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Buletinul Ştiinţific al Universităţii “Politehnica” din Timisoara, ROMÂNIA Seria CHIMIE ŞI INGINERIA MEDIULUI

Chem. Bull. "POLITEHNICA" Univ. (Timişoara) Volume 50 (64),1-2,2005

148

Studies Concerning the Aluminium Ions Removal from Waste Water

Adina Negrea, P. Negrea, Lavinia Lupa, Adriana Dinu, Georgeta Burticǎ

University “Politehnica” of Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, P-ta Victoriei No.2, 300006

Timisoara

Abstract: In the paper were established the optimum conditions of aluminium removal from residual industrial waters. In the frame of the experimental study the parameters of the initial water have been determinate. In the situations in which the residual effluents volume is small, the proceeding being discontinuous it’s applied the physical – chemical independent treatment of this. The precipitation agents used in classic mod are lime milk, sodium hydroxide or sodium carbonate. The neutralisation process of the effluent must be lead in the pH limits = 6.5 – 8.5 in accordance with the indicators of the waste waters quality evacuated in sewerage system. Keywords: precipitation, aluminium ions removal, waste water

1. Introduction

The large quantities of the surface waters and residual ones which need treated by coagulation impose a major important problem the obtaining of the inorganic coagulants.

Like coagulant agents are used the aluminium, iron, calcium and magnesium salts.

These salts action by the hydrolyse products which constitute the active element of the coagulation. For a certain metal ions concentration, the pH is the most important parameter in establishment of the predominant hydrolyses products.

The optimum pH domain of these products formation vary between: 4.5 – 7.5 for aluminium sulphate, 8.5 – 11 for iron sulphate, 5 – 11 for iron chloride and 10 – 11.5 in the case of magnesium chloride use.

The use of aluminium based coagulants in the treatment waters may lead to this concentration increasing in the treated water affecting the water quality to the consumer. For example a higher concentration of the residual aluminium could increase the water turbidity in the distribution retea by aluminium hydroxide precipitation. The hydroxide floccules can interfere with the disinfection process by micro organism protection. Also the residual aluminium can lead to the formatting of the storages in the ell of the alimentation pipes leading to the decreasing of the transport capacity of these. The studies show that the residual aluminium from drinkable water is responsible of the Alzheimer ill apparition to consumers.

The most agencies established standards for aluminium control from drinkable water. These values are variable and reflect the difference of opinion regarding the effects of this presence [1-8].

2. Experimental In this paper was established the optimum conditions

of aluminium removal from residual waters resulted from industry.

In the frame of the experimental study were determinate the parameters of the initial water. In the situations in which the residual effluents volume is small, the proceeding being discontinuous it’s applied the physical – chemical independent treatment of this.

The precipitation agents used in classic mod are lime milk, sodium hydroxide or sodium carbonate. The neutralisation process of the effluent must be lead in the pH limits = 6.5 – 8.5 in accordance with the indicators of the waste waters quality evacuated in sewerage system.

The pH was measured with a pH-meter Denver 250 and in the decanted water we determinate the aluminium ion by the atomic absorption spectrophotometer method with SpectrAA 110 Varian.

3. Results and discussion Studies regarding the aluminium removal from residual

waters 3.1. The physical – chemical characterisations of the residual effluent

In the frame of the experimental study were determinate the parameters of the initial water.

In table 3.1. are presented the parameters values for the initial water sample.


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TABLE 3.1. Parameters values for the initial water sample

No. Parameter Value 1 pH 3.1 2 Acidity, mval/L (m=p) 3.25 3 Aluminium, g/L 1.35 4 Magnesium, mg/L 5.05 5 Sulphates, g/L 2.02 6 Chlorine, mg/L 350

3.2. Studies regarding the acid residual effluent neutralisation and physical–chemical analyses of the treated effluent

In the situations in which the residual effluents volume is small, the proceeding being discontinuous it’s applied the physical – chemical independent treatment of this.

The precipitation agents used in classic mod are lime milk, sodium hydroxide or sodium carbonate. The neutralisation process of the effluent must be lead in the pH limits = 6.5 – 8.5 in accordance with the indicators of the waste waters quality evacuated in sewearage system (the normative NTPA 002).

In this situation the residual effluent pH is 3.08. Were made neutralisations studies using lime milk,

NaOH, and Na2CO3. 100 ml of water samples were titrated from 0.5 in 0.5 ml with 1N concentration alkaline solutions.

The obtained results are presented in figure 3.1 for NaOH, Na2CO3 and respective for lime milk.

From the neutralisations studies realised for these 3 used reagents result a reaction time comprised between 15 and 30 minutes.

Figure 3.1. Neutralisation curves

The optimum dose of the precipitation reagent is presented in table 3.2 for those 3 situations.

TABLE 3.2. The optimum dose of the precipitation reagent

pH 6 7 8,5

mL Na2CO3 1N/100 mL residual water

14

16

22 Na2CO3 optimum

dose Kg Na2CO3 / 1 m3 residual water

7.56

8.64

11.88 mL NaOH 1N/100 mL

residual water 12 13 14 NaOH optimum

dose Kg NaOH/ 1 m3 residual water 5 5.4 5.8

mL Ca(OH)2 1 N / 100 mL residual water

14.5

15.5

17 Ca(OH)2 optimum

dose Kg Ca(OH)2 / 1 m3 residual water

5.36

5.74

6.30

For pH values of 6, 7 and 8.5 we determinate the sedimentation velocity of the samples for each precipitation agent.

The experimental data regarding the sedimentation velocity determination are presented in figures 3.2 – 3.4.

Figure 3.2. The sedimentation velocity in the case of use of the 1N

Na2CO3 solution for different values of pH.

Figure 3.3. The sedimentation velocity in the case of use of the 1N NaOH

solution for different values of pH.


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Figure 3.4. The sedimentation velocity in the case of use of the 1N lime

milk solution for different values of pH.

Following the figures 3.2 – 3.4 we can observe that the sedimentation velocity is higher in the case of use of Na2CO3 1 N solution at pH = 8.5 when the sedimentation devolved in optimum conditions. The optimum time was of 30 minutes.

In the case of using of 1N NaOH solution, having in mind that NaOH is expensive we consider the use of this unprofitable, and in the case of use of lime milk the sedimentation is hard, the time sedimentation being very big.

In these conditions taking into account that the residual effluents is very acid (pH=3) is adequate to use like neutralisation agent of the Na2CO3 solution, how result from the experimental data. The use of the lime milk presents the advantage that realise the simultaneous separation of the aluminium and sulphate, respective the acidity neutralisation.

The operation and removal of the sludge resulted at the chemical precipitation in this case with lime milk determine the obtaining of a big quantities of sludge, which settle very hard reason for what from made experimental studies results very clear the efficiency of Na2CO3 using for residual effluent neutralisation resulted in the Al2(SO4)3 obtaining technology.

For those three situations of neutralisation agent we determinate the sediment volumes at three value of pH (6, 7, 8.5) and for the sedimentation times of 30, 220, 400 and 1300 minutes. The results are presented in tables 3.3 – 3.5. TABLE 3.3. The sediment volume obtained in the case of use like neutralisation agent of the 1 N Na2CO3 solution at different times.

The sediment volume [m3 / 1 m3 residual water] Sedimentation

times [min] pH=6 pH=7 pH=8.5 30 1.06 1.05 0.48 220 0.63 0.75 0.30 400 0.55 0.67 0.29

1300 0.54 0.62 0.28

TABLE 3.4. The sediment volume obtained in the case of use like neutralisation agent of the 1 N NaOH solution at different times.


times [min] pH=6 pH=7 pH=8.5 30 0.99 1.04 1.13 220 0.62 0.70 0.53 400 0.60 0.70 0.49

1300 0.55 0.65 0.42 TABLE 3.5. The sediment volume obtained in the case of use like neutralisation agent of the 1 N lime milk solution at different times.


times [min] pH=6 pH=7 pH=8.5 30 0.95 0.97 0.95 220 0.65 0.90 0.80 400 0.61 0.85 0.74

1300 0.60 0.82 0.70 It can be observed that in the sedimentation phase the velocities are dissatisfactory, the precipitate volume which is not compact is big (would need an adding of sludge conditioning adjuvant), we consider that in this situation is not indicated the precipitation decantation. From this reason we determinate the suspensions content in effluent, according to STAS 6953-81 for those three optimum situations. The quantity of total materials in suspension is presented in table 3.6. TABLE 3.6. The quantity of total materials in suspension

Neutralisation agent Parameter Measure Units NaOH Na2CO3 Ca(OH)2

The formed sludge

quantity

mg/dm3 7100 7200 14000

Comparing with the NTPA 002 normative the suspensions quantity is over in all the cases, the limit value admitted being 300 mg/dm3. For those three precipitation agents used at pH=6.5 where established the aluminium residual concentration dependence, respective the dependence of the separation degree function of the precipitation agent nature. The experimental data are presented in table 3.7.


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TABLE 3.7. The aluminium residual concentration, respective the separation degree dependence functions of the precipitation agent nature at pH=6.5

Precipitation agent nature

Aluminium residual

concentration, mg/l

Separation degree,

(%)

NaOH 60.5 95.5 Na2CO3 0.33 99.9 Ca(OH)2 1.8 99.8

From the experimental data we can observe that in all the cases we obtain a separation degree over 90% but in the case when we use as precipitation agent Na2CO3 we obtain the smallest residual concentration of aluminium.

4. Conclusions

In this paper was established the optimum conditions of aluminium removal from residual waters.

In the frame of the experimental study were determinate the parameters of the initial water.

In the situations in which the residual effluents volume is small, the proceeding being discontinuous it’s applied the physical – chemical independent treatment of this.

The precipitation agents used in classic mod are lime milk, sodium hydroxide or sodium carbonate.

The neutralisation process of the effluent must be lead in the pH limits = 6.5 – 8.5 in accordance with the indicators of the waste waters quality evacuated in sewearage system (The NTPA 002 normative). In this situation the residual effluent pH is of 3.08.

Were made neutralisations studies using lime milk, NaOH, and Na2CO3. 100 ml of water samples were titrated from 0.5 in 0.5 ml with 1N concentration alkaline solutions.

For pH values of 6, 7 and 8.5, we determinate the sedimentation velocity of the samples and the sediment volume for different sedimentation times.

Also we determinate the dependence of the aluminium residual concentration, respective the separation degree function of the precipitation agent nature at pH=6.5.

From the neutralisations studies realised for these 3 used reagents result a reaction time comprised between 15 and 30 minutes.

In the case of using of 1N NaOH solution, having in mind that NaOH is expensive we consider the use of this unprofitable.

The use of the lime milk presents the advantage that realise the simultaneous separation of the aluminium and sulphate, respective the acidity neutralisation, but in this case we obtain a big quantity of sludge which settle very hard.

The optimum precipitation agent used for aluminium removal from waste waters is sodium carbonate. The optimum sedimentation time was of 30 minutes and in this case we obtained the smallest residual concentration of aluminium. Comparing with the NTPA 002 normative the suspensions quantity is over in all the cases, the limit value admitted being 300 mg/dm3.

References 1. Roques, H., Fondements Theoriques du traitement chimique des eaux, vol. II, Ed. Technique et Documentation - Lavoisier ii, Rue Lavoisier-F 75384, Paris,1990. 2. Degremont, Water Treatment Handbook, Vol. I, Sixt edition, 1991. 3. Moşoarcă, G., Residual aluminium in drinkable water, Ed. Politehnica, Timişoara, 2004. 4. Negulescu M., Quality waters protection, Ed. Tehnica, Bucuresti, 1982. 5. Tehobanoglous, G., Franklin L. Burton, Wastewater engineering - Treatment, Disposal and Reuse, McGraw-Hill, Inc. New York, 1997. 6. Gruzado, A., Self-purification processes in natural waters, Chişinău, 113, 1995. 7. Burtică, G., Inorganic salts, Ed. Politehnica Timişoara, 2002 8. Wassmann, P., Journal of Marine Systems, Elsevier Science, 23 ,1,1999. 9. Humenick, M.J., Water and Wastewater Treatment, Marcel Dekker, New York, 1977. 10. Tardat-Henry, M., Sciences et technique de l`eau, Vol. 22, No. 4, 1989.

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