Within the plant, two more factors can influence coagulation.Mixing effects and coagulant effects will both influence thecoagulation/flocculation process.
We will discuss each of these factors in more detail in the sectionsbelow.
pH, Salts, and Alkalinity
The levels of pH, salts, and alkalinity in water are all ways ofmeasuring the amounts of positively charged particles (cations) andnegatively charged particles (anions) in the water. As a result,all three factors influence the amount of coagulants which must be usedto remove the turbidity in the water.
The pH range of the water may be the single most important factor inproper coagulation. The vast majority of coagulation problems arerelated to improper pH levels. The optimum pH range variesdepending on the coagulants used, but is usually between 5 and 7.These lower pH values mean that there are more positively chargedparticles loose in the water to react with the negatively chargedcolloids. To read more about pH, refer to ENV 110'sLesson 3.
Salts are compounds whichcontain both a cation and an anion. In water, the cation and theanion come apart and can interact with other charged particles in thewater. All natural waters contain some concentration of cationsand anions, such as calcium, sodium, magnesium, iron, manganese,sulfate, chloride, phosphate, and others. Some of these ions mayaffect the efficiency of the coagulation process.
The alkalinity of water is related to both the pH and the salts in thewater. Alkalinity is thecapacity of the water to neutralize acids, based on the water's contentof carbonate, bicarbonate, hydroxide, borate, silicate, andphosphate. Water with a high alkalinity is preferred forcoagulation since it tends to have more positively charged ions tointeract with the negatively charged colloids.
Turbidity
The amount and type of turbidity in the water has an obvious effect onthe coagulation process. Turbidity found in natural surfacewaters is composed of a large number of sizes of particles found indifferent concentrations. The types and amounts of particles inthe water varyconstantly, based on precipitation and man-made factors. Forexample, heavyrains will raise turbidity levels, which will result in the waterrequiring more coagulant, though not in a linear manner.
Low turbidity waters can also be very difficult to coagulate due to thedifficulty of inducing collision between the colloids. As aresult, floc formation is poor and much of the turbidity is carrieddirectly to the filters.
Temperature
Coldwater temperature hinders coagulation in two ways.First, aswater temperature cools, almost all chemical reactions in the wateroccur more slowly. The cold water can also make it more difficultto evenlydisperse the coagulants in the water. As a result, coagulationbecomesless efficient and higher coagulant doses must be used tocompensate.
Second, cold water influences floc settling. At nearfreezingtemperatures, water is very dense, which keeps the floc suspended inthe water.
Mixing
Thorough mixing is an essential part of anychemical reaction. The chemicals which cause the coagulationreaction to occur are mixed into the water in the flash mix chamber, sopoor or inadequate mixing there will result in a variety ofproblems. Fish eyes arelarge clumps of polymer which are not properly mixed into thewater. Inadequate flash mixing can also result in small floc,high turbidity in the treated water, and frequent filter backwashing.
Coagulant
The proper typeand concentration of coagulant are essential to the coagulationprocess. The coagulant choice will depend on the conditions atthe plant. The concentration of coagulant also depends on thewater conditions, and a jar test can be used to determine the correctconcentration to use at any given time.
Coagulants are usually fed into the water using a gravimetric feeder ora metering pump. A gravimetricfeeder 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 eachstroke or rotation.
Improper coagulation related to coagulant may result from:
- Using old chemicals
- Using the wrong coagulant
- Using the wrong concentration of coagulant. This mayresult from setting the wrong feed rate on the gravimetric feeder ormetering pump or from a malfunction of the equipment.
