Coagulation and Flocculation

Many substances in wastewater vary greatly in size, from a few angstroms for soluble solids to a few hundred microns of suspended materials. Consider a force balance upon a clay particle with diameter of 1 micron; in the absence of electrostatic forces, the terminal settling velocity of this particle in water is approximately 10—4 cm/s based on the following expression (Equation 3.6):

18ju where (ps — pf) is the density difference between the particle and fluid (water), and ^ is the viscosity of the fluid (water). This is obviously too low for any practical sedimentation process. To remove a large portion of these substances from wastewater by sedimentation or filtration, smaller particulates need to be aggregated into large and more readily settleable or filterable particles. This process of forming aggregates from smaller par-ticulates is called coagulation. Many unsettleable small particulates are colloids that can exist stably in water under favorable conditions. The objective of coagulation is to destabilize the colloidal dispersion in wastewater by using either chemical/polymer agents or hydrodynamic forces. The aggregation of colloidal particulates can be visualized as a two-step process: particle contacts/collisions brought up by hydrodynamic forces and particle destabilization to allow attachment of particles when collided.

There are two terms describing aggregations of colloidal particles: coagulation and flocculation. Depending on the industry that employs the unit operation, they may mean different things or they are synonymous. In environmental engineering, particularly a wastewater treatment field, floc-culation refers specifically to destabilization of colloidal particles by forming aggregates of colloids with added water-soluble polymers (polymer bridges); coagulation is caused by destabilization of colloids through compression of electrical double layers of the particles. However, this review is not universally shared. Some experts refer to the initial step of adding chemicals (coagulants or flocculants) to the wastewater in an intensely stirred tank as coagulation and subsequent slow stirring of the destabilized colloidal suspension in another tank to promote floc growth as flocculation.

The conventional practice of initial rapid mixing followed by slow stirring of coagulation-flocculation intends to maximize the extent of the formation of aggregates of colloids; this does not always have desirable outcomes. The slow stirring of the later stage of flocculation produces many large, fluffy flocs with high interstitial water; although these large flocs are settleable, they could severely strain the operation of dewatering of sludge and ultimately result in large amounts of sludge destined for landfill (Liu, 1995). Glasgow and Liu (1995) proposed a scheme of coagula-tion-flocculation that involves slow stirring interspersed with short bursts of highly intensive mixing in a flocculator in order to produce flocs with more compact structures.

Many food wastewaters contain large amounts of organic materials, such as proteins that are of colloids in nature; they tend to be charged, a result of ionization of carboxyl and amino groups or their constituent amino acids and therefore stabilizing in the streams. Other organic substances, also common in some wastewaters, may contain grease and oil and become charged due to adsorption of anions such as hydroxyl ions. Destabilization of the colloidal suspension containing these charged colloids requires overcoming the zeta (£) potential of the colloid dispersion in order to form aggregates. Zeta potential refers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle that is organized into an electrical double-layer consisting of the immovable Stern layer and the diffuse layer. The usefulness of the vaunted zeta potential as a process parameter is questionable in real-world situations because it varies with the composition of the suspension and is hardly repeatable.

Destabilization of colloids can be achieved through addition of chemical agents, including charged or nonionic water-soluble polymers. Depending on the conditions under which the agents are used and the characteristics of the agents, destabilization of colloids in water may be achieved through one or more of four distinct methods: (1) compression of the diffuse layer of the electric double layer, (2) adsorption of agents to produce charge neutralization, (3) enmeshment of colloids in a precipitate, and (4) adsorption of polymeric agents to allow interparticle bridging. The electric double layer is the result of dynamic charge equilibrium between the particle and water, resulting in a zero net electrical charge in the colloidal dispersion (particles plus water). The double layer consists of the charged particles and counterions in water that are attracted to the charges on the particles. The concentrations of counterions near the particles are determined by not only the charges on the particles but also the diffusional force due to the concentration gradient in water. The result is a concentration distribution of counterions near the charged particle with the highest concentration of counterions near the particle surface, decreasing gradually with increasing distance from the particle surface. When concentration of counterions is low, the electric double layer is extended (because large volume of the diffuse layer is needed in order to maintain the electrical neutrality of colloidal dispersion). Destabilization of colloidal dispersion by adding counterions in the form of chemical agents is achieved by reducing the volume of the diffuse layer needed in order to maintain electrical neutrality of the colloidal dispersion. The compression of the double layer enhances the likelihood of aggregation among colliding particles because the attractive forces (van der Waals forces) between particles are short-distance forces and operate only during collisions or near misses.

Adsorption of counterions on particles to neutralize the charges on the surface eliminates electrostatic repulsions among colloids making aggregation possible.

Enmeshment of colloids in wastewater is mainly attributed to precipitation of the insoluble Fe(OH)3 or Al(OH)3 when common coagulants, FeCl3 or AlCl3, are added into wastewater under alkaline conditions.

When water-soluble polymers are used for destabilizing colloidal dispersion, the mechanism of destabilization is not of charge neutralization because the most effective polymeric coagulants are the anionic polymers—even the majority of colloids are negatively charged. The bridging theory stipulates that the chemical groups of the polymer chains bond to the sites of colloidal particulates forming particle-polymer-particle aggregates (hence the name bridging in reference to the mechanism). Many common polymers used for wastewater treatment are classified as poly-electrolytes because they contain ionizable groups such as carboxyl, amino, and sulfonic groups; polyelectrolytes can be positive, negative, or ampholytic (with both positive and negative groups), which can be prepared from acidic and basic vinyl monomers, from sulfobetaine monomers, from ion-pair co-monomers, or from charged anionic and cationic monomers mixed in varying proportions.

Flocculation is not limited to colloids destabilized by coagulant chemicals and polymers. Aggregation of microorganisms is common in biological wastewater treatment plants. It is evident that natural polymers either excreted by microorganisms or exposed at the surface of the microbial cells are responsible for enabling the bioflocculation. These natural polymers are also responsible for destabilizing organic colloids in wastewater. It has been demonstrated that synthetic polymers can also destabilize the microorganism suspension causing it to aggregate.

The selection of the optimum type and dosage of coagulant can be made only after judicious experiments with wastewater samples. Many substances can be used as coagulants. For food wastewater containing high proteinaceous substances, it is sometimes required to adjust pH by adding acids or alkali. For protein-rich wastewater, coagulation of the proteins can be started with denaturing. Denaturing is a process of changing structural conformation of proteins under heat or shear or chemical addition. The downside with denaturing as a way of coagulation of proteins is high cost associated with energy requirement; it is cheaper to use chemical agents as coagulants. If the recovered sludge from coagulation-flocculation treatment is to be used for animal feeds, the toxicity of chemical agents as coagulants is a very important issue.

Because of the complexity in wastewater compositions and operational policy, no single set of operating conditions will meet the treatment criteria of food and agricultural wastewater. It is therefore necessary to evaluate coagulants, pH, coagulant dosage, and operational procedures using a laboratory test that simulates the operation of a full-scale coagulation-flocculation called a jar test. A jar test is a scripted lab testing conducted in a series of beakers and stirrers in a jar test apparatus (Fig. 3.9). Jar tests have been used to evaluate the effectiveness of various coagulants and flocculants under a variety of operating conditions for water treatment. The procedures and evaluation process have been adapted to dredged material. However, conducting jar tests and interpreting the results to determine design parameters are not simple tasks because there are many variables that can affect the tests. Only experience can assist in applying the following jar test procedures to a specific project.

Jar tests are used in these procedures to provide information on the

Figure 3.9. A schematic diagram of jar test apparatus.

most effective coagulant, optimum dosage, optimum feed concentration, effects of dosage on removal efficiencies, effects of concentration of the suspension on removal efficiencies, effects of mixing conditions, and effects of settling time:

1. Fill the jar testing apparatus containers with sample wastewater from a stock suspension (either real sample or synthetic one with composition similar to the wastewater) of known turbidity, color, alkalinity, and pH. Calculate the amount of alkalinity required to react with the maximum dosage of aluminum or ferric sulfate. If necessary, augment the natural alkalinity by the addition of 0.1 N Na2CO3 so that the alkalinity will be at least 0.5 meq/l (25 mg/l as CaCO3). One container will be used as a control while the other containers can be adjusted depending on what conditions are being tested. For example, the dosage of coagulants, pH, and settling time in the containers can be adjusted to determine the optimum conditions.

2. If it is a test for an existing coagulation-flocculation process, the procedure should reflect the actual conditions of the specific plant in terms of rapid mixing RPM and time, slow mixing RPM and time, and finally settling time. If not, choose a set of appropriate stirring speeds for rapid and slow mixing, mixing times, and settling time for flocs to settle completely; add chemicals (aluminum or ferric sulfate) to each beaker near the vortex at high RPM for a minute and follow the actual or proposed operating conditions. Next, look at the beakers and determine which one has the best results (if any). An underdosed suspension will cause the sample to look cloudy with little or no floc. An overfeed suspension will cause fluffy flocs to occur and will not settle well. The beaker with an appropriate dosage of coagulant will have floc that has settled to the bottom, and the water above it will be clear determined either by vision or a nephelometer. If none of the beakers appear to have good results, the procedure should be rerun using different dosages until the correct dosage is determined.

The jar test procedure described follows a conventional empirical experiment design where variables are explored one factor at a time, keeping the other factors constant. This is not always effective or practical because the optimum conditions identified by the conventional approach may not be the true optimum if interactions between factors are present. Variables such as pH, mixing, and stirring speed may be important factors and should be included in the experiment design; a factorial design method may yield a better test result.

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