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

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

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

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.

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.

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

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

Water Treatment: basis of operation

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

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

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

Water Treatment: basis of operation

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

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.

Water Treatment: basis of operation

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

Water Treatment: basis of operation

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 200-400% Different designs both plate and modular

Water Treatment: basis of operation

Water Treatment: basis of operation

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 (0.5-2.0m 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

Water Treatment: basis of operation

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

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

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

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

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

Chlorination Water Treatment: basis of operation Degree of dissociation is pH dependent: >pH9.0 -100% chlorine as chlorite <pH5.0 -100% chlorine as hypochlorous acid pH 7.5 - 50% 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. 0.2-0.5 mgL-1 after 30 min)

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; 0.45 mgL-1 at pH9

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

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. http://www.sodis.ch/index_EN

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 6F- + 6H+ + SiO3

Water Treatment: basis of operation Modern chemical dosing system

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

Water Treatment: basis of operation

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

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

Water Treatment: basis of operation

Water Treatment: basis of operation

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.

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.