Water Treatment by Coagulation-Adsorption with Dolomite

Water Treatment by Coagulation-Adsorption with Dolomite

205 WATER TREATMENT BY COAGULATION-ADSORPTION WITH DOLOMITE A. M. DZIUBEK and A. L. KOWAL Institute of Environment Protection Engineering, Technical...

507KB Sizes 8 Downloads 233 Views

205

WATER TREATMENT BY COAGULATION-ADSORPTION WITH DOLOMITE A. M. DZIUBEK and A. L. KOWAL

Institute of Environment Protection Engineering, Technical University of Wrociaw, 50-370 Wrociaw, Poland

ABSTRACT Experiments were run on natural surface water samples for removal of turbidity, coloured matter and organics by coagulation-adsorption with soft-burned and wet-slaked dolomite as coagulant. The optimum coagulant dose was found to depend on initial alkalinity provided that turbidity and coloured matter concentration are moderately high. Empirical relationships are developed between optimum dolomite dose and alkalinity, and between pH and dolomite dose. The optimum pH for dolomite coagulation ranges from 10.2 to 10.8 and is lower than that for lime coagulation with precipitation of magnesium. The relationship between removals of TOC and CODp and dolomite dose can be described by generalized BET adsorption isotherms.

1. INTRODUCTION

During the past decades chemical methods have become a vital part of practically all technological systems for advanced water and wastewater treatment. By far the most common reagent to be encountered in water and wastewater technologies is lime, which has found considerable application in softening processes, and can also be successfully used for removal of algal nutrients. Since lime is amongst the most common reagents, it is also amongst the ones which receive serious attention of many investigators who described the relationship between the presence of magnesium hydroxide precipitated during lime coagulation and the removal efficiency in wastewater treatment. The results reported earlier [ 11 indicate that the degree of clarification increases when the lime dose employed is sufficient to precipitate magnesium in the form of Mg(OH)*. This phenomenon has later been explained [ 2 ] as being due to the adsorption of precipitated CaC03 particles on the Mg(OH)2 flocs, a process which improves the degree of clarification. Taking into account the coagulating and adsorbing capacities of magnesium hydroxide, Black and Thompson [3, 41 developed a water treatment method involving magnesium carbonate as coagulant, which is precipitated to magnesium hydroxide by lime addition. The method was quite successful in producing good treatment effects both in soft and hard waters. It is also claimed to have the advantage of recirculating the coagulant which has been recovered from sludges. A recent study [ 5 ] substantiates the great ease of mag-

206 nesium hydroxide in coagulating and adsorbing organic substances present in water and wastewaters. It is generally known that the precipitation of Mg(OwZ from aqueous solutions is significant at a pH of about 10.5; a practically complete precipitation takes place at a pH range between 11.2 and.. 11.5, which can be achieved in a &h-lime treatment. On the other hand, it is obvious that, in this process, the pH level in the water or wastewater to be treated should be decreased to the pH of stabilization. Comparative investigations [6, 71 on the use of pure COz and atmospheric air in the recarbonation process have revealed that atmospheric air employed for decreasing pH in alkaline wastewaters yields CaC03 particles which show a greater settleability than those obtained with pure COz. Furthermore, when air is used for pH adjustment, the precipitated CaCO3 does not solubilize. Based on the results of both theoretical and experimental studies, the following generalization can be made: the total effect of water treatment involving lime coagulation is made up of two effects, the effect of CaC03 and the effect of Mg(OH)z on the pollutants occurring in the water to be treated. The objective of the present study was to determine the efficiency of turbidity, colour and organic matter removal in a coagulation-adsorption process with dolomite as coagulant. The results reported here form a part of a large research programme (sponsored by the Polish Government) which is concerned with the applications of domestic dolomites in water and wastewater treatment systems, as well as with the usability of dolomites in water renovation. 2. EXPERIMENTAL

The experiments were run with soft-burned and wet-slaked dolomite as coagulant. The parameters of coagulant preparation from crude dolomite have been reported in an earlier study [8] dealing with thermal dissociation and hydration. The results show that a highly reactive product will be achieved when crude, grinded dolomite is burned for three hours at 1073 K, and then slaked at a water to dolomite ratio of 2: 1. With these parameters, the total degree of hydration (for CaO and MgO) equaled 98.6 percent. The experiments reported in this paper involved 2-percent dolomite suspensions containing about 60 percent of Ca(Ow2 and 30 percent of Mg(OW2. Dolomite doses varied from 40 to 600 gm-3. The experimental water was surface water; its composition is given in Table 1. The water samples had a volume of 1.5 dm3. The experiments were conducted using the jar-test method. The process parameters were the following: rapid mix with the use of a 3 x 8 cm paddle mixer at a speed of 80 rev min-' (G = 158 s-' ) during two minutes; slow mix (flocculation-adsorption) with the same paddle, at a speed of 20 rev min-' (G = 20 s-'), during 20 minutes, and sedimentation, during 30 minutes. After completion of the sedimentation process, the samples were analyzed for turbidity, coloured matter, pH, alkalinity, presence of Ca2+and Mg" ions, and concentration of organics; the latter in terms of total organic carbon (TOC) and chemical oxygen demand-permanganate (COD,). The determinations were carried out according to Standard Methods (TOC being determined with the use of a Beckman Analyzer, TOC 915 A). The criterion for optimizing the dolomite dose was the decrease of turbidity and coloured matter to a level of 10 gm-' and 20 gm-3 (Pt-Co) respectively after sedimentation.

207 Tab, 1. Chemical composition of the raw water (except pH, other values are in gm-') Parameters

Average'

Range

Turbidity Colour (PtCo) PH Alkalinity as CaCO, Total hardness as CaCO, Chlorides as C1Sulphates as SO:TOC CODp as 0, TDS Calcium as Ca Magnesium as Mg Ammonia as N Nitrates as N Nitrites as N

30 35 1.2 120 220 150 130 12.8 10.2

15-20 20-45 6.9 -1.4 60-150 200-260 125-185 125-165 1.2-24.5 5.6-21.5 520-920 64.3-83.6 9.4-13.3 0.6- 1.4 0.1-3.9 0.015-0.200

I00 61.8 12.0 0.9 2.0 0.050

3. RESULTS AND DISCUSSION

The removal of turbidity and coloured matter showed a typical behaviour, which is usually observed during water coagulation. The dolomite doses employed in the experiments enabled a practically complete decolorization and clarification to be achieved. The optimum dolomite dose depends primarily on the initial alkalinity level. No correlation was found between initial turbidity or coloured matter content and the optimum quantity of dolomite to be used. Thus, the optimum dolomite dose is a function of alkalinity provided that the levels of turbidity and coloured matter in raw water are moderately high. Based on a statistical analysis of the experimental results (for alkalinity range of 50 to 150 gm-3 as CaC03), the relationship of interest can be described by the following empiricial equation: D o p t = 0.014 A*

+ 0.8 A

where D o p t = optimum dolomite dose, gm-3, = alkalinity of raw water, gm-3 as C ~ C O ~ . A

The calculated correlation coefficient, r, equals 0.975. Thus, the optimum dolomite dose ranges from 100 to 400 gm-3, depending on the alkalinity level in raw water. Analysis of the pH behaviour during dolomite coagulation indicates that the optimum pH equivalent to the optimum dolomite dose is between 10.2 and 10.8. At this pH range, precipitation of magnesium contained in the water was insignificant. Fig. 1 shows the relationship between pH and dolomite dose for water of various alkalinity levels. Hence,

208

a

11

P" 10 initial alkallmty. g ~ nas - ~CaC03

9 7

I

0

m

1

I

I

zm 300 mo dolomite dose D.

0-0

60-90

0-0

110-120

&-A

I

50.3

gG3

150 I

600

7 0 ~

Fjg. 1. pH versus dolomite dose.

for alkalinities ranging from 60 t o 90 gm-3 as CaC03, the relation pH = f(D) becomes pH = D(0.083 D l.l)-' (when D E ) 50 gm-3), and for alkalinity levels varying from 110 to 150 gm-3 as CaC03, this relation takes the form pH=D(0.078 D t 5.4)-' (when D 2 200 gm-' ), where pH indicates the value obtained after coagulation, and D denotes dolomite dose, gm-3 . The calculated correlation coefficients are 0.999 for both equations. This means that the correlation between pH and dolomite dose is very good from the pH level of about 9.5 on (which conforms with water softening). For comparative purpose, another experimental series with two coagulants, dolomite and lime, was run. While there was almost no difference in the range of the optimum dose between the two coagulants, this difference became more pronounced in the ranges of optimum pH. The oprimum pH for lime coagulation is always above 11.O (it usually falls between 11.1 to 11S ) , whereas that for dolomite coagulation never exceeds 11.0 (it varies, at most, from 10.2 t o 10.8). This difference in optimum pH between lime and dolomite (even though the water contained in the samples and the dosage of both coagulants were identical) can be attributed to the fact that in dolomite Ca(OH)? accounts for some 60 percent, whereas the remainder consists predominantly of Mg(OH)?. As the optimum pH for lime coagulation (particularly of water with low or moderate alkalinity levels) exceeds 11.O, there is a need to precipitate all of the Mg" ions in the form of Mg(OH),, which is an important factor affecting the removal efficiency, as well as the degree of water clarification. When the coagulation process involves dolomite, Mg(OH)2 is contained in the coagulant. Conducting coagulation at a pH of about 10.5 is advantageous in that it not only brings about a softening of the water and an increase in the degree of flocculation of CaC03 particles, but it is also sufficiently high to prevent Mg(OH), from dissolving (together with dolomite) in the water. The decrease of opt% mum pH level in dolomite coagulation as compared to lime coagulation is a problem of great significance for the following two reasons: (1) When pH is higher than 10.5, the concentration of hydroxide ions OH- increases rapidly. Thus, increasing the pH level from 10.5 to 11.5 gives an almost tenfold increase in OH- ions concentration in the water. In other words, to convert OH- to COi- at pH 11.5 the consumption of COz is ten times higher. Hence, water subjected to high-lime

+

209 8

0

u

e

dolomite dose D, gni3 Fig. 2. TOC and COD, versus dolomite dose.

coagulation will need much more COz for recarbonation than when subjected to a coagulation process using dolomite. (2) Water treatment carried out at a pH less than 11.O prevents magnesium ions present in the water from a complete precipitation, and this is also important to the human organisms. In addition to turbidity and coloured matter removal achieved predominantly by coagulation, water systems involving chemical methods may yield removals of organics, due to adsorption. In Fig. 2, the removals of TOC and COD, are plotted against dolomite dose. For TOC, the relation Ce = f D takes the form CFoc = D (0.19 D - lo)-', and for COD, this relation becomes CZ0!' = D (0.27 D - 22)-' in which CToc and CZoDP are residual concentrations of TOC and COD,, respectively, which persist in the water after the process, and D is dolomite dose greater than 200 gm-3. The calculated correlation coefficients are 0.997 and 0.977 for TOC and COD,, respectively. This indicates that TOC and COD, and dolomite dose are highly correlated, beginning from > 200 gmF3 doses, above which TOC and COD, removal efficiencies are no longer dependent of their initial concentration in the water. As can be seen from the plots of Fig. 2, TOC and COD, removal tends asymplotically to certain values which are referred to as non-removable concentrations (Cn). The values of Cn, calculated from the equations of the relation Ce = f(D) for an assumed coagulant dose of 1000 gm-3, are C;foc = 5.6 gm-3 and CEOD, = 4.0 gm-3 as 02, this accounts for some 40 percent of the initial TOC and COD, concentrations. Analyzing TOC and COD, concentrations removed per unit mass of dolomite as a function of equilibrium concentrations (C,) permitted respective curves to be plotted. The shape of the curves is similar to that of the BET multilayer adsorption isotherms. In these considerations the effect of Mg(OH)z precipitated from water at pH > 11 on TOC

210

and COD, removal efficiencies is insignificant as compared to the amounts af Mg(OH)* entering the water together with the dolomite dose. Based on the assumption that the solution contains certain amounts of C,, and that the process of TOC and COD, removals satisfies the model of multilayer adsorption, a

generalized equation of the BET isotherm was derived. The final formula with the introduction of Cn has fhe form:

where:

of grams of solute adsorbed per gram of dolomite at adsorbate concentration C,) Xm - number of grams of solute adsorbed in forming a complete monolayer on the adsorbent surface K - constant expressing the energy of interaction with the surface C o - initial adsorbate concentration Ce - equilibrium adsorbate concentration Cn - non-removable adsorbate concentration.

X

- adsorption capacity (number

Figure 3 gives the isotherms of adsorption for TOC and COD, along with respective equations. The calculated correlation coefficients are 0.997 and 0.916 for TOC and COD,, respectively. Having these in mind, it may be concluded that the removal of organics by coagulation-adsorption with dolomite obeys the model of multilayer adsorption and can be described by adsorption isotherms.

n nn

equilibriun concentration

lo .g$

Fig. 3. Adsorption isotherms for TOC and COD,. Co values as in Fig. 2.

11

a

21 1 4. CONCLUSIONS

The experimental results show that dolomite prepared in an appropriate manner is an effective coagulant which may be successfully employed in the treatment of surface waters. In this method, the process of water softening has been combined with the coagulation-adsorption process. The optimum dose of coagulant depends on the alkalinity level in raw water, but only if turbidity and coloured matter content are moderately hgh. Optimum dolomite doses range from 100 to 400 gm-3, depending on initial alkalinity. With these doses, hgh degrees of decolorization and clarification are achieved; TOC and COD, removal efficiencies may be almost complete. In dolomite coagulation optimum pH varied from 10.2 to 10.8 and was lower than that recommended for lime coagulation with precipitation of magnesium. Dolomite coagulation conducted at optimum pH has the inherent advantage that the quantities of COz required for recarbonation are considerably lower than those needed in hgh-lime coagulation, and magnesium present in the water will not be precipitated. The main mechanism governing TOC and COD, removals is adsorption both on CaC03, which has been precipitated in the course of the process, and on Mg(OH)2, which enters the water together with the dolomite dose. The non-removable fractions persisting in the water after completion of the dolomite coagulation-adsorption process were up to 40 percent of the initial TOC and COD, values. TOC and COD, removal efficiencies can be plotted as isotherms of adsorption, and may be described by a generalized equation of BET isotherm. The overall efficiency of adsorption on CaC03 and Mg(OH), was found to be considerably lower than that on typical activated carbons. Coagulation-adsorption involving dolomite as coagulant can be successfully applied to the treatment of surface waters irrespective of their hardness and initial magnesium concentration. Current studies of dolomite coagulation-adsorption deal with the application of sludge blanket clarifiers, the modification of the Mg(OH)2 structure, and the management of precipitation sludges. REFERENCES

1 2 3 4 5 6

M.E. Flentje, J. Am. Wat.Wks. Ass., 17,1927,253-260. A. P. Black and C. G. Thompson, Grant Project 12120 ESW, EPA, 1971. C. G. Thompson, J. E. Singley and A. P. Black, J. Am. Wat. Wks. Ass., 64, 1972, 11 -20. C. G. Thompson, J. E. Singley and A. P. Black, J. Am. Wat. Wks. Ass., 64, 1972, 93-100. J. Leentvaar and M. Rebhun, Water Res., 16,1982,655-662. B. Dziegielewski, A. M. Dziubek and A. L. Kowal, in L. Pawlowski (Ed.), Physicochemical Methods for Water and Wastewater Treatment, Pergamon Press, Oxford and New York, 1980, pp. 283-289. 7 A. M. Dziubek and A. L. Kowal, in P. S. Hansen (Ed.), Proc. Int. C o d . Coal Fired Power Plants and the Aquatic Environment, Copenhagen, 1982, pp. 330-338. 8 A. M. Dziubek and A. L. Kowal, Government Project PR 7 - 03.04.02.02.120., Inst. of Env. Prot. Engng., Wroclaw Technical University, 1980 (in Polish).