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Polymers for Coagulation, Flocculation and
Dewatering
Sludge Dewatering
Jar Testing Product Line
Jar
Testing Dosage Chart
Tramfloc Polymer Technology:
The Process And Jar Testing
A. COAGULATION - FLOCCULATION
The purpose of water clarification is to remove the suspended particles and colloidal materials from water supplies or from wastewater. Dense suspended solids such as sand can be easily removed by sedimentation. Problems in clarification begin with the less dense suspended particulate matter and the colloidal materials. Small, less dense particulate matter may be removed by sedimentation only with extended detention times not available in many situations. Colloidal materials form very stable water suspensions. Colloidal particles may consist of clay and silt, color bodies, precipitated iron or manganese oxides, and bacteria and algae. Treatment processes such as lime softening may produce colloidal calcium carbonate or other precipitates requiring clarification. Often, many colloidal species are formed during the secondary biological waste treatment process. The need for water and wastewater clarification exists in most every type of situation involving water renovation.
Coagulation and flocculation are the mechanisms by which particulate and colloidal materials are removed from water in the process of clarification. Coagulation can be defined as charge neutralization which results in the destabilization of suspensions of particles in the colloidal size range (1-500 millimicrons) allowing agglomeration to occur. Colloidal particles have a negative electrical charge. This net negative charge results in stable suspensions due to the repulsive forces between each particle. As the halo of net negative charge surrounding each particle is neutralized, it is stripped away reducing the effective particle diameter. One may easily visualize the particle agglomeration that may occur through natural particle collision, helped by slow mixing, once the charge is neutralized and the effective diameter of the particles is reduced.
Theoretically, the term coagulation may also be applied to the neutralization of a net positive charge surrounding particles suspended in water. However, in most waters requiring charge neutralization, it is a net negative charge that must be neutralized. Therefore, the term coagulant is applied only to cationic chemicals.
The extent of particle agglomeration due to coagulation cannot be predicted. In some circumstances very small numbers of particles may agglomerate to form microfloc. The coagulated material may or may not be agglomerated enough to achieve good settling without flocculation. A high density macrofloc may not need further treatment, while microfloc and low density macrofloc will likely require flocculation for optimum clarification.
Flocculation can be defined as the mechanism by which microfloc or low density macrofloc particles are further agglomerated resulting in rapid settling floc bodies and enhanced finished water quality.
Inorganic coagulants have been used to clarify water for years. Trivalent ions such as aluminum and ferric iron coagulate colloidal suspensions by charge neutralization and by promoting agglomeration. Therefore, in addition to their coagulating ability, they are also capable of further flocculation through their ability to form hydrated gelatinous hydroxides, at appropriate pH levels. These gelatinous hydroxides entrap destabilized particles as they sweep through the water under the force of gravity.
Polymeric coagulants, or that class of polyelectrolyte with relatively low molecular weight (compared to flocculant polymers) and a high cationic charge density, are finding wide application as coagulants in water clarification. Polymeric coagulants also have some ability to cause flocculation through a mechanism quite dissimilar to that of the inorganic coagulants. Flocculation by polymer coagulants is brought about via their molecular weight (M.W.) which is very high as compared to inorganic coagulants.
Molecular weight, for the purposes of this discussion, may be pictured as representing polymer chain length. The greater the M.W., the greater the chain length and the greater the flocculating ability. Longer chain length allows bridging, or attaching to, greater numbers of particles. However, coagulant polymers have relatively low molecular weights and flocculating ability when compared to the high molecular weight cationic, nonionic and anionic flocculant polymers. Therefore, polymeric coagulants have the ability to enhance flocculation only to a limited degree.
Many water and wastewater treatment facilities using either inorganic or polymer coagulants may require enhanced flocculation to meet water quality requirements. Enhanced flocculation may be brought about via the higher molecular weight cationic, nonionic, or anionic flocculants. Flocculants may be pictured as working mainly by their ability to bridge microfloc particles producing larger, more dense, and faster settling floc while also producing a clearer, solids free, supernatant.
It is difficult to predict whether a cationic, nonionic, or anionic flocculant will produce the best result. Due to this non-predictability, there can be no substitute for exhaustive jar testing when it comes to selecting coagulants and flocculants for an economic system of water clarification.
The preceding discussion was slanted toward gravity sedimentation carried out in clarifying equipment of various designs. This same model can be used when developing a chemical treatment system for enhanced clarification via dissolved air flotation, D.A.F.. First, there must exist solid, particulate material in the form of microfloc to separate from the aqueous phase. These particles can generally be produced or maximized by charge neutralization and destabilization of the suspension with coagulant materials. Once the cationic demand is satisfied, further flocculation of the particles may be obtained with polymer flocculants. These materials are used to produce floc to which the minute air bubbles will attach and float to the surface intact.
In facilities presently using inorganic coagulants to satisfy a high cationic demand, it may not be economically possible to replace the inorganic with a polymeric coagulant. However, partial replacement of the inorganic with polymer may be beneficial in terms of chemical handling ease and lower solids production. Total replacement of inorganic coagulants with polymer material is becoming commonplace with the continuing development of coagulants with higher charge density.
B. SLUDGE TREATMENT - Thickening/Dewatering
The solid material, separated from water in the process of clarification, is called sludge. Great quantities of polymeric flocculants are used to dewater sludge in order to produce a product that can be disposed of economically. The mechanical equipment used to dewater sludge includes sludge thickening tanks, centrifuges, vacuum filters, and filter presses.
After a dewatered sludge cake is obtained, the sludge is generally either trucked to a land fill site or burned in an incinerator. In either case, a product that will produce a drier sludge cake can be translated into many dollars of savings.
Again, it is difficult to predict which flocculant will do the job most economically. Therefore, bench testing the flocculants in cylinder or Buchner Funnel tests is of prime importance before proposing a material for limited full scale tests. Generally, the cationic flocculants (high M.W.-low charge density) have been very successful in treating domestic and industrial sludge. Some sludges respond best to nonionic or anionic flocculants. In some cases it may be advantageous to further satisfy cationic demand, when present, with the use of coagulant prior to flocculant addition.
C. TRAMFLOC POLYMER PRODUCT LINE
D. POLYMER SELECTION - INTRODUCTION
Jar tests are used to best advantage in the study of clarification. Type of equipment most generally simulated in jar tests are the following:
When jar testing it is convenient to have the following reagents and equipment with you, or access to same at plant site.
a. Gang Stirrer or jar tester capable of simultaneously stirring several beakers of test water.
b. Beakers for gang stirrer large enough to conveniently hold 500 ml water samples, i.e., 800-1000 ml beakers.
c. Graduated 1 ml and 5 ml pipettes.
d. Syringes (without needles) for the viscous liquid polymers. (3 ml size)
e. A 500 or 1000 ml graduated cylinder.
f. A balance to weigh out 0.5 or 1.0 gm samples of powdered polymer.
g. A mixing device (not a Waring Blender) to solubilize powdered polymer.
h. A 10% solution of alum. 100,000 ppm.
i. A 10% slurry or solution of other inorganic coagulants such as ferric sulfate, ferric chloride, sodium aluminate, etc., as required.
j. Tramfloc Polymers
k. Jar test Data Sheets to record results.
a. Liquid - Liquid polymers are easily prepared by dilution with water.
Several of the liquid anionic polymers are very high molecular weight products that will allow solution strengths to 2.0% maximum.
Liquid cationic products can be made up in virtually any concentration.
Typically, 0.1% to 0.5% solutions are prepared for clarification tests. Higher solution strengths are recommended for sludge dewatering studies. Solutions must be made weekly. If solutions are stored in a hot auto trunk they should be made up daily. When polymer solutions can be made up in concentrations of 1.0% or greater they may be stored for several weeks. All solutions of polymer store best at refrigerator temperature.
Syringes are conveniently used to measure out viscous liquid polymers to obtain positive volume displacement not obtained with a pipette.
b. Dry Products - Dry polymer test solutions may be prepared by two different methods. Typical solution strengths of 0.1% are employed in clarification studies, whereas solution strengths up to 0.5% are typically used in sludge dewatering studies.
Test Solution Preparation - 0.1%
NOTE - Violent agitation and high mixer blade tip speeds as produced by Waring Blenders are not recommended for any polymer solution preparation.
c. Make-Up
Remember that:
a.
And that:
b.
Calculations are made as follows:
c. Using a 1 ml quantity of a 0.25% solution in 500 ml of test water, how many ppm are present in the treated water?
2,500 ppm x 1 ml = 5 ppm
500 ml
| TABLE I | |
|---|---|
|
DOSAGES IN 500 ml of TEST WATER USING VARIOUS % STOCK SOLUTIONS |
|
| Stock Solution | ppm dosage per ml stock solution added |
| 2 | 0.1 |
| 5 | 0.25 |
| 10 | 0.5 |
| 20 | 1.0 |
a. Coagulant: Rapid mix for 2 min. at 80-100 rpm. Slow mix for 1 min. at 10-20 rpm.
b. Flocculant: Rapid mix for 30 sec. at 80-100 rpm. Slow mix for 3 min. at 10-20 rpm.
Because of water quality variations and unknown factors influencing coagulation and flocculation, it is impossible to predict the types and amounts of reagents necessary to achieve the desired result economically. Therefore, trial and error experimentation via the jar test procedure must be carried out, preferably with fresh water samples at the job site.
The important thing is to plan the work to permit reaching a logical, definite conclusion with a minimum of time and effort. In laying out your plan, you will want to ask yourself several questions and then seek answers to these questions from jar test data. Generally, the questions are as follows. They are listed in logical order and are generally answered in the same order.
If you have found that a coagulant is necessary, you will next want to determine whether or not an organic coagulant can totally or partially replace the inorganic coagulant demand economically. E-mail Tramfloc for classification of cationic coagulants vs. cationic flocculants.
Can you totally replace inorganic coagulant with organic coagulant? If not, what combination of inorganic and organic coagulant is most economical, remembering that inorganic coagulant can be adding significantly to the amount of solids produced.
Remembering that alum can act simultaneously as a coagulant and a flocculant, can a total replacement of alum be economically achieved with organic coagulant followed by organic flocculant.
In many instances good clarification and settling can be attained with an organic coagulant, alone, no flocculant being required.
If it has been determined that an inorganic coagulant will have to be used or recommended at your account, is alum the best choice? Perhaps another inorganic will work as well or better. Perhaps the procurement situation at your account makes a particular inorganic coagulant less expensive than others.
When it appears that very little or no inorganic or organic coagulant is necessary, in other words, that very little if any charge neutralization is required, then it may be possible to achieve clarification with a flocculant alone. Without prior knowledge, it may take a cationic, nonionic, or anionic high molecular weight, low charge density, flocculant to give the desired economic effect.
Again, determine the following in jar tests:
On the jar test data sheets record floc size, settling rate, and supernatant clarity on a scale of 1 to 10. The best is represented by 10. Untreated control samples are represented by 5. Good results as compared to the control are then graded 6-10. Results poorer than control are graded 1-4. No visible effect, as compared to the control, is graded as 5.
In some cases you may also be interested in the volume of sludge produced and how this volume may vary depending upon the treatment. This can easily be done using graduated jar test beakers after a given period of settling time. Record in "Comments" section.
What is your proposed program remembering that the coagulants need thorough, rapid mixing followed by slow mixing and that the flocculants require some rapid mixing for adequate dispersion in the water to be treated followed by longer periods of quiescence.
a. How much and where will coagulant be applied to system? Will the coagulant be inorganic, organic, or a combination?
b. If flocculant is required, what is the proposed dosage and where will it be applied? (Always following coagulant.)
c. What pumps, tanks, and mixing equipment will be necessary?
d. How will you measure success of the program?
e. How will you determine whether or not the plant operators are using your prescribed dosage?
After you have decided what your proposed dosage will be in ppm from the jar tests, you will have to calculate your pounds/hr. or pounds/day dosage in the water flow to be treated.
Water flow: gal/mm. X 60 = gph
gal/mm. X 1440 = gal/day
and: gph X 8.34 = lbs. of water/hr.
gal/day X 8.34 = lbs. of water/day
Remembering that ppm or parts/million parts is the same as pounds/millions pounds; and you want to treat a given water flow at 5 ppm, then:
1,000 gpm flow X 1,440 min/day X 8.34 lbs/gal =
12,009,600 pounds of water/day, or simplified:
1,000 X 12,000 = 12,000,000 pounds of water/day.
therefore:
lbs. of product/day = ppm product dosage X gpm water flow X 12,000
lbs. of product/day = 12,000 X 1,000 X 5 = 60 lbs/day
1,000,000
If the specific gravity of the polymer is 1.12 this means it is 1.12 X the weight of water, which is 8.34 pounds/gallon. Therefore, this hypothetical polymer weighs:
1.12 X 8.34 = 9.34 pounds/gal
and: 60.05 lbs polymer/day = 6.43 gals. polymer per day
9.34 lbs/gal
and: 6.43 gals polymer/day = 0.2679 gals/hour
24 hr/day
or: Since 1 U.S. gallon = 3.785 liters
then: 0.2679 gals/hr. X 3785 = 1,014 milliliters/hr.
but: All of the above is based on neat polymer, and since you
are going to dilute the polymer before addition to the
system, you will have to take the following into consideration.
For instance: You have made up a polymer solution in a day tank at 0.5%.
This means that the polymer has been diluted 200X.
This means: Your application rate of the diluted polymer must be 200 times
the gal. or milliliter figure given above in order to have the
application rate in neat polymer, as previously calculated.
Therefore: 0.2679 gals/hr. X 200 = 53.58 gals/hr. of a 0.5% polymer solution.
Alternatively: 0.2679 gals/hr. = 53.58 gals/hr.of a 0.5% polymer solution
.005
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Last updated October, 2009
