Coagulation

Coagulation 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 coagulation/flocculation process. 

We will discuss each of these factors in more detail in the sections below.



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 Lesson 3

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. 



Turbidity


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 example, heavy 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. 



Temperature

Cold water temperature hinders coagulation in two ways.  First, as water temperature cools, almost all chemical reactions in the water occur more slowly.  The cold water can also make it more difficult to evenly disperse the coagulants in the water.  As a result, coagulation becomes less efficient and higher coagulant doses must be used to compensate. 

Second, cold water influences floc settling.  At near freezing temperatures, water is very dense, which keeps the floc suspended in the water. 



Mixing

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.



Coagulant

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: