is the process in which particles in water are clumped together into
larger particles, called
floc. In a well-run water treatment plant,
adjustments are often necessary in order to maximize the
coagulation/flocculation process. These adjustments are a
reaction to changes in the raw water entering the plant.
Coagulation will be affected by changes in the water's pH, salt
content, alkalinity, turbidity, and temperature.
Within the plant, two more factors can influence coagulation.
Mixing effects and coagulant effects will both influence the
We will discuss each of these factors in more detail in the sections
pH, Salts, and Alkalinity
The levels of pH, salts, and alkalinity in water are all ways of
measuring the amounts of positively charged particles (cations) and
negatively charged particles (anions) in the water. As a result,
all three factors influence the amount of coagulants which must be used
to remove the turbidity in the water.
The pH range of the water may be the single most important factor in
proper coagulation. The vast majority of coagulation problems are
related to improper pH levels. The optimum pH range varies
depending on the coagulants used, but is usually between 5 and 7.
These lower pH values mean that there are more positively charged
particles loose in the water to react with the negatively charged
colloids. To read more about pH, refer to ENV 110's
Salts are compounds which
contain both a cation and an anion. In water, the cation and the
anion come apart and can interact with other charged particles in the
water. All natural waters contain some concentration of cations
and anions, such as calcium, sodium, magnesium, iron, manganese,
sulfate, chloride, phosphate, and others. Some of these ions may
affect the efficiency of the coagulation process.
The alkalinity of water is related to both the pH and the salts in the
water. Alkalinity is the
capacity of the water to neutralize acids, based on the water's content
of carbonate, bicarbonate, hydroxide, borate, silicate, and
phosphate. Water with a high alkalinity is preferred for
coagulation since it tends to have more positively charged ions to
interact with the negatively charged colloids.
The amount and type of turbidity in the water has an obvious effect on
the coagulation process. Turbidity found in natural surface
waters is composed of a large number of sizes of particles found in
different concentrations. The types and amounts of particles in
the water vary
constantly, based on precipitation and man-made factors. For
rains will raise turbidity levels, which will result in the water
requiring more coagulant, though not in a linear manner.
Low turbidity waters can also be very difficult to coagulate due to the
difficulty of inducing collision between the colloids. As a
result, floc formation is poor and much of the turbidity is carried
directly to the filters.
water temperature hinders coagulation in two ways.
water temperature cools, almost all chemical reactions in the water
occur more slowly. The cold water can also make it more difficult
disperse the coagulants in the water. As a result, coagulation
less efficient and higher coagulant doses must be used to
Second, cold water influences floc settling. At near
temperatures, water is very dense, which keeps the floc suspended in
Thorough mixing is an essential part of any
chemical reaction. The chemicals which cause the coagulation
reaction to occur are mixed into the water in the flash mix chamber, so
poor or inadequate mixing there will result in a variety of
problems. Fish eyes are
large clumps of polymer which are not properly mixed into the
water. Inadequate flash mixing can also result in small floc,
high turbidity in the treated water, and frequent filter backwashing.
The proper type
and concentration of coagulant are essential to the coagulation
process. The coagulant choice will depend on the conditions at
the plant. The concentration of coagulant also depends on the
water conditions, and a jar test can be used to determine the correct
concentration to use at any given time.
Coagulants are usually fed into the water using a gravimetric feeder or
a metering pump. A gravimetric
feeder feeds dry chemicals into the water by weight. A metering pump feeds a wet solution
(a liquid) into the water by pumping a volume of solution with each
stroke or rotation.
Improper coagulation related to coagulant may result from:
- Using old chemicals
- Using the wrong coagulant
- Using the wrong concentration of coagulant. This may
result from setting the wrong feed rate on the gravimetric feeder or
metering pump or from a malfunction of the equipment.