Closer Look At The Mechanical Clarification Process And The Chemistry Of Clarification

So by now it should be clear that what the process of clarification is all about is removing suspended solids from water. Important concepts that we have eluded to, but maybe not spelled out so clearly up to now are:

1. Stable solids suspensions in water- The mechanisms involved in keeping solids suspended in water,

2. Chemical treatments -How organic polymers and inorganic coagulants work to counteract solids stabilization mechanisms and enhance removal of solids from water, and

3. The function of clarification unit operations- How these units work and how chemical treatment enhances their performance.

A term that we should get into our vocabulary is "subsidence". This term essentially means settling. While a degree of clarification can be accomplished by subsidence, most industrial processes require better quality water than can be obtained from settling only. Most of the suspended matter in water would settle, given enough time, but in most cases the amount of time required would not be practical. As we have shown from our derivations of expressions describing the classical theory of sedimentation, settling characteristics depend upon the :

1. Weight of the particle,

2. Shape of the particle,

3. Size of the particle, and

4. Viscosity and/or frictional resistance of the water, which is a function of temperature.

The settling rates of various size particles at 50° F (10° C) is illustrated in Table 1.

Table 1. Some Settling Rates for Different Particles (assumed spherical) and Sizes

Particle Diameter (mm)

Particle Type

Time to Settle One Foot

10.0

Gravel

0.3 sec.

1.0

Coarse sand

3.0 sec.

0.1

Fine sand

38.0 sec.

0.01

Silt

33.0 minutes

0.001

Bacteria

35.0 hours

0.0001

Clay particles

230 days

0.00001

Colloidal particles

65 years

Look closely at the settling times in Table 1 - the times span from a fraction of a second to almost a lifetime! A great deal of the suspended matter found in waste waters fall into the colloidal suspension range, so obviously we cannot rely on gravitational force alone to separate out the pollutants.

WHAT COAGULATION IS ALL ABOUT

The term coagulation refers to the first step in complete clarification. It is the neutralization of the electrostatic charges on colloidal particles. Because most of the smaller suspended solids in surface waters carry a negative electrostatic charge, the natural repulsion of these similar charges causes the particles to remain dispersed almost indefinitely. To allow these small suspended solids to agglomerate, the negative electrostatic charges must be neutralized. This is accomplished by using inorganic coagulants, which are water soluble inorganic compounds), organic cationic polymers or polyelectrolytes. The most common and widely used inorganic coagulants are:

• Alum-aluminum sulfate-Al2(S04)3

• Ferric sulfate-Fe^SOJj

• Ferric chloride-FeCl3

• Sodium aluminate-Na2Al204

Inorganic salts of metals work by two mechanisms in water clarification. The positive charge of the metals serves to neutralize the negative charges on the turbidity particles. The metal salts also form insoluble metal hydroxides which are gelatinous and tend to agglomerate the neutralized particles. The most common coagulation reactions are as follows:

A12(S04)3 + 3Na2C03 + 3H20 = 2A1(0H)3 + 3Na2S04 + 3C02

A12(S04)3 (NH4)2S04 + 3Ca(HC03) = 2A1(0H)3 + (NH4)2S04 + 3CaS04 + 6C02 A12(S04)3 K2S04 + 3Ca(HC03) 2 = 2A1(0H)3 + K2S04 + 3CaS04 + 6C02 N^AIA + Ca(HC03)2 + H20 = 2A1(0H)3 + CaC03 + Na2C02 Fe(S04)3 + 3Ca(OH)2 = 2Fe(OH)3 + 3CaS04 4Fe(OH)2 + 02 + 2H20 = 4Fe(OH)3

The effectiveness of inorganic coagulants is dependent upon water chemistry, and in particular — pH and alkalinity. Their addition usually alters that chemistry. Table 2 illustrates the effect of the addition of 1 ppm of the various inorganic coagulants on alkalinity and solids concentration.

Table 2. Coagulant, Acid and Sulfate - 1 ppm Equivalents.

1 ppm Formula or Chemical

Increase

Increase

ppm Total Solids Increas e

A12(S04)318H20

0.45

0.45

0.64

0.40

0.16

A12(S04)3(NH4)2S04 •24H20

0.33

0.44

0.63

0.29

0.27

A12(S04)3K2S04 -24H20

0.32

0.43

0.60

0.28

0.30

FeS04 -7H20

0.36

0.36

0.61

0.31

0.13

FeS04 -VHzO + iSCy

0.54

0.36

0.51

0.48

0.18

Fe2(S04)3

0.76

0.76

1.07

0.64

0.27

H2S04 - 96%

1.00

1.00

1.42

0.88

0.36

H2S04 - 93.2% (66° Be)

0.96

0.95

1.36

0.84

0.34

H2S04 - 77.7% (66° Be)

0.79

0.79

1.13

0.70

0.28

NaS04

-

0.64

0.95

-

1.00

Na2Al204

Increase 0.54

-

-

Reduces 0.47

Note that the use of metal salts for coagulation may increase the quantity of dissolved solids. One must consider the downstream impact of these dissolved solids. In addition, the impact of carryover of suspended Al ... and Fe... compounds and their related effect on downstream processes must be considered.

Aluminum salts are most effective as coagulants when the pH range is between 5.5 and 8.0 pH. Because they react with the alkalinity in the water, it may be necessary to add additional alkalinity (called buffering) in the form of lime or soda ash. Use the values in Table 3 to guide you. Iron salts, on the other hand, are most effective as coagulants at higher pH ranges (between 8 and 10 pH). Iron salts also depress alkalinity and pH levels; therefore, additional alkalinity must be added. Sodium aluminate increases the alkalinity of water, so care must be taken not to exceed pH and alkalinity guidelines. As is evident from the reactions discussed above, a working knowledge of the alkalinity relationships of water is mandatory. By using inorganic coagulants we can wind up producing a voluminous, low-solids content sludge the is difficult to dewater and dries very slowly. The properties of the sludge to be generated and estimated quantities needs to be carefully determined, in part from pilot-scale and bench testing prior to the design and construction of a plant.

Polymers are often described as long chains with molecular weights of 1,000 or less to 5,000,000 or more. Along the chain or backbone of the molecule are numerous charged sites. In primary coagulants, these sites are positively charged. The sites are available for adsorption onto the negatively charged particles in the water. To accomplish optimum polymer dispersion and polymer/particle contact, initial mixing intensity is critical. The mixing must be rapid and thorough, Polymers used for charge neutralization cannot be over-diluted or over-mixed. The farther upstream in the system these polymers can be added, the better their performance. Because most polymers are viscous, they must be properly diluted before they are added to the influent water. Special mixers such as static mixers, mixing tees and specially designed chemical dilution and feed systems are all aids in polymer dilution. Static or motionless mixers in particular are popular for this application. Refer to Figure 20 for an example of an in-line static mixer.

Table 3. Recommended Alkali and Lime 1 ppm Equivalents.

Chemical -lppm

Formula (1 ppm)

Alkalinity Increase

(1 ppm)

Hardness as CaCOj Increases

Sodium bicarbonate

NaHCOj

0.60

-

-

Soda ash (56% Na2, 99.16% Na2C03)

Na2C03

0.94

0.41

Chemical -lppm

Formula (1 ppm)

Alkalinity Increase

(1 ppm)

Hardness as CaC03 Increases

Caustic soda (76% Na20, 98.06% Na2C03)

NaOH

1.22

1.09

Quicklime (90% CaO)

CaO

1.61

1.41

1.61

Hydrated lime (93% Ca(OH)2)

Ca(OH)2

1.26

1.10

1.36

Figure 20. Example of an in-line static mixer for polyelectolyte additions.

Once the negative charges of the suspended solids are neutralized, flocculation begins. Flocculation can be thought of as the second step of the coagulation process. Charge reduction increases the occurrence of particle-particle collisions, promoting particle agglomeration. Portions of the polymer molecules not absorbed protrude for some distance into the solution and are available to react with adjacent particles, promoting flocculation. Bridging of neutralized particles can also occur when two or more turbidity particles with a polymer chain attached come together. It is important to remember that during this step, when particles are colliding and forming larger aggregates, mixing energy should be great enough to cause particle collisions but not so great as to break up these aggregates as they are formed. In some cases flocculation aids are employed to promote faster and better flocculation. These flocculation aids are normally high molecular weight anionic polymers. Flocculation aids are normally necessary for primary coagulants and water sources that form very small particles upon coagulation. A good example of this is water that is low in turbidity but high in color (colloidal suspension).

A final are we should discuss is color removal. This is perhaps the most difficult impurity to remove from waters. In surface waters color is associated with dissolved or colloidal suspensions of decayed vegetation and other colloidal suspensions. The composition of this material is largely tannins and lignins, the components that hold together the cellulose cells in vegetation. In addition to their undesirable appearance in drinking water, these organics can cause serious problems in downstream water purification processes. For examples:

1. Expensive dernineralizer resins can be irreversibly fouled by these materials.

2. Some of these organics have chelated trace metals, such as iron and manganese within their structure, which can cause serious deposition problems in a cooling system.

There are many ways of optimizing color removal in a clarifier. The three most common methods are:

• Prechlorination (before the clarifier) significantly improves the removal of organics as well as reducing the coagulant demand.

• The proper selection of polymers for coagulation has a significant impact on organic removal.

• Color removal is affected by pH. Generally, organics are less soluble at low pH.

A REVIEW OF EQUIPMENT OPTIONS

Although we have discussed the major hardware, it is still worthwhile reviewing these in relation to the major classes of clarifier processes. The major categories of this process are:

• conventional

• solids-contact

• sludge-blanket

Conventional clarification is the simplest form of the process. It relies on the use of a large tank or horizontal basin for sedimentation of flocculated solids. Figure 21 provides a sketch of the basic configuration. The basin normally contains separate chambers for rapid mix and settling. The first two steps critical in achieving good clarification. An initial period of turbulent mixing is needed for contact between the coagulant ans suspended solids. This is followed by a period of gentle stirring which helps to increase particle collisions and floe size. Retention times are typically between 3 and 5 minutes, 15 to 30 minutes for flocculation, and 4 to 6 hours for settling. Coagulants are added to the wastewater in the rapid mix chamber, or sometimes immediately upstream. The water passes through the mix chambers and enters the settling basin. Refer again to Figure 21, which is a classical large-tank clarifier. The water passes out to the circumference, while the flocculated particles settle to the bottom. Accumulated sludge are scraped into a sludge collection basin for removal and disposal (sometimes post processing, as discussed in Chapter 10). The clean water flows over a weir and is held in a tank , which is referred to as a clearwell. A rectangular version of a conventional clarifier is illustrated in Figure 22. This unit is referred to as a horizontal basin clarifier.

Drive

Drive

Mud Clarifier Diagram
Figure 21. Sketch of a large tank or circular clarifier.
Clarifier Clearwell
Figure 22. Sketch of a horizontal basin clarifier.

It is often advantageous to employ a zone of high solids contact to achieve a better quality effluent. This is accomplished in an upflow clarifier, so called because the water flows upward through the clarifier as the solids settle to the bottom. Most upflow clarifiers are either solids-contact or sludge-blanket type clarifiers, which differ somewhat in theory of operation. Cross-sections of these two types of units are illustrated in Figures 23 and 24. Both units have an inverted cone within the clarifier. Inside the cone is a zone of rapid mixing and a zone of high solids concentration. The coagulant is added either in the rapid mix zone or somewhere upstream of the clarifier.

Agitator

Agitator

Homemade Laboratory Filter Paper
Figure 23. Sketch of a solids-contact clarifier.

Agitator

Agitator

Sketch Flocculator
Figure 24. Sketch of a sludge-blanket clarifier.

In the solids-contact clarifier, raw water is drawn into the primary mixing zone, where initial coagulation and flocculation take place. The secondary mixing zone is used to produce a large number of particle collisions so that smaller particles are entrained in the larger floe. Water passes out of the inverted cone into the settling zone, where solids settle to the bottom and clarified water flows over the weir. Solids are drawn back into the primary mixing zone, causing recirculation of the large floe. The concentration of solids in the mixing zones is controlled by occasional or continuous blowdown of sludge.

Some Application Pointers for Clarifiers l. 2.

Primary coagulants need good mixing Carefully read over all equipment manuals and know the operating ranges and limitations Experiment with different feed points Don't feed polymers too close to chlorine or other oxidants Use dilution water with polymers Split up feeding of polymers Watch blowdown carefully Watch the centerwell for troubleshooting and to observe the need for any chemical dosage changes Optimize the operating variables (e.g., sludge bed depth, turbine speed, chemical feed rate, etc.)

The sludge-blanket clarifier (Figure 24) goes one step further, by passing the water up from the bottom of the clarifier through a blanket of suspended solids that acts as a filter. The inverted cone within the clarifier produces an increasing cross-sectional area from the bottom of the clarifier to the top. Thus, the upward velocity of the water decreases as it approaches the top. At some point, the upward velocity of the water exactly balances the downward velocity of a solid particle and the particle is suspended, with heavier particles suspended closer to the bottom. As the water containing flocculated solids passes up through this blanket, the particles are absorbed onto the larger floe, which increases the floe size and drops it down to a lower level. It eventually falls to the bottom of the clarifier to be recirculated or drawn off.

Although these processes seem relatively simple, especially in relation to many chemical manufacturing operations or unit processes, there are a number of operational problems that can make the life of an operator miserable. Excessive floe carryover is a very common problem. This is most often associated with hydraulic overload or unexpected flow surge conditions. You can tackle this problem by relying on equalize flow (metering the flow of the clarifier), which will help to dampen out surges. Unfortunately, hydraulic overload conditions are not the only causes of excessive floe carryover. Other reasons many be thermal currents, short-circuiting effects, low density floe, chemical feed problems. Another common operator problem is simply no floe in the centerwell. This can result from underfeeding of chemicals or a loss of the sludge bed recirculation. Refer to the sidebar discussion on this page for some general corrective actions you can try.

You will have to investigate and apply trial and error field tests to resolve some of these problems. When new equipment are installed, it is wise to spend time during a shake-down and start-up period to explore the operational limitations of the process and train operators on how to handle these types of problems.

Corrective Actions for Floe Problems

Excessive Floe Carryover

Cause: Hydraulic overloading; flow surges.

Corrective Action: Equalize flow to eliminate surges.

Cause-. Thermal currents.

Corrective Action: Equalize flow.

Cause: Short-circuiting.

Corrective Action: Use tracer dye to confirm and identify exact nature of problem. Check clarifler internals such as mixing baffles and modify as needed. Cause: Low density floe.

Corrective Action: Increase coagulant/flocculant doses. Cause: Chemcial feed problems.

Corrective Action: Check pump and meter settings. Confirm thast setting correspond with anticipated chemical feed rates. No Floe in Centerwell Cause: Chemical underfed.

Corrective Action: Check operation of feed pump. Check pump settings. Cause: Sludge bed recirculation lost. Corrective Action: Increase recirculation slowly.

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Responses

  • colomba
    How does the water temperature affect the mixing speed of a upflow solids contact clarifier?
    2 years ago

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