1. Water Technology Lecture 1: Introducing Water Technology
Professor Nick Gray
Centre for the Environment
Trinity College
University of Dublin
© Tigroney Press

2. Water Treatment Basis of operation
Professor Nick Gray
Centre for the Environment
Trinity College
University of Dublin
© Tigroney Press

3. Water Treatment Basis of operation
Learning Objectives:
To understand the operational basis of water treatment
Selection of Unit Processes
To understand the process of disinfection and fluoridation

4. Water services cycle Water Treatment: basis of operation
Water technology deals with the processes and mechanisms that are required to manage the water services cycle. It’s function is to provide continuous and sufficient quantities of safe palatable drinking water for both domestic and industrial consumers and dispose of the used water to prevent environmental damage and to protect public health.

5. Objective of Water Treatment
Water Treatment: basis of operation
Objective of Water Treatment
To produce an adequate and continuous supply of water that is:
Palatable
Safe
Clear
Colourless and odourless
Reasonably soft
Non-corrosive
Low organic content
Originally the prevention of disease
Must conform to EU Drinking Water Directive (98/83/EEC) as well as National Standards.

6. Unit Processes Unit process selection depends on:
Water Treatment: basis of operation
Unit Processes
Unit process selection depends on:
Raw water quality and seasonal variability
Finished water quality
Process selection more complex than wastewater treatment.
Pre-treatment may be required if raw water quality is poor
All water requires coarse screening to remove fish, natural debris, litter.
Normal waters only require conventional treatment
Advanced treatment aimed at removing problematic contaminants or for high purity

7. Main unit Processes Sedimentation Catchment control
Water Treatment: basis of operation
Main unit Processes
Sedimentation
Secondary treatment
Rapid sand filtration
Slow sand filtration
Disinfection
Advanced treatment
Adsorption
Activated carbon
Fe and Mn removal
Membrane processes
Softening
Fluoridation
Distribution
Catchment control
Raw water storage
Intake
Pre-treatment
Coarse screening
Pumping
Storage
Fine screening
Equalization
Neutralization
Aeration
Chemical pre-treatment
Primary treatment
Coagulation
Flocculation

8. Water Treatment: basis of operation

9. Generalized layout of Water Treatment plant
Water Treatment: basis of operation
Generalized layout of Water Treatment plant

10. Preliminary screening
Water Treatment: basis of operation
Preliminary screening
Prior to transport to plant: Coarse screens (100mm spacing);-branches, litter etc.
Prior to treatment: Fine screens (25000 apertures cm-2) - algae, fine solids.
Drum screens are mainly employed

11. Storage Water Treatment: basis of operation
Improves quality and ensures continuation of supply
Processes during storage:
Sedimentation
UV radiation
Dilution
Photosynthesis
Problems:
Algal development
Pollution from birds
Thermal stratification
Atmospheric pollution and fallout

12. Water Treatment: basis of operation

13. Aeration Water Treatment: basis of operation
Groundwaters, stratified lakes/reservoirs, polluted rivers
Water must be aerobic for treatment
Reduces corrosiveness
Removes Fe and Mn
Cascade aeration
Step aeration

14. Coagulation/Flocculation
Water Treatment: basis of operation
Coagulation/Flocculation
After fine screening colloidal solids remain
Negatively charged-prevents aggregation and settlement
Coagulant added to induce destabilization of solids
Rapid mixing (<60 sec) produces microflocs
Coagulants (salts or synthetic organic polymers):
Aluminium sulphate (alum) / Aluminium hydroxide
Ferric chloride / Ferric sulphate
Lime
Polyacrylamide
Polyacrylic acids
Mixture is mixed to induce microflocs to coalesce and grow (i.e. flocculation) ensuring efficient removal by subsequent settlement and filtration.

15. Water Treatment: basis of operation

16. Sedimentation/clarification
Water Treatment: basis of operation
Sedimentation/clarification
Sedimentation clarifies the water hence the name clarification
Upflow clarifier design
encourages flocculation
Sludge is constantly bled off from the top of the sludge blanket

17. Water Treatment: basis of operation

18. Lamella designs Water Treatment: basis of operation
Inclined submerged plates placed in settlement zone set at critical angle
Parallel compartments act as individual settlement chambers
Encourage settlement – reduces settlement volume needed
Critical angle 60o
When retrofitted can increased throughput by %
Different designs both plate and modular

19. Water Treatment: basis of operation

20. Water Treatment: basis of operation

21. Filtration Rapid gravity filters Slow sand filter
Water Treatment: basis of operation
Filtration
Only fine solids remain (<10 mgL-1) and soluble material
Filters contain layers of graded sand and gravel
Rapid gravity filters
Coarse grade of quartz sand, large interstices so water passes quickly
Gravity or pressure operated.
Backwashed
High rate of loading (5-6 m3m-2h-1) – comparatively cheap
Slow sand filter
Surface layer of fine sand ( m deep) over coarse sand/gravel (1-2m)
Biological treatment (Biofilm/schmutzdecke)
Top 2mm autotrophic bacteria/algae (N, P removal, O2 release)
Below 300mm heterotrophic layer (organic matter removal)
Dirt layer removed mechanically
Low rate of loading (<0.1 m3m-2h-1) - expensive

22. Water Treatment: basis of operation

23. Water Treatment: basis of operation
Rapid gravity filters must be backwashed at least daily, often more frequently.
First air scoured then backwashed to remove solids

24. Water Treatment: basis of operation
Operational success very dependent on sand quality and specification
High quality quartz sand used – expensive, often from Netherlands
Different grades can be used
Backwashing grades into layers
Can incorporate anthracite for specific adsorption
Anthracite sg 1.5
Silica sand sg 2.5
Garnet sand sg 4.2

25. Slow sand filtration Biofilm/schmutzdecke
Water Treatment: basis of operation
Slow sand filtration
X40 slower than pressurized
rapid gravity systems
Biofilm/schmutzdecke

26. Water Treatment: basis of operation
pH control to prevent corrosivity of water using lime.

27. Main disinfection methods
Water Treatment: basis of operation
Main disinfection methods
Ozone
Powerful oxidation properties.
No residual action but also reduces colour, taste and odour.
Dose of 1 ppm destroys all bacteria within 10 minutes.
Must be manufactured on site, costly to produce. UV
Used for point of entry only.
Requires pre-filtration as surface action only
No residual effect
Chlorination
Not as powerful as O3 but has residual effect…disinfection continues through the distribution main to the consumers tap
Chlorine gas or hypochlorite used

28. Chlorination Water Treatment: basis of operation
Degree of dissociation is pH dependent:
>pH % chlorine as chlorite
<pH % chlorine as hypochlorous acid
pH % OCL and 50% HOCl
HOCl more effective so more effective under acidic conditions
Chlorine reacts with organic matter and reducing agents so residual may not last long
Dosage rate depends on rate of flow and residual required (e.g mgL-1 after 30 min)

29. Chlorine taste threshold is
Water Treatment: basis of operation
Residual forms of chlorine
Ammonia reacts with chlorine to form chloramines
Mono- and di-forms have long lasting disinfecting properties
Provide a combined rather than free residual effect
Much less effective – require 100 times longer contact time
Chloramines more odorous than free chlorine (tri- worst)
Chlorine taste threshold is
0.16 mgL-1 at pH7; mgL-1 at pH9

30. Breakpoint Chlorination Curve
Water Treatment: basis of operation
Breakpoint Chlorination Curve

31. Water Treatment: basis of operation
SODIS is short for solar water disinfection and is perhaps one of the simplest and safest ways of treating water in developing countries.
Transparent plastic bottles (PET) are filled with settled water and left in the sun for 6 hours. A mixture of temperature and UV-A rays inactivate viruses, bacteria and protozoa making the water safe for consumption.
It also works in cloudy conditions although it is recommended that the bottles are exposed to light for 2 consecutive days.
5 million people rely on this method for safe water.

32. Fluoridation SiF6 + 3H2O 6F- + 6H+ + SiO3
Water Treatment: basis of operation
Fluoridation
Fluoride occurs in many groundwaters, rare in surface waters
Prevention of DMF teeth if concentration raised to 1 mgL-1
Sodium silicofluoride cheapest and most convenient to use
Dissociates when added to water to release fluoride ions
SiF6 + 3H2O F- + 6H+ + SiO3

33. Water Treatment: basis of operation
Modern chemical dosing system

34. Advanced Water Treatment
Water Treatment: basis of operation
Advanced Water Treatment
Softening
Ion-exchange
Adsorption
Membrane filtration (reverse osmosis)
Chemical oxidation
Desalination

35. Water Treatment: basis of operation

36. Sludge production and disposal
Water Treatment: basis of operation
Sludge production and disposal
Different to wastewater sludges
Rich in pathogens especially Cryptosporidium oocysts
Origin:
Suspended solids in raw water
Colour removal
Dissolved chemicals that precipitate during treatment
Coagulation chemicals
Other chemicals (e.g. activated carbon)
Biological growth
Treatment:
Normally thickened to 5-10% then dewatered to 40%.
Centrifugation increasingly used although traditionally plate presses used
Disposal:
Landfill for solids or discharge to sewer for liquid sludge

37. Filter press for dewatering sludges
Water Treatment: basis of operation
Filter press for dewatering sludges

38. Water Treatment: basis of operation

39. Water Treatment: basis of operation

40. Conclusions Conventional treatment normally adequate.
Water Treatment: basis of operation
Conclusions
Conventional treatment normally adequate.
Advanced systems used to control specific problems or produce high purity water
The cleaner the water the less steps.
Household systems based on physical filtration and use either UV or hypochlorite tablets/solution for disinfection.

41. Further reading: Water Treatment: basis of operation
Read sections 17.1 and 17.2 of the 2017 course text:
Gray, N.F. (2017) Water Science and Technology: An Introduction. CRC Press, Oxford.
Further reading:
Binnie, C, Kimber,M. and Smethurst, G (2009) Basic water treatment 4th edn. Thomas Telford, London.
Twort, A.C. et al. (2008) Water Supply 5th edn, Arnold, London.
Percival, S.L., Yates, M. Williams, D.W., Chalmers, R.M. and Gray, N.F. (2014) Microbiology of Waterborne Diseases: Microbiological Aspects and Risks (2nd edn.) Academic Press, London. pp.695.