The Modern Student lcrborcrtoru w
Chemistry and the Environment
A Small-Scale, Easy-To-Run Wastewater-Treatment Plant The Treatment of an Industrial Water That Contains Suspended Clays and Soluble Salts Mercedes ~lvaro,' Mercedes Espld, J e s k Llinares, Ramdn Martinez-~diiez,2and Juan Soto Departamento de Quhica de la Universidad Politbcnica de Valencia, Camino de Vera sin, 46071 Valencia, Spain For many years there has been growing awareness that high population densities and advanced levels of industrialization have made it necessary to remove the pollutants emitted in order to lessen their detrimental effect on the environment. There have also been important advances in combating the threat to the environment. Many industrial processes have been modified to decrease emissions. New methods of removing pollutants have been developed, and old ones have been improved. Stricter waste-disposal standards have been implemented. Broadly speaking there are two media in which pollutants and waste products are predominately found-air and water-although there is also soil contamination in many places. Principles of Plant Design Chemistry students are often interested in the chemical principles involved in industrial processes, as well a s in which pollutants and waste products are generated and how they can be removed for disposal or reuse. In this article we describe the design and operation of a small-scale wastewater ~urification~ l a n tIts . size makes it both easv to use and &dagogicall~usefulin illustrating some of th; most imoortant ~hvsicaland chemical ~rinciolesof sewaw disposac This experiment introduces students to the following theoretical concepts as they apply to a real physical and chemical waste-treatment process. &
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colloidal solutions coagulation flocculationand adsorption mechanisms precipitation reactions colorimetric analysis Normally, water-treatment processes seek to remove suspended solids, colloids, and soluble matter (organic or inorganic), using several methods involving physical, chemical, or biological principles. Suspended aolida are eliminated by sedimentation or filtra-
tion.
Finely divided solids, such as colloids, are eliminated using
coagulants and floeculants.
Solubk matter can be removed bv ion exchange. - . adsomtion
processes, and (sometimes)precipitation reactions.
'Author to whom correspondence can be addressed. 2Alternate author to whom correspondence can be addressed.
The Treatment of an Industrial Wastewater In this experiment, the water to be treated is basically composed of suspended clays and low concentrations of soluble salts of metallic cations. This water can be in the laboratory, or water with a composition can be obtained from a ceramic factory. To treat such water, the basic plant design stages.
First, a primary sedimentation pmess removes the greater part of the heavy, suspended solids. Then a physieochemical process eliminates colloids and, indirectly, some of the metals or coloring matter in solution. Coagulation of ColloidalParticles The settling velocities for h e l y divided solids like colloidal clay are so small that their removal in a settling tank is impossible under ordinary conditions because very long settling times would be required. For example, typical colloidal particles (about 0.1-0.01 p in diameter) would require 2-20 years to settle 1m. Thus, the very small particles must be coagulated into larger particles with higher settling velocities. Coagulation of colloidal particles in water is highly complex and involves physicochemical processes that are outlined here only in simplified form.
Artificial'IFansfonation This coagulation is not a natural process because repulsion forces maintain the high stability of the colloidal particles over long periods of time. Thus, the colloidal suspension must be transformed artificially This transformation is produced by two different actions: destabilization of the colloids due to aggregation or adsorption mechanisms that cancel the repulsion forces acting on the colloidal particles mowth of unchareed colloids to form lamer aarticles ~~~~~~~-----
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Generally the term coagulation refers to the destabilization process, and flocculation refers to colloidal growth. The coagulation-flocculation process is carried out by adding certain chemicals~oagulantsand flocculantsto the water. First, a coagulant is added to the water. The most common coagulants are aluminum sulfate * iron(I1)sulfate iron(II1)chloride
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The coagulant action of a cation depends on its valence, which has the opposite charge :as the colloidal particles. (Continued on page A132) Volume 70 Number 5 May 1993 A129
The Modern Student laboratoru:
Figure 1. Equilibrium solutions in contact with precipitated hydroxides AI(OH),, Fe(OH),, Cu(OH),, or Cd(OH),, calculated using values for equilibrium constants for solubility and hydrolysis equilibrium.
F ~ g ~2r eScneme ot tne plr~flcatlon p ant snowing tne dinerent compartments ana the correspond ng add rlon of cnemicals. Thus, coagulation is most effective when the valency is high. Flocculation Floe Formation The addition of one of these coagulants is followed by the addition of hvdroxide. which brines the DH close to the value that results in minimum solu6ility oithe metallicion of the coagulant used. When aluminum sulfate is used, hydroxide is-added until a pH of about 7 is reached because a t this pH the solubility of the A13+cation in water is minimum (see Fig. 1). A130
Journal of Chemical Education
Chemistry and the Environment Thus, aluminum hydroxide is obtained, with colloidal particles aggregated or adsorbed on the large surface area provided by the hydroxide particles. This step transforms the colloidal ~articles.which are now adsorbed or aggregated on thealuminim hydroxide, into larger called flocs. On the other hand, under these conditions (pH about I),other common metallic ions can be eliminated~by precipitation (hydroxide formation) (see Fig. 1). Floc Growth The flocsobtained can be increased in size by adding floeculants. The most commonly used are synthetic polymers called polyelectrolytes. Polyelectrolytes, when used with the common metal coagulants, usually yield large, dense flocs that settle rapidly, even though their effectiveness depends on the nature of the water pollutants. This may reduce flocculation time. Although polyelectrolytes are usually expensive, experiments show that use of small amounts results in an important reduction in the coagulant concentration required. Experimental The ex~erimentaldesien consists of a sincle tank divided in& several compartments in which trhe different omcesses described above take dace. It is made of methaciylate plastic and mounted on metallic frame. Figure 2 illustrates the system design. The actual experimental treatment plant used in the laboratory is shown in Figure 3. Flow Rates and Tank Sizes
The water flows from one compartment to another by gravity, as in real purification plants. The plant is sized for a flow rate of 60 L/h. To take into account the head loss produced by the weirs and submerged passages, a drop in water level of about 5 cm has been assumed. Assuming steady and uniform flow, each compartment can be sized according to the retention time t required in the different stages. V=Qt
(1)
where Q is the flow rate and V is the volume of the compartmeit. The plant was designed assuming t values of 8 min for compartments B and C (8L) 2 6 min for compartment E (26 L) obtaining values of 8 and 26 L for these tanks, respectively, for a working flow rate of60 Lh. The purification plant as designed is large, but it is difficult to work with flow rates larger than 60 Lh in the laboratory, especially because experiments last about 3 h. On the other hand, a lower-scaled plant is not used because it is too small to work properly. Also, concentration errors made by the students can be minimized when the compartments used are large with respect to the flow rate. Dosing
Chemicals are added by hand using dropping funnels. Typical solutions were prepared: 0.5 M AI&3O4), 2 M NaOH 1-gr/Lpolyelectrolyk (Praestol-2540)
Volume 70 Number 5 May 1993
A131
The Modern Student laboratory: Chemistry and the Environment The wmpartment dosage was varied; it was matched with the kind and concentration of the colloids in the water at the time. The normal values were
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Ala(S04)3at 0.16 Lm NaOH at 0.12 Lm polyeleetmlyte at 0.1Lm
Operating Mode
The Settling Tank (4 The wastewater is introduced in the plant in comparbnent A, which is a srnalI settling tank (not scaled) for removing large, heavy, suspended solids. Cwgulation (B). The finely divided solids and soluble metal salts pass to wmpartment B where the wagulantaluminum sulfat-is added. Floe Formation (C). Compartment B is connected to C, where the dosage of NaOH is set to maintain a pH of approximately 7. Rapid stirring is advisable in order to obtain a fast andintimate mixture of the chemicals added. At this stage the flocs are formed. Floc Growth (D). From C the water overflows to wmpartment D where the floes are grown by addition of an ~ N o n i polyeledmlyte. c Slow stirring, to avoid floe rupture, could be introduced here. However, stirring does not seem essential, as indicated by the experiments carried out in the plant. The Static Settling Tank (E). From D the water and floes go to E, a static settling tank where the floes are separated to produce clarified water. Removal by sedimentation is based on the difference between the specific gravity of the suspended solid particles and that of the liquid. Flow Rates and Concentrations In the experiment the students are responsible for regulating the flow rate and adjusting the concentration of chemicals. They also must note what happens if the polyelectrolyte is not added or if a wrong wncentration of NaOH is used, resulting in high or low pH values. Decolomtion Indirectly, removal of colloids also allows some wmmon cations or colored organic molecules to be eliminated from the water. Under t& working wnditions, these two pmcesses can easily be observed. It is well-known that removal of color-producing particulates appears to occur by adsorption of the organic molecules in the amorphous precipitate. Students may see this effectby adding a wloring agent, such as methylene blue, to the water to be treated. During the treatment process, blue coloration on the flocs and decoloration of the solution are clearly detected. Precipitation of Cations Cations like Cu%, Fe3+,or Cd2+can also be removed from the solution. At the working pH. the copper and iron are eliminated during the p m c e ~ ~ b ~ h y d r o xformation ide (see Fie. 1). Removal of these ions can be observed using auantiGtiGe analytical colorimetric methods before and-&er the purifying process. The Cd& cation does not precipitate
A132
Journal of Chemical Education
Figure 3. Experimental treatment plant working in the laboratory. at a working pH of 7, but its analysis may be included in the experiment to evaluate wprecipitation effects. When imn(II1) chloride is used a s the coagulant, the optimum working pH is about 8.5, providing better wnditions than aluminum sulfate for removing metals in solution by hydroxide formation. However, we use aluminum sulfate instead of the iron(II1) chloride because flocs formed by aluminum are white. Thus, changes in color produced by aggregation or adsorption of clay colloids or organic coloring are more clearly observed. ~cknowled~ment We thank the UNversidad Polit6cNca de Valencia (Proyedo P.1.D.-60) for financial support. Literature Cited I. Imhoaa.K HW of uldan ~ m i ~ ~ ~ wamewdef.r o a n d ~iewsol., John wiiqlod Sona: NewYorlr, 1989. 2. MehalfEddy Wsrteunt#,Engineeri"~:CdMion,lkomMlSDiepasol;MffirawHjll, 1971
3. Qasim, S. R.Woatpuxlrer hatment Plants; CBS College, IssS. 4. Ramalho, R.8. h t d d i m to W ~ o t o r h f m nPmzas=s;AEademic, t 19s
