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Chapter: Environmental Engineering : Water Treatment

Water Treatment: Selecting Technology

When selecting technology and systems of treatment it is vital that as full a picture as possible of the source water quality is available.

Water Treatment: Selecting Technology


When selecting technology and systems of treatment it is vital that as full a picture as possible of the source water quality is available. It is important to know what is in the water before trying to design appropriate treatment systems. It is equally important to maintain a thorough monitoring programme through the plant to ensure that each stage of treatment is working effectively and efficiently.


All waters may need treatment before they are fit for human consumption, although surface waters tend to be more vulnerable to contamination than groundwater. All surface waters will require treatment prior to consumption. Furthermore, all water supplied through distribution systems should be disinfected to provide a residual disinfectant which provides ongoing protection from bacterial growth and survival.


Water Treatment


Presentation Plan


Section Key points OHP


Introduction - need to treat all surface waters and microbiological, chemical or physical microbiological contamination is most important as it


causes highly infectious disease with short-term impacts chemical contamination tends to have

longer term effects on health suspended solids affect microbial survival and the acceptability of


water, always disinfect water supplies and maint contamination during distribution and storage



Multiple Barrier Principle


need to have more than a single process during treatment prevents breakdown in one process leading to complete treatment failure source must be well protected


Treatment processes


many processes available, the suitability of each is a function of source quality, operator capacity and financial resources technology selection must be made on the basis of the above to ensure sustainability often need to reduce turbidity before treating water as this may interfere with treatment prefiltration is a physical process which removes suspended solids prefilters can be horizontal, vertical upflow or vertical upflow-downflow main advantage is limited working parts

and doesn't use chemicals  disadvantages include poor ability to remove fine material, microbial


removal poor and may need frequent cleaning sedimentation is achiev particles in slow moving water simple sedimenters do not use chemical coagulants and are


not effective in removing fine material


Section Key Points OHP


settling is improved through addition of co

settling and removes fine material

modular and plate settlers improve settling alum is the most common coagulant, others i


polyelectrolytes and ferric salts such as sulphate and



advantages include removal of fine particle disadvantages include expense, need for good monitoring capacity, need trained operators


Treatment processes

ndsafiltration can be rapid or slow

slow sand filtration is a biological proces

filtration a physical process

slow sand filters a biologically active top


predatory bacteria schumtzdeckekillsbacteria and viruses

require cleaning-4 days@ toeveryrecover 2 months, take 3 rapid sand filters work at much faster rate advantages of slow sand filtrationy simpleinclude


to operate

disadvantages include large land requiremen advantages of rapid sand filtration include


Treatment plant assessments

assessments of treatmentriedoutfor   plants may be car

number of different reasons

routine assessments often carried out by wa


assessments may also be undertaken when th

produce water of adequate quality

assessments involve the evaluation of each

and to identify any process failures and causes of failures

assessments should also evaluatehnologies the(e.g. suit sometimes find simple sedimenters combined with slow sand filters when turbidity

was relatively high - led to failure)

assessments should be linked to performance


Section Key Points OHP Conclusion


both surfacewatermay requireand treatmentgroundbeforedistribution

source water quality (and likely variations technologies should be used which reflect


adequate treatment

multiplea barrier principle should always be used when treating water

source protection is also vital


The Multiple Barrier Principle of Water



Upflow-Downflow Prefilter


Horizontal Flow Prefilter




Coagulant Dosing

Slow Sand Filter


Rapid Sand Filter

Water Treatment Plant Assessments


When and why the should be carried out:

Routine assessment of operational efficiency and state of equipment


When contamination is found When disease outbreaks occur


If disinfection dosing requirements suddenly change


Water Treatment Plant Assessments Parameters


Raw Water:


turbidity, pH, alkalinity, coliforms, major ions, nutrients, known problem substances


Coagulation-flocculation-settling: turbidity, pH, residual aluminum, residual acrylamide, coliforms


Prefiltration: turbidity, pH, coliforms


Sand filtration (rapid/slow): turbidity, pH, coliforms




Residual (usually chlorine), pH, turbidity, coli forms (thermotolerant and total)

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