Coagulation and filtration in water and wastewater treatment become more efficient by modify,ing the chemical variables to improve the efficiency osf collisions between particles and particles and filter grains Werner Stiimm Swiss FederalInstitute! of Technology Zurich, Switzerland The solid-solution interface is of particular importance in natural waters and in water treatment systems. Suspended particles in natural and wastewaters valy in diameter from 0.005 to about 100 p m (5 X lo-$ m). For particles smaller than 10 pm, terminal gravitational settling will be less than about lo-* cm per s. Filter pores of sand filters, on the other hand, are typically larger than 500 pm. The smaller particles (colloids) can become seDarated either by settlina if they. aaareaate - or by filItration if they attach to filter grains: Particle separation. is of importance in the following processes: aggregation of suspended particles (clays, hydrous oxides,
-
unit operations: innovation and process improvement, especially improvement in process dynamics, had to come from a better engineering design rather than from new tools. Progress in engineering design of coagulation (followed by sedimentation, flotation, or filtration) and filtration processes has been achieved from a more thorough understanding of the process concepts. Considering separately the chemical (esDeciallv colloid and surface chemical) and the physical (includingmasstransportand fluid mechanic) aspects is (If areat helD in assessing the variables and parameters that influc?niethese processes.Coagulation and filtration may oe renaerea more aynamlc or more efficient by modifying the chemical variables in such a way as to improve the efficiency of.the collision between particles . ~ . . . ... . .~ . ana particles and filter grains. Higher filtration rates may often be achieved without impairing the quality of the filtrate if a reduction in contact opportunities between suspended particles and filter grains (by a decrease of filter depth and media diameter) is compensated for by a chemical improvement of the collision efficiency. This article is organized along the following lines:
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Solution variables may influence the charge of colloidal surfaces; a simple case history will serve for illustration: The specific adsorption of such cations as Mg2+, Ca2+, heavy metal cations and H', and of such anions as Sod2-,HP042-, Si0 (OH)3-, OH- on the surface of hydrous oxides of Si, AI(III), or Fe(lll) affects the resultant net charge of these oxide particles. The effectiveness of particle aggregation in coagulation and particle removal in filtration, both in natural and in treatment processes, are compared; this illustrates how they 'depend on contact opportunities, on one hand, and on the efficiency of the contacts on the other; the designer of a treatment process can choose among various chemical or physical variables. Finally, an application in practice; the removal of phosphate in complementary biological and chemical sewage treatment exemplifies some of the concepts discussed.
Specific chemical interactions and colloid stability The presence of an electric charge on the surface of particles is often essential for their existence as colloids; the electric double layer on their surface hinders the attachment of colloidal particles to each other and to filter grains. The resultant net charge is sensitive to the composition of the aqueous phase, because adsorption or binding of solutes to the surface of the colloids may increase, decrease, or reverse the effective charge on the solid. Adsorption occurs as a result of a variety of binding mechanisms: electrostatic attraction or repulsion, covalent bonding, hydrogen-bond formation, van der Waals interaction, hydrophobic interaction. One speaks of specific chemical interaction if binding mechanisms other than electrostatic interaction are significantly involved in the adsorption process. Chemical destabilization of colloids is achieved in coagulation (or flocculation) and contact filtration by adding substances that enhance the aggregation or attachment tendency of these colloids. Natural and synthetic macromolecules (ofter polyelectrolytes) have a strong tendency to accumulate at interfaces and have been used successfully as aggregating agents.
The Fe(lll) and AI(III) salts used as coagulants and destabilizers belong also to this category because they form polynuclear hydrolysis products, Me,(OH),Z+, which are adsorbed readily at particle-water interfaces. That specific chemical interactions contribute significantly to the adsorption and colloid destabilization is evident from the observation that these coagulants, at proper dosage, can cause reversal of the charge of the colloid. Colloids also become less stable with increasing concentrations of indifferent (not specifically interacting) electrolytes because the diffuse part of the electric double layer becomes compressed by counter-ions. Oxides, especially those of Si, AI, and Fe, are abundant components of the earth's crust; they participate in many chemical processes in natural waters and often occur as colloids in water and waste treatment systems. The colloid stability of hydrous oxides may be affected by electrolytes in a different way than that of hydrophobic colloids. Specific adsorption of cations and anions on hydrous oxide surfaces may be interpreted as surface coordination reactions. Hydrous metal oxides exhibit amphoteric behavior and can, at least operationally, be compared with amphoteric polyelectrolytes: EMeOH2+ ~t GMeOH H+; Ki,
+
EMeOH
2
GMeO-
t H+;K&
Since H+ and OH- ions are primarily the potential determining ions for hydrous oxides, alkalimetric, or acidimetric titration, curves provide a quantitative explanation for the manner in which the charge depends on the pH of the medium. Operationally, there is a similarity between H+, metal ions, and other Lewis acids. The OH group on a hydrous oxide surface has a complex forming 0-donor group like OH- or OH groups attached to other elements (phosphate, silicate, polysilicate). Proton and metal ions compete with each other for the available coordinating sites on the surface: EMeOH + Mz+ 2EMeOH
+ Mz+
=MeOM(Z-')
+ H+.I "KS1
(=Me0)2M(Z-2)
+ 2H+; $'i*
The effectiveness of coagulation and filtration depends on a related dimensionless product cy
Coagulation:
Collision efficiency
Filtration:
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t
Volumetric concentration of suspended particles
Velocity gradient
Time
1-f
Volumetric concentration of filter medium
L d Number of collectors
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Single collector efficiency (v, d) V
Contact opportunities Design and operational variables
Chemicals
Coagulation aids, media size, sludge recirculation
Energy input, mass transport
Residence time, filter lengths, and media diameter
Volume 11, Number 12, November 1977
1067
The extent of coordination is related to the exchange of Hf by Mz+ ions; and it can be measured by the displacement of alkalimetric titration curve. Similarly, ligand exchange with coordinating anions leads to a release of OH- from the surface and a displacement of the titration curve in the other direction:
+ 4z- p =Me-AZ-l + OH-; K: + 20H-; pi 2zMe-OH + AZ=Me-OH
$
For protonated anions the ligand exchange may be accompanied by a deprotonation of the ligand at the surface. For example, in the case of HP04*-: kMeOH
+ HP04*- F! =MeHP04- + OHe =MeP0d2- + H20
How the variables determine the Coagulation efficiency
cesses in natural waters and in water and waste treatment. As pointed out by O’Melia and Stumm and others, filtration is analogous to coagulation in many respects. This is illustrated by juxtaposing the basic kinetic equations on particle removal: coagulation: -dn/dt = (4/7r)aG4n
*)
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(1 f) agn filtration: -dn/dL = (312)d where n is the concentration of particles (number per cm3), a is the collision efficiency factor that reflects the chemistry of the system, 4 is the volume of solid material (suspended particles) per unit volume of solution, G is the mean velocity gradient [time-l] that reflects the rate at which contacts occur between particles by mass transport, t is the time, (1 f) represents the volume of filter media per unit volume of filter bed (f = porosity), p is a “single collector efficiency” that reflects the rate at which particle contacts occur between suspended particles and the filter bed by mass transport, L is the bed depth, and d the diameter of the filter grain. [ * ) [The rate of coagulation for particles of smaller diameter (d < 1 p) occurring mainly by Brownian interparticle contact is given in the next equation.] The singlecollector efficiency is an inverse function of filter medium size, filtration rate, and water temperature, and also depends on the size and density of the particles to be filtered.] The effectiveness of particle aggregation in coagulation and particle removal ’in filtration depends upon a dimensionless product of the variables
-
a4Gt (coagulation)
ap(1 - f)(L/d) (filtration)
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Velocity gradient G
concentration (vol./vel.)
It is also conceivable that in a high pH condition, adsorption of a metal ion may be accompanied by hydrolysis. The process described above can be characterized by equilibrium constants. These constants can be used to estimate the extent of adsorption and the resultant net charge of the particle surface as a function of pH and solute activity. Some equilibrium constants with cations and anions have been determined for a few representative oxides. The net resultant charge at the surface of an oxide is experimentally accessible from the proton balance at the solid-solution interface (obtainablefrom an alkalimetric or acidimetric titration curve in presence of specifically bound cations and/or anions) and from a measurement of the bound unhydrolyzed cation and deprotonated anion. Specifically adsorbed cations will shift the proton condition in such a way as to lower the pH of zero point of charge (ZPC), the pH at which that portion of the charge which is due to H+ or OH- ions or their complexes becomes zero. Because of the binding of MZ+,the resultant net charge increases (or becomes less negative)and the pH at which the resultant net charge becomes zero, the isoelectric point (IEP) is shifted to higher pH values. Correspondingly, specifically adsorbing anions increase pH of ZPC but lower pH of IEP.
Particle attachment In coagulation and filtration The kinetics of particle transport and particle attachment determine to a large extsnt the various particle removal pro1088
Environmental Science & Technology
that-as shown by O’Melia-have comparable meanings for both processes. Either equation can be used to evaluate the performance of upflow sludge blanket clarifiers. Particle transport is required in both processes either to move the particles toward each other or to transport the particles to the surface of the filter grain or to the surface of a previously deposited particle. The contact opportunities of the particles to collide with one another or with filter grains depend upon Gt or p (L/d). The detention time, t, is somewhat related to (L/d), the ratio of bed depth to media diameter. One can compare how detention time, particle concentration and collection efficiency’ influencecoagulation in natural systems and in water and waste treatment systems. In natural waters, long detention times may provide sufficient contact opportunities despite very small collision frequencies (small G and small 4). In fresh waters, the collision efficiency is usually also low Le., only 1 out of lo4 to lo6 collisions leads (a = to to a successful agglomeration). In seawater, colloids are less stable (a a 0.1-1) because the salinity compresses their double layer. Estuaries with their salinity gradients and tidal movements represent gigantic natural coagulation tanks where much of the dispersed colloidal matter of the rivers becomes settled. In water and waste treatment systems we can reduce detention time (volume of tank) by adding coagulants at proper dosage (a 1) and by adjusting power input (G).If the concentration, 4, is too small, it can be increased by adding additional colloids as so-called coagulation aids. O’Melia has shown that similar trade-offs in design and operation exist for optimizing particle removal in filtration. We restrict our discussion to the efficiency of particle removal although other considerations, especially avoidance of excessive build up of head loss in the filter bed, are of course also very important. In the development from very slow filtration in ground water percolation to ultra high rate-contact filtration, a relatively Constant efficiency in particle removal (constant product aV(L/d)) is maintained despite a dramatic increase in filtration rate. This is achieved by counterbalancing decreased contact opportunities (decreasing 7, L, and increasing d) by improving (through addition
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of suitable chemical destabilizing agents (aids that increase a up to 1) the effectiveness of particle attachment.
Rates Kinetic considerations determine the type of process to be used for particle removal. The ideas discussed above and some other considerations permit us to select the particle removal process most suitable for a practical problem. As shown by Kavanaugh and Boller and others, the ranges of possible application of various particle-removal processes depend upon concentration of suspended particles, particle number concentration, and particle diameter. (See art for suitable range for application of various processes of particle removal.) The various boundaries can be readily explained: If particles are sufficiently large (d 30-50 pm) they can-in accordance with Stokes’ law-readily be removed by sedimentation (large concentrations) or by straining, e.g., microstrainer (small concentrations). Coagulation, followed by sedimentation, can be accomplished within reasonable detention times only if the particle concentration is sufficiently high so as to provide sufficient contact opportunities. This is especially important in perikinetic coagulation [ i.e., coagulation where colloids are sufficiently small (d < I pm) that interparticle contact occurs predominantly by Brownian motion (diffusion)]. In this case, the rate of particle removal may be given by
Suitable ranges for application of various processes of particle removal Particle concentration per cm3
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+ (4akT/3p)n2
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’
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where k is Boltzmann’s constant, T is the absolute temperature, and p is the fluid viscosity. With this equation one can calculate suitable processes at suspended solids concentrations larger that the detention time becomes very large (t 1 h) even for a = 1, when the concentration is smaller than I O9 particles ~ m - ~ ; than about 50 mg/L. at these low concentrations, there are not enough interparticle Phosphate removal collisions. Contact filtration however can, over the depth of the Phosphate can be removed from sewage by precipitation with filter bed, provide sufficient contact opportunities in this case Fe(lll) or AI(III) salts; precipitation may be done in the aeration (n IO9 ~ m - ~ If) particles . are larger (d > 0.1 to 1 pm), shear tank of the biological treatment. The following processes occur forces enhance the interparticle contact (orthokinetic coaguupon addition of Fe(lll) to the wastewater: lation). These larger size particles may also conveniently be precipitation of soluble phosphate to a non-stoichiometric removed by filtration; orthokinetic coagulation, followed by solid phosphate of the composition Fe(P04)x(0H)3-3x. sedimentation, or upflow sludge blanket clarification are more formation of particular Fe(OH)3or FeOOH to the surface of which phosphate becomes strongly bound coagulation of suspended solids and of suspended P by Marked increase in filtration rate can be achieved by polynuclear Fe(lll) hydrolysis products. counterbalancing a reduction in contact opportunities These processes are very efficient for the elimination of P from by chemically improving the contact efficiency, with solution, but because of the strong adsorbability of phosphate similar efficiency in particle removal to precipitated iron phosphate and to Fe(lll) oxide hydroxide, some highly dispersed P-bearing colloids are formed; these colloids do not settle and may not even be retained by a membrane filter. Pilot experiments done in the experimental station of our Institute by Boller and Kavanaugh have shown that contact filtration for removal of P precipitated with Fe(lll) following mechanicalbiological treatment is a process of practical feasibility, leading to very low residual P and suspended solids concentrations. Contact filtration with appropriate chemical doses and media design appears to be a potential alternative to coagulation/ sedimentation or flotation systems for phosphorus removal following conventional treatment. The permissible solids loading must be determined for each case. For the chemical conditions tested, and filter design constraints, the solids limit was about 50 mg TSS/L, at flow rates up to 4 gpm/ft2 (10 m/h) equivalent to about 3-4 mg P/L dissolved total phosphorus in the filter influent.
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