Understanding Separation Essentials For Wastewater Treatment
By Dr. J.H. Wakefield
Overview
In wastewater treatment, all wastestreams consist of solid and liquid components — all of them. The first step in the treatment process is to remove as much as is feasible of the solid components (hereafter referred to as the solid phase) from the liquid ones. These steps may involve bar screens, various filtering technologies, sedimentation basins of one sort or another, clarifiers, other separation technologies such as centrifugation, other physical means (impaction, for example), and even chemical treatment.
The second step involves the treatment of the liquid components (wastestream) by means of chemical and biological methods. A commonly used method involves the application of activated sludge and ancillary procedures including oxygenation and chemical "charging" to result in the destruction of pollutants that have deleterious effects on the environment (toxic ions and organic compounds, for example) or that have both a direct and an indirect environmental effect by lowering aqueous oxygen levels.
All ensuing steps in wastewater treatment are relegated to the category of tertiary treatment. These usually include methods for removal of additional components of the liquid phase, advanced treatment technologies for the further separation of difficult-to-remove solid components, and remediation and treatment of engendered sludges resulting from the solid-phase components. There are others such as the removal of radioactive isotopes/elements from these wastestreams, but they (at this time) are not a widespread problem, and we hope that they will never be.
The solid and liquid phases of the wastewater stream both contain mixtures of various substances which are physically combined but may, or may not, be chemically combined. Furthermore, these mixtures may differ in chemical and physical properties from the individual substances from which they originated. These solid-phase wastestream components may be categorized as settleable solids, suspended solids, or colloidal solids. The liquid-phase components are designated as colloids, soluble compounds, gases, or ions. In fact, ions are charged solvated solids that have chemically reacted with the solvent (usually water) in the wastestream to enter into the solution. Colloids are solids of such small size that they are dispersed with an adsorbed charge to maintain stasis in the liquid phase. These colloids are actually mixtures (either homogenous or heterogeneous) in which the particles are invisible to the naked eye, cannot be removed by filtration, but can be contained within a semi-permeable membrane. Solids may be encountered in liquids under several forms: solutions, colloidal dispersions, and solid suspensions. As previously mentioned, the nature of these states depends on the size of the solid particles. Although we keep repeating ourselves on much of this, it can be a fruitful field for consideration, and much knowledge of the true state and mechanisms involved are slowly realized as we delve into them with increased knowledge.
Solids Settling
Settleable solids include those that are quite large and can be removed via screening procedures, as well as those that are somewhat smaller which can be removed by settling them out in appropriate structures. Settleable solid particulates may be defined as those larger than 1 micrometer (1 μm) and with a specific gravity greater than 2.6. Particulates smaller than this are relegated to colloids if their sizes range from >1 nanometer (1 nm.) to <1 micrometer (1 μm). Colloids are considered dispersed — that is, not chemically bound — but they are characterized by an adsorbed surface charge which enables them to maintain a stable dispersion. As the particulates reach even more miniscule levels, they are relegated to chemical compounds (solvated) or ions; the size range for these is <1 nanometer (1 nm.).
Separation would be more easily attained between the solid and liquid phases if the solid-phase material and particulates were larger and heavier to allow gravity separation of the solid phase. As these particulates get smaller (and lighter) as colloids, significant separatory problems arise. And, of course, solvated particulates and ions, which are even smaller (and lighter), require chemical changes to separate them. By using appropriate coagulants and flocculants, we are able to increase the density of the treated mixture components and thus let our old friend gravity assist us in their separation. Some of this involves collapsing electrostatic charge fields that stabilize some of the colloids and by using chemical changes to induce chemical reactions to form precipitates in the dissolved solutes (or otherwise change them into more manageable forms).
At this point it would be advantageous to realize that coagulation and flocculation are not the same thing (synonymous, that is), but they are frequently considered as synonymous. As a rule, coagulation is the process by which colloids are removed from a liquid phase by "precipitation" — essentially, an induced aggregation. These colloidal particles (and very fine solid suspensions as well) are physically destabilized so that they can agglomerate under the appropriate conditions. This is usually accomplished quickly with a maximum amount of vorticity (stirring); whereas, flocculation facilitates the settling of suspended solids in solutions (or in dispersions which have been "collapsed") and results in the conglomeration of these aggregated particulates into even larger aggregates for ease in separation. Flocculants are considered bridge-formers among the agglomerated clumps of solids, making them larger, and the flocculants are usually (if not always) applied with very gentle mixing to allow the growth of the flocculant particulates. These dispersed solids (colloids) suspended in wastewater streams are stabilized (for the most part) by negative electric charges on their surfaces. This causes them to repel each other. Because this action prevents these charged particles from colliding to form larger masses, called flocs, they do not settle. To assist in the removal of these colloidal particles from suspension (dispersion), chemical coagulation and flocculation are required. These processes, usually performed sequentially, are a combination of chemical and physical procedures. Chemicals are mixed with the wastewater streams to promote the aggregation of the suspended solids into particles of sufficient size to settle or be removed.
Coagulation And Flocculation
Coagulation literally means to curdle and refers to the process of blood clotting and the formation of clots or large lumps or even to sour milk forming curds. It is the process by which colloidal particles and very fine solid suspensions initially present in a wastewater stream are combined into larger agglomerates that can be separated by means of sedimentation, flocculation, filtration, centrifugation, or other separatory methods. This refers to a chemical process in which destabilization of non-settleable particles is realized. These non-settleable particles can refer to most colloids as well as extremely small solid particles and all solvated (dissolved) particles. These particles form clumps with the help of a coagulant. It is commonly achieved by adding different types of chemicals (coagulants) to the wastewater stream to promote destabilization of any colloid dispersion present and the agglomeration of the individual resultant colloidal particles. Coagulation is the destabilization of these colloids by neutralizing the electrostatic forces that keep them apart. Cationic coagulants provide positive electrostatic charges to reduce the negative electrostatic charges (zeta potential) of the colloids. As a result, these particles collide to form these larger floc particles. Rapid mixing is a requirement of the coagulation process in order to disperse the coagulant throughout the liquid. Care must be taken to minimize an overdosing of the coagulants as this can cause a complete electrostatic charge reversal and a restabilization of the colloid complex. Flocculation refers to the coming together of particles (floc means flake). At times, this needs to be observed under a microscope. It is a physical or mechanical process resulting in the joining together of large aggregated clumps (or flocs) to form larger masses and eventually to precipitate them from the liquid phase and thereby convert them into the solid phase for further separation. In coagulation, these forces responsible for keeping the particles suspended and dispersed after they contact each other are reduced. This is usually referred to as collapsing the colloid and/or precipitate formation in the case of solvated particles. Flocculation joins these de-established colloidal dispersions into large aggregates that enter the solid phase.
Both water treatment and wastewater treatment methods use coagulants and flocculants to achieve the ends of removing these particles from the liquid phase and rendering them components of the solid phase. Water and wastewater (always) commonly contain colloidal particles including non-settleable organic matter, clay particles, plankton, bacteria, small particles of decayed plant material, and other dispersed moieties in the colloidal range. The addition of some common coagulants to a wastewater stream not only produces the coagulation of colloids, but also results in the precipitation of soluble compounds, e.g., phosphates that can be present in the wastewater stream. This is known as precipitation and sometimes is also referred to as sequestration, particularly if the phosphate is bound into some carrier clay such as Bentonite. Yet another advantage of coagulation for removal of wastestream entities resides in the removal of particles larger than colloidal ones as a result of entrapment of such particles in the gelatinous products formed during coagulation — think of liquid flypaper. Once suspended particles are agglomerated/flocculated into larger particles, they can usually be removed by means of sedimentation, that is, if a sufficient density differential exists between the suspended matter and the liquid phase. Such particles can also be removed or separated by media filtration, straining, or flotation. When a filtering process is used (particularly the PeriFilter®, depending on the total suspended solids [TSS] levels), the addition of a flocculant may not be required in as much as the particles formed by the coagulation reaction(s) may be of sufficient size to allow removal. Some flocculation reactions also not only increase the size of the floc particles to allow them to settle faster, but also can affect the physical nature of the floc, making these particles less gelatinous and thereby easier to dewater.
Colloids, which are physical dispersions, may be either hydrophilic or hydrophobic. Hydrophilic colloids are typically formed by large organic molecules that become hydrated (solvated) when they are in the presence of water. These molecules are thermodynamically stable in their solvated forms. The electrostatic charge on such molecules originates from the presence of ionizable groups on the molecule that transforms the molecule into a "macro-ion" when placed in solution. As a consequence of these electrostatic charges, hydrophilic colloidal particles are significantly hydrated when placed into an aqueous solution. Agglomeration of hydrophilic colloids typically involved the addition of significant amounts of ions, which compete with the colloids for water molecules. This results in the dehydration of the colloidal particles, which is commonly referred to as salting out. Hydrophobic colloids, on the other hand, are comprised of small colloidal particles having little or no affinity for water (the solvent). Their stability results from the presence of an electrostatic charge, which attracts other ionic species present in the water. The result is a formation of an electrically charged layer around the colloidal particles. These colloidal dispersions are thermodynamically unstable. If the charge layer is removed, the particles tend to agglomerate spontaneously and can be removed from the wastewater stream.
Let us delve into this topic in greater detail. As a general rule, we have seen that coagulation refers to the use of chemicals, whereas flocculation is normally relegated to the action of polymers to form bridges among the floc particles and to bind them into large agglomerates or clumps. Bridging occurs when segments of the polymer chains adsorb on different particles and help these particles to aggregate. An anionic flocculant will react against a positively charged colloidal dispersion (or suspension, as the case may be), adsorbing on the particles and causing destabilization either by bridging or electrostatic charge neutralization. This process is sensitive so that slow and gentle mixing is called for to allow these particles to agglomerate into larger particles. These newly formed particles are quite fragile and can be broken apart by excessive shear forces during the mixing process. It is also incumbent to ensure that overdosing of the polymer does not occur as so doing will result in settling/clarification problems. Anionic polymers themselves are lighter than water. As a result, increasing the dosage will increase the tendency of the floc to float and not to settle. All is not lost, however, in this situation, as skimming can remove the floating floc.
Physical Chemistry
It is now time for us to examine the physical chemical structures of particles (focusing on colloids) and, to this end, please consider the following information — disjointed though it may appear. There is a double layer associated with colloidal particles. As a colloidal particle is electrically charged, it attracts ions as well as other colloidal particles of the opposite electrostatic charge. These ions are tightly attached by electrostatic forces to the colloidal particle forming an inner layer of charges. This layer is referred to as the Stern layer and has the thickness of a single, hydrated ionic layer. This colloidal particle and the attached ions of opposite electrostatic charge form an electrostatic double layer. Additional ions of opposite sign to that of the colloidal particle also accumulate next to the Stern layer. These form the diffuse layer. Within the diffuse layer there is typically another layer of ions which are not so tightly attached to the particle as the Stern layer, but which cannot be removed by the presence of any external velocity gradient either. These ions bound to the particle even as the particle moves, delimit the shear plane, that is to say, the plane of ions which are unaffected by fluid motion.
Neutralization of the colloid charge is the basis for collapsing these colloid fields. This mechanism is based on the addition of large organic molecules containing ionizable groups to the colloidal dispersion. The charge of the dissociated molecule must be opposite to the electrostatic signature of the colloid. As the organic molecules dissociate the resulting charge, charged molecules can effectively interact with the Stern layer, thereby replacing the counter ions originally present in this layer. Because of the size of the molecule and the low charge on its organic "tail," the potential around the particle is decreased, allowing (even making) the colloidal particles to interact, agglomerate, and coagulate.
Collapsing a colloidal dispersion can be accomplished by means of "bridging" by a polymer addition. The addition of large polymeric molecules having charged or ionizable sites (polyelectrolytes) to a colloidal dispersion results in the attachment of the polymer to the colloid (just as in the charge neutralization process examined previously). Because of the polymeric chain, the motion of the colloid-polymer particle is hindered, thereby promoting contact with other particles. The polymer chains protruding from colloid-polymer particles can interact with similar particles forming "bridges." This results in polymer agglomeration and eventual coagulation. If significant amounts of aluminum or ferric salts are added to a solution, the hydroxides of these metals will precipitate (depending on the pH). During this precipitation process, the hydroxides form large, three-dimensional polymeric structures (floc formation). As these polymeric structures form, the colloids can become entrapped in it and can be precipitated by a "sweeping floc" mechanism. Large amounts of sludges are formed as a result of this action.
Another variable that we must be cognizant of is the zeta potential. The zeta potential is defined as the electric potential difference between the shear plane of a colloidal particle and the bulk of the solution. The zeta potential can be considered an indirect measure of the electrical charge of the colloidal particle. The zeta potential can be experimentally measured (using a microscope) by determining the velocity of a particle moving under an electric potential of known intensity. (The actual potential between the surface of the particle and the solution cannot be measured experimentally.)
Quantitative Determination of Zeta Potential from Experimental Data in which the zeta potential is defined as:
ζ=4Πν/εVx = 4ΠμEM/ε
Where: ζ = zeta potential v = particle velocity ε = dielectric constant of the medium Vx = applied potential per unit length EM = electrophoretic mobility.
Quantitative Determination of Zeta Potential from Experimental Data At 25°C in water, the zeta potential can be directly calculated from:
ζ = 12EM
Where: ζ = zeta potential in mV, EM = electrophoretic mobility in (μm/s)/ (V/cm).
Average zeta potential for wastewater colloids: -16 to -22 mV (rnge: -12 to -40 mV)
Another related parameter to consider when examining the zeta potential is the isoelectric point; in fact, that is the purpose of determining the zeta potential. When colloidal particles are in the presence of sufficient counter ions, they become electrostatically neutral. This point is known as the isoelectric point. The zeta potential at this point is zero. If the particles are not any more "shielded" by the double layer (as they are at this time), they can interact with each other. Therefore, this isoelectric point is the point in which the particles have the highest potential for agglomeration.
If the repulsive forces produced as a result of the presence of the double layer are too significant, the colloidal particles will not be able to come in close contact for the attractive forces to produce agglomeration and thereby coagulation. In such a case, the effects of the double layer must be neutralized (for example, by increasing the ionic strength of the solution) for coagulation to occur.
Repeating ourselves, coagulation is a process of destabilization of colloids. Coagulation of colloids occurs when a stable colloid (because of the intrinsic stability of the colloidal dispersion) or a stabilized colloid (because of the presence of a double layer) is destabilized. Destabilization, as previously mentioned, results when colloidal particles are brought close enough to each other for agglomeration to occur. Several of these destabilization mechanisms are possible. These mechanisms depend on the type of colloidal suspension (dispersion) that undergo different coagulation mechanisms, to wit: repression of the double layer; neutralization of the colloid charge by adsorption of counter ions on the surface of the colloidal particles; "bridging" of colloidal particles by means of polymer addition; and, entrapment of colloidal particles by sweeping floc. We have examined these previously to add to our understanding of these phenomena as they relate to wastewater and water treatment processes.
Let us now more closely examine the repression of the double layer. The potential generated by a charged particle decreases rapidly with the distance from the particle's surface. As a consequence, the electrically charged layer surrounding the particles also decreases with the distance from the particles. If the particle is surrounded by a large number of added ionic species, their presence will interfere with the potential generated by the particles. Consequently, the potential will decrease even faster with the distance from the particles. The thickness of the electrically charged layer surrounding the particles is arbitrarily taken as that distance at which the potential falls to 37 percent (=1/e) of the surface value. From the Debye-Hückel Theory, it is possible to calculate that the thickness of this layer is given by the equation:
Z=0.33 X 10-2 (ε/I)1/2
Where: Z= thickness of layer (in cm.); ε = dielectric constant for the solution (C/ (V cm); I = ionic strength (moles/liter). For water at 20°C, it is:
Z= 3.0 X 10-8
Where: Z is in cm and I is in moles/liter. Example: for I=0.001M, then Z ≈ 100 Å; for I=0.1M., then Z=10 Å.
The results from the previous equations indicate that double-layer repression can be accomplished by increasing the ionic strength of the solution. This increase does not alter the charge of the colloidal particles, but reduces the extent to which the same charge affects the potential around the charge. Double-layer repression can therefore be achieved by: increasing the ionic strength of the solution by adding additional ionic species; even more effectively increasing the ionic strength of the solution by adding ions of high valence in as much as:
I= ½ Σ Cj Zj2
Where: Cj = concentration of ionic species and Zj = charge of the ionic species.
The typical chemicals used in double-layer repression are those that produce cations with a large charge such as Al+++ and Fe+++. As a consequence, chemicals such as Al2(SO4)3 · 14 H2O (alum) and FeCl3 (ferric chloride) are often used as coagulants. These salts also produce coagulation because of their charge suppression and bridging capability. As an example, Al+++ is ≈ 560 times more effective in this regard than is Na+ or any other monovalent cation such as K+.
Parting Words
I hope this has been useful in establishing a perspective and understanding of some of these issues. I have concentrated on presenting the cationic actions, but the anionic ones may be just as important (even more so) on specific encountered wastestreams and water sources. Just remember, cationic species affect negatively charged particles and anionic ones affect positively charged particles. This is "across the board" and may directly affect species that aren't even colloidal in nature.
Dr. J.H. Wakefield is a consulting scientist/engineer with more than 30 years of experience in water/wastewater treatment. He holds advanced degrees in microbiology and physical/analytical chemistry, and has been a practicing chemical and environmental engineer for many years.