CHAPTER 11 PHYSICAL AND CHEMICAL METHODS OF WASTEWATER TREATMENT
Physicochemical methods play a significant role in wastewater treatment. They are used both independently and in combination with mechanical, chemical and biological methods.
The physicochemical methods of wastewater treatment include coagulation, flotation, adsorption, ion exchange, extraction, rectification, evaporation, distillation (evaporation), hyperfiltration (reverse osmosis) and ultrafiltration, crystallization, as well as methods associated with the application of electrical fields - electrocoagulation, electroflotation, electrolysis, etc. These methods are used to remove finely dispersed suspended solid and liquid particles, soluble gases, mineral and organic substances from sewage.
11.1. Coagulation and Flocculation of Wastewater Contamination
The use of physicochemical methods for wastewater treatment in comparison with biochemical has several advantages: the possibility of removing toxic, biochemically non-oxidizable organic contaminants from sewage; achieving a deeper and more stable degree of purification; smaller sizes of structures; less sensitivity to changes in loads; possibility of full automation; more in-depth study of the kinetics of some processes, as well as modeling, mathematical description and optimization, which is important for the correct selection and calculation of equipment; methods are not related to the control of the activities of living organisms; the possibility of recuperation of various substances.
The choice of this or that method of purification (or a set of methods) is made taking into account the sanitary and technological requirements for purified wastewater for further use, as well as the amount of waste water and the concentration of contaminants in them.
In the process of mechanical purification, particles of 10 μm or more are easily removed from the wastewater, fine particles and colloidal particles are practically not removed. Thus, sewage after mechanical treatment facilities is an aggregate-stable system. For their purification, coagulation methods are used; the aggregative stability is thereby impaired, larger aggregates of particles are formed which can be removed from the waste water by mechanical means.
Coagulation is the process of coalescence of particles of a colloidal system as a result of their interaction under the action of molecular forces of cohesion with mixing or directed movement in an external force field. As a result of coagulation, aggregates are formed - larger (secondary) particles consisting of a cluster of small (primary) particles. Primary particles in such aggregates are joined by forces of intermolecular interaction directly or through a layer of the surrounding (dispersive) medium. Coagulation is accompanied by a progressive coarsening of the particles and a decrease in their total number in the volume of the wastewater. The adhesion of homogeneous particles is called homocoagulation, and heterogeneous is called heterocoagulation.
In the purification of waste water, coagulation is used to accelerate the process of precipitation of fine impurities and emulsified substances. It is most effective for the removal of colloidal-dispersed particles from water; particles with a size of 1 ... 100 μm. Coagulation can occur spontaneously or under the influence of chemical and physical processes. In the process of wastewater treatment, coagulation occurs under the influence of special substances - coagulants added to them.
Coagulants in water form flakes of metal hydroxides that precipitate rapidly under the action of gravity. Flakes have the ability to capture colloidal and suspended particles and aggregate them. Since colloidal particles have a weak negative charge, and flocculation of coagulants is a weak positive charge, mutual attraction arises between them.
The main process of coagulation wastewater treatment is heteroagulation - the interaction of colloidal and finely dispersed sewage particles with aggregates formed when coagulants are introduced into the waste water.
Colloidal particles are characterized by the formation of a double electric layer on the surface of the particles. One part of the double layer is fixed at the interface, and the other creates an ion cloud; one part of the double layer is fixed, and the other is mobile (diffusion layer).
A particle, together with a diffusion layer, is called a micelle. The change in the electric field of the micelle is shown in Fig. 11.1. The potential difference arising between the fixed and moving parts of the layer (in the volume of the liquid) is called the ξ-potential or the electrokinetic potential, which is different from the thermodynamic potential E, which is the potential difference between the particle surface and the liquid. The zeta potential depends both on E and on the thickness of the double layer. Its value determines the amount of electrostatic repulsive forces of particles that prevent particles from sticking together. The small size of colloid particles of contaminants and the negative charge distributed on their surface, causes high stability of the colloidal system.
Fig. 11.1. The structure of the micelle: a - ξ & gt; 0.03; b - ξ ~ 0; A - adsorption layer; D is the diffusion layer; I - the core
When the electric charge of particles decreases, i.e. as the ξ-potential decreases, the repulsive forces decrease and it becomes possible for particles to coalesce-the coagulation of the colloid. To cause coagulation of colloidal particles, it is necessary to reduce the value of their ξ-potential to a critical value by the addition of ions having a positive charge. Thus, when coagulation occurs, destabilization of colloidal particles due to the neutralization of their electric charge. The forces of mutual attraction between the colloidal particles begin to predominate over the electric repulsive forces at the ξ potential of the system less than 0.03 V. At a potential of zero, coagulation passes through a maximum intensity.
With coagulation, flocs are formed first by a part of suspended particles and a coagulant or only a coagulant. The resulting flakes of the latter sorb the substances that pollute the wastewater and, settling with them, purify the water.
One of the methods for reducing the ξ-potential of a colloidal system is the increase in the concentration of electrolytes in water. The ability of the electrolyte to cause coagulation of the colloidal system increases with the valence of the coagulating ion having a charge that is opposite to the charge of colloidal particles.
For the beginning of coagulation, the particles must approach each other for a distance at which attractive forces and chemical affinities act between them. Approximation of particles occurs as a result of Brownian motion, as well as in laminar or turbulent flow of water.
Bentonite, electrolytes, water soluble salts of aluminum Al2 (SO4) 3, iron salts FeCl3 or mixtures of them, polyacrylamide, which hydrolyze form floccous hydrates of metal oxides are used as coagulants. Various clays, aluminum-containing production waste, pickling solutions, pastes, mixtures, slags containing silicon dioxide can also be used for wastewater treatment.
When aluminum and iron salts are used as coagulants, as a result of the hydrolysis reaction, water-soluble hydroxides of iron and aluminum are formed which adsorb suspended, fine-dispersed and colloidal substances on the developed flocculate surface and settle under the favorable hydrodynamic conditions to the bottom of the sedimentation tank, forming a precipitate:
Sulfuric and hydrochloric acid formed during hydrolysis should be neutralized with lime or other alkalis. Neutralization of acids formed during the hydrolysis of acid coagulants can also occur due to an alkaline reserve of the wastewater:
When using mixtures of aluminum sulfate A1 2 (SO 4 ) 3 and ferric chloride FeCl 3 in ratios from 1: 1 to 1: 2, the best coagulation result is achieved than with the separate use of reagents.
The coagulation rate depends on the electrolyte concentration (Figure 11.2).
Fig. 11.2. Dependence of the relative coagulation rate on the electrolyte concentration
At small concentrations of the electrolyte, the particle collision efficiency is ψ, i.e. the ratio of the number of collisions ending in clumping to the total number of collisions is close to zero (ψ = 0). As the concentration increases, the coagulation rate increases, but not all collisions end in the coalescence of particles-this coagulation is called slow. When ψ = 1, rapid coagulation occurs, in which all particle collisions end in the formation of aggregates.
The rapid coagulation rate for a stationary medium for Brownian motion of particles according to the Smoluchowski theory is:
The number of particles per unit volume of water in a time/for fast and slow coagulation is determined by the formulas:
where k is the coagulation constant; n x - the number of aggregates of particles; n 0 is the initial concentration of particles; T 1/2 is the coagulation time, during which the number of particles per unit volume is halved; ψ is the coefficient of particle collision efficiency.
Coagulation occurs in polydisperse systems faster than in monodisperse systems, as larger particles entrain smaller ones during settling. The shape of the particles also affects the coagulation rate. For example, elongated particles coagulate faster than spherical particles.
The flake size (within 0.5 ... 3 mm) is determined by the ratio between the molecular forces that hold the particles together and the hydrodynamic tearing forces that tend to destroy the aggregates.
The density of flakes px is determined taking into account the water densities p0 and the solid phase pT, as well as the volume of solid matter per unit flake volume δ r :
The strength of the flakes depends on the granulometric composition of the aggregates of particles formed and the plasticity. Agglomerates of particles that are heterogeneous in size are stronger than homogeneous ones. Due to gas evolution from water, as well as aeration and flotation, gas flocculation takes place, which is accompanied by a decrease in flake density and a decrease in the deposition rate.
The ratio of the rate of cramped deposition to the velocity of free particle deposition is
where φ is the volume concentration of particles; ζ 0 and ζ ст are the particle drag coefficients for free and restricted deposition.
Coagulation of waters containing finely dispersed and colloidal particles can occur when waste water passes through an electrolytic cell with an anode made of aluminum or iron. The anode metal under the action of a direct current is ionized and passes into the waste water, the particles of contamination of which are coagulated by the formed hardly soluble hydroxides of aluminum or iron.
The method of electrochemical coagulation can be applied to the treatment of sewage containing emulsified particles of oils, fats and oil products, chromates, phosphates.
Flocculation is one of the types of coagulation, in which fine particles suspended in the suspended state under the influence of specially added substances (flocculants) form intensely settling loose flocculate aggregates. Unlike coagulation in flocculation, aggregation occurs not only when the particles are directly contacted, but also as a result of the interaction of the molecules of the flocculant adsorbed on the particles.
The mechanism of action of flocculants is based on the phenomenon of adsorption of flocculant molecules on the surface of colloidal particles, on the formation of the network structure of the flocculant molecules, on the adhesion of colloidal particles due to van der Waals forces. When flocculants act between colloidal particles, three-dimensional structures are formed, capable of more rapid and complete separation from the liquid phase.
Flocculation is carried out to intensify the process of flocculation of aluminum and iron hydroxides in order to increase the rate of their precipitation. The use of flocculants can reduce the dose of coagulants, reduce the duration of the coagulation process and increase the rate of precipitation of the flocs formed.
For natural wastewater treatment, natural and synthetic flocculants are used. Natural flocculants include starch, dextrin, cellulose ethers, etc. Active silicium dioxide (xSiO 2 · yH 2 O) is the most common inorganic flocculant. Of the synthetic organic flocculants, polyacrylamide (PAA) was most widely used. When choosing the composition and dose of flocculant take into account the properties of its macromolecules and the nature of the dispersion particles. The optimum dose of PAA for industrial wastewater treatment ranges from 0.4 to 1 g/m.
When dissolved in waste water, flocculants may be in a non-ionized and ionized state. In the second case, they are called soluble polyelectrolytes. Depending on the composition of the polar groups of flocculants are:
non-ionic - polymers containing non-ionic groups: -OH, -CO (starch, hydroxyethylcellulose, polyvinyl alcohol, polyacrylonitrile, etc.);
anionic polymers containing anionic groups: -COOH, -SO 3 H, - OSO 3 H (active silicic acid, sodium polyacrylate, sodium alginate, lignosulfonates and etc.);
cationic polymers containing cationic groups: -NH 2 , -ΝΗ (polyethyleneimine, copolymers of vinyl pyridine, etc.);
amphoteric - polymers containing simultaneously anionic and cationic groups: polyacrylamide, proteins, etc.
The speed and efficiency of the flocculation process depends on the composition of the waste water, its temperature, the intensity of mixing and the sequence of the introduction of coagulants and flocculants. Doses of flocculants are usually 0.1 ... 10 g/m, and an average of 0.5 ... 1 g/m. The efficiency of any flocculant is calculated using the formula
where w cf and w 0 - the deposition rate of the flocculated and nonflocculated sludge, mm/s; q - flocculant consumption per 1 ton of solid, g.
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