faster,
·
Back rinsing with water, air, and water/air
·
Pressure filtration for smaller filter areas
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 18
Open fast filter
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 19
Filtration effect and resistance


critical filtrate conc. ccr
t1
Filter operation time t


Urban Water
Filter resistance p (hF in m)
Filtrate concentration c
in TE/F or mg/l
Threshold values pcr (hF,cr)
t2
Filtrate concentration
Filter resistance
Chapter 7 Water purification
© PK, 2010 – page 20
Back rinsing
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 21
Slow filtration (i)
 Removal of
particles
microbial load
bio-degradable substances
• Sieve effect
• Adsorptive effect over entire filter depth
• „Mud cover“, a few cm, biologically active
Organic compounds are degraded
Ammonia is nitrified
• Prerequisite
O2-concentration is sufficient
small TSS-concentration
• Surface overflow rate 0,1 – 0,2 m/h, pressure head ca. 1 m
• 3 – 24 months removal of „mud cover“
• large area necessary
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 22
Slow filtration (ii)
Filtration
Raw water
Mud layer,
bio-active !
Sand filter,
  0.6 mm, H > 0.5 m
Sand filter, layered with
increasing diameter
towards bottom
Drainage
bottom
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 23
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 24
Removal of Ferric and Manganese
Contact with O2
Fe2+
Fe3+
High
solubility
Low solubility
Rosty precipitation
 Filtration
Manganese
Similar to ferric substances, Oxidation through micro-organisms
or catalytic reaction
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 25
Active carbon adsorption
 Removal of dissolved organic compounds
microscopic pores through glowing of coal at 650°C, as a
significant part is oxidised and volatilised as CO2
huge internal surface: 1000 – 2000 (m2/g active carbon)
active carbon is sensitive on mechanical stress
rinsing as seldom as possible via pre-treatment and
separation of particles before the activated carbon process
microorganisms on the surface  bio-degradation of organic
compounds
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 26
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 27
De-acidification
Drinking water treatment
Neutralisation
CO2
CaCO3
Acid
Lime, calcium carbonate
Aggressive, corrosive
Precipitation, encrusting
 Aeration
 Filtration through lime
 Adding hydroxides, soda
Urban Water
Chapter 7 Water purification
 Precipitation
 Reduction of solubility
 Ion exchange
© PK, 2010 – page 28
lime – carbonic – acid – system
KS4,3
Urban Water
KB8,2
Chapter 7 Water purification
© PK, 2010 – page 29
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 30
Disinfection
 Inactivation of pathogenic germs
Chemical
oxidation
O3
fast decay, no by-products, Oxidation
Cl2
cheap, Network safety, reactive
ClO2
Network protection, local production
 possibly production of by-products
UV-radiation
In 1 cm layer, for some seconds
No by-products
No network protection
Only for small plants, expensive
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 31
Effect of disinfection means
Chlorine
Chlorine dioxide
Ozone
UV-Radiation
Urban Water
Oxidation
Disinfection
long term effect
Satisfactory
good
good
Bad
satisfactory
satisfactory
Good
good
negative
Bad
satisfactory
negative
Chapter 7 Water purification
© PK, 2010 – page 32
Peter Krebs
Department of Hydro Sciences, Institute for Urban Water Management
Urban Water
7
Water purification
7.1 Concept, overview
7.2 Gas exchange
7.3 Removal of particles
7.4 Removal of colloids and solubles
7.5 Chemical stabilisation
7.6 Disinfection and network protection
7.7 Sludge disposal
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 33
Sludge production
Process
Residues
Sieves, grills,
coarse filters
Rinsing water with gross solids
Flocculation and sedimentation,
filters
Sludge-water mixture with content of e.g. Fe, Mn,
organics and Flocculation aids, e.g. Al, Fe CaCO3
Dosing stations
Dosing chemicals
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 34
Mass in Germany
(Source: Wichmann and Akkiparambath 2001)
Others
(13.000 t, 11 %)
Fe and Mn
Sludges
(13.000 t, 11 %)
CaCO3
residues
(55.000 t, 44 %)
Flocculation sludges
(42.000 t, 34 %)
 ca. 4 % of annual WWTP sludge production (related to DM)
Urban Water
Chapter 7 Water purification
© PK, 2010 – page 35
Disposal in Germany
(Source: Wichmann and Akkiparambath 2001)
WWTP
application (5 %)
Environmental
Technology (6 %)
Disposal (35 %)
Receiving water (7 %)
Industrial
use (7 %)
Land and forestry management
(9 %)
Urban Water
WWTP
treatment (31 %)
Chapter 7 Water purification
© PK, 2010 – page 36