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
chloride
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
optimised
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
Treatment
Upflow-Downflow Prefilter
Horizontal Flow Prefilter
Flocculator
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
Disinfection:
Residual
(usually chlorine), pH, turbidity, coli forms (thermotolerant and total)
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