1. WATER TEATMENT AND DESALINATION
Chapter 6
WATER TEATMENT AND DESALINATION

2. 1- Water Treatment Processes
Dr Salam Al-Dawery

3. Water treatment
It is common to treat raw water to produce safe drinking water for the protection of public health, as most raw water quality does not meet safe drinking water standards.
The most cited reasons for treatment water are:
• To remove disease-causing pathogens;
• To remove potentially toxic natural or synthetic substances;
• To remove dissolved and gaseous radioactivity;
• To improve organoleptic quality of water to prevent consumer
rejecting water due to its physical appearance, taste or odour;
• To prevent bacterial after-growth in the distribution system;
• To prevent deposition and silting up of pipes;
• To prevent corrosion and dissolution of pipes and fittings; and
• To comply with local, national and international law on water
quality.

4. Disinfection Any process to destroy or prevent the growth of microbes
Intended to inactivate (destroy the infectivity of) the microbes by physical, chemical or biological processes. Use one of the following:
Free chlorine: HOCl (hypochlorous) acid and OCl- (hypochlorite ion)
strong oxidant; relatively stable in water (provides a disinfectant residual)
ozone, O3: strong oxidant; provides no residual (too volatile, reactive)
Chlorine dioxide, ClO2,: strong oxidant; unstable (dissolved gas)
UV radiation
low pressure mercury lamp: low intensity; monochromatic at 254 nm
medium pressure mercury lamp: higher intensity; polychromatic nm)
Boiling: efficient kill; no residual protection; fuel/environmental costs

5. Summary of Mainline Water Treatment Processes
Storage
Disinfection
Physical: UV radiation, heat, membrane filters
Chemical: Chlorine, ozone, chlorine dioxide, iodine, other antimicrobial chemicals
Filtration
Rapid granular media
Slow sand and other biological filters
Membrane filters: micro-, ultra-, nano- and reverse osmosis
Other physical-chemical removal processes
Chemical coagulation, precipitation and complexation
Adsorption: e.g., activated carbon, bone char, etc,
Ion exchange: synthetic ion exchange resins, zeolites, etc.

6. Typical Surface Water Treatment Plant
Alum
Some of the main treatment processes can be found in Table in following slide. Typical water treatment processes usually comprises of pre-treatment, main treatment and disinfection.

7. Screening

8. Chemical Coagulation-Flocculation1/2
Removes suspended particulate and colloidal substances from water, including microorganisms.
Coagulation: colloidal destabilization
Typically, add alum (aluminum sulfate) or ferric chloride or sulfate to the water with rapid mixing and controlled pH conditions
Insoluble aluminum or ferric hydroxide and aluminum or iron hydroxide complexes form
These complexes entrap and adsorb suspended particulate and colloidal material.

9. Chemical Coagulation-Flocculation1/2
Slow mixing that provides for a period of time to promote the aggregation and growth of the insoluble particles (flocs).
The particles collide, stick together and grow larger
The resulting large floc particles are subsequently
removed by gravity sedimentation (or direct filtration)
Smaller floc particles are too small to settle and are
removed by filtration

10. Granular Media Filtration
Used to remove suspended particles
(turbidity) including microbes.
Two types of granular media filters:
Slow sand filters: uniform bed of sand;
low flow rate
Rapid sand filters: 1, 2 or 3 layers of sand/other media
Diatomaceous earth filters
siliceous sedimentary rock that is easily
crumbled into a fine white to off-white
powder used as filtration aid
fossilized skeletons of diatoms
(crystalline silicate); powdery deposit;

11. Roughing Filter
gravel: rock that is of a specific particle size range
Widely Used
inexpensive
low maintenance
local materials
Remove large solids
Remove microbes
90% turbidity reduction

12. Membrane Filters More recent development and use in drinking water
Microfilters: several tenths of μM to μM diameter pore size
nano- & ultra-filters: retention by molecular weight cutoff
Reverse osmosis filters: pore size small enough to remove dissolved salts; used to desalinate (desalt) water as well as particle removal
High >99.99% removal of cellular microbes
Virus removals high >9.99% in ultra-, nano- filter.

13. Adsorbers and Filter-Adsorbers
Granular activated carbon adsorption
remove dissolved organics
poor retention of pathogens, esp. viruses
biologically active; develops a biofilm
can shed microbes into water
Filter-adsorbers
Sand plus granular activated carbon
reduces particles and organics
biologically active
microbial retention is possible

14. Water Softening and Microbe Reductions
”Hard" Water: contains excessive amounts of calcium and magnesium ions
iron and manganese can also contribute to hardness.
Hardness ions are removed by adding lime (CaO) and sometimes soda ash (Na2CO3) to precipitate them as carbonates, hydroxides and oxides.
This process, called softening, is basically a type of coagulation‑flocculation process.
Microbe reductions similar to alum and iron coagulation when pH is <10
Microbe reductions >99.99% possible when pH is >11
microbial inactivation + physical removal

17. Rapid Gravity Filtration

18. Pressure Filters

21. Membrane Micro-filtration

22. Activated Carbon Adsorption

24. Ultra- violet (UV) Disinfection

25. 2- Desalination Industry

26. Introduction to Desalination
97% of the earth’s water is seawater
salt content > 30,000 mg/liter
Domestic water supply requires dissolved solids content<1000 mg/l
Drinking water < 500 mg/l
Desalination refers to the wide range of technical processes designed to remove salts from water.

27. Consumers
Desalination has developed rapidly, is an important source in parts of :
Middle east
Arabian Gulf
North Africa and some islands

28. Simplified Box Diagram for Desalination
Feedwater (seawater or brackish water)

29. Desalination Processes
Thermal processes:
Multi Stage Flash
Multi effect distillation
Vapor compression distillation
- Thermal vapor compression (TVC)
- Mechanical vapor compression (MVC)
Membrane Processes:
Reverse osmosis (RO)
Electrodialysis
Combined Processes
Minor processes: freezing, solar humidification

30. 1-Multi Stage Flash Technology1/4
Well proven track record
Large capacity units
Low operating & maintenance cost
High quality product water
Used where adequate steam and power is available
DESALINATION Page 6
Dr Salam Al-Dawery

31. 1-Multi Stage Flash Technology2/4

32. 1-Multi Stage Flash Technology3/4
Seawater feed passes through tubes in each evaporation stage where it is progressively heated. Final seawater heating occurs in the brine heater by the heat source.
Subsequently, the heated brine flows through nozzles into the first stage, which is maintained at a pressure slightly lower than the saturation pressure of the incoming stream. As a result, a small fraction of the brine flashes forming pure steam.
The heat to flash the vapour comes from cooling of the remaining brine flow, which lowers the brine temperature.

33. 1-Multi Stage Flash Technology4/4
Subsequently, the produced vapour passes through a mesh demister in the upper chamber of the evaporation stage where it condenses on the outside of the condensing brine tubes and is collected in a distillate tray.
The heat transferred by the condensation warms the incoming seawater feed as it passes through that stage.
The remaining brine passes successively through all the stages at progressively lower pressures, where the process is repeated.
The hot distillate flows as well from stage to stage and cools itself by flashing a portion into steam which is re-condensed on the outside of the tube bundles.

34. Boiling Temperature of Water at Different Pressures
Pressure (bar) Temperature (deg. C)

35. 2-Multi Effect Distillation Technology1/3
Well proven track record
Mid-size units
Low Operating&Maintaince cost
High quality product water
Used in IWPPs where adequate steam is available but may be some constraints on power
DESALINATION Page 8
Dr Salam Al-Dawery

36. 2-Multi Effect Distillation Technology2/3

37. 2-Multi Effect Distillation Technology3/3
In each effect, heat is transferred from the condensing water vapour on one side of the tube bundles to the evaporating brine on the other side of the tubes.
This process is repeated successively in each of the effects at progressively lower pressure and temperature, driven by the water vapour from the preceding effect.

38. 2-Multi Effect Distillation Technology3/3
In the last effect at the lowest pressure and temperature, the water vapour condenses in the heat rejection heat exchanger, which is cooled by incoming seawater.
The condensed distillate is collected from each effect.
Some of the heat in the distillate may be recovered by flash evaporation to a lower pressure.
As a heat source, low pressure saturated steam is used, which is supplied by steam boilers or dual-purpose plants (co-generation of electricity and steam).

39. 3-Vapor compression1/2 diesel engines in 1940
VC plants were first used in naval vessels powered by
diesel engines in 1940
Vapor compression is generally integrated with MED plant
They are used for small or medium scale applications
Two types of vapor compression
Thermal vapor compression
Mechanical vapor compression
Developments in the technology are:
Unit size increased by 5 times
Energy consumption is reduced; it is 8-14 kWh/m3

40. 3-Vapor compression2/2

41. Evaporation plant with mechanical vapour compression

42. Reverse Osmosis Process (RO)
Reverse osmosis is a membrane separation process in which pure water is “forced” out of a concentrated saline solution by flowing through a membrane at a high static transmembrane pressure difference.
This pressure difference must be higher than the osmotic pressure between the saline solution and the pure water.
The saline feed is pumped into a closed vessel where it is pressurised against the membrane. As a portion of the water passes through the membrane, the salt content in the remaining brine increases. At the same time, a portion of this brine is discharged without passing through the membrane.

44. Components of a Reverse Osmosis Plant

46. Reverse Osmosis Process
Dr Salam Al-Dawery

47. Two Types of Reverse Osmosis Membrane

48. Reverse Osmosis Low capacity units Higher O&M Cost
Preferred option for stand alone water plants
Easy O&M
Lower installation cost

49. Electrodialysis1/3 What is electrodialysis (ED)? General Principles
Electrodialysis is a membrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of an electrical potential gradient.
ED is generally applied for brackish water desalination
General Principles
Salts dissolved in water forms ions, being positively (cationic) or
negatively (anionic) charged.
These ions are attracted to electrodes with an opposite electric
charge.
Membranes can be constructed to permit selective passage of
either anions or cations.
Dr Salam Al-Dawery

50. Electrodialysis2/3

51. Electrodialysis3/3

52. Combined Process: Solar Distillation1/2

53. Combined Process: Solar Distillation2/2