Water Treatments Methods
Screening
The first step in purifying surface water is to remove large debris like
sticks, leaves, trash and other large particles which may interfere with
subsequent purification steps. The smaller the holes in the screen, the smaller
the debris must be to pass through. Thus a screen with small holes will filter
out more debris but will become clogged quicker and require cleaning more
frequently. Groundwater does not need screening before other purification steps.
Storage
Water from rivers may also be stored in bankside reservoirs for periods
between a few days and many months to allow natural biological purification to
take place. This is especially important if treatment is by slow sand filters.
The filtered water is then treated to remove or inactivate remaining potentially
harmful microscopic organisms including viruses, protozoa and bacteria. This
removal step comprises part of a multistep process of disinfection which is
completed by chemical and/or ultraviolet light treatment which damages and makes
non-infectious any remaining viable harmful microbes. For waters that are
particularly difficult to treat such as from catchments with intensive
agriculture, extra physical, chemical and biological treatment steps may be
necessary.
Coagulation and flocculation
Together coagulation and flocculation is a traditional purification method which
works by using active chemicals called coagulants that effectively "glue" small
suspended particles together so that they settle out of the water or stick to
sand or other granules in a granular media filter. In a relatively new and
economically attractive development polymer film with chemically formed
microscopic pores called micro or ultrafiltration membranes can be used in place
of granular media to filter water effectively without coagulants. The type of
membrane media determines how much pressure is needed to drive the water through
and what sizes of microbes can pass.
Coagulation normally works by eliminating the natural electrical charge of the
suspended particles so they attract and stick to each other. The joining of the
particles so that they will form larger settleable particles is called
flocculation. The larger formed particles are called floc.
Coagulation
Many of the suspended water particles have a negative electrical charge. The
charge keeps particles suspended because they repel similar particles.
Coagulation processing reduces the surface charge to encourage attraction which
forms floc which can settle. The coagulation chemicals are added in a tank
(often called a rapid mix tank), which typically has rotating paddles. In most
treatment plants, the mixture remains in the rapid mix tank for 10 to 30 seconds
to ensure full mixing. The amount of coagulant that is added to the water varies
widely due to the different source water quality. It is often easiest to mix
varying amounts of coagulants with samples of the source water to see which
dosage creates the best floc. The chemicals also act as additional particles
which the suspended solids can bond to form floc.
The most common coagulant used in the United States is aluminum sulfate,
sometimes called filter alum. Aluminum sulfate reacts in water to form aluminium
hydroxide, which attracts smaller suspended particles, forming floc. The water
being purified must be alkaline for the aluminium hydroxide reaction to occur.
If the water is not buffered to resist acids, lime or soda ash is added to raise
the pH. Lime is the more common of the two additives because it is cheaper, but
it also adds to the resulting water hardness. Aluminum sulfate is an inexpensive
coagulant, but it produces sulfuric acid that may cause rapid corrosion of water
mains if soda ash or lime is not used in enough quantity to counteract the acid.
Iron(III) sulfate or chloride are other common coagulants. They also needs a
buffered water, so lime or soda ash is often added to the water. Iron(III)
coagulants work over a larger pH range than aluminum sulfate but are not
effective with many source waters. Other benefits of iron(III) are lower costs
and in some cases slightly better removal of natural organic contaminants from
some source waters.
Cationic and other polymers can also be used as coagulants in water treatment.
They are often called coagulant aids used in conjunction with other regular
coagulants. The long chains of positively charged polymers can help to
strengthen floc making it larger, faster settling and easier to filter out. The
main advantages of polymer coagulants and aids are that they do not need the
water to be alkaline to work and that they produce less settled waste than other
coagulants, which can reduce operating costs. The drawbacks of polymers are that
they are expensive, can plug up sand in filters and that they often have a very
narrow range of effective doses.
Flocculation
Flocculation is the clumping together of small particles to form larger
particles, called floc, which is more readily settled out of the water.
Flocculation is the main method to decrease turbidity. After charge
neutralization of suspended particles, they will stick to each other and to the
coagulant chemical particles. To aid in the flocculation, water is slowly mixed
in a large tank called a flocculation basin. Unlike a rapid mix tank, the
flocculation paddles turn very slowly to minimize turbulence. The idea is to
gently mix the water so particles contact as many others as possible becoming as
large as possible without breaking up. Generally, the retention time of a
flocculation basin is at least 30 minutes with speeds between 0.5 feet and 1.5
feet per minute (15 to 45 cm / minute). Flow rates less than 0.5 ft/min cause
undesirable floc settlement within the basin.
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Information source:
Wikipedia
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