CIV911 Water SupplyEngineering

Dr P J SallisDr C A Weatherell

Dr D WernerProf T Donnelly

Dr G Parkin

Introduction to Moduleand Coursework

• Main components of water supply

• Why water needs treating

• Main water treatment unit processes

• Summary and scope of design coursework

• Further reading

Main Components of Water Supply

Water Treatment

Plant

River

Impounding Reservoir

Service Reservoir

Distribution Network

Trunk Main

Feeder Main

Distribution Main

What needs to be removed from the water?

• Colour

• Suspended solids

• Turbidity

• Pathogens

• Hardness

• Taste and odour

• Harmful chemicals

Drinking water should be:

• Palatable• Safe• Clear• Colourless• Odourless• Reasonably soft• Non-corrosive• Low organic content

Main Water Treatment Unit Processes • Screening and storage• Aeration• pH adjustment• Coagulation• Flocculation• Clarification• Filtration• Disinfection• Tertiary processes• Monitoring• Sludge removal

Screening and storage

• Preliminary screening

• Storage

• Fine screening

• Microstraining

Aeration

• Why?– Reduces taste and odours– Reduces corrosiveness– Oxidises iron and manganese

• Types– Bubbled air– Cascade– Fountain– Packed towers– Diffusers

pH Adjustment

• Why?– To ensure maximum efficiency of processes– To avoid metal corrosion– To avoid salt deposition

• Acids used to decrease pH• Alkalis used to increase pH

– Lime– sodium carbonate– Caustic soda

Coagulation• Small particles left after screening

– Negatively charged– Repel each other– Settle very slowly

• Adding a coagulant– Destabilises the particles– Induces aggregation into larger flocs– Larger flocs settle quickly

• Common coagulants– Aluminium sulphate (alum)– Aluminium hydroxide– Iron chloride– Lime

• Rapid mixing required– Flash-mixer– Mixing channel

Flocculation

• Gentle mixing after coagulation– Paddle mixer– Turbine mixer

• Increases chance of particles colliding

• Larger flocs formed

Clarification• Settlement of flocs

– Water flows upward– Flocs settle downwards– Thick sludge blanket develops– Further flocculation occurs– Clarified water flows over weir– Sludge bleeding regularly

• Many designs– Inclined plate, parallel plate, tube settlers

Filtration• Removal of residual fine solids by

– Downward flow of water through layers of sand and gravel• Filter must be cleaned when

– Particles clog the surface– Flow rate becomes too low

• Rapid sand/gravity filters– Coarse sand– High flow, small– Regular back-washing

• Slow sand filters– Fine sand over course sand or gravel– Low flow, large– Biological as well as physical treatment– Remove pathogenic bacteria, taste, odour, nitrogen and phosphorus– Top layer of sand replaced every few months– Expensive

Disinfection• Residual pathogen bacteria and viruses• Complete sterilization not feasible• Ozone disinfection

– Efficient– Expensive– Required on-site manufacture– Lack of residual disinfection

• Ultraviolet radiation disinfection– Effective if exposure time is adequate– Used in small scale– Lack of residual disinfectiom

• Chlorination disinfection– Readily available– Cheap– Provides residual disinfection– Not as aggressive as ozone– Very reactive (e.g. with organic compounds and ammonia)

Tertiary Processes

• Soluble inorganic residuals– Precipitation softening

• Lime, soda ash

– Ion exchange softening• Natural zeolite resins

• Non-biodegradable organic residuals– Activated carbon

• Granular (GAC)• Powdered (PAC)

Monitoring

• Routine monitoring– of pH, residual chlorine, fluoride, aluminium,

iron, dissolved oxygen, colour, turbidity, ammonia, total organic carbon, nitrate, flow

– before, after and between processes

• Fish monitors– Before and after treatment– For trace contaminants– Fish response triggers alarms

Sludge Removal

• Large amounts of thin slurry produced in water treatment– Gelatinous hydroxide sludge from coagulation and clarification– Precipitation sludge from water softening– Back-wash water from sand filters– Wash-water from microstrainers– Spent fine sand and PAC

• Sludge treatment– Dry in shallow lagoons– Dewater in filter press

• Sludge disposal– Landfill site– Spread on land– Incinerated

Summary and Scope of Design Coursework• Screening and storage• Aeration• pH adjustment• Coagulation• Flocculation• Clarification• Filtration• Disinfection• Tertiary processes• Monitoring• Sludge removal• Water supply infrastructure

Further Reading

• Twort, A.C., Ratnayaka, D.D. & Brandt, M.J. (2000) Water Supply, 5th Edition. Arnold / IWA Publishing, London.

• Hammer, M.J. & Hammer, M.J.Jr. (2001) Water and Wastewater Technology, 4th Edition. Prentice Hall, New Jersey.

• Kiely, G. (1998) Environmental Engineering, International Edition. Irwin/McGraw-Hill

• Viessman, W. Jr. & Hammer, M. J. (1998) Water Supply and Pollution Control, 6th Edition. Addison Wesley Longman, California.